PAPERS PRESENTED AT THE INTERNATIONAL GEOPHYSICAL SYMPOSIUM AND TECHNICAL EXHIBITION
(BUCHAREST, OCTOBER 22-25, 1995)

TRENDS IN EXPLORATION TECHNOLOGY — A REVIEW PAPER MAXIMISING EXPLORATION REWARDS

G. R. PARRY, MICHAEL J. PINK
Shell Romania Exploration BV, 155 Victoriei Avenue,
Block D. Floor 4, Bucharest, Romania

Oil remains the world's major source of energy. It accounts for almost 40% of primary energy demand, while coal and gas account for some 27% and 21%, respectively. The occurrence of hydrocarbons is widespread and reserves of oil have been discovered in some 90 countries. However, these reserves are unevenly distributed with just four Middle Eastern countries containing over half the current 1000 billion barrels of total world's reserves. These reserves now correspond to some 45 years of supply at current production levels, as compared to about 30 years in 1973.
The challenge for the oil industry is therefore to identify strategies. Successful strategies for maximising the upside potential value of the discovered reserves and for optimising future investments in both exploration and exploitation activities will rely increasingly on:
- focused application of advanced technologies, often enhanced by proprietary techniques or design, to find additional reserves;
- integrated exploration and production activities, based on a clear vision that a cross-- disciplinary approach must prevail from field discovery to abandonment;
- "innovative thinking" to reduce the exploitation costs of particularly small discoveries, as well as of existing fields;
- continuous dialogue with governments to arrive at realistic and stable tax and fiscal regimes, to enable the industry to take up the required technological challenges in a climate of sustained economic growth with confidence in long-term investment.
This paper outlines key aspects related to these challenges and illustrates, with selected examples, how Shell is already addressing some of them.
Key words: oil, exploration, strategies, trends

AN OVERVIEW OF EXPLORATION RISKS
IN THRUST BELTS AND FORELAND BASINS

J. A. COOK1, J. L. COLEMAN1, M. DAVIS1, L. EGGERS1, K. ENZOR2, M.D. TASKER3
1AMOCO Worldwide Exploration, 501 Westlake Park Boulevard, Houston, Texas 77079-2696, USA
2AMOCO Romania
3AMOCO Italy

Hydrocarbon exploration within thrust belts and their associated foreland basins is definitely challenging and can be very rewarding. But to many, it can also be a very frustrating and disappointing endeavor. What "works" in one thrust belt may not work in another, and what "worked" ten years ago may not work today. With the new technologies available to the geophysicists (i.e., high capacity digital recording systems, 3D and pre-stack depth migration), the temptation to focus on resolving the detail structural complexities within thrust belts must be tempered with the need to fully understand all components of the petroleum system in order to be economically successful. Assessment of these components (exploration risks) requires, at first, an understanding of one's perspective or focus: wildcat, frontier play definition, or field development drilling. Additionally, one should determine one's commitment to make the play. Thrust belts do not give up their treasures easily.
A common list of exploration concerns will includes source rock distribution and effectiveness; timing of source rock maturation and migration of hydrocarbons, and identification of migration pathways; trap timing, definition, and integrity; reservoir volume and performance; fluid type (oil, condensate, gas, or other); the assorted factors governing profitability. All are important and need to be addressed in both frontier and mature exploration. The emphasis of an explorer involved in frontier exploration will probably begin at the source rock end of this list, whereas the exploiter involved in mature exploration and field development might begin at the other.
Understanding the conditions for effective source rock distribution, generation, and migration is critical for the analysis of hydrocarbon potential in frontier basins. These source rock conditions may expel hydrocarbons into several migration paths at different times during thrust belt and foreland basin development, leading to an even higher number of potential traps. The usefulness of regional seismic transects is discussed.
As one approaches the prospect drilling stage, emphasis is placed on a slightly different set of elements: trap definition (i.e., seismic imaging, structural style, trap type), prospect integrity (time–depth accuracy), and volume estimation (reservoir heterogeneity and performance). Detail seismic surveys become essential for the assessment of these more localized elements.
In general, exploration risk will increase toward the more highly deformed portions of thrust belts and foreland basins. However, attention to detail in all aspects of the exploration phase can improve the potential for economic success, whether it be plate tectonic reconstruction or meander-belt point bar sandstone reservoir modeling. Understanding and, if necessary, compensating for the inadequacies of the various tools of exploration are likewise, essential. We conclude that exploration in thrust belts and foreland basins is, in reality, no different than exploration in any other petroliferous basin. If the basic ingredients are present, then oil and/or gas will be found. Whether it will be profitable will depend upon local and regional market conditions and concession obligations.
Key words: hydrocarbon exploration, exploration risks, thrust belts, foreland basins.

CONVERGENT ASPECTS IN ENGINEERING GEOPHYSICS
AND ENVIRONMENTAL GEOPHYSICS

G.P. MERKLER1, D.K. HANNICH2
1 University of Karlsruhe, Kaiserstrasse 12, 76 128 Karlsruhe, Germany
2 Federal Institute for Research and Waterway Engineering, Karlsruhe, Germany

It is increasingly necessary to develop industrial and hydrotechnical engineering structures under unfavourable geological and geotechnical conditions. Furthermore, it becomes more and more important to build effectively and economically and to find optimal solutions for a long-term steady function of the structures. This demands comprehensive information on the structural situations and engineering parameters of local site assessments by areal investigations of the sites and their in situ petrophysical parameters. However, this requires the use of geophysical techniques. During the last two or three decades, international applied geophysics has systematically developed new possibilities and methods for site investigations to determine petrophysical parameters in situ, as well as to observe the system building and site. These new methods have come to be known as engineering geophysics. Geophysical methods make it possible to develop areal models of subsurface conditions of building sites, to quantify relevant in situ engineering parameters as well as to analyze the long-term behaviour of the buildings, which are influenced by internal or external factors.
Because of the increasing pollution of the environment brought about by complicated problems, in connection with soil and groundwater contamination, engineering geophysics has been applied more and more often in seeking solutions to these problems. And so, an independent branch of engineering geophysics developed, that deals specifically with environmental problems – the environmental geophysics.
Relatively low depths of penetration, high resolution of measurements, taken in or near cities, often with an accompanying high noise level, characterize not only the scope of engineering and environmental geophysics, but also the common difficulties.
The detection and definition of polluted and contaminated areas, the determination of preferred migration paths and the exploration of the geological subsurface of existing or future waste site deposits, can be seen as the fundamental and chief tasks of environmental geophysics.
In the present paper there will be discussed goals, possible solutions and characteristic difficulties and there will be defined the convergent aspects in engineering and environmental geophysics. The two independent branches of applied geophysics, i.e., engineering geophysics and environmental geophysics, should both be considered as aids to the geosciences, structural engineering and environmental research. The key to a successful application of engineering or environmental geophysics is the conversion of the results of geophysical measurements and the meaning of the determined petrophysical parameters of the underground into the language of structural engineering and into geotechnical and hydrogeological terms.
Examples from both branches will be given and the above-mentioned convergent aspects in measuring techniques and interpretational difficulties will be discussed.
Key words: engineering geophysics, environmental geophysics.

INTERPRETATION OF GRAVITY FIELD
AT THE BANAT AREA – SERBIA

MIROSLAV STARCEVIC
Mining and Geology Faculty, Geophysics Department, Djusina 7,
11000 Belgrade, Yugoslavia

The Banat area belongs to the south-east edge of Pannonian basin with extension to the east from the Tisa and Danube rivers. Morphology of this area distinguishes itself by plane relief to mean heights of about 80 m.
The Bouguer gravity map for this area is presented and interpretation has been performed to obtain a regional geological view of the area, at first to establish the depths of Neogene sediments and main faults.
Gravity minima indicate the areas with very thick Neogene sediments. In north and middle Banat, this thickness amounts to more than 300 m, while in south Banat, the sediments thickness is less than 1,000 m. In the south-east of Banat, the metamorphites appear to the surface (the Vrsac mountains). They belong to Serbo-Macedonian massif. The Bouguer anomalies in that area are characterized by gravity maxima.
To the west of the Vrsac mountains there is a remarkable negative anomaly, in the south-north direction, starting from 60-80 km on the X-axis. This anomaly is caused by a Neogene trench elongating toward south in Serbia and toward north in Romania. The depths of sediments in this area are between 1,000 and 3,000 m.
The north Banat is also characterized by negative anomalies, especially in the vicinity of the Romanian boundary. The thickness of Neogene sediments in this area is sometimes more than 4,000 m. This is an area which has a great number of oil and gas deposits.
Belgrade area distinguishes itself by a positive gravity anomaly caused by an uplifted block of a triangular form, situated on the south part of the area.
The positive gravity anomalies in the central part are caused by the eastern edge of Fruska Gora mountain.
The form of gravity field at the Banat area shows that the geological features of the Neogene sediments base are not simple. In some parts, for example in north and middle Banat, these sediments are lying directly on the Paleozoic rocks and in some areas, especially in south Banat, the thickness of Mesozoic sediments is significant.
The tectonic features of the Banat area are very complicated, particularly in the south part. This is due to the collision of three great tectonic units: Dinarides from the west, the Serbo-Macedonian unit from the south-east, and Carpatho-Balkanides from the east. These tectonic units can be clearly visible on the Bouguer gravity map observed on the great area (main part of Serbia).
Key words: gravity, Bouguer map, interpretation, Banat, Serbia.

ON THE INTERPRETATION OF THE BOUGUER MAP OF ROMANIA

VLAD ROSCA1, JUSTIN ANDREI1, CONSTANTIN CALOTA2, ADRIAN NICOLESCU1, MARIUS VISARION1
1 Geological Institute of Romania, 1 Caransebes St., 78344, Bucharest, Romania
2 Bucharest University, Romania

An attempt to interpret the Bouguer map of Romania by correlating this map with a tectonic-structural map has been made.
The main features of the Bouguer image of Romania are closely related to the Carpathian Arc, since the axis of a dominant low crosses the whole country together with this arc. Relative heights with amplitudes definitely smaller than those of the main low are to be outlined on the Black Sea coast and along the Danube river, as well as in the western and eastern border regions of the country.
The eastern high area represents the foreland of the East Carpathians. The very Carpathian low is sited on the flysch nappes and a strong gradient separates this low from the foreland high. The model proposed for explaining this gravimetric relation takes into account the thickening of the crust by the overthrusting of a folded cover over a foreland basement as well as the result of an isostatic process. Accordingly, a 6-7 km thick nappes cover lies on a westward dipping slope of the foreland and the crust itself evolves from a 40 km thickness in the eastern border area to 46 km in the folded region and shows a drastic thinning beneath the Neogene volcanoes.
For the South Carpathians, the Bouguer anomaly is not very different from that of the East Carpathians. The foredeep is represented by a most remarkable low, that includes the least value observed on the Romanian territory. Northward of this low, a relative high, developed mainly in the central area of the ridge corresponds to this relief feature and shows the presence of the dense crystalline and Mesozoic rocks at the surface.
In the Apuseni Mountains the Bouguer image is a rather complicated one. At north, two major lows submeridianly orientated are both representing large plutonic plugs. The west one is a banatitic granodiorite and the east one is a Paleozoic granitoid.
In South Apuseni Mountains, Jurassic basic igneous rocks are the source of a major high extended along the Mures valley. Arguments for an obduced position of these rocks have been revealed by modelling.
On the Black Sea coast, viz. in Dobrogea, the highest Bouguer value for the Romanian territory is to be noticed. It pinpoints a central high corresponding to a Precambrian high density green schist horst.
For a general view of the mass distribution inside and outside the Carpathian Arc, a profile with a WNW-ESE orientation has been analyzed by correlating potential field data with deep seismic sounding data and heat flow data.
Key words: Bouguer map, interpretation, Romania.

ON THE NORMAL GEOMAGNETIC FIELDS WITHIN CONFINED AREAS. THEIR MODELLING AND UNDERSTANDING

LUCIAN BESUTIU, MARIUS ROGOBETE
Geological Institute of Romania, 1 Caransebes St., 78344 Bucharest, Romania

Despite their obvious advantages, the geomagnetic reference field models (IGRF or DGRF), systematically adopted by IAGA, still remain less used in the exploration practice, mainly due to computation inconvenients and lack of representativeness for the studied areas.
The basic theory of normal geomagnetic fields (NGF) modelling within confined areas belongs to Kautzleben. Whatever the adopted method, such NGF models will have some basic defficiencies: the missing of the physical support, the determination of the Taylor series coefficients under false circumstances (i.e. flat surface hypothesis), the "smoothing" degree mastered by the "personal factor". Besides, the achieved models for the adjacent areas will not fit each other.
Starting from data sets provided by the secular variation (SV) network and the national magnetic control network (NMCN), for various epochs, NGF models based on both second-order polynomials and harmonic analysis were performed and analyzed.
Taking into account the meaning of the geomagnetic models provided by spherical-harmonical functions, attempts have been made to understand the images offered by the second-order polynomials NGF models. The analysis has been performed by structuring the dipole field (offered by Lagrange polynomials series up to n=1) and quadrupole field (n=2) from the second order polynomials NGF models.
Comparisons made for the Romanian territory have proved that second order polynomials models, used so far in our country, mainly reflect the geomagnetic quadrupole field.
Due to the neglecting of the Earth curvature, increasing errors through the national borders appear, making unsuitable the use of those models for such areas. Not taking into consideration the vertical gradient of NGF may lead to errors up to 20-30 nT/km.
Concerning the long wavelength geomagnetic anomalies gathered by subtracting the dipole field for the epochs 1967.5 and 1980.0, a westward drift of about 0.24°/year has been pointed out.
Key words: geomagnetism, normal field, westward drift

A STRUCTURAL AUTOMATIC ANALYSIS OF
THE GRAVIMETRIC AND MAGNETIC DATA OF NW ROMANIA

RADU CRAHMALIUC, ANCA CRAHMALIUC
Geological Institute of Romania, 1 Caransebes St., 78344 Bucharest, Romania

The paper presents a new manner of studying and interpreting the gravimetric and magnetic data in an area of great geological and structural complexity. A special visualization program, based on the vertical and horizontal derivative properties of the potential fields has been used. This program points out automatically the discontinuities and anomalous peaks.
The computed structural analysis represents a modern method of interpretation, which displays the transformed images of the initial gravimetric and magnetic maps, with the help of GMI-Pack, a package of interactive graphic interpretation programs, designed by the French companies BRGM Orléans and TOTAL.
Because of the regional geological structural specific features and methodological reasons, the area referred to was investigated within three perimeters: Gutai Massif (70 x 40 km), Oas Mountains (51 x 32 km) and the NE extremity of Pannonian basin zone (83 x 38 km).
Within each of these areas, the gravity and aeromagnetic maps were digitized in a square grid of 1 km edge and stored in ASCII files. These primary data served to the processing of vertical and horizontal gradient maps. The transformed images together with Bouguer and total reduced-to-pole anomaly provided the entry data for structural automatic analysis. The method allows the outlining of linear discontinuities and anomalous alignments classified on intensity degrees. In the plotted images used for the drawing of some sketches the following elements can be recognized: discontinuities with density and petromagnetic contrast meanings, in fault zones, geological or structural boundaries, peak surfaces and their anomalous axis, representing areas of maximum or minimum intensity.
The outlined structural sketches were useful to the already existent hypothesis of reinterpretation, correlated with direct geological data and with those provided by other geophysical investigation methods. The structural map, obtained through regional data complex interpretation, reflects, in a semi-quantitative-qualitative way, the tectonic-structural elements of the basement and the specific features of Neogene magmatic structures.
Gravimetric, magnetic and seismic data analysis highlights:
– linear discontinuities, corresponding to the main fractures system;
– anomalous zones according to the block elevation;
– an anomalous axis, according to EW-trending horst- or graben-type structures.
Magnetic data, correlated with geological and gravimetric ones, reveal:
– the presence of some major plutonic bodies, providing regional aeromagnetic anomalies;
– discontinuities, representing petromagnetic limits and growing thickness of the structures;
– local peaks, indicating igneous bodies (necks, subvolcanic bodies, dikes).
The use of this structural automatic analysis in the Baia Mare mining zone and NE Pannonian basin determined the performing of transformed images and a new way of capitalization of the structural elements, partially outlined by other interpretation methods. The reevaluation of some structural parameters, directly correlated with polymetallic mineralized zones or specific accumulation areas of oil, gas or thermal waters, led to the objective growth of knowledge degree and also precise economic implications.
Key words: potential fields, transformed images, interactive graphic interpretation programs, tectonic-structural elements, Baia Mare mining zone.

ON THE ACHIEVEMENT OF THE COMPOSITE GEOMAGNETIC MAPS. THE MAGNETIC MAP OF ROMANIA

LUCIAN BESUTIU, GEORGETA BESUTIU
Geological Institute of Romania, 1 Caransebes St., 78344, Bucharest, 32, Romania

The paper deals with problems related to the achievement of the composite geomagnetic maps, offering the example of the Romanian vertical component intensity scalar ground magnetic map (RVCISGMM).
RVCISGMM has been compiled on the basis of about 60,000 magnetic stations belonging to various magnetic surveys conducted for more than 30 years (from 1950 until 1982). It has been achieved without the support of any national magnetic control network (NMCN). Consequently, there have been no secular variation (SV) corrections applied to the data. Map sheets at the scale 1:200,000 have been performed, step by step, by simply linking their base stations and visually fitting the contours at their borders.
Studies dedicated to the SV influences of the Romanian territory have led the authors to the following remarks:
(i) SV has considerable values and different time/space evolution within the Romanian territory;
(ii) there are pointed out singular points in its evolution, meaning that it would not be a suitable way to attempt to remove its influences by using a time-continuous function for longer periods of time;
(iii) the only way to avoid such effects would be to refer all the survey data to a datum provided by a national magnetic reference network (performed in a short-enough period of time not to be distorted by SV phenomena).
As the data set provided by the SV studies network was too sparse for this purpose, during the years 1983-1984 a national magnetic control network (NMCN) was accomplished. Then old stations belonging to each geomagnetic survey from RVCISGMM were linked to the base stations of that magnetic control network.
By comparing the geomagnetic level provided by the base stations of NMCN with the datum of RVCISGMM, important deviations were pointed out, which required the re-drawing of RVCISGMM by taking into account the necessary SV correction.
Key words: composite geomagnetic map, secular variation, correction.

EXTENSION OF RADIOMETRICAL INTERPRETATIVE MODELS USING OTHER GEOPHYSICAL METHODS

MIHAI MAFTEIU, ADRIAN PRIPOAIE
R.A.M.R. – Sectia Prospectiuni,
51 Atomistilor St. MG-25, 76911, Bucharest, Romania

The "Cinderella" role of the radiometric methods from the viewpoint of the depth penetration is wellknown around the geophysical world. Because of this, some radiometric anomalies situated near the subsurface do not appear in the geophysical image. Passing over this hindrance, the authors have tried to improve the interpretation model obtained from the radiometric data by using some other geophysical methods. Moreover, in addition to the radiometric methods depending on the type of signal reception, related to the radiation type and also to the investigation depth, there were used a series of indirect geophysical prospecting methods (resistivity and magnetics). After all, the authors have been interested in emphasizing some favourable structures instead of elaborating an inventory of radiometric anomalies, without any geological significance.
The results obtained during the last several years of investigation allow us to consider that this complex of geological and geophysical prospecting methods create a favourable informational background for geological interpretation. All these methods have been successfully used in some specific geological environments and in various types of radiometric anomalies.
The network optimization and the choice of specific survey methods, related to an accurate geological hypothesis, represent, without doubt, very important tools in the field investigation. They lead to an efficient interpretative model.
Some case histories are presented in this paper, showing both the technical investigation and the possible interpretative model.
The aim of this endeavour is the opening of new perspectives for the radiometric survey, towards a deep investigation in radioactive structures. In addition, the geophysical information of various investigation levels is filled in with more appropriate prospecting and interpretation methods. All these will have a significant impact on future field development plans.
Key words: radiometry, anomaly, interpretation.

DEEP STRUCTURE OF THE LITHOSPHERE
IN RHODOPE AND SERBO-MACEDONIAN MASSIFS
AND ITS RELATIONSHIP TO THE METALLOGENIC ZONING

G.N. SIDERIS1, S. NICOLAOU1, A. ANGELOPOULOS1, D. IOSIFOV2
1 Institute of Geology and Mineral Exploration, 70 Messoghion St., 11527Athens, Greece
2 S.R.I.M.R. – Bulgaria

One of the most significant norms for the distribution of the endogenous ore deposits, in the earth crust, is the determination of the relationship among the various scales of the developed groups of these deposits and the types of their structures.
This is expressed through the tectonic features of the various stages and grades which are responsible for the creation not only of the regional but also of the local structures. This norm derives from the "basic law" of the geological procedure of the differentiation of the earth substance, determining the zoning distribution of metallic elements throughout their various levels.
The results of this study came out of a large scale multidisciplinary geophysical exploration program approach with gravity, aeromagnetic, aeroradiometric, deep seismic reflection as well as of the results of shallow seismic activity, in the area under study, which have been obtained both in Bulgaria and Greece.
When we compare various types of ore deposits on the basis of statistical analysis we conclude that their spatial growth is related to the thickness of the earth's crust, which is expressed as follows:
a) At the eastern Rhodope massif with a mean thickness of about 35 km the post-Alpine ore deposits have a polymetallic composition (Pb-Zn) with secondary ores the so called precious metals (for instance, Au). The polymetallic ore deposits are placed at volcano-tectonic three-dimension-type structures, like caldera, etc. Here we can also find Cr ore deposits which are connected with serpentinite type bodies.
b) At the southern side of the Serbo-Macedonian massif (mean thickness of about 36 km) there have been mainly developed polymetallic ore deposits like Pu, Zn, Cu, Au. These deposits have the same geological and ore deposit peculiarities as those of eastern Rhodope massif. In this area we can distinguish ore deposits of Devonian age, within amphibolite- and chloride-type schists.
c) A larger potential of ore deposits has been discovered within the zones of eastern Rhodope massif as well as within the zones of the western Rhodope and Serbo-Macedonian massifs. However, in the ore deposit zones of central Rhodope massif and the zone of Osogovo-Blagodatska (in the Bulgarian side) significant Pb-Zn ore deposits have been discovered. They frequently include Au at economic concentrations. These zones have a characteristic variation of the thickness of earth's crust, their mean values ranging from 42 to 48 km.
d) In western Rhodope massif, where we meet intense and dense granitic magmatism and where the thickness of earth's crust is ranging from 42 to 50 km we meet rare earth- and radioactive-type deposits. Within their central part there have been developed quartzitic veins with wolframite and serisite (Grintsavitsa and Babiak). At the periphery of this area fluorite ore deposits have been located (Mihalkovo, Palat and Kozium, etc.). On the boundaries and western Rhodope massif some uranium deposits have been also discovered (Smolian, Dospat, Elesnitsa, Iskir, etc).
We must stretch our attention to a few metallic ores such as Mo, Au, Ag, and Cu which have been discovered as secondary ore minerals and are practically located in the whole area under study.
From the practical point of view, the most significant characteristic proved by our geophysical exploration activities, for the localization of economic ore deposits, is the thickness of the earth's crust. This is the most important argument which proves that the regional metallogenic zoning of the Rhodope and the Serbo-Macedonian massifs is directly correlated with the thickness of the earth's crust.
Key words: deep structure, metallogenesis, ore deposits, the Rhodope massif, the Serbo-Macedonian massif.

TECTONIC CONFIGURATION OF CARPATHIAN FOREDEEP ILLUSTRATED BY SEISMIC PROFILES
AND HYDROCARBON PROSPECTS

OPREA DICEA
" Prospectiuni" S.A., 20 Coralilor St., 78449 Bucharest, Romania

The most researched seismic zone from the Romanian Carpathian belt was the external part, namely, the foredeep zone.
According to Sandulescu (1984) the inner limit of the foredeep is represented by very heterogeneous elements: the Peri-Carpathian Fault, to the north of Trotus Valley, Casin-Bisoca Fault, south of Trotus Valley, and, westward from the Slanic Valley (of Buzau), the limit is marked by an erosional contour of Sarmato-Pliocene formations.
On the north side of Trotus Valley, the Peri-Carpathian Fault is well documented both tectono-stratigraphically and by seismic profiles. Along this line the lower molasse formations overthrow on upper molasse formations.
Beneath the overthrust plane, the Sarmatian formations are closed.
On the south side of Trotus Valley, the Peri-Carpathian Fault is hidden below the upper Sarmato-Pliocene formations. In this case, the inner limit of the foredeep is represented by the Casin-Bisoca Fault. South and south-westward from the Slanic Valley the Sarmato-Pliocene molasse cover, all the Mio-Pliocene zone and Getic Depression and, also, the main tectonic lines are hidden.
Besides the detailed structure, the main tectonic lines were evidenced. Peri-Carpathian Fault has a characteristic configuration. Many other structures are revealed and onlapping, toplapping in downlapping configurations are taking into account for subtle traps delimiting.
By correlation of seismic profiles, many prospective traps were delimited.
Key words: Romanian Carpathians, foredeep zone, seismics, hydrocarbon prospects

NEOGENE STRUCTURAL EVOLUTION OF THE EASTERN CARPATHIAN BEND AREA, ROMANIA

KEITH ENZOR2, GARY WEIR1, MARGARET DONELICK1
GABOR TARI1, MIHAI STEFANESCU2, IONEL OLTEANU2, ALEXANDRU BENZAL2
1 AMOCO Corporation, R.M. 2.368, 501 Westlake Park Boulevard Houston, Texas 77079-2696, USA
2 AMOCO Romania

Details of the Neogene deformation history in the Carpathian Bend Area have been derived from 600 kilometers of recently acquired 2D seismic data, surface geologic maps, limited subsurface information, and surface geologic traverses. Integration of these datasets has led to an improved understanding of the depositional and deformation history of the shallow Neogene section.
In the Eastern Carpathian Bend, a major change in structural trend occurs between the north-south trending Eastern Carpathians and the east-west trending Southern Carpathians. Several thrust sheets, including the Tarcau, Marginal Folds, and Sub-Carpathian Nappes, have undergone at least three phases of deformation during the Neogene. These include the New Styrian phase (Middle Miocene), the Moldavian phase (Late Miocene), and the Wallachian phase (Pliocene-Pleistocene). These phases have been dated on the basis of widespread unconformities, which are regarded as tectonically enhanced sequence boundaries.
During the New Styrian phase, east-southeast to southeast directed thrusting continued in the Tarcau and Marginal Folds thrust sheets. During this time, the deformational front propagated into the Sub-Carpathian thrust sheet. This produced a low-amplitude wave train of folds across the area and created sub-basins in the broad intervening synclines. This tectonically induced topographic relief restricted deposition of some Middle Miocene units to the synclines. These sub-basins were transported as "piggy-back" basins during subsequent phases of deformation.
During the Moldavian phase of deformation, east-southeast to southeast directed thrusting continued as the Sub-Carpathian thrust sheet was transported onto and across the Moesian Platform. As observed on seismic data, this produced continual folding and growth of structures within the thrust sheets. Paleotopography continued to restrict deposition of some Late Miocene units to the "piggy-back" basins in the synclines. Late Miocene syntectonic units thickened rapidly into the synclines, recording the growth history of the adjacent anticlinal structures. Erosional truncation of Late Miocene and older deposits then occurred, and younger stratigraphic units were deposited uniformly across most of the structures in the area.
The Pliocene Wallachian phase of deformation was the last major event affecting structures in the Carpathian Bend Area. Late Miocene and younger units were deformed along the trend of pre-existing structures. Folds in the Neogene section became even more pronounced, as large unconformities related to this event can be observed on the seismic data. Kinematic data suggest a north-south shortening during this phase. The Sub-Carpathian thrust sheet north in the Bend Area exhibits north-northeast oriented strike-slip faults in the Neogene section. These faults become dominantly dip-slip to the southwest as the thrust sheets curve through the Bend Area. The Lower Miocene Salt Formation could have acted as a secondary detachment surface during deformation. Therefore, movement along these faults may be restricted to the section above the Salt Formation.
Key words: Bend Area, Eastern Carpathians, evolution, Neogene.

RESULTS OF A SEISMIC INTERPRETATION
IN THE GREAT HUNGARIAN PLAIN

PETRU M. KISS-PARCIU
Eotvos Lorand Geophysical Institute of Hungary, Columbus U. 17 23
1145 Budapest, P.O. Box 35, 1440 Budapest, Hungary

A complete study of the structural development has been carried out in the Tiszaföldvár-Kiskunfélegyháza area. The study represents a part of an integrated structural modelling of the Pannonian basin and is based on the interpretation of 2D reflection seismic time sections, lithological data from boreholes and some well-logs.
Several TWT structural maps have been generated: Lower Cretaceous Top, Base Neogene, Middle Miocene Top (Base Pannonian). In the Upper Miocene-Pliocene megasequence, three third-order time stratigraphic sequence boundaries were mapped. Maps, interpreted seismic sections in combination with regional sections prove the polyphase tectonic evolution of the area.
The tectonic phases are the following: (a) pre-rift tectonism: (1) Mesozoic thrust;
(b) synrift tectonism: (2) Middle Miocene extension; (c) post-rift tectonism: (3) Upper Miocene compression, (4) Pliocene extension, (5) Pliocene-Quaternary wrench.
The Mesozoic thrust faulting is present in the south-western part of the area, where basement-involved thrusts result in the overthrusting of Paleozoic metamorphics over Cretaceous sediments. The age of the faulting is Upper Cretaceous (Turonian) but it is possible that many of the thrust surfaces were reactivated during the Miocene. Thrust direction is NW-oriented, and in this part it probably represents the contact between the Mecsek and Villány-Bihar subunits within the Tisza unit. Recently, many studies performed in the Great Hungarian Plain have demonstrated the connections between the Apuseni Mountains nappe system, the Villány-Bihar subunit or nappe, and the imbricate structure of the Villány Mountains.
The main activity of the tectonism, as in fact in the whole Pannonian basin, was in the Middle Miocene. This phase was an extensional one and it was represented by planar or listric normal faults. Earlier tectonic events were often reactivated as normal faults. In the eastern part of the area, where the Paleocene flysch is present, strike-slip faults (positive flower structures) are frequently associated with normal faults.
The tectonic movements have continued on a smaller scale in the Upper Miocene, when a part of the faults generated in the earlier tectonic phases were reactivated as reverse faults.
A few listric normal faults occurred in the Pliocene extension phase.
In the non-marine post-rift sedimentary fill (Upper Miocene-Pliocene), based on the seismic stratigraphic analysis and well control, three third-order sequence boundaries were identified. The sequence stratigraphic profiles and the mapping of the sequence boundaries permitted us to determine the age and size of the post-rift tectonic events, especially the wrench faulting. Two Pliocene-Quaternary strike-slip fault zones were detected, with a SW-NE direction and a left-lateral character for the first and a right-lateral character for the second. Their horizontal displacement is estimated at 5-8 km. The first zone is linked with the flysch, being a reactivation of the Middle Miocene wrench system. We mention the fact that some hydrocarbon traps could be associated with these tectonic zones.
Key words: seismics, interpretation, Great Hungarian Plain.

THERMAL AND BURIAL HISTORIES OF SOURCE ROCKS
IN THE TRANSYLVANIAN AND PANNONIAN DEPRESSIONS

CRISAN DEMETRESCU, GABRIELA POLONIC, MARIA ANDREESCU, MIREL ENE
Romanian Academy, "Sabba S. Stefanescu" Institute of Geodynamics,
19-21 J. L. Calderon St., 70201 Bucharest, Romania

Thermal and burial histories of sediments are key factors in understanding the evolution of the organic matter which is responsible for the generation of hydrocarbons.
The tectonic evolution of the two largest intra-Carpathian molasse depressions of Neogene age, namely, the Pannonian (PD) and Transylvanian (TD) Depressions, is comparatively discussed, and thermo-mechanical models of the lithosphere and sedimentary cover evolution are presented, based on subsidence data from boreholes and heat flow data.
As a result of different tectonic evolutions, the two units are characterized by contrasting values of parameters defining the lithosphere, namely: the Pannonian Depression is characterized by a high heat flow (80-100 mWm-2), a hot and thin lithosphere (70-80 km), a thin crust (25-27 km), whereas the Transylvanian Depression is distinguished by its low heat flow (30-60 mWm-2), a normal lithosphere thickness (~100 km), and an abnormal crust of 30 km in the central area and 34-36 km at margins, with an attenuated upper crust.
The tectonic subsidence of the basement and the burial history of the sedimentary formations were studied by backstripping and decompaction of sediments for borehole data in the Romanian sector of PD and in TD. The results show a quasi-continuous subsidence since the Badenian to Present, in case of PD, reaching 1,200-1,400 m, and a subsidence beginning at the end of the Early Miocene, interrupted by uplift episodes in case of TD. The tectonic subsidence reached about 1,500 m and stopped 5.1 Ma ago in TD.
Thermal modelling of tectonic processes characterizing the two Neogene depressions, namely, the pre-Badenian instantaneous extension of the lithosphere, followed by thermal subsidence, in case of PD, and three extensional episodes – in the Senonian, at the Eocene/Oligocene limit and in the Badenian –, followed by subsidence, uplift and erosion episodes, in case of TD, allowed the evaluation of thermal conditions of kerogen evolution in various sedimentary formations and the working out of kinetic models of kerogen degradation and of the resulting hydrocarbons.
Key words: source rock, Transylvanian Depression, Pannonian Depression, burial, thermal evolution.

SEISMIC AND GRAVITY DATA FOR VARIOUS GEOLOGICAL FEATURES IN THE EAST SREDNOGORIE REGION

EKATERINA SOULEVA, RAINA IVANOVA, KAMELIA DIMITROVA
Geology and Geophysics Corporation, 23 Sitniakovo Avenue, 1505 Sofia, Bulgaria

The territory of Bulgaria is composed of two main tectonics structures, namely, the Moesian platform and the Alpine-Himalayan belt. Sredna Gora structural zone is a part of an extremely fractured and declining (in the earth's upper crust) area and to the north and south it is bordered by deep fractured structures – the Balkan Fracture and the North Strandja Flexure. This zone is characterized by rather complex combinations of Paleo-volcanic formations and folds deposited at a later time. An analysis of the bulk density and magnetic susceptibility of the rocks reveals that they vary considerably in the different lithological types. This is probably due to a great diversity in the composition of rocks and to the extent of their metamorphism, tectonic deformation, evaporation, etc. Sedimentary rocks have the lowest bulk density and intrusive rocks have the highest one. The Upper Cretaceous volcanic main and ultramain materials – andesites, andesito-basalts, basalts, etc. – which filled up one considerable part of the east of East Srednogorie region were classified as not having oil and gas potential.
It is considered that Triassic, Jurassic and Cretaceous sediments are possible oil and gas stratigraphic units with a proved industrial value in the Moesian platform and Fore-Balkan zone. The existence of oil and gas shows in the region of Balkan ore basin (40 km to the west of the region of interest) and of those near Varbista (to the north of the region) provides a possibility to divide them into two groups – one from the sediments of Jurassic and another from Senonian and Turonian. If we accept that, independently of the volcanic centers in Late Cretaceous, Triassic, Jurassic and Cretaceous sediments preserve their potential, then the observed anticline structure according to the correlation of the two crossing seismic lines is of a great interest. The interpretation of the instantaneous seismic amplitude and phases enables us to find out the fluid contacts reflections under the arch of the anticline structure. The dependence of a porous material upon the fluid contained in the pore spaces has been studied both theoretically and experimentally. The represented velocity information for the crossing seismic lines shows that the sand saturated with gas from the hypothetical gas and oil trap has a lower interval velocity and the contrast is considerable. The detection of gas-saturated sand due to the differentiation of velocities in the represented geological setting where the anticline structure is shallow – the fluid contact is approximately at a depth less than 2,000 m is quite easy. The interpretation of seismic and gravity data over the geological features of anticline and cryptobodies provides a possibility to identify potential sedimentary complexes in the proximity of volcanogenic complexes.
Key words: seismics, gravity, interpretation, Srednogorie, Bulgaria.

STRUCTURE AND EVOLUTION OF SOUTHERN
PRE-ADRIATIC DEPRESSION SEDUCED FROM WELL
AND SEISMIC DATA INTERPRETATION

FATMIR FEZGA, REXHEP KOSI, TRIFON SOTA
Oil and Gas Institute, Fier, Albania

The Pre-Adriatic Depression extends on about 150 km in a North-South direction in the western part of Albania. We describe in this paper the southern part of the Pre-Adriatic Depression or the Panaja Region.
The combination of drilling results with multichannel seismic reflection data has helped us to understand the structural model of terrigenous deposits in the southern part of the Pre-Adriatic Depression. So, from the interpretation of seismic sections, three anticline structures are delimited: Zvernec Structure, Panaja Structure, and Babica Structure. All these structures are overthrusted from east to west and separated from each other by listric faults.
Finally according to all data available and the stratigraphic analysis, we have vdeduced that sedimentation of the terrigenous deposits in the southern Pre-Adriatic Depression began in the Lower Miocene. At the end of Burdigalian an East-West motion occurred. This motion is associated with some thrust faults and three anticline structures formed in this time (Zvernec Structure, Panaja Structure and Babica Structure). The general trend of thrust faults is SE-NW and, in addition, the thrust from East to West is represented as a common feature of southern Pre-Adriatic Depression. During Seravalian time these anticline structures were covered by the sea. During the Messinian time the eastern part of the studied area was elevated and came over the sea level. During Pliocene the sea covered all Panaja region and Pliocene sediments were deposited transgressively over the Messinian Sequence. At the end of Pliocene the East-West motion was reactivated and this tectonic event affected all sediments from Tortonian to Pliocene.
Key words: structure, evolution, seismics, well data, Pre-Adriatic Depression.

INTERPRETATION OF SEISMIC DATA FOR EXTERNAL ALBANIDES

VILSON BARE, JANI SKRAMI
Oil and Gas Institute, Fier, Albania

This paper briefly deals with the seismic contribution in the study of geologic structures in the External Albanides and of relationships between tectonic zones. There are three external tectonic zones: Kruja, Ionian, and Sazani. The Pre-Adriatic Depression overlies in the central and northern part of these zones.
The Kruja zone is the eastern part of External Albanides. It is characterized by narrow and linear anticlines associated with thrusts on their western flanks. In the seismic lines we can clearly see that the structures thrust on the Ionian structures, and we can also see that all structures of the Kruja zone are thrusting in each other. In general, the anticlinal structures are eroded on their top and, consequently, are well reflected in the seismic line.
The Ionian zone is formed of three anticlinal belts: Berati, Kurveleshi, and Cika. This zone is characterized by many longitudinal thrust type faults. For the Ionian zone, the seismic data effectively distinguish the covered target for oil and gas exploration.
The Sazani zone outcrops on the Sazani Island and the Karaburuni Peninsula as well as on Mali i Kanalit. The Sazani zone is involved in a very active fault, creating a developed uplift of powerful horsts and grabens. The seismic lines performed in this zone have been highly effective. By the interpretation of the seismic lines, onshore and offshore, there have been identified a clear continuation of the platform to the east and a transition of the platform to the basin, where carbonates are recorded poorly and without distinct features. In the western part, the carbonate platform and the overlying deposits are clearly recorded. In the transition sector, the platform slope, by the seismic lines we have interpreted an interesting object for further exploration. There are also many diffraction waves which must be evidenced in order to be used in seismic interpretation.
The Pre-Adriatic Depression (PAD) is located over the folded structure of the external zones and is formed of anticlines and synclines of structural belt. According to the geologic mapping, the Miocene-Pliocene anticlinal folds do not outcrop with all their elements, whereas synclinal folds are buried under Quaternary deposits, which are extended horizontally and are placed in unconformity over the folded structures of the Kruja, Ionian and Sazani zones. The contribution of seismic exploration is paramount not only for the morphology of anticlinal folds that outcrop partially, but also for the synclinal and anticlinal folds buried under Quaternary deposits. A correct relation between seismic data and well data has made possible the determination even of the age of fault formation. The faults are relatively new, in general Pliocene and Post-Pliocene, whereas some others are reactivations of old faults or their consequences.
Key words: seismics, interpretation, External Albanides.

SEISMIC CONTRIBUTION TO THE EXPLORATION WITHIN CARBONATE SEQUENCES IN EXTERNAL ALBANIDES

STAVRI DHIMA, TODI KOROVESHI, SAZAN GURI
Oil and Gas Institute, Fier, Albania

Exploratory activity in Albania has tackled the hydrocarbon prospecting of carbonate sequence targets from Upper Triassic to Lower Cretaceous, in addition to sandstone targets of Miocene-Pliocene and limestone targets of Cretaceous-Eocene.
Authors of the studies carried out so far have tried to regard all the factors which determine the hydrocarbon prospecting within carbonate section. In this respect, the attention is focused on the following: 1. stratigraphical and petrographical data; 2. existence of source rocks, seals, reservoirs and traps; 3. migration and accumulation; 4. tectonic style.
Based on geochemical as well as stratigraphic and petrographic studies, there is drawn the conclusion that within the carbonate section in question several levels of source rocks and reservoir rocks must exist.
Meanwhile, the structural and tectonic studies have evidenced promising data for the existence of traps. The most critical factor which determines the hydrocarbon prospecting within carbonate section in External Albanides remains the existence of seals.
Seismic works carried out so far have aimed at giving the structural shape, continuity of deposits and hiatuses, dolomization, presence of faults and flexures, etc. As regards the complex processing of seismic data, one of the most important problems is conversion of time sections into depth sections. In this respect we have used some programs and methods that consider the construction of reflection horizons for multibed environments by finding the velocity changes of seismic waves within carbonate rocks. (The velocity ranges from 4,800-5,500 m/sec. in Upper Cretaceous-Paleocene-Eocene deposits to 5,800-6,000 m/sec. in Triassic-Lower Cretaceous deposits.)
Despite a nonconcrete discovery within carbonate sequences up to now in Albania, there are obtained some promising complex data on possible prospects which require a special attention in the future.
Further improvement of the contribution of seismic method to the exploration within the carbonate sequences will aim at recording deeper levels down to Triassic section and at using more available processing programmes.
Key words: seismics, interpretation, carbonate sequence, External Albanides.

TOMOGRAPHIC DECONVOLUTION
OF REFLECTION SEISMOGRAMS

ENDERS A. ROBINSON
Columbia University, New York, NY 10027, USA

This paper gives a new method of deconvolution called "tomographic deconvolution".
Two types of records, the reflection seismogram and the tomogram, provide critical information about the internal structure of a body. Usually either the reflection seismogram or the tomogram, as the case may be, is used to depict the internal structure of the body. In earthquake seismology the tomogram is customarily used, whereas in exploration seismology the reflection seismogram is used almost exclusively. However, the recorded seismic data is smeared and clouded by two effects. One is the effect produced by the shape of the source wavelet. The other is the effect produced by reverberations due to multiple reflections within the body.
In this paper, a new deconvolution method, called "tomographic deconvolution", is proposed. This method makes use of the information both in the reflection seismogram and in the tomogram. Specifically, the tomogram is used as an operator to deconvolve the reflection seismogram. The proposed deconvolution process can be described as follows. Divide the z-transform on the reflection seismogram by the z-transform of the tomogram. The coefficients of the result give the deconvolved signal. This deconvolved signal, so computed, except for a constant scale factor, gives the desired approximation to the internal structure of the body. The deconvolved signal is readily computed because neither complicated statistical estimation methods nor sophisticated numerical approximations are required. Because the proposed deconvolution method does not require knowledge of the source wavelet, the source wavelet does not have to be recorded or otherwise measured. Also, it is not necessary that the source wavelet be minimum-phase in order for this deconvolution method to work. This proposed deconvolution process automatically removes the unknown source wavelet entirely, and, in addition, clears up a significant component of the reverberations due to internal multiples. The tomographic deconvolved reflection seismogram gives a better picture of the internal structure of the body than either the reflection seismogram by itself or the tomogram by itself can possibly give.
Key words: seismics, reflection, tomographic deconvolution.

DIP SELECTIVE GATHER

MIHAI BARBU
" Prospectiuni" S.A., 20 Coralilor Str, Sector 1, 78449 Bucharest, Romania

It is well known that the actual reflection point on a limit has the same position as the CDP only in the case of plane horizontal limits. If a surface of acoustical impedance contrast is dipping, the reflection point on the limit is no more coincident with the Common Mid-Point (CMP) projection on the limit. The situation is even worse if the limit is not plane.
In the standard processing of seismic data this situation is "solved" by the velocity analysis, which yields an average velocity that will maximize the stack. This velocity is not correct and, for plane limits, a simple formula can be used to get the true average velocity as a function of dip. However, the stacked time section, even if nice-looking is not correct, because we have added information coming from different points of the limit and this can have unwanted effects mainly for AVO studies.
An alternative to the standard processing is DMO, which will partially make the seismic data migrate to the correct CDP (true Common Reflection Point – CRP) before the stack. The DMO is applied either on constant offset or on common shot ensembles and is a process which is not dip-dependent. Also, DMO is not too sensitive to velocity variations. But DMO is a computer resource and time-consuming process and cannot be easily applied on singular points on a seismic line. Another weak point is that DMO is applied on the unstacked data, where the signal-to-noise is low and strong noise can build important migration artifacts.
In the actual seismic reflection technology, the multiple coverage recording method is giving us a large amount of regularly distributed seismic traces. It is then possible to select from these traces the ones which come from the true CRP of a dipping limit. The selecting rules are no more the same as for normal CDP gathers (common mid-points rule).
Using a simple model, consistent with the models used for velocity analysis and DMO and elementary geometry, it is easy to determine a new rule for selecting the seismic traces for a true CRP. The selection is depth- and dip-dependent and allows the computation of templates of depth intervals for the selection of traces, which can be transformed into time intervals by using the average velocity.
This method of selecting traces for a true common reflection point can be used for an accurate velocity-dip analysis in points of interest on a seismic line. All the computations involved are very simple as compared to the algorithms of DMO; however the fact that the selection is dip- and velocity-dependent is not a favourable condition for it to be used as a general trace-gathering method before stack. Nevertheless, it could be comparable as efforts to the Post-stack DMO, which also involves several stacks for different dips.
Key words: CDP, CRP, DMO, dip, multiple coverage, trace gathering.

DETECTION OF DIFFRACTED WAVES FROM SEISMIC DATA

EVGENY LANDA1, SHEMER KEYDAR1, BORIS GELCHINSKY2
1 Institute for Petroleum Research and Geophysics, P.O. Box 2286, Holon 58122, Israel
2 Tel Aviv University, Tel Aviv, Israel

The diffracted waves contain valuable information with regard to both the structure and composition of seismic media. Usually, the presence of diffractions on seismic sections indicates discontinuities in the acoustic impedance of the subsurface. Diffractions can be produced by various geological discontinuities, e.g., faulting, a pinchout, a wedgeout, a reef edge, or any sudden change of facies. Diffractions can be also associated with seismic bright spots. However, in standard processing the field data, the diffracted waves are usually regarded as noise and information contained in these waves is not used. Several attempts have been made to detect diffracted waves and to use them to interpret seismic data. In this work we propose to use a new type of correlation procedure, namely, homeomorphic imaging, for diffracted waves wavefront description.
Firstly, let us consider the main kinematic and dynamic properties of the diffracted waves. In a simple case of a single diffraction point and homogeneous overburden, the diffraction traveltime curve is hyperbolic with apex at the receiver location vertically above the diffracting edge. The sign of the diffraction amplitude changes phase on the border between light and shadow regions. The amplitude of diffracted wave decreases rapidly as the distance from the border of geometrical shadow increases.
The parameter of the new curve is a radius of curvature of a wavefront, associated with the diffracted wave. A local time correction for a ray of a diffracted wave is determined (in spherical approximation) by the relationship
,
where Xim and Rim are the coordinates of the image point, V0 is the velocity of wave propagating just below the surface.
We use an efficient optimization procedure for the determination of the coordinate of the image point. The detection algorithm has been successfully applied to field data. We believe that the proposed algorithm can serve as an efficient tool in interpretative processing for the reliable detection of diffraction objects.
Key words: seismics, interpretation, diffracted waves.

COMPARISON ANALYSIS OF SEISMIC MIGRATION BEFORE AND AFTER SUMMING AND THE POSSIBILITY OF THEIR COMBINED APPLICATION

SLOBODAN STANIC
Faculty of Mining and Geology, 7 Dusina St., Belgrade, Yugoslavia

Technology method of multicoverage common mid-point (CMP) enables us to follow: angle of boundary reflection, resolution, seismic data, and all the effects which depend on defined seismic signals. The results of compared analysis should be connected with: gather traces of the same sources, receivers and offset.
From the theoretical point of view, the problem of migration can be best solved by application of seismic migration before summing applied in gather traces of the same source. But it is confirmed in practice that applying of this method does not always get good results.
Defining crossed appearing seismic waves and their separation before application of summing are achieved by application of migration before summing and partial migration. The most successful results are given by application of partial migration.
It can be concluded that in the case when given data of seismic exploration are good, in case of single geology investigated environment, one of the programs for migration after summing should apply. On the contrary the solution of application of migration should be used in combined application of partial migration and the migration after summing and before summing.
Solving the problem of seismic migration does not mean only the application of one method of seismic migration, but implies applying also the seismic migration before and after summing and partial migration before summing.
The same problem can be solved in different gather traces of seismic data. That advantage permits combined application of seismic migration before and after summing and partial migration before summing during the process.
The advantages of this application of the seismic migration are multiplied because it gives possibility of achieving more information on the basis of recorded seismic-geological data under condition that the combined application of method of seismic migration is possible. Besides, the model of combined application of two different methods of seismic migration can be used in other studies in seismics, geophysics, and geology.
Compared analysis seismic migration before and after summing is able to remove many of disadvantages which are present in the application of standard schema of processing in reflective seismic explorations.
Key words: seismics, interpretation, migration, summing.

SALT AND GAS VOLUMETRIC DETERMINATION THROUGH THE USE OF NEW NEUTRON LOGGING TECHNOLOGY
P.A.A. CROSSOUARD

Schlumberger Petroleum Services C.V., Laan van Meerdervoort 55, 2517 AG The Hague, The Netherlands

Salt can in some reservoirs partially plug the initial effective porosity by diagenesis. When those reservoirs are also gas-bearing, two problems occur: (a) the accurate volumetric determination of salt deposited in the pore space to evaluate the permeability; (b) gas detection for testing purposes, and later its accurate volume determination for reservoir purposes.
Neutron porosity logging plays an important role in the evaluation of newly drilled wells when used in combination with other measurements such as density logging. The basic design of commercial neutron tools used for open hole logging typically has a continuously emitting source of neutron and uses either one or two neutron detectors.
An improved neutron porosity measurement has been designed, using an electronically controlled miniature (minitron) neutron source which reduces radiation hazards. The Acceleration Porosity Sonde (APS) measurement has the following advantages over conventional compensated neutron porosity logs:
– The porosity response is affected primarily by the hydrogen index of the formation and is relatively insensitive to changes in formation atom density.
– The vertical resolution of the measurement is improved by the close spacing between source and the combined two near epithermal and array epithermal detectors.
– In carbonates, the response to limestone and dolomite is almost identical.
– Combining appropriately spaced measurements allows gas detection without the use of other logs.
– Environmental effects are reduced by the back shielding of the detectors towards the borehole and their eccentricity towards the formation.
– An epithermal neutron slowing-down time measurement is provided.
– A thermal neutron capture cross-section measurement is also provided.
Due to the very low environmental corrections of the APS neutron measurements and the incidental fact that a mixture of salt and gas has a similar but inverse effect on porosities derived from density and neutron, a well-site porosity equal to the average of density and neutron porosities is found to match the porosity obtained from core analysis.
As a summary survey of this new neutron porosity tool significantly improves the determination of the following: (a) an accurate volume of salt (up to 25 PU corresponding to total porosity plugging). This can be calculated by using the near to array porosity (APLC) and the measurement from APS. This is due to the specific response of salt in both these measurements (21 PU for APLC and 750 cu for ); (b) gas detection in shaly sands is helped by the different excavation effect on the near to array porosity (APLC) and near to far porosity (FPLC) of APS, and is enhanced by their lower reading in shale than the conventional CNT porosity. The gas detection is evident with the combination of LDS/APS tools or the sourceless (no radioactive source) combination of SONIC/APS tools (an added safety in the case of tough hole conditions).
Key words: neutron logging, porosity, gas, salt.

THE HYDROCARBON FORMATION EVALUATION
BY MEANS OF CONVENTIONAL WELL LOGS

ALEXANDRU BABSKOW, ION BUCATARU, VIOREL POPESCU, IULIA DINCA
" Petrom" R.A. - I.C.P.T. - Campina,
29 Culturii Avenue, 2150 Campina, Romania

The paper presents a procedure of conventional log quantitative interpretation, in order to compute petrophysical parameters of both the clean and shaly collector rocks, that is, the porosity and the fluid saturation. The conventional logs are represented by the so-called "standard" and "lateral" electrical logs, known as the BKZ, and the DRR methods. The input data are the SP and the apparent resistivities, as well as several auxiliary parameters concerning the geological information and fluid characteristics.
The interpretation procedure involves two steps: the semi-quantitative stage, consisting in invaded and uninvaded zone resistivities computation, and the quantitative stage, which uses as input data the results of the first stage, and computes the petrophysical parameters. Two interpretation models are considered: clean formations and temperate shaly formations.
The interpretation principles are the following:
– resistivity/porosity/saturation dependence relationship use;
– the shale surface conductivity coefficient and the shale content evaluation;
– the apparent and real formation factor and porosity concept utilization;
– several variants for porosity and saturation computation are used: the Archie and ratio methods, for clean and shaly formations, the Rocky-Mountain method, for clean formation, and the apparent porosity method for shaly formation;
– the permeability and fluid volumes computations are performed.
The results are compared with the complex log synergetic interpretation and with core measurements, the errors being included in an acceptable range, as a function of input data quality.
The application conditions of the procedure, as well as the disturbance factors of the results are revealed.
A FORTRAN PC software for quantitative interpretation has been elaborated.
Key words: logging, interpretation, PC software.

GEOLOGICAL AND GEOPHYSICAL DATA INTEGRATION
FOR ABNORMAL PRESSURE EVALUATION
IN MOESIAN PLATFORM

PANDELE NECULAE, ION BALEANU
" PETROM"- R.A. - I.C.P.T. Campina
29 Culturii Avenue, 2150 Campina, Romania

Romanian Oil Corporation, "Petrom", a State-owned company, is involved in oil and gas exploration and production on Romanian onshore and offshore. Also, "Petrom" is involved in international activities of exploration, production, and services.
Moesian Platform is one of the most important structural onshore unit from Romania as regards hydrocarbon production and prospect.
The goal of our paper is to present geological and geophysical data integration for abnormal pressure determination and the main results obtained during the time spent on Moesian Platform.
At first, the drilled stratigraphic sequence of the Moesian Platform is briefly presented. The tectonic features, sedimentary facies and fluids content, as well as the geological evolution of this platform are related to formation pressure. Further, types and sources of geological and geophysical data are described as input data for formation pressure determination.
There are also presented the ways for identification of sedimentary formations with abnormal pressure by well logs, drilling data, in correlation with pressure values obtained while drilling and at production tests.
The integration of these data, including the seismic data, allows us to accurately determine abnormal pressures, which are used both for the optimal drilling program establishment in new areas and for the preliminary evaluation of the possible behaviour of the adjacent hydrocarbon bearing formations.
Finally, the paper presents some estimated abnormal formation pressure values, their origin, all related to overpressured or underpressured sedimentary formations. Also, it is shown the pressure relation with production flows (recent flows over 150 t oil/day and 500,000 cu m gas/day) and presented the exploration perspective of the Moesian Platform.
Key words: abnormal formation pressure, data integration, Moesian Platform.

SOURCE ROCK OIL WINDOW REVEALED
BY BOREHOLE PHYSICAL MEASUREMENT

VICTOR NEGOITA
" Petrom" R.A. – I.C.P.T. – Campina
29 Culturii Avenue, 2150 Campina, Romania

The customary technique used to know the organic matter quantity per rock volume unit as well as the organic matter maturation stage is based on geochemical analyses accomplished on a preselected number of samples and cuttings drawn from boreholes during the drilling period.
But the same objectives can be approached without any extracost, by using the continuous measurements of well logs recorded in each well from the ground surface to the total depth.
During the diagenetic stage of organic matter evolution, the identification of potential source rocks out of which no hydrocarbon has been generated may be carried out using a well logging suite including Gamma Ray Spectrometry, the Compensated Neutron (Litho Density combinations and a Dual Induction/Sonic Log).
During catagenetic stage, the onset of oil generation brings some important changes in the organic matter structure as well as in the fluid distribution throughout the pore space of source rocks. The replacement of electric conductive water by electric nonconductive hydrocarbons, together with water and oil expelling from source rocks represents a process of different intensities dependent on time/temperature geohistory and kerogen type.
The different scenarios of generation and expulsion of hydrocarbons taking place during catagenetic and metagenetic stages of source rocks are very well revealed by Induction and Laterolog investigation.
They make possible a better source rock characterization regarding the hydrocarbon distribution, bitumen redistribution, and the drainage efficiency through the neighbouring rocks.
With this aim in view, the electric resistivity of main source rocks was compared to other two important parameters, namely, the total organic carbon (TOC) reflecting organic matter quantity in the source rock-bulk volume and the vitrinite reflectance (Ro), generally accepted as a basic parameter of the organic matter maturation degree.
Several crossplots relating vitrinite reflectance, total organic carbon, and log-derived physical parameters are illustrated and discussed.
Key words: source rock, well log, vitrinite reflectance, total organic carbon.

SEDIMENTOLOGIC FACTORS THAT HAVE CONDITIONED OIL BEARING AT KUCOVA SUITE IN THE EASTERN PART OF MARINZA FIELD

SPIRO KOZMA, VLADIMIR DODBIBA
Logging Service Enterprise, Albpetrol, Patos, Albania

Marinza oil field is located in central-western part of Albania, near Fieri city. Marinza oil field is the biggest one in Albania in sandstone deposits. From the stratigraphic point of view the hydrocarbons are found in Tortonian and Messinian sandstone section. This section is represented by sandstone layers interbedded with shales and siltstones filling the erosion valley of limestone and flysch deposits of Jurassic and Late Oligocene.
Seismic and sedimentologic studies as well as paleological restoration are effected for solving many exploration and development problems.
The exploitation of Marinza field has begun in 1957 and continues to present. The main sandstone suites in exploitation named Marinza, Driza, and Gorani are composed of several distinguishable sand beds. The thickness of suites are 170 m, 120 m, and 100 m respectively.
Recently, based on geologic and sedimentologic studies, oil beds have been also discovered in this suite which is located over Gorani, with a thickness of about 250 m. There are 13 distinct sand beds in Kuçova suite. Most of the beds on some determinate areas have resulted as being oil saturated from interpretation of logs and from recent tests made in some of them. Oil saturation varies from 60% to 80%, their effective porosity being about ??= 23% and shale content Vsh = 10% to 15%. These beds have been considered nonproductive with heavy oil and have not been cemented. Thickness of the single beds varies from 5 m to 15 m and in some cases up to 80 m, when they are grouped together.
The geologic reserves of Kuçova suite are evaluated at about 4 million tons.
The oil beds are located in a narrow belt area, 2.7 km long and about 300 m wide, north-south direction, which determines a higher sand percentage from 0.4-0.6, and good reservoir characteristics like porosity and permeability. The lack of correlation between beds in this area is caused by high wave action during deposition age of sediments.
Oil accumulation in these beds is due to migration from other suits as result of bed contact with them.
The oil density, examined by laboratory analyses, is 0.989 g/cm3, and the content of oil fractions is 4.5% at 200°C and 20% at 300°C.
The most important characteristics of oil bearing beds of Kuçova suite on the eastern part of Marinza field are as follows:
- Kuçova suite is located at the upper part of Tortonian deposits, normally over Gorani suite.
- There are 13 sand beds in this suite.
- Oil-bearing part of Kuçova suite is limited to a restricted area, where high sand percentage and good reservoir properties are present.
- Lithological changes of sand on this area caused by high wave action during the process of deposition make difficult the correlation of sand beds.
Key words: sedimentary, oil bearing, Marinza field, Albania.

APPLYING THE INDUCED POLARIZATION METHOD
IN THE CROSSED TRANSMITTERS VERSION

ION STOICA
" Prospectiuni" S.A., 1 Caransebes St., 78344 Bucharest, Romania

In practising resistivity measurements and subsequently induced polarization (IP) it has been noticed that the anomalies of apparent electrical parameters are not similar but, in particular cases, they are similar to the real parameter distribution. For example, the bodies with vertical symmetry axes are outlined in apparent parameters maps derived through gradient profiling measurements by anomalies stretched on the directional orthogonal to the power line (formed by two current electrodes).
S.S. Stefanescu and P. Tanasescu (1965) have shown that anomalies may become similar to real distribution if two fixed powerslines, in orthogonal crossed fixed position, according to axes Ox and Oy and a mobile system to measure the potential differences, which is formed of two potential lines, also in crossed position, parallel to axes Ox and Oy, respectively, are used for measurements. In this case, instead of resistivity, in each point the following parameter is determined: , where and are the electrical field vectors, measured in the case of injecting the current in line A1B1 and A2B2 respectively, while and are the electrical fields calculated in the case of an infinite homogeneous halfspace. The authors have denominated this version of applying the resistivity method the "crossed transmitters" (CT) method.
In the present paper this procedure is extended to the IP method in time domain. Due to the fact that the IP secondary potential is a proportion generally lower than 1/10 of the primary field potential, the power lines form a 45° angle with the potential lines, which are parallel to the coordinate system axes.
The measurement procedure is the following: the primary field is generated by injecting current in line A1B1, while line M1N1 measures the and parameters, and line M2N2 measures and . In a similar way, by injecting current in power line A2B2, are derived. Notations beginning with "ma" mean an apparent polarizability (mV/V), and those beginning with "ra" mean an apparent resistivity (m).
The following vectors are built:
and
respectively, where ma is the notation for apparent polarizability (mV/V), and a is the apparent resistivity (m).
For this CT version, instead of apparent resistivity and apparent polarizability derived through the classical four-point electrode system, the following new parameters have been estimated:
;
;
;
;
;
.
Applying the IP and resistivity methods in the CT version has been achieved through scale physical modelling and numerical modelling for spheres and finite plates.
The anomalies of the parameter pairs – – are very similar. These anomalies are quasi-independent of CT electrode array and their forms suggest the forms of the anomaly sources, unlike the gradient electrode array cases, when we face a strong dependence of the anomaly on the power line orientation.
Key words: crossed transmitters version, induced polarization method, resistivity method, physical modelling, numerical modelling.

BOREHOLE VELOCITY MEASUREMENTS USING VSP METHOD

VERICA SUNARIC
Nis-Naftagas, Serdar Janka Vukotica 8, 11000 Belgrade, Yugoslavia

Vertical Seismic Profile (VSP) is a method which has been used in our country since 1989. The physical property measured is the formation velocity. The velocities of the formations are determined by measuring the time. Measuring the time requires a wavelet which is generated by a surface energy source. This wavelet reaches the seismic tool, which is positioned at various depths in a well.
Borehole seismic records can be obtained by recording several seconds of waveform data at one level. The process is repeated at frequent downhole intervals. The type of this recording is known as the Vertical Seismic Profile (VSP).
The purpose of VSP measurements in the "Indija" well in Vojvodina is to order the depth of the base of serpentinites and average and interval velocities of seismic waves spreading.
VSP measurement in the "Indija" well is done by using "Geolock H3" downhole tool, made by AMG France. The cylindrical downhole tool contains seismic sensors (three-component geophones) and is lowered into the borehole and clamped to the borehole. One geophone lies along the package (Z axis), the other two geophones (X and Y axes) form an angle of 45° relative to the clamping direction. The shallow wells stay from the wellhead at a fixed position of 100-105 m. The depth of the wellhead is 1,594 m. Seismic wave trains are recorded digitally. The primary response of earth is enhanced by standard signal processing techniques. The wavefield recorded in the well consists of both downgoing and upgoing events. These events will be separated by computer processing. Processing of zero-offset VSP contains pre-processing and processing steps. Pre-processing includes: field data, time break, edit and stack, time pick, static shift and two-way time, interval velocity and time-depth curve. Processing includes: wavefield separation, F-K method, median filter, deconvolution, mute and stack, impedance log. From the first arrivals we calculate the average and interval velocities.
Therefore, the velocities which we have measured in situ are very important for stratigraphic and lithologic analyses.
Key words: formation velocity, Vertical Seismic Profile, Vojvodina, Serbia.

INTEGRATED SEISMIC AND WELL LOGGING DATA INTERPRETATION FOR RESERVOIR GEOLOGICAL MODELLING

ALEXANDRU BABSKOW1, DESDEMONA POPA1, VALERIU PLATON1,
VASILE BADIU1, EVELINA PARASCHIVOIU1, ION BALEANU2
1"Petrom" R.A. - I.C.P.T. - Campina, 29 Culturii Avenue, 2150 Campina, Romania
2"Petrom" - R.A., Bucharest, Romania

The 3D seismic data interpretation has been performed by OASIIS software package. The data treated are as follows: migrated time seismic profiles, borehole seismic interpretation logs, integrated acoustic velocity logs, well inclination surveys, geological and lithological data, etc.. The results of interpretation are represented as seismic profiles which reveal, besides the formation boundaries, the structural and tectonic features which allow a more accurate characterization of the reservoirs.
The well log data interpretation performed by EXPRESS software package, by environmental correction of the logs, and "optimizing" procedures using the OPTIMA program, allows the analysis of the well log processing, in correlation with core data, with the result of formation evaluation, reservoir, zonation, and petrophysical parameter determining, such as porosity, fluid saturation, permeability, lithologic components, including shales.
The average log and core porosities and permeability along the structures are correlated, and statistically analyzed, and the test estimates are considered.
The results obtained from seismic and well log data allow us to create a 3-D geological model of the structure by means of a Stratigraphic Geocellular Model software, taking over the seismic and well log data processing features and creating a geocellular model with the distribution of certain attributes determined by well logs on the background of the geological image offered by the seismic data.
Such statistical geological models are further used in reservoir simulation. Several structures, both offshore and onshore, are analyzed.
Key words: seismics, well logging, integrated interpretation, reservoir, modelling.

THE APPLICATION OF HORIZONTAL DIRECTIONAL DRILLING ECHNOLOGIES FOR GEOTECHNICAL AND ENVIRONMENTAL PROBLEMS

INGO SASS
Flow Tex GmbH, Kleiner Plom 4, 76275 Ettlingen, Germany

The rapid evolution of the horizontal directional drilling technology (HDD) in the last years has climbed a lot of technical hurdles like environmentally-friendly mud technologies and extremely precise location and navigation techniques in the underground. All these enable us today to successfully apply HDD in difficult environmental and geotechnical settings.
HDD does not affect roadway surfaces or buildings and rivers. Such controlled horizontal drilling permits remediation work in contamined areas and in strongly populated zones without disturbing the industrial production or the public traffic. Moreover, health and safety control is much easier using HDD in polluted areas than using conventional techniques.
In the last time these systems have been also successfully used for in situ containment of hazardous waste disposals by a three dimensional encapsulation with a combination of jet grouting and HDD. Overlapping horizontal diaphragms will be installed until the three dimensional barrier is completed. For realizing these horizontal imprevious walls, this system uses a natural wax extracted from lignite, called "Romonta Montan Wax". In this way high pressure injections are performed with a mixture of resins and waxes. The advantages of this system of horizontal high pressure injections consist not only in the variable geometry of the sealing diaphragms, but also in the fact that the Montan Wax seales even cohesive grounds. The final plasticity and permeability can be adapted to the technical or hydraulic requirements. This new technology named "Flowmonta" can be also realized under the groundwater level.
The installation of curved horizontal wells (Schumaflow-Technology) with special filters made of sinter porous polyethylene makes additional pump and treat methods possible.
This paper presents the new technology of horizontal directional drilling as well as new applications of this system in environmental and geotechnical problems, especially in encapsulation jobs for contaminated areas.
For an appropriate use of HDD environmental technology one needs proper geophysical survey data. Seismical or geoelectrical tomographic methods can also be used as well as more conventional logging methods. Such horizontal applications of geophysics offer opportunities to detect the basement contours of waste disposals or landslides. In hard rocks it can be used for the exploration of tunnel projects.
Key words: drilling, directional, environment, geotechnique.

GPR MEASUREMENTS WITH DIRECTIONAL ANTENNAE, EVALUATION AND THREE-DIMENSIONAL REPRESENTATION

DIETER EISENBURGER, VOLKER GRUNDELACH
BRG-Hannover, Bundesanstalt für Geowissenschaften und Rohstoffe
2 Stilleweg St., 30655 Hannover, Germany

GPR measurements are made in both boreholes and drifts for exploration. Measurements with directional antennae not only allow the distance to the reflection points to be determined but also the angle with respect to the azimuth of the plane defined by the reflection point and the transmitter and receiver antennae. If a directional antenna cannot be used, the location of the reflection point can be determined by a special placement of the antenna in a perpendicular plan to the axis of the drift. For borehole measurements, a directional antenna developed in cooperation with Prakla-Seismos is used. This antenna is constructed from two loop antennaes. If connected, these loop antennaes function as a result of a dipole antennae. The signal received by the dipole antenna is used to determine the distance to the reflectors. The direction A of the reflected signal can be calculated from the signals received by the two loop antennas
,
where xi and yi are the voltage induced in the two loops during successive time intervals i (i1,...., in).
The migration method we use makes it possible to determine the point of reflection from the traveltime and the angle defined above.
A planar element perpendicular to the normal to the reflection surface can be assumed through each point of reflection. If these elements are connected with each other, a zone is obtained representing the location of the reflector.
The calculation of the angle, the filter processes, signal attenuation compensation, and other kinds of data processing, including determination of traveltime (“picking"), are done using an interactive computer program that we have developed. A software interface has been developed to transfer the reflection points to a 3-D graphics system so that projections, cross sections, and spatial representation can be prepared.
Key words: GPR, directional antennae, software.

APPLICATION OF GROUND-PENETRATING RADAR
IN SALT AND POTASH MINES

R. THIERBACH
Niedersachsisches Landesamt für Bodenforschung
2 Stilleweg St., 30655 Hannover, Germany

Radar probing has been implemented successfully for investigating structural and stratigraphic nature of media involving fairly low attenuation to radio wave propagation like coal, ice, granite, and salt.
In this paper emphasis is given on the investigation of salt layers since they are not only exploited for mining of rock salt and potassium, but also provided for economic storage cells for liquid and gaseous hydrocarbons. Moreover, saltdomes are considered to be nearly ideal dump for radioactive waste as they are considered to be stable.
For exploration preparatory to mining, various probe systems have been developed in order to be used in drillholes to suit the requirements of different applications. For vertical drillholes it is used a probe which can withstand high pressures and can be used down to thousands of meters. Several salt domes have been explored with this probe. For investigations in mines different probes for horizontal, inclined and vertical drillholes are available. Light weight probes with a diameter of 40 mm have been specially designed for slim hole investigations. These work at frequencies between 30 and 300 Mhz. The higher frequency range covers close range detection and has a high resolution. To extend the range of performance of underground probing, a completely flame proof drillhole radar system has been developed to be used in mines in which there is the risk of explosions.
The receiving antennae in the probes were previously exclusively dipole antennae, not providing any information on the azimuthal position of the individual reflectors. A considerable step forward has been achieved by the development of a direction finding drillhole antenna which the probes have now. With this antenna the spatial orientation of the reflecting planes can be determined.
The technical aspects of the equipment are discussed and the results of exploration in salt deposits by using the radar method are used as examples.
Key words: GPR, salt, potash, application.

ALGORITHMS AND PROGRAMMES FOR GRAVITY, MAGNETIC AND IP 3-D NUMERICAL MODELLING

ION STOICA
" Prospectiuni" S.A., 1 Caransebes St., 78344 Bucharest, Romania

The paper introduces algorithms and programmes for computing gravity, magnetic and IP anomalies for a 3D geological model made of irregularly shaped bodies. The geological model is finite, that means it is limited by a vertical prism of rectangular section.
It is shown that for the calculation of gravity and magnetic anomalies in an arbitrary point located inside the prism it is necessary to extend (to know) laterally the geological model for each body up to a distance L from the prism of about 4 times the maximum depth reached by each body (layer). For IP modelling, L depends on the geometry of the electrodes layout.
Excluding the computing relations of the anomalous effect generated by an elementary volume of the model, which are typical, three types of modelling are achieved according to formally similar algorithms.
If the anomalies to be interpreted include regional components generated by sources located very deep as compared to the local anomalies source, it is advisable, in order to lower the amount of computation, that the model be split into one model for the local anomalies and one model for the regional anomaly.
As a rule, any model includes only the top limit surface for the last deepest body, as it is assumed that its extension to depth is unlimited. In this case, the body will be limited at its bottom by a plane Z = Zm, where Zm is the maximum depth of its limit surface.
If a geological model comprises N bodies, each body is delimited by a top surface and a bottom surface. These surfaces are numerically represented by 2D matrices derived through a rectangular discretization grid.
The anomalies are calculated in a rectangular grid located on the topographic surface, centrally as against the discretization grid of the surfaces delimiting the bodies. The IP modelling is an exception, as its topographic surface must be planar.
If for a calculation point a body is not known up to the above-mentioned maximum integration limit, the programmes automatically extend the integration matrices by extrapolating the known surfaces. This is the only way to keep comparable the anomalous values calculated in various points.
The calculation points may also be inside the geological model, such as it is required for interpreting the data derived through measurements in mining works and boreholes.
The anomalous effect of each body is calculated by summing the effects of elementary quasi-cubes derived from dividing a prism which is generated by the nodes of the body surfaces discretization. The division is made in such a way that the quasi-cube inside be at least 5 times smaller than the distance to the calculation point.
It is noticeable that gravity and magnetic modelling in this work is quite different from those of M. Talwani et al. (1960, 1965, and 1966), actually considered as classical ones.
IP modelling extends the procedure (I. Stoica and M. Ianas, 1985), being employed in the models of low resistivity contrasts to the case where electrodes may be inside the model, and the topographic surface may be a random plane. In the calculation algorithm the nonlinearity of the IP phenomenon is implemented, as it is very important in the surroundings of the current electrodes.
For gravity and magnetic modelling there are programmes developed that take into account the sphericity of the earth. The coordinate systems employed are the topographic and the geographic ones.
The paper presents the validation of the algorithms and programmes by comparing with the Talwani 2D modelling procedures. There are also presented applications, such as the calculation of the gravity anomaly given by the Moho discontinuity for Romania's territory by using the isobath map of this discontinuity (Fl. Radulescu, 1988), and a modelling of the magnetic anomaly of the Transylvanian basin.
Key words: numerical 3D modelling, gravity method, magnetic method, IP method.

FILTERING POTENTIAL FIELDS ABOVE ROUGH SURFACES

FLORENTIN COJOCARU
Bucharest University, 3 Hulubilor St., 74132 Bucharest, Romania

Gravity and magnetic data generally verify Laplace equation. Therefore, we can find the distribution of the field at a certain altitude depending on the observation values at the inferior elevations. This type of filtration can be accomplished either for measurements performed on the horizontal plane or in case of the data which are distributed in a rough area. The procedure leads to the attenuation of noise and of the geometric relief effects which are given by the particular position of the source with respect to the observed surface.
For filtration it is used here the method of the equivalent dipole layer with a polyhedral approximation of the relief. Also, it is used the method of the frequency analysis, taking into account the medium elevation of the relief.
In rough areas, the total magnetic field cannot be precisely determined because of the sources buried in the relief. In this case, we measure the vertical component of the field. The algorithms have been studied for the computation of the total magnetic field from the vertical component. Processing of gravity data or of the vertical component data by upward continuation is a particular case. This fact makes a problem of maximal importance either for the geological modelling or for the aeromagnetism.
The testing of algorithms has been made in some synthetic cases and in one real case. The synthetic cases, in a number of four, are modelling different rough areas with a spherical source introduced into the relief. The real case is represented by the vertical component anomaly of the total magnetic field in the area of Rosia Poieni, Metalliferous Mountains, Romania.
From the testing we draw the conclusion that very good results are obtained, which can be used for a quantitative interpretation with the equivalent dipole layer method. Some good results that can be used for a quality interpretation with the frequency analysis method are also obtained.
Key words: potential field, filtering, rough surface, software.

OPTIMIZED GRIDDING AND INTERPOLATION
OF ONSHORE / OFFSHORE ROMANIAN COAST GRAVITY DATA
AS DEDUCED BY ITS FRACTAL DIMENSION ANALYSIS

R.G. DIMITRIU, C.G. EUFROSIN
Romanian Center for Marine Geology and Geoecology
23-25 Dimitrie Onciu St., Bucharest, Romania

The scientific interest for marine geophysical surveys is, naturally, focused both on the continental shelf of the Black Sea and on the adjacent onshore area of Dobruja. The acquisition of the regional terrestrial gravity data (1:200,000 scale gravity mapping) has been carried out mostly in the 1952-1966 period. Lately, during the '80s, detailed onshore gravity surveys have been carried out especially in northern Dobruja.
Offshore gravity surveys, consisting both in on-bottom (GD-K gravity units) and in on-board (GMN-K) meters measurements, have started in 1980. Gravity mappings of the continental shelf have been carried out at the scales of 1:500,000 to 1:50,000 on different zones. Since 1989, on the "transition zone", located between the onshore and ofshore domains, on-bottom and terrestrial gravity measurements have been performed.
The processing of the geophysical data, in order to obtain gravity maps, and the development of specific databases require a proper statistic analysis of the information density of gravity data for any different region. This analysis has to take into account the technical specifications of the gravity meters and positioning systems, the detailing degree of the survey, the access facilities in the deltaic and shallow marine and lacustrine water zones, and so on.
For a random spatial distribution of the data set, like most of real gravity data sets are, there is no exact analytical method to extract the appropriate interpolation interval. In such cases, it is preferable to apply the spectral analysis of cumulative distribution of all the interstation intervals in order to determine the correlation distance Dc, or to determine the fractal dimension Df by fractal analysis.
Three different cases have been selected, as follows: one area located in the "transition zone", analyzed for two different data distribution sets, and two offshore-located areas corresponding to L-35-131-B and L-35-143-C maps, which have high, respectively low density of gravity networks.
In order to compute the gravity data sets, we are trying to find out the optimum regular gridding interval for the inhomogeneously distributed gravity stations. The value of the fractal dimension Df is obtained directly from the negative slope of the curve logN(d) against log(d). The smallest length d at which the scaling regime ceases to be constant is the optimum gridding interval for the data set. The optimum interpolation interval can be found by taking into account the specifications of the gravity meters and of the surveying methodologies involved.
Key words: marine geophysical survey, gravity, processing, gridding, fractal dimension analysis.

NEW ASPECTS OF ZLOT MAGNETIC ANOMALOUS AREA

ALEKSANDR DORDEVIC, NATASA MERIC
Faculty of Mining and Geology, 7 Dusina, 11000 Belgrade, Yugoslavia

In this paper we will pay our attention to the quantitative interpretation considering one part of Zlot anomalous area. The anomalous area with the greatest intensity of total vector (T) of Earth's magnetic field (the greatest intensity of our state) is placed south-west from Bor, the place that, together with its surroundings, makes a central part of eastern Serbia. The anomalous area covers a surface of 4 squared km.
The geological explanation given up to now about Zlot anomalous area relies on the hypothesis and material evidence against structure-geological development of Timok eruptive complex and its metallogenies.
In order to qualitatively interpret Zlot geomagnetic anomaly in the 1981-1984 period there were effected many geological-petrological and geophysical examinations. The results of geophysical examination showed that the intensity of natural remanent magnetization varied in very wide limits and that it was 70 to 100 times larger than the intensity of natural remanent magnetization of similar rocks of Timok eruptive complex. Originators of the anomalous area were not found.
Due to the development of geophysical methods and the computers progress, in the last 20 years, concerning the above mentioned explanations on Zlot anomalous area, we are able today, by the application of the contemporary programmes regarding the laboratory conditions, to find out some new quantitative results on the possible originators as concerns the mentioned anomalous area.
Using the programme MAG 86 along the two profiles A-B over the borehole B1 and C-D over the borehole B2 we have a mathematical 2D model of the possible originators for each profile.
On the profiles A-B and C-D it can be seen that the boreholes B1 and B2 do not cross the originator not even with one part.
For the quantitative interpretation we have also used the results based on sines detections, which have allowed us to locate regional and local ruptures and then to perform the correlation of these results with those of mathematical model-making. Taking into account these considerations, we have concluded that the location of intrusions is completely real and possible, and that these ruptures have enabled the penetration of intrusions near the surface of the ground (20-100 m under the surface), and, at the same time, its pressure has caused the formation of new ruptures of local significance. It is obvious that the boreholes (under the hypothesis that intrusions exist) based only on the qualitative explanation of geophysical data are not well positioned.
The results of quantitative interpretation have shown that new questions are opening on the cause of Zlot anomalous area.
Key words: geomagnetism, anomaly, interpretation, Zlot area, Yugoslavia.

GEOPHYSICAL-GEOCHEMICAL MODELLING
OF AN ACID, POSTMAGMATIC AFFECTED INTRUSIVE STRUCTURE

(A way of interpreting geophysical and geochemical data for magmatic intrusive structures)

D. ONESCU1, M. NEDELCU1, AL. PATRUTI1, ALICE TARLEA2, B. BALTAI2
1"Prospectiuni"- S.A., 1 Caransebes St., Bucharest, Romania
2 Bucharest University, Romania

This paper deals briefly with the geophysical and geochemical contribution in the study of an intrusive magmatic body (mode of emplacement and further postemplacement features).
Our example is an intrusive granitic-dioritic body situated in the Southern Apuseni Mountains, in Cerbia area, which pierced the Mesozoic ofolitic rocks during Lower Cretaceous time.
The whole rock chemical analyses and magnetic-bearing minerals analyses allowed us to estimate the emplacement Curie temperature at around 450°C. Using Kawai graphics, it was also estimated the depth of emplacement at about 7 km.
Based on the knowledge of depth of emplacement, its spatial shape (based on 2 D modelling profiles, using gravity and aeromagnetic data), assuming an initial cooling temperature at about 800°C and considering a conservation-nonstationary-thermoconduction, it was determined the temperature distribution around the intrusive structure. An unequal distribution was found with a large zone to the northwest side.
The postmagmatic stage characterized by an important metallic content is also asymmetric, both in intensity and in areal extent, and corresponds to the thermal asymmetry area.
Inside the structure the correlation between magnetization, density with postmagmatic alteration ratio and SiO2, K2O, Fetotal, shows a relative polarization of postmagmatic processes in the northwest side of the body.
This kind of interpretations allows us to characterize the inside and outside structure of such kind of bodies or to individualize the distribution of postmagmatic processes.
This method which uses susceptibility, density and whole rock and mineral-bearing chemical analyses on acid intrusive magmatic structures can easily solve geological situations with an important economic impact.
Key words: intrusive structure, magnetization, chemical analyses, thermal distribution, postmagmatic processes.

MAGNETOMETRIC RESEARCH EMPLOYED
IN THE ARCHAEOLOGICAL SITE SCÂNTEIA

MIHAELA GHITA, GINA MANEA
Geological Institute of Romania
23-25 Dimitrie Onciu St., Bucharest, Romania

This paper deals with the magnetic mapping in locating certain archaeological sites of Cucuteni culture in Romania, Scanteia Village, Iasi district. A team from the Geomagnetism and Petrophysics Laboratory of the Geological Institute of Romania carried out the magnetometric survey and the interpretation.
Providing a quick information about the subsurface and locating the buried platforms, are advantages of geomagnetic method in archaeological work.
A magnetic susceptibility contrast between the buried structures and the environment is required by the magnetometric method; the Cucuteni platforms are made of burnt clays respecting this condition.
Information about the depth, size, and dimensions of buried structures is provided by interpretation procedures of the magnetic anomalies.
Key words: magnetism, archeology, Cucuteni culture, Romania.

SOME EXAMPLES OF APPLYING THE GEOGRAPHICAL INFORMATION SYSTEM FOR ENVIRONMENT STUDIES

ANCA VAJDEA, VASILE VAJDEA, CONSTANTIN NITICA, IOAN POPESCU, ALEXANDRA MARINESCU
Geological Institute of Romania
1 Caransebes St., 78344 Bucharest, Romania

This paper aims at evaluating and demonstrating some possible remote sensing applications in support of environmental policy (land protection), inclusively of integrating the resulted data in some other information obtained by classical means, using for this purpose the modern geographical information system (GIS).
Starting from the database built up for the whole territory of the country both within the framework of the national programme "Differentiating the ecoregions of Romania in order to assess their state" and within the framework of the international project "CORINE Land Cover" financed by the Commission of the European Union, to which many institutes had cooperated, it was selected for exemplification the area corresponding to Sibiu County.
The methodology of establishing the ecoregions in Romania consisted in digitizing and processing, using the facilities offered by ARC/INFO and ERDAS, the maps at 1:1 000 000 scale referring to relief as elevation and morpho-petrography, climate as annual average temperature and precipitation, water resources, soils and potential vegetation. From the ecoregion maps built up for the whole territory of Romania, an area corresponding to LANDSAT-TM scene 184/28, including Sibiu and Copsa mica, was extracted by means of ERDAS software for exemplifying the distribution of CORINE Land Cover categories within the ecoregions of the chosen region.
The CORINE Land Cover methodology consisted in the computer-aided photointerpretation of LANDSAT-TM images (false color composites-band 4 in red, band 5 in green, and band 3 in blue) in order to identify and separate on the satellite image maps at 1:100,000 scale the 44 land cover classes of the CORINE nomenclature. When the interpretation of one 1:100,000 map sheet was completed, the respective map sheet was digitized, the topology was built for the arcs which made up the polygons representing the CORINE Land Cover classes, and the borders of the map sheet were edgematched with the surrounding sheets. Then the map sheets were joined in a single file.
In order to see the distribution within the above mentioned area of the land cover categories with respect to the ecoregions, the latest had to be extracted from the 1:1,000,000 map and rescaled with the pixed size of 100 meters or 25 meters if the comparison with the LANDSAT-TM images was requested. Now, having the ecoregion and the land cover map, the modifications occurred in the respective components and their consequences due to human activities could be analyzed.
The data processed and integrated in a GIS were presented in a graphical form (as maps and diagrams) and as statistics, showing the new results obtained referring to land use, the distribution of the respective categories with respect to relief, potential vegetation, etc. There were also presented some 3D perspectives, as well as the shaded relief of the study region.
Another examples referred to the polluting effects of gas emissions in the atmosphere due to industrial units and to the human impact upon the environment caused by the mineral-extracting activity.
Key words: GIS, land protection, application, Romania.

INTEGRATION OF LANDSAT AND GEOPHYSICAL DATA AS AN AID TO THE STRUCTURAL ANALYSIS OF VOURINOS REGION IN NORTHERN-CENTRAL GREECE

G. MEMOU, M. STEFOULI
Institute of Geology and Mineral Exploration
70 Messoghion St., 11527 Athens, Greece

The paper comments on the usefulness of remotely-sensed data (Landsat TM in digital format – geophysical, both aeromagnetic and gravimetric data) in the tectonic analysis of seismically active areas. The Vourinos region has been used as a case study area, where the recent, least-expected earthquake has occurred (May 1995 – Grevena-Kozani earthquake, of 6.5 magnitude).
Both image processing (spectral and spatial analysis) of the Landsat CCT and the computer assisted integration of the features mapped on the images and the geophysical data have been carried out. Structural interpretation is improved by the study of enhanced Landsat images, aeromagnetic and Bouguer gravity maps, while the use of various computer techniques makes the analysis of the patterns easier and more accurate.
The combined interpretation of geophysical and satellite data adds several significant features previously unrecognized from the separate interpretation of aeromagnetic data and Landsat images or from general purpose seismotectonic maps. The analysis can assist in interpreting the relationship between fault geometry and the occurrence of the recent earthquake epicentres within the Pelagonian zone in Vourinos region in northern-central Greece.
Key words: Landsat, ground data, image processing, interpretation, Vourinos region, Greece.

BOUGUER GRAVITY MAPS, BASINS STRUCTURE
AND THE DISTRIBUTION OF MAJOR OIL AND GAS RESERVES

J.C. PRATSCH
Integrated Frontier Exploration
13711 Butterfly Lone, Houston, Texas 77079, USA

In many geological basins across the globe their regional basin structure is clearly evident on regional Bouguer gravity maps if three basic conditions are met: 1) quantitatively and qualitatively, the gravity stations must be sufficient, 2) basin topography should be minimal in order to avoid complex corrections of terrane-related gravity anomalies, and 3) a sufficiently thick low-density section should overline deeper high density rocks.
One of the most valuable usages of regional Bouguer gravity maps lies in their usage in regional oil- and gas-migration analysis in oil and gas exploration.
Vertical hydrocarbon migration has no (lateral) map expression. Lateral migration of oil and gas, though, can be mapped: It is controlled by subsurface pressures existing across the basin. On a regional basin-wide scale, regional subsurface isobars are parallel to regional structure contours. Oil and gas migrate laterally in directions perpendicular to isobars or regional structural contours, and parallel to pressure gradients. Therefore, regional structure can be used to predict the preferred directions and positions of lateral oil and gas migration pathways. Most-prospective traps will be located along such regional lateral migration pathways; no or only minor oil and gas reserves will be found away from the preferred lateral migration pathways.
There are several reasons why the usage of gravity data in oil and gas exploration will have considerably financial and technical advantages: 1) Gravity data already exist or can be obtained early in an exploration cycle. 2) Gravity data are low-cost data. 3) Interpretation of regional gravity data in oil and gas exploration encourages integrated exploration efforts including other geophysical data, well data, surface-geological, and geochemical data. 4) Past basin evaluations have revealed that 75% or more of oil and gas reserves in a basin are located on just 25% or less of the basinal area. Realizing these numbers could lead to major savings in exploration cost, efforts, and time in most basins.
There are numerous excellent examples in the literature of the value and practicality of the sketched approach to regional hydrocarbon exploration. We can, therefore, predict that a modern re-evaluation of gravity data in Romania too will lead to interesting, practical, and cost-saving results. Examples from producing basins elsewhere can be readily applied to the petroleum-geological conditions found in the Carpathian foreland, intra-mountains basins, and on ex-Carpathian structural platforms. Here, even at the present advanced stage of geological knowledge, detailing regional gravity maps will illustrate the location and presence of major oil and gas migration conditions not yet fully explored.
Key words: gravity, Bouguer map, basin structure, oil and gas reserves.

SEISMICITY OF SOUTHEAST TURKEY AND DETERMINATION OF RATES OF CRUSTAL DEFORMATION

GUNRUN BAGCI
General Directorate of Disaster Affairs, Earthquake Research Department 06530 Ankara, Turkey

The seismicity and recent deformation type of southeast Turkey are investigated starting from the earthquakes that occurred in the region between 37.5° N - 38.5° N latitude and 37.0° C - 42.0° E longitude.
The epicenter maps of earthquakes with 4.0 magnitude and higher occurred in region between 1900 and 1990 have been prepared as functions of magnitude and depth. In order to determine the seismic activity in this zone, time distributions of earthquakes and energy releases are investigated. The "a" value as 6.50 and "b" value as 1.06 are estimated from the magnitude-frequency relationship.
In this study, fault plane solutions of 12 earthquakes occurred in the investigated region with a magnitude of 5.0 and higher are made. It is also found that earthquakes have reverse fault mechanisms with strike slip component and oblique fault mechanism. It is also found that the dominant directions of the planes of effect and maximum P axes are toward NE-SW, and the directions of T axes are toward E-W.
Asymmetric moment tensor method is first applied to determine the crustal deformation type and to calculate deformation rates for southeast Turkey by using the fault plane solution results. In southeast Anatolian Thrust, the deformation rates are obtained in N-S contraction at a rate of about 0.18 mm/yr and in E-W contraction at a rate of 6.00 mm/yr. As a result of N-S contraction, the thickening is found to be 0.36 mm/yr.
Key words: seismicity, crustal deformation, deformation rate, Turkey.

MAGNETOTELLURIC STUDIES IN THE TETHYAN SUTURE ZONE ON THE INTERNATIONAL PANNONO-CARPATHIAN GEOTRANSECT

D. STANICA, MARIA STANICA
Geological Institute of Romania
1 Caransebes St., 78344 Bucharest, Romania

The 20 magnetotelluric (MT) soundings carried out on the Lipova-Lugoj profile, belonging to the Pannono-Carpathian Geotransect, were analyzed in order to determine both the regional conductivity distribution and the internal conductivity structure of the ophiolitic and nappe system environments. The distortion effects on the data are removed by supposing that the site gain at each location can be estimated by requiring that the long-period asymptotes of the E-polarization phase curves be the same. The geotectonic model elaborated for the study area showing the deep structure of the crust is based on the data supplied by MT surveys as well as by geomagnetic, gravity, and geothermal information. This model emphasizes a narrow zone of enhanced electrical conductivity below Carpalnas-Techereu Nappe, with a characteristic width of about 4 km. The cause of this crustal zone of enhanced conductivity is thought to be the free saline water possibly released by metamorphic devolatilization of the oceanic crust belonging to the Transylvanides during the obduction processes. This area is placed between two continental crusts (Inner Dacides towards north and Median Dacides towards south) and it has been interpreted as being the Major Tethyan Suture (the ophithitic complexes). From north to south there are two distinct compartments of the crust, one of them situated under the development area of the Inner Dacides (with thicknesses at about 24-26 km) and the other one in the part of Median Dacides (with thicknesses at about 32-33 km). For this geotectonic model we have calculated the electromagnetic response by using 2D forward modelling (a numerical method based on the finite element), and the results have been compared with the regional observed electromagnetic characteristics. One of the interesting results, taking into account the data obtained on the other MT profile situated towards east, is that along geotraverse the top of asthenosphere lies at a depth of about 78-80 km, indicating the existence of a shallower lithospheric plate.
Key words: magnetotellurics, Tethyan suture zone, Pannono-Carpathian Geotransect.

STRUCTURAL MODELLING BY MEANS OF MAGNETOTELLURIC DATA IN SOUTH BESSARABIA ON THE PROFILE CENACU-ALUAT-CIUMAI

L. ASIMPOLOS, CORNELIA FURNICA, H. NISTOR, C. VISARION
Geological Institute of Romania
1 Caransebes St., 78344 Bucharest, 32, Romania

The purpose of this paper is to show the magnetotelluric researches carried out along the Cenacu-Aluat-Ciumai profile by means of data achieved in 1994.
The profile with a north-south direction is situated in the northern Bessarabia and, geologically, crosses the Eastern European Platform – in north – and the Scythian Platform – in south.
The two principal geologic units are individualized by the resistivity ratio between them, such as the high resistivity values that characterize Eastern European Platform in contrast to the low resistivity values that are predominant in the Scythian Platform; this aspect is shown in the resistivity and phase pseudosections.
By means of qualitative interpretation of MT data, taking into account the frequency dependence of the penetration depth of electromagnetic waves, a depth section of real resistivity is obtained, on which layers with different geoelectric characteristics, horizontally correlated, are figured.
Taking into account the relationship between the polarization state parameters of electromagnetic waves, the polarization state of medium in the framework of section can be described.
The sections of dimensionality indices, specific for the method, are also pictured.
Another approach used in interpretation of MT data along the mentioned profile is the computation of the group velocities and penetration depth of electromagnetic waves for each frequency found as a consequence of the data processing. So, in this purpose, a computing program in BASIC, based on the relation of Miecznik and Tomaszkiewicz (1980), which transforms the amplitude and phase curves into group velocities vs depth diagrams, is made out.
The paper also presents the similarity between MT responses over the profile and the images obtained by group velocities. For example, there are included for MTS-29 diagrams which are determinated in four directions.
Some conclusions are to be drawn up from the integration of such information:
– pointing out of some major structural features (such as the basement of Pre-Dobruja Depression, containing tectonic blocks deepening in a steep structure from south to north, the Vaslui-Cetatea Alba crustal northern type fault);
– determination of the presence of subvolcanic intrusive bodies less extended at the surface;
– determination of a structure south of the Biruinta locality that might favour the hydrocarbon accumulation.
Key words: magnetotellurics, south Bessarabia, software.

DISTRIBUTION OF SEISMIC WAVES VELOCITIES IN THE ROMANIAN CARPATHIANS CRUST

DORU MATECIUC, ALEXANDRU POMPILIAN, FLORIN RADULESCU, DAN STIOPOL
National Institute for Earth Physics
P.O.Box MG-2, Magurele-Bucharest, Romania

This seismic study presents the results of the processed information provided by large quarry blasts which took place in some regions of Romania during the 1978-1985 period.
The recordings on these quarry blasts were used on some conventional and telemetered seismological observatories.
All recorded points were considered along the chosen profiles that allowed us to treat them as observable data by the aid of well-known methods, both in seismic and seismological studies.
The P and S waves arrivals considered in these analyses were obtained from a list of seismic events elaborated by Smalbergher (1986). Raw data were processed in order to make a better relocalization of the seismic events, while the geographic coordinates of the quarries from topographical maps are well-known. The relocalization program consists in the minimization of the time residuals square sum. Plots on time-distances curves, drawn before and after the relocalization method, are presented.
The seismic velocity distribution of P and S waves with depth was computed by using the Wiechert-Herglotz method, applied on time-distance curves for the above-mentioned profiles. Bartlesen's (1970) method was applied in order to diminish the local variation of the apparent velocities to obtain a growing distance-time function.
The information obtained by the aid of the above mentioned method was extended to 30 km depth, in the lower crustal area. Reduced relative distances at which recordings were performed of up to 200-250 km did not allow us to search deeper domain such as the transition zone between the crust and the mantle, which in this orogenic area appeared at a depth, of more than 40 km. So, the seismic information referring only to the lower crustal zone and a conclusive argument of this idea may be the P and S medium seismic velocities.
As regards the southern Carpathian area, the velocity-depth function was computed as shown above, pointing out some greater velocities in the eastern Carpathian zone as well as a greater increasing ratio of the velocities vs depth.
The profile recorded in the Eastern Carpathian Arc Bend showed a different velocity structure, especially on the Racos-Galati profile.
Key words: seismic value, velocity, structure, Carpathians, Romania.

DEEP SEISMIC IMAGE OF THE SOUTHERN CARPATHIAN FORELAND

M. DIACONESCU1, V. RAILEANU1, C. DIACONESCU1
F. RADULESCU1, C. DINU2, V. MOCANU2
1 National Institute for Earth Physics
P.O.Box MG-2, Magurele-Bucharest, Romania
2 Bucharest University, 6 Traian Vuia St., Bucharest, Romania

The sedimentary cover of the Southern Carpathian Foreland (SCF) shows a reflectivity which decreases with depth, the most contrasting level being the Neogene deposits. The transition from Tertiary to Mesozoic deposits is very reflective, with a reflection coefficient of 0.13 – 0.36. Under these levels, the reflectivity decreases with depth and a suite of reflections can be seen at the erosion surface levels within the Mesozoic and Paleozoic stacks.
The western part of SCF shows a transparent upper and middle crust with some diffractions and/or some short and dipping reflections that evidence a brittle medium. The reflectivity increases with depth and crust-mantle transition zone with distinct and prominent reflections has been identified. This pattern of the western part of SCF and MP points to reworked basement during the Alpine orogeny. Thus, in our opinion the mid crustal zones marked by short and subhorizontal events are crustal decoupling zones.
The crust from the central part of SCF shows a decreasing reflectivity with depth, the upper crust presenting zones with strong and long correlate reflections alternating with zones void of reflections. At the same time, some short and diverging reflections like "seismic crocodiles" occur in the middle crust (6-8 s TWT). They can be correlated with compressional belts and seem to characterize young and old identation or interwedging processes.
The lower crust shows specific features in the eastern and western parts. The top of the lower crust appears to be relatively transparent or/and crossed by some diffractions, while the base of the crust contains two multicyclic reflection bands over the 1.5-2.0 sec TWT interval designated as a crust-mantle transition zone. This reflectivity type could be specific to the Paleozoic extensional areas. The features of the subcrustal reflectivity from western and central parts have a possible cause in an isostatic adjustment which moves the Moho from an older position (13-16 TWT) to the present one (9-12 s TWT) or a reduced viscosity interval where the horizontal shear stress can produce laminated zones.
Key words: seismics, deep structure, Southern Carpathians, foreland, Romania.

DEEP SEISMIC REFLECTION SOUNDING IN THE TRANSYLVANIAN DEPRESSION

VICTOR RAILEANU, MIHAI DIACONESCU
National Institute for Earth Physics
P.O. Box MG-2, Magurele-Bucharest, Romania

Two deep seismic reflection lines (17 s TWT – two-way time) – NW Tg. Mures-N Sighisoara (I), and W Copsa mica-N Sighisoara (II) – cut the central area of the Transylvanian Depression, an area underlaid by ophitic rocks. The first 4 s TWT of each seismic sections show many coherent levels of energy belonging to the molassic deposits and the post tectonic cover. The deepest part of the depression is marked by dipped reflections on the ends of the seismic section and by horizontal reflections in its central part.
The seismic section of line I points out an upper and a lower crust almost devoid of coherent events, like a "transparent crust". Some very short subhorizontal events, possibly reflections or diffractions suggest a brittle tectonized and fractured crust. In the 9.5- 14.5 s TWT interval dense coherent subhorizontal or slightly dipped events appear. It seems that over this depth interval the traces of the important tectogenetical processes developed in the Cretaceous time have been preserved. On the seismic section there is not visible any sharp boundary for Conrad, and Moho can be correlated only with the top of the reflective/diffractive zone that begins from 9.5 s TWT and extends down to 14.5 s TWT.
The seismic section of line II shows a near transparent crystalline crust from 4 to 17 s TWT. Some very short subhorizontal or slightly dipped events cut the whole section. On the ends section a lot of events having the dip toward the central part appear from 6 – 9 down to 16 s TWT. The origin of the latter one has to be diffractive. The studied area is considered to be a suture zone, where two continental margins have been collided. The direction of the suture would be about N–S, and the line II cuts it on the E–W direction. We cannot see coherent events aligned on that direction because the crustal rocks are very fractured. The coherent events over the 9.5–14 s TWT interval of line I come from a direction parallel to the strike of the suture, and their nature can be either reflective or diffractive. In our opinion, this suture area has suffered a delamination process, which has affected the last part of the lower crust and the uppermost mantle part. While these lithospheric parts have been sunk, the weakness zones have been penetrated by the hot mantle material which has formed sheets (sills and dykes) of denser magmatic rocks. The continuity of these sheets seems to be better preserved on the N–S direction over the 9.5–14.5 s TWT interval, while on the W–E direction they are fractured. The ends of the fragmented sheets could generate diffractions or even reflections visible only into a N–S vertical plane.
Key words: seismics, reflection, deep structure, Transylvanian Depression, Romania.

CORRELATION IN SIZE AND SPACE DISTRIBUTIONS PERTAINING TO FRAGMENTED STRUCTURES

CRISTIAN SUTEANU, DOREL ZUGRAVESCU, CRISTIAN IOANA, FLORIN MUNTEANU
Romanian Academy, "Sabba S.Stefanescu" Institute of Geodynamics,
19-21 J.L. Calderon St., Bucharest 37, RO-70201, Romania

While fragmentation phenomena enjoy a large interest in studies regarding the solid earth, there are still open problems and striking contradictions. They stem from approaches that highlight, on the one hand, fractal distributions (no characteristic sizes – in fracture sets, in size distributions for rock fragments – Turcotte, 1993) and, on the other hand, distributions emphasizing accumulations of fragments on intervals of dominant dimensions (Sadovskii, 1983, 1985), while maxima of the multimodal size distributions obey a geometric progression, with a ratio between 2 and 5, irrespective of the rock type, of space and time scale and of fragmentation processes involved.
Our fragmentation experiments have shown that, even when point-like stress concentrations are avoided, dominant fragment sizes appear; the same dominant size intervals are present in distributions for different distinct regions of the same sample, demonstrating that such multimodal distributions are not markings of superpositions of different "local" distributions. We also highlight correlations between the sizes of fragments and their spatial distribution, showing a clear tendency of fragments of comparable sizes to form groups of neighbouring pieces.
From our experiments which have implied very different mechanisms (fragmentation by shock applied on a large area, cracking of drying suspensions), we conclude that, in size distributions of fragments, clustering occurs on correlated dominant size intervals. We suggest that we are faced with an overall self-similar distribution that emphasizes an internal fractal structure, expressed by the discrete, power-law-governed series of scales corresponding to dominant dimensions. Since this clustering on intervals corresponding to "dominant" dimensions does not destroy the overall fractal pattern (visible in cumulative log-log distributions), we call it "weak clustering". We consider these features to be important for studies regarding geodynamic systems, although it is not easy to detect them without a proper methodology.
Key words: fragmentation, size distribution, space distribution, clustering.

FRACTAL DOMAINS AND DOMINANT SIZES IN CRACK PATTERNS

FLORIN MUNTEANU, CRISTIAN SUTEANU, CRISTIAN IOANA, DOREL ZUGRAVESCU
Romanian Academy, "Sabba S.Stefanescu" Institute of Geodynamics
19-21 J.L. Calderon St., Bucharest 37, RO-70201, Romania

Geodynamic structures emphasize a particularly high complexity and strong variability, implying difficulties for a quantitative approach. It is the aim of this work to highlight new results obtained by the application of fractal analysis techniques to complex crack patterns. A rigorous, quantitative pattern analysis applied to surface features can offer valuable information for their evaluation, enabling comparisons between different structures. It may lead to useful classifications and to hints regarding the pattern-generating conditions for structures as cracks on the earth crust or on rock samples, desiccation mud-cracks, with patterns depending on parameters like slope, humidity, desiccation velocity, etc. (discovered in older strata, they can give hints about the generating conditions), polygonal ground (stone polygons that solifluction phenomena can turn into relatively short block stripes or, if they are more active, in long ellipses that become stone stripes or striped ground), albedo features on other planets or satellites. The scale invariance of the overall patterns has been studied by box-counting techniques applied to digitized images of crack patterns generated in desiccation experiments. We have found that, while a global fractal character can be observed, by means of a computer program conceived to search the best fit possibilities, one can notice distinct size domains (with correlations above 99.99%): i) at small scales, under a size threshold (e.g., 3 mm in starch suspensions), with a fractal dimension D1 = 1.55; this dimension characterizes in our opinion, the fractal profile of the crack curves themselves; ii) above the size threshold, with D2 = 1.97, expressing the tendency of the structure to fill the plane.We consider that the "global" dimension (here we should have D = 1.76) often used is less relevant. We found the same behaviour for structures of the highly fragmented surface of the Jovian satellite Europa (resolution analysis: 1 km; scale threshold: 24 km, D1 = 1.74, D2 = 1.95, while the overall – less relevant – dimension is D = 1.87).
Key words: fractal analysis, crack pattern, dominant size.

Lucrari prezentate ca poster (Paperworks - Posters)

A NEW APPROACH TO THE PROSPECTS ON OIL AND GAS BEARING OF THE BLACK SEA REGION

R.F. TYAPKIN1, V.N. GONTARENKO1, V.S. GOTYNYAN2
1 State Mining Academy of Ukraine
320600 Dniepropetrovsk, 19 K. Marx Avenue, Ukraine
2"Priroda" State Enterprise, Ukraine

A new approach to the appreciation of the Black Sea region prospects on oil and gas bearing consists in the following:
1. As the conceptional base for study of the region tectonics was taken the new rotational hypothesis of structure formation (K.F. Tyapkin, 1974). There is an interconnection between the orientation of the basement faults systems and the time of their activation.
2. Another interconnection takes place between the activation of the system of basement faults and the structure formation in the cover. The orientation and location of structures in the cover have to be subordinated to the orientation of corresponding system of faults. So, an opportunity appears to restore the history of tectonic development of regions by using two parameters: the time of activation of the system of basement faults and the age of deformed rocks of the cover.
3. The methods to appreciate the prospects of a region on oil and gas bearing are based on these regularities. Two variants are possible. A more simple case is to compare the known local structures or mineral sites with the maps of the systems of the basement faults and thus to determine the faults of certain systems, with the activation of which it was connected the formation of such structures. The fragments of faults not sufficiently studied can be considered as perspective areas.
Such an approach to the study of the Black Sea region was first used by A.Ya. Krasnoschek. Good perspectives were outlined but as he left the region the works stopped. They were renewed last year by the group of authors of this report. In 1994 we composed a map of the system of basement faults of the Black Sea and Ukrainian Black Sea coast in the scale 1:500,000, eventually on the basis of gravitational and magnetic methods. In the continental part of the Black Sea coast there were used the results of distance methods of investigation and in the aquatorium, the bathimetric maps. On all these maps 5 systems of faults are clearly seen with the strike azimuths of 0 and 270°, 12 and 282°, 35 and 305°, 62 and 332°, 77 and 347°. The comparison of these maps with the situation of known local structures leaves no doubts as to their close interrelation to some systems of faults (it is demonstrated in the report).
Key words: oil, gas, prospects, the Black Sea region.

DECIPHERING THE GEOLOGICAL STRUCTURE OF THE DANUBE DELTA BASEMENT BY A GRAVITY SURVEY:
THE GOLOVITA-SOUTH RAZELM SECTOR

R.G. DIMITRIU, C.S. SAVA
Romanian Center for Marine Geology and Geoecology
23-25 Dimitrie Onciu St., Bucharest, Romania

The paper presents the main results of the gravity survey carried out during the past few years in an area located in Dobruja, between the continental and marine domains. This area, named "the transition zone", includes the Danube Delta, the Black Sea coast, and the lacustrine and marine shallow-water zone. The gravity survey consists of both on-bottom and terrestrial gravity measurements carried out with GD-K, respectively Scintres CG-2G gravity meters. The research is carried out in order to improve the geophysical data set in a poor gravity measurements density area. Unpolluting, environmental friendly methodologies are performed, inducing no anthropic effects.
The Golovita-South Razelm sector is located in the central part of the Razelm-Sinoie Lagoon Complex, in the southern Danube Delta. The sedimentary deposits of the Babadag Basin underlie the deltaic poor consolidated sediments. The basement of the Babadag Basin consists of sedimentary and crystalline rocks belonging to the central and northern Dobruja mega-units.
The limits between the main geological units, known previously by geological and geophysical data, are more accurately pointed out. By deciphering the deep geological structure, the existence of two main fracture systems is emphasized. These fracture systems contribute to the genesis of a "tectonic blocks structure" of the Babadag Basin basement. The geological structure pointed out extends over the continental shelf beneath the Paleogene-Neogene sedimentary cover of the Black Sea.
A close correlation between some of the elevated tectonic compartments of the basement, tectonic lines – emphasized by geophysical surveys – and some geomorphological features of the Razelm-Sinoie Lagoon Complex is also pointed out. We assume that the uplifted pre-deltaic tectonic compartments, pointed out by gravity data, may have contributed to the appearance of those spits which have influenced the genesis and evolution of the Danube Delta.
Key words: gravity survey, basement, the Danube Delta, Romania.

STRESS ORIENTATION DETERMINATION IN ROMANIA
BY BOREHOLE BREAKOUTS. THE GEODYNAMIC SIGNIFICANCE

AURELIAN NEGUT1, CORNELIU DINU1, IOAN SAVU2, RADU BARDAN2
MIHAELA LIANA NEGUT3, IULIANA CRAICIU3
1 Bucharest University, 6 Traian Vuia St., P.O. Box 37, 70139 Bucharest, Romania
2 Atlas Gip S.A. Ploiesti
3 Petrom R.A., Bucharest

For defining regional and local stress fields, besides other information sources, the data from borehole breakouts identification are at present used on a wide scale.
Borehole enlargements and elongation, as a result of the spalling of rock fragments of the wellbore in a direction parallel to the minimum horizontal stress, may be identified by borehole geophysical measurements including 4-Arm Continuous Dipmeter Tool, oriented calipers (BGT–Borehole Geometry Tool), acoustic imaging tool (BHTV–Borehole Televiewer), electrical imaging tool (FMS–Formation Microscanner).
The preoccupation for tectonic stress orientation determination using borehole breakouts started in Romania in 1994 after our implication in the PANCARDI Project as a part of the International Scientific Cooperation Program – EUROPROBE.
Due to the fact that borehole breakouts identification seems to become the primary method for evaluation of in situ stress orientation, the following aspects, in a synthetic manner, are presented in the paper: borehole breakouts and tectonic stress orientation relationships; borehole breakouts identification criteria on continuous dipmeter diagrams and practical aspects of stress orientation determination; optimum applicable conditions, perturbing factors, limitations; main applications of the knowledge of regional and local field orientation stress; the results of stress orientation obtained in Romania using borehole breakouts identified from primary recordings of the continuous high resolution dipmeter tool.
For borehole breakouts identification the data of 4-Arm Continuous Dipmeter Tool in deep hydrocarbon boreholes were used. Data from about 20 wells located on different geological structures were analyzed and processed.
Borehole breakouts were identified at different levels on the depth interval 500-5,800 m. In some cases, the borehole ellipticity is associated with fractures (Baicoi: 5,700-5,805 m; Stoienita: 4,200-4,350 m; Darmanesti: 1,930-2,000 m) in compacted rocks. Also, the elongations of borehole were observed (separation of the two calipers) as a result of tectonic stress, an effect superimposed on washouts for shaly formations. In some cases, the tendency of forming key-seat borehole is evident.
Finally, a preliminary geodynamic significance of resulted stress directions according to the major geological structure of Romania and their correlation with present tectonic active elements are discussed in the paper.
Key words: stress, borehole breakout, geodynamics, Romania.

USE OF ELECTRICAL AND ACOUSTIC MEASUREMENTS FOR EVALUATING ABNORMAL HIGH FORMATION PRESSURES IN POVELCA GAS FIELD

XHEMIL HOXHA
Oil and Gas Institute, Albpetrol, Patos, Albania

Povelca gas field, with abnormal high formation pressures (AHFP), is located in the western part of Peri-Adriatic Depression near the Adriatic Coast. It was discovered in 1987, from the first wildcat well Pov-1, that blew out dry gas during drilling at a depth of 2,030 m. The formation pressure gradient (FPG) of the gas bearing stratum that blew out was 1.53 Bar/10 m. The shale and sandstone formations that are penetrated by the wells belong to the Tortonian age.
Detection and evaluation of AHFP in this gas field are made by means of wireline logging and technological methods (shale resistivity, shale inductiveness, acoustic servey, D-exp, Dc-exp, S-log, etc.).
The paper deals with the use of resistivity surveying and acoustic measurement for detecting and evaluating AHFP in this gas field. By using these methods for all drilling wells (40 wells), the top of AHFP, the transition zone, that is, the interval of interest, the top of superpressures under the interval of interest were determined and the FPG profiles of every well were constructed.
On the basis of the above problems, that were solved correctly by means of wireline logging methods, the profiles of FPG for all the wells, spreading maps of FPG tops, and superpressure top maps were drawn up.
Quantitative and qualitative evaluations of FPG made by these methods led on the right way of reducing the drilling costs (by improving the casing plan, drilling muds, etc.).
Comparison of FPG values, calculated by geophysical methods, with direct measurements that are made during well testing indicates an effective use of the geophysical methods for evaluating geopressured formations in Povelca gas field and in the other structures of Albania.
The paper includes some graphs and figures that illustrate the efficiency of geophysical methods for detecting and evaluating geopressured formations in Povelca gas field.
Key words: formation pressure, abnormal, resistivity survey, acoustic measurements, Povelca gas field, Albania.

FORMATION EVALUATION USING PULSED NEUTRON CAPTURE LOG

ADRIAN ZETT, FLORIN POPESCU
" Petrom" - R.A., 109 Victoriei Avenue, Bucharest, Romania

Taking into consideration the high number of wells in Romania which were not completely investigated (they had only the minimal electric survey), for decreasing expenditure with perforating/well testing operations, as well as for fluids saturation monitoring, "Petrom" - R.A. has begun investigations for this kind of wells for the two purposes mentioned above.
Investigations were made in areas with good conditions for such measurements (we took into consideration the restrictions imposed by water salinity for oil fields), in both oil fields and gas fields. In many situations, this method of investigation was used like a unique method to determine shale volume, porosity and fluids saturation, the synthetic curves confirming in this way the computation parameter of the model.
When recording Compensated Neutron Log we found an opportunity to calibrate/verify the porosity which was determined from ratio curves, RATO or RICS.
We will expose the final presentation of four wells called here A, B, C, and D, each of them having different lithologies, different borehole fluids (water, gas, or oil), eruptive or gaslift-producing, perforated or unperforated. All evaluation steps, parameters selection (frequency crossplots or selection criteria), and more details for each well will be presented on posters.
– Well A, producing in gaslift/5 1/2" -casing well flowing record / open hole data not available.
– Well B, gas producing/5 1/2"-casing/well flowing record/available only C.N. recorded in cased hole.
– Well C, oil producing-well killed/5 1/2"-casing/available only C.N recorded in cased hole.
– Well D, unperforated/5 1/2"-casing/open hole data not available.
Each case was interpreted with its own features, the flow tests confirming the result of interpretation.
Key words: logging, pulsed neutron capture, formation evaluation.

SEISMIC SEQUENCE ANALYSES OF THE EOCENE FORMATIONS FROM LOTUS STRUCTURE (THE BLACK SEA)

ION MOROSANU
" Prospetiuni" - S.A., 20 Coralilor St., 78449 Bucharest, Romania

The studied area comprises a part of south Histria Depression. This depression is a sedimentary basin developed during the Oligocene epoch on the Romanian Continental Shield. Limited southward by the Moesian Platform, the depression lies on the North Dobruja Orogene.
The origin of this depression is complex: tectonic and erosional.
Several wells have been drilled in Histria Depression.
In the northern part of Histria Depression many hydrocarbons shows, including commercial or noncommercial oil, gas and gas-condensate accumulations, occur within Eocene sandstone or limestone formations deposited in prograding costa – plain and transitional – marine environments.
The most significant structural closure on the southern part of this area is the Lotus structure, which is localized, in fact, at the Cretaceous level.
This structure is believed to be associated with the evolution of the northern margin of the Moesian Platform.
The upper part of this structure reveals a large prograding structure in Eocene formations.
This study is based on the stratigraphic and structural relationships interpreted from both seismic reflection profiles (3D) and well data. Interpretation of seismic data was mainly important for establishing the structural and stratigraphic framework of the study area.
A particular emphasis is put on the relationships between the traps, reservoirs, and source rocks.
The results help us to evaluate the likelihood that the south margin of Histria Depression can become a future petroleum province.
Key words: Histria Depression, hydrocarbons, prograding structure, seismic reflection.

SEISMIC SEARCH LEVEL IN THE GEOLOGICAL EXPLORATION IN ALBANIA

CAUS XHUFI
Oil and Gas Institute, Fier, Albania

1. Geological setting. Geological exploration activity for oil and gas in Albania involves the western part of the country which belongs to the External Albanids. Based on lithologic and tectonic characteristics, the External Albanids are divided in three tectonic zones: 1) Sazani zone, 2) Ionian zone, and 3) Kruja zone. The Peri-Adriatic Depression (PAD) is another geological unit which partially covers the above tectonic zones. The formations taking part are: evaporites, carbonates, flysch, and molasse. In the Ionian zone nearly all the oil fields are discovered, whereas in PAD molasse the gas fields are located.
2. The seismic method used. Seismic activity analysis in Albania originated in the mid 1950s with oscillographic recording. It continued during the 1970s with magnetic (analogue) and after 1980 chiefly with digital recording. Processing began after 1973.
Some of the main acquisition parameters are:
– Average grid density 500-1,000 m, but not homogeneous.
– Folding 10-24 fold.
– Wave generation chiefly with single holes of 20-30 m and a charge of 4-8 kg dynamite.
– Receiver array of range n=36, dx=1.5-3 m linear and plane.
– Processing with the program system EXPLORER-2000 up to 1992, and after 1992 with MEGASEIS system.
– Estimation of weathering velocities by the refraction method, approximately every 1,000 m.
– A great deal of velocity curves of wells are used for interpretation, etc.
As regards the concrete carbonate structures outlined by seismic analysis, as the most important target for hydrocarbon exploration in Albania, the conclusion that not all the known carbonate structures of oil fields are reflected by seismic analysis is inferred. The structures with considerable dimensions (over 4-5 km), with good surface conditions (a very soft relief, clay on the surface, water bearing horizons, etc.) are reflected clearly by seismic analysis (Patos-Verbas, Kucova, Tirana, Ishmit, etc.). The structures of relatively lesser dimensions and bad surface conditions (the presence of sandstone, marl and breccia and rough relief on surface) are partly reflected by seismic analysis (Ballshi, Cakran, Amonica) or not reflected at all (Delvina, Karbunara, etc.).
In these conditions, in order to improve the seismic image, it is necessary a comprehensive analysis of the factors affecting seismic information (i.e., seismic surface conditions, acquisition parameters, processing flow and the expected nature of the geological structure).
The fact that not all carbonate structures of the known oil fields are reflected by seismic analysis leaves open the opportunity for exploration, as satisfactory up-to-date seismic information is not yet achieved.
All the above inferences are illustrated by an abundance of graphical materials (tectonic sketches, seismic and geological sections).
Key words: seismics, geological exploration, Albania.

EXPLORATION PROSPECTIVENESS OF TIRANA – ISHMI DEPRESSION (MOLASSIC DEPOSITIONS)

HETEM SEITI, FATMIR SAZHDANAKU
Oil and Gas Institute, Fier, Albania

The Tirana-Ishmi Depression extends over the northeastern part of the Pre-Adriatic Depression. The molassic deposits (Tortonian-Pliocene) are transgressively overlaid on older deposits (flysch-carbonate) of Kruje tectonic zone. In this area, the carbonate anticlinal structures are eroded up to Upper Cretaceous. In general, they are extended in a linear SE-NW trend and overthrusted towards west. The Ishmi carbonatic structures are the western extreme of Kruja tectonic zone. The map of eroded surface is represented as a monocline form with small undulations, while the northeast part is represented as an erosional trough.
The deposits of the Tortonian megasequence are formed as a result of a transgressive cycle and a regressive one, which are interpreted in the paper.
The sedimentation of northeast (erosional trough) has more favourable conditions for hydrocarbon exploration than other parts of Tirana-Ishmi Depression.
The main arguments are:
1. The presence of source rocks.
2. The favourable conditions for oil-gas generation in these levels.
3. The existence of some migration phases (pre-Tortonian, during Tortonian, and post-Pliocene).
4. Tortonian deposits overlaid transgressively on eroded limestone surface.
5. The presence of gas-oil shows in depth (drilled wells) and at outcrops.
Seeing that exploration results in Tirana-Ishmi Depression have been actually a failure, in this study we conclude: The quantity and quality of seismic data have not succeeded in resolving and mapping the possible traps for oil-gas exploration.
Based on all recent geological and geophysical data, a Neogene trap for hydrocarbon exploration is mapped.
Key words: hydrocarbon exploration, conditions, Tirana-Ishmi Depression, Albania.

INTERPRETATION OF SEISMIC AND GEOLOGIC DATA ON TIRANA DEPRESSION

AHMET COLLAKU
Albseis, Lagja "Liri Gero" Pallati 7/24 Fier, Albania

The study area is situated between Kruja zone and Ionian zone. The surface relief is especially developed from 50 m to 1,000 m over the sea level.
Carbonate mountains are exposed on the eastern part of the area. All the wells drilled in this area are located out of prospects and are of interest in this paper. Thus the area can be considered still unexplored and there exist all the possibilities for oil and gas exploration. The main oil fields of Albania are located in the Ionian zone, whereas in Kruja zone there are a lot of oil seeps.
The seismic grid is not regular, and there are zones not covered by seismic lines. The lines are short and they rarely intersect.
The seismic acquisition in this area, which has begun since 1985, is with magnetic recording. The coverage is not uniform, being from 3- to 10-fold and the system is not homogeneous. The irregular seismic grid and the low coverage influence the quality of data, making it difficult to get a good seismic image of subsurface.
Considering all possible data in this area, and combining the surface geologic data with subsurface information from wells and seismic analysis, several leads or prospects may be predicted. Those are located all over this area and the possible depth is ranging between 0 and 3,000 m.
Further, there are given the directions for improving the quality of data and I believe that there are other areas in Albania where the present information is an interesting prospect for oil and gas exploration.
Key words: seismics, geology, Tirana Depression, Albania.

THE SOUTH-WESTERN LIMIT OF EAST EUROPEAN PLATFORM IN ROMANIA AND THE MOLDOVA REPUBLIC

M. VISARION1, V. NEAGA2, C. VISARION3, LIGIA ATANASIU3
1 Romanian Academy
2 Institute of Geophysics and Geology, Chisinau
3 Geological Institute of Romania

The present paper deals with the study of the metamorphic folded basement of the Epi-Karelian Platform, in the southern Ukrainian Shield. According to the results provided by drillings and by other geographical methods, a structural sketch map with isobaths has been achieved for the folded basement of the platform. The folded basement in the southern and south-western part is covered by a sedimentary cover increasing in thickness to about 1,500 m and even more. Although the Epi-Karelian Platform basement in the western Ukrainian Shield has been studied by many researchers, the present paper takes into account Jeru's works (1987).
Taking into consideration the petrographic, chemical, and genetic data, in this paper we have distinguished 24 petrographic associations which are organized into three principal groups. This succession is characteristic for north Moldova, where the folded metamorphic basement is situated at -200 m, 350 m depth, and it has been intercepted by many drillings in Coonita, Golosnita, Soroca zone. On the territory of the Moldova Republic the drilling has intercepted the platform basement at different depths: Hartop (-358 m), Edineti (-363 m), Balti (-523 m), Prodanesti (-493 m), Cornesti (-925 m), Ungheni (-1,065 m), Chisinau (-1,076 m). On the Romanian territory only some drillings have crossed the basement: Iasi (-1,060 m, -1,076 m), Bãtrânesti (-1,210 m), Todireni (-859 m) and Popesti (-1,210 m).
Several types of rocks, even ultrabasic rocks, intruding the basement are associated in time and space with gothian events (1,750 ± 50 Ma). This fact is based on radiometric investigations which reflect the last tectono-metamorphic activity.
Finally, another problem with an important implication in this area is the presence of a gravity anomaly with a particular morphology, specific to the Epi-Karelian Platform, associated with magnetic anomalies. In this zone, the executed drillings have intercepted on a 30 m thickness, quartzite with magnetite. The study of the drilling data from Veliko-Poloka zone proves a similarity with those of Palazu Mare. Bilikis et al. (1967) studied this zone and concluded that it was extended in the southern part, just near Frunzorka town, its edges being broadened by crustal earthquakes.
Key words: basement, East-European Platform, SW limit, Romania, the Moldova Republic.

THE ROLE OF ALTERATION ZONE ON THE PRODUCTION OF SELF-POTENTIAL ANOMALIES OVER MINERALIZED BODIES

T.D. PAPADOPOULOS, G.A. SKIANIS, D.A. VAIOPOULOS
University of Athens, Trakis 44 Helioupolis, GR-16342 Athens, Greece

Inhomogeneities in earth conductivity may seriously affect the conformation of self-potential anomaly produced by a polarized body. The authors have recently studied the simple case of a single vertical contact. The present paper attempts a mathematical description of the self-potential (SP) curve over a mineralized body, which is embedded in an alteration zone with resistivity 2. This zone has vertical contacts relative to the host rock with resistivity 1. The mineralized body functions as a geobattery and is simulated by a vertical dipole with infinite length.
The mathematical expression for the electric potential (SP) is derived by applying the image technique in the case of a model where a point-pole is laid in a vertical zone of resistivity 2 which is surrounded by a host medium of resistivity 1.
The self-potential V(r) at ground surface can be expressed in terms of a series of images taken by successive reflections across the boundaries of the alteration zone. For a point P out of the alteration zone, V(r) is equal to the sum of potentials produced by the primary source S plus the image sources at the opposite side of the alteration zone. The potential at point P inside the alteration zone is formed by the primary source S plus all the image sources at the left and right sides of it.
Studying the SP field for various resistivity contrasts 2/1 and depth for the mineralized body, the following conclusions can be deduced:
a) An alteration zone with a high resistivity contrast favours the development of strong and rather sharp SP anomalies. On the contrary, over an alteration zone with low resistivity contrasts weak and rather broad SP anomalies are expected to be developed.
b) Deformations in the shape of the SP curve may introduce errors in the quantitative interpretation of SP anomalies, if the ground inhomogeneity is not taken into account.
Finally, it is concluded that whenever an alteration zone with a relatively high resistivity is present, the calculated depth is lower than the true one, if a homogeneous ground is assumed. On the other hand, if the alteration zone has a relatively low resistivity, the calculated depth is larger than the true one.
Key words: self-potential anomaly, alteration zone, mineralized body, analytical modelling.

QUO VADIS BOUGUER GRAVITY ANOMALY?

C.S. SAVA1, P.C. SAVA2
1 Romanian Center for Marine Geology and Geoecology
P.O. Box 72-145, 74600 Bucharest, Romania
2 Schlumberger-GeoQuest, Hannover, Germany

Convinced as we are, just like J.L. Bible, that the "old gravity should never die" and also agreeing with T.R. LaFehr that "by the end of this century geophysicists will need a better understanding of all geophysical subdisciplines", we are trying to contribute to solving the present-day problems of gravity prospecting, actually to modernizing of this geophysical subdiscipline.
The inclusion in gravity prospecting of certain concepts that occurred in geodetic gravimetry, such as Free-air anomaly and Bouguer anomaly, caused a series of problems that compelled the researchers to bring numerous contributions with a view to solving them. Among these it is sufficient to mention the attempt to improve the building of the Bouguer gravity anomaly map by using available densities or the anomalous vertical gravity gradient as well as specifications concerning the significance and the applications point for values of Bouguer gravity anomaly. This leads to a situation of confusion where the same concept – Bouguer gravity anomaly – is used in two distinct domains with different significations.
The authors of this paper plead for removing the confusion that has persisted for such a long time in gravity prospecting, by abandoning the use of terms characteristic for geodetic gravimetry. We consider that it would be better to use the term "gravity anomaly" as such, defined as the difference between the observed gravity and the computed gravity at the same point, the computed value being the effect corresponding to the theoretical Earth identical in shape with the real one, but with a homogeneous density distribution. This value is obviously applied at the measuring point. The simplification of the situation is noticeable because it is superfluous to justify the reduction or corrections, the only necessary operation being the building of the theoretical Earth model.
Our belief is that it is advisable to abandon the term "Bouguer gravity anomaly" to our colleagues geodesists and to adopt that of "gravity anomaly" for gravity prospecting.
Aiming at modernizing the "old" gravity prospecting, we suggest in our paper the use of gravity maps with the contour lines drawn in accordance with the relief, i.e., together with height contours, as well as the use of vertical sections of gravity anomaly along with conventional representations as profiles or plane maps.
We also envisage the generalization of the upward continuation of gravity data from the measuring surface to a plane by using effectual algorithms, e.g., the procedure involving the boundary element method.
Key words: Bouguer gravity anomaly, concepts, terms.

DEEP GEOLOGICAL STRUCTURE OF THE COAS-FIRIZA VALLEY ZONE AS DEDUCED FROM MAGNETOTELLURIC DATA

AL. BALEA, A. IVANOV
Geological Institute of Romania, 78344, 1 Caransebes St., Bucharest 32, Romania

The magnetotelluric researches carried out along two profiles crossing the Coas-Baia Mare Depression – the volcanic areas from the central part of the Gutai Mountains and the northern area of the Trans-Carpathian Depression – have aimed pointing out new elements to solve the regional geological structure.
In order to obtain information up to 10-15 km depth, the electromagnetic field micropulsations have been recorded in the frequency range 0.01-20 Hz. For the increasing of investigation depth, the range of the useful frequencies has been extended up to 0.001 Hz, in a number of ten soundings.
The data processing is based on spectral decomposition of recordings, by using Fast Fourier Transform, the impedance tensor being statistically determined by spectral averaging, made on the least means squares principle. The range of the Fourier spectrum used for data processing has been selected in order to assure the impedance tensor values for all frequencies of the equipment. As the operation frequency ranges cannot be covered by the harmonics scale used, the "decimal cascade" is applied. The final stage of the data-processing program offers the parameters used in data interpretation.
The 1-D inversion of the magnetotelluric data is based on the Marquardt algorithm and uses the resistivity and phase curves. The geoelectrical sections drawn up on the basis of magnetotelluric data interpretation, associated with gravimetric, magnetic and drilling data, have emphasized several new elements, illustrative for the investigated zone, such as:
– emphasizing of the Coas-Baia Mare Basin, having molassic deposits with thicknesses from 500 m up to 1,000 m, probably greater in Baia Mare region;
– estimation of the thickness of the Neogene volcanic formation (800 m in MTS.8-1,200 m in MTS.1);
– development beneath the Neogene volcanic formation of the Pieniny Nappes, unseparated electrically, with an average thickness of 2,000 m, and resistivity values ranging in a large scale;
– development in depth, up to 11 km, of the Neogene intrusive body from southern Gutai, identified in aeromagnetic data, too;
– presence under Baia de Aries Nappes, which are characterized by high resistivity values, of a conductive horizon, which might correspond to the specific structural elements of the Transylvanides (south of the Coas locality);
– a normal character of the transcrustal Preluca Fault, along which it is made the suture between the formations belonging to the Inner Dacides and those belonging to the Median Dacides;
– existence of the Transition Zone, identified by low values of the apparent resistivity (13.8-52 ohm.m) and having a thickness of about 5-6 km. The appearance of this zone may be explained by the brittle-ductile transition at temperature conditions of 350°-400°C.
Key words: magnetotellurics, deep structure, Coas-Firiza Valley, Romania.

PETROELECTROGRAPHY FOR PERMEABLE ROCKS WITH APPLICATIONS IN THE GEOLOGICAL AND GEOELECTRICAL DOMAINS

D. SVORONOS, A. NICOLESCU, C. NITICA
Geological Institute of Romania, 78344, 1 Caransebes St., Bucharest 32, Romania

The electrographic effect for rocks was observed in 1992 as a photoluminescent phenomenon, when a sample of sandstone with mica was subjected to a high voltage electrical field.
The fundamentals of petroelectrography can be easily understood by placing a sample of rock, with a film attached around it, between the two conductors of a Leyden bottle. Such a bottle was used mostly in the 18th and 19th centuries as a capacitor, in order to produce an electric spark. For certain high electric potentials, the rock sample and the dielectric are penetrated. As a result, the film attached around the rock sample will show photoluminescent effects. Certainly, this will completely discharge the capacitor.
In our working method, the Leyden bottle is replaced by a monoimpulse generator. The rock sample, the dielectric, and the film are all part of our recording system.
The water content and the amount of free water are essential for the electrographic picture of a given rock sample.
We have created various working techniques to generate information either from the rock surface or from the rock sample interior. In this way there have been studied various sedimentary rocks such as sandstone, limestone, and marl. Petroelectrographic pictures have been then analyzed on the basis of a computer program.
Petroelectrography is a nondestructive method for investigating permeable rocks, which enables us to analyze how water penetrates the rock pores and also the morphology of pores.
Key words: petroelectrography, permeable rocks, pores.

INTERPRETATION OF MEASUREMENTS WITH "SWIFT MULTIELECTRODE SYSTEM" BY MATHEMATICAL MODELLING

DAN CAPRITA
Research Institute for Environmental Engineering
294 Splaiul Independentei, Bucharest, Romania

This paper intends to present a methodology allowing the interpretation by mathematical simulation of the measurements supplied by "Swift multielectrode system".
The Swift multielectrode system is designed for a fast and flexible automatic survey in the field. A complete system consists in: one interface box and up to 254 electrode switches placed on each electrode and connected by a multilead cable to the central interface unit. The switches are capable of connecting any combination of the Sting terminals (A, B, M, N) to the electrode.
The Swift system is controlled either directly by the Sting or by an external MS DOS type computer. Measurements are taken by the Sting and stored in the internal memory or on the PC hard disk if a computer is controlling the electrode array. The Sting is capable of directly controlling the most cases, there being no need to bring a computer to the field. If there is a requirement for a special type of array and performance of measurement, the Swift-PC software is run on an MS-DOS type PC.
Unfortunately, a simple methodology to interpret the measurements carried out by Swift system does not exist. For this reason, we propose a methodology to interpret the data supplied by Swift system through mathematical simulation by means of a finite element method.
The proposed algorithm has in view the fact that any electrometric device assumes at least two switch plugs (to inject and to take out) and one or more measurement points. From the viewpoint of modelling a current electrode represents a primary source and for this source the electric potential is supplied in all discretization points of the simulated field. Thus, for every source a vector of potentials is obtained in all discretization points.
Finally, all types of excitation devices which are linear combinations of primary sources (the same of the 254 electrodes) and for each excitation device any combination of measurement electrodes can be simulated.
There exists no restriction concerning the simulated measurements field, that could have any shape, size, or specific features.
Key words: Swift system, mathematical simulation, interpretation.

STUDY OF DUMREA DIAPIR BASED ON GEOPHYSICAL
AND GEOLOGICAL DATA

KRISTOFOR JANO, TELO VELAJ, HETEM SEITI
Oil and Gas Institute, Fier, Albania

Evaporitic diapir of Dumrea extends on internal subzone of External Albanides (Ionian Zone). Its surface is about 210 squared km and its volume about 2,000 cubic km. Thus, it is the biggest diapiric outcrop of Albanides and Hellenides.
The Dumrea Diapir, which has an ellipse form, is surrounded by ring shaped flysch deposits (Pg13, Pg23, Pg33). According to geological data, it is represented as an ordinary salt dome such as Moreni Diapir in Romania, or Golf Coast in USA, or Pricaspik Diapir in Russia.
At the beginning of exploration in this area, the external parts of diapir were considered by many authors as the most perspective part. The reservoir type was prognosed the tectonic one. We underline that this conclusion is based only on the interpretation of geological services only.
On the basis of the above studies, the result of deep drilling has been a failure but it may clear up and explain the geological phenomena related to the tectonic model and its origin. Under these circumstances, it was decided to carry out geophysical surveys (seismic analysis, gravimetry, electrometry), in order to clarify the geological model and perspective play, etc.
A dense grid of seismic lines with about 20 fold (digital recording) including surrounding zones was carried out. There are plenty of good seismic marks under the salt sequence by which there is possible to throw light on the tectonic model of diapir and on the relationship with carbonatic structure around and to map buried carbonatic structures under.
Thus the Dumrea diapir may be represented as an old salt dome which in the course of the compressional stage was overthrusted towards west loosing its roots.
Many check shots were used to convert from time to depth the seismic horizons because there are dramatic changes of velocities vertically and laterally.
Based on electrometric and gravimetric surveys it was possible to establish the contact of evaporite with surrounding deposits.
The evaporitic facies is represented chaotic, but under the evaporitic facies there are plenty of seismic markers which are interpreted as reflections from carbonatic uplifts.
Using all integrated geological and geophysical data it was possible to establish the geometric form of the diapir and the geological structure under it, which was interpreted with the presence of carbonatic structures. These structures are expected to be more favourable for oil and gas exploration.
Key words: seismics, salt diapir, Dumrea, Albania.

TENTATIVE UNDERGROUND GRAVITY WITHIN ALTAN TEPE MINE.
A FEASIBILITY STUDY

LUCIAN BESUTIU, ADRIAN NICOLESCU, VLAD ROSCA
Geological Institute of Romania, 78344, Bucharest 32, 1 Caransebes St. Romania

Problems connected with underground gravity have been treated for a long period of time all over the world. To implement this method, as a help in mining exploration, during the years many attempts have been also made in Romania. But, as a rule, they simply consisted of routine measurements followed by classical procedures in data processing. Tentative models for the anomalous sources were rarely performed.
Unlike the above mentioned works, this paper is intended to answer the basic questions related to the gravity noise range within such circumstances and to accordingly evaluate the confidence level for the anomalies pointed out.
Consequently, a feasibility study related to the applicability of the method in the exploration of the Altan Tepe copper mine area was performed.
By summing up the evidenced errors, a general confidence level was set to ± 0.3 mgals.
Based on these results, a study examining the resource of the method was then attempted by comparing the confidence level with gravity signals provided by various ore bodies (as known from the previous mining activity).
One of the most surprising results of the researches carried out was the revealing of the so-called "blind zones", namely, the areas in which the exploration geophysicist cannot "see" the target bodies. The performed models showed that there could be such an ineffective gravity area even in the close neighbouring of the ore bodies. Obviously, the shape and size of such zones mainly depend on the geometry and size of the target.
The paper also includes a brief review of some practical results gathered within the Altan Tepe mining area. Various forms of data presentation (including 3D projections) are reviewed and critically analyzed.
Key words: gravity, underground survey, noise, Altan Tepe, Romania.

A NEW HYPOTHESIS OF OIL FORMATION UNDER NATURAL CONDITIONS

V.V. BERKOVSKAYA
Ukraine State Geological Prospecting Institute
78 Avtozavodskaya St., 254114 Kiev, Ukraine

A brief general information is given on a new hypothesis of oil formation.
The emerging problem is equivalent to the well-known hypothesis of organic and inorganic oil formation.
For the first time the author has grounded and shown the oil synthesis process as:

It is shown on the example of south-eastern part DDV that the depletion perspective of oil and gas structures at small depth and a transition to prospecting on deeper horizons require a special methodological approach to their study using new achievements in the fields of geology, geophysics, geochemistry, nuclear physics, cybernetics, etc..
The following problems have been solved, based on the synthesis of theoretical developments, experimental investigations in laboratories and borehole observations:
1. The effect of the change hydrocarbon under the influence of radiolysis reaction is found.
2. Oil deposits are timed to reservoirs of increased natural radioactivity.
3. The analysis of exposed anomalous high pressure zones shows that they are concentrated in the catagenes zone in which top compression of hydrocarbon deposit results in the difficulty of both gaseous and liquid fluid distribution.
4. One of the reasons of the anomalous high-pressure zones related to secondary processes taking place in reservoir under the influence of increased radioactivity is shown.
5. The study of geochemical situation of some facies shows that the most favourable situation for the formation of low ohmic productive reservoirs is that of lake and marsh facies with the biggest iron ion mobility.
6. Definitions of low ohmic productive reservoirs and geophysical model of facies are given.
The results obtained permit us to propose a new scheme of oil- and gas-bearing evolution which gives a logical explanation to the majority of incomprehensible facts from geochemistry and geophysics on oil and gas.
As an example, it is shown that the application of this hypothesis to the oil-origin theory significantly improves its explanatory and predicting possibilities.
Key words: oil, synthesis, geochemistry, evolution.

EVALUATION OF THE EXPECTED GROUND ACCELERATION FROM EARTHQUAKES ON LANDSLIDE TERRAIN

A. BOYKOVA, ST. SHANOV
Bulgarian Academy of Sciences, Geological Institute
G. Bonchev St., Bl. 24, 1113 Sofia, Bulgaria

Methodology for evaluating the seismic hazard for dams, power plants, industrial and civil buildings has been elaborated in the Laboratory of Seismotectonics of the Geological Institute, Bulgarian Academy of Sciences. It gives the possibility of seismic microzoning by using geological and geophysical data and of determination of the maximum possible intensity (scale MSK-67) and acceleration (in units g) according to the real ground conditions of the studied terrain. The seismic microzoning for every concrete region needs a preliminary synthesis of the available information on the seismic velocity characteristics of the upper ground layer, where the building foundation will be placed, its density characteristics, as well as its thickness. This type of studies is very important for landslide terrain in seismic active regions. Special investigations on such types of terrain have been performed on the northern Black Sea Bulgarian coast in 1994 and 1995.
From the Catalogue of the earthquakes for the Bulgarian territory, an extract of 67 earthquakes recorded since 1900 up to now in a 100-km radius around the studied territory was selected. The strongest shaking was in 101 (Shabla earthquake, M=7), leaving considerable landslide deformations along the sea coast. All calculations were performed for an earthquake with the same foci and the same magnitude with a probability of realization of 0.3% per year.
The acceleration and intensity were evaluated for each geophysical point taking into account the real geological conditions. These 48 geophysical points were irregularly situated on the studied territory and the geostatistical analysis was performed (in the sense of Matheron, 1970), through variogram analysis of parameters, choice of mathematical model, reflecting their intrinsic structure and interrelations and resolution of the kriging system of linear equations. The final results were maps of the studied parameters. All these procedures were performed with the geostatistical software GEO-EAS.
The maps of the maximum possible intensities and accelerations from earthquakes reflect correctly the seismic risk and give to constructors the tendencies for changing the standards towards the construction security. The maximum horizontal acceleration values for the studied landslide terrain are between 0.37 and 0.46 g.
Key words: earthquake, landslide terrain, ground acceleration, Bulgaria.

LOCAL AND GLOBAL ANISOTROPY IN TWO-DIMENSIONAL STRUCTURES.
A NEW ESTIMATION METHOD

CRISTIAN IOANA, FLORIN MUNTEANU, CRISTIAN SUTEANU, DOREL ZUGRAVESCU
" Sabba S. Stefanescu" Institute of Geodynamics of the Romanian Academy 19–21 J.L. Calderon St., 70201 Bucharest, Romania

Among the properties emphasized by the complex patterns of the earth's crust, anisotropy is particularly interesting and important, since it can offer precious information on the generating conditions of crack patterns. It may be essential for the dynamic properties of the structures comprising cracks. In the study of polygonal ground, one should be able to appreciate quantitatively the anisotropy implied by solifluction phenomena, that may lead first to block stripes and then to striped ground. To overcome the disadvantages and difficulties implied by the usual methodology (Orientation Distribution Functions, by which cracks with direction changes, width fluctuations, bifurcations and intersections, are difficult to evaluate in an unambiguous way, while ODF say nothing about the contiguity of cracks, and the weight of crack widths in the structure anisotropy is hard to estimate), we propose a new method for anisotropy evaluation, called Angle Distribution of Percolation Length (ADPL). The idea of the proposed method is to consider the length L = L(,) over which a crack structure percolates inside a window of a given width w and having the orientation . The evaluation is performed – for a given width w – in different positions of the window and for angles between 0 and 2, with a step . One can obtain:
i) a global evaluation, expressed by a rose diagram representing the mean percolation length for each direction and for all positions;
ii) a local characterization, highlighting the spots and directions with a particularly high anisotropy (local maximum percolation lengths are retained in this case).
When a power law dependence of L upon the width w is found, then its exponent characterizes the structure in a qualitatively new way. The weight of the crack widths in anisotropy is estimated via a "standard image" built by erosion/dilation techniques. The effectiveness of the proposed method is proven on crack structures found in nature (including surfaces of other planetary bodies) or produced in our laboratory experiments.
Key words: structures, anisotropy, local, global, crack structures.

A METHOD FOR THE ANALYTICAL DETERMINATION
OF DECLINATION FROM THE Z-MEASURED VALUES

D.V. STEFLEA, VALERIA ROSIN
Geological Institute of Romania
23-25 Dimitrie Onciu St., Bucharest, Romania

We propose in this paper an analytical method for the determination of magnetic declination based on Z- or Z measured values.
The first step is represented by the elimination from the Z data of the normal field, calculated with the formulas for IGRF (International Geomagnetic Reference Field) for the observation point. In this mode it is obtained the Z value, the anomalous part of the vertical component.
These Z values are interpolated in a regular network (quadratic or rectangular) with a number of points compatible with FFT (Fast Fourier Transform). This is the next step in which the data are transformed in spectral domain with the direct Fourier transform.
Through multiplication by adequate operators, S(Z) (Z spectrum) is transformed in X and Y spectra, which are the spectrum of North magnetic component and the spectrum of East magnetic component.
By applying the inverse Fourier transform, the values X and Y are obtained.
Another step is the calculation of XN and YN, the normal IGRF values for the North and East components, and the summing of the total values X and Y.
The magnetic declination is D = artg(Y/X); D = D-DN,
where DN is the normal IGRF declination.
Thus, we obtain for every point of the network the D value and D value.
If the distribution of data is in a two dimensional form, by interpolation the maps with isogones are obtained.
The method has been successfully tested on theoretical models and an example of its application in a real case is presented in this paper.
It is presented the map of magnetic declination at 1990.0 for Romanian territory obtained on the basis of Z values measured in 550 points of national absolute measurement network. The comparisons with the D values obtained with direct measurements indicate a good precision of the order of 5 minutes.
The presented method may be used for regional or local networks, the images obtained being suitable for the improvement of interpretation.
Key words: magnetic declination, vertical component, maps, Romania.

REGIONAL AND LOCAL ASPECTS SEPARATED BY PROCESSING
OF GDS (GEOMAGNETIC DEPTH SOUNDING) DATA

A.A. SOARE, D.V. STEFLEA, GABRIELA CUCU, VALERIA ROSIN
Geological Institute of Romania
23-25 Dimitrie Onciu St., Bucharest, Romania

For the geomagnetic method (GDS) the most frequently transfer functions used are the induction vectors resulted from a linear relation between the variational magnetic components. The geomagnetic induction vector determined for a certain observation point is contained in a preferential plane. Taking into account that the force lines of the induced magnetic field tend to be parallel to a contact surface between two media of different conductivities, the orientation and inclination of the inductor vector can be indicators for the existence and geometry of such a contrast in the subsoil.
The Romanian territory has been covered by a relatively dense station network, where the induction vectors obtained on the basis of temporary geomagnetic recordings present a feature which can be correlated with some major elements of the geological structure.
In order to try the separation of effects produced by different regional and local causes, we propose a method which offers the possibility to obtain information on the "regional" degree of different sources of observed anomalies.
The general form of the induction vector (Wiese) represents the relationship between Z, H and D variations: Z = aD + bH.
We represent in the form of maps the separate distributions of a and b values for all the vectors measured in the studied zone.
These maps are analyzed by polynomial approximations of different orders:

The coefficients K1 are determined and the surface which fits better the distribution of data is established. The images of different orders offer information about the position and the depth of the anomalous sources.
This kind of analysis has been realized for all Romanian territory and for selected zones. In this paper there are presented the results for Dobruja zone and we have obtained images indicating the most important faults and structural blocks. At the same time, it is remarked a "coastal effect" produced by the Black Sea.
Key words: GDS, Wiese vectors, maps of transfer functions, Romania.

MAGNETOTELLURIC SOUNDINGS IN DECIPHERING
THE GEOLOGICAL STRUCTURE OF THE EAST CARPATHIANS

C. DAVID, A. COVRIG, M. MICU
" Prospectiuni" S.A., 1 Caransebes St., 78344 Bucharest, Romania

During the last five years magnetotelluric soundings (MTS) have been carried out for a detailed investigation programme of the East Carpathians Flysch Zone, in order to evaluate the hydrocarbon perspectives and depths over 5,000 m.
The processing of more than 600 MTS performed by us in the Flysch Zone up to now shows that the dominant phenomenon is the electrical polarization of the electromagnetic field. It develops in the same direction as the main structure of the Carpathian Flysch.
The relations between the impedance tensor elements have proved that the 3-D effects are relatively low up to the contact zone between the Carpathian Nappes and their underthrusted foreland. Therefore, the general structure may be approximated as a 2-D model whose uniformity axis has the same direction as the structures of the Carpathian Flysch. This aspect has been also reinforced by the polar diagrams of the components belonging to the impedance tensor.
Consequently, the quantitative interpretation of MTS has been made by using 1-D inversion. These data, together with geological and geophysical surveys, have enabled us to obtain interpretative geological sections over the studied area. These sections have mainly pointed out the relations between the overthrusted units and their substratum, but valuable and very useful data concerning the internal structure of different tectonic units, especially at great depths, have been obtained too.
As for the deep structure of the East Carpathians Flysch, we realized MTS profiles which cross entirely the mountain area approximately W-E, starting from the eastern border of the Transylvanian Basin up to the foreland zone. They are very useful in deciphering the deep structure of the underthrusted basement connected with the important compressional phenomena occured at the end of the Upper Cretaceous.
Key words: magnetotellurics, flysch zone, polarization, East Carpathians, Romania.

TELLURIC MEASUREMENTS AT THE SURLARI OBSERVATORY

CARMEN DUMITRESCU
Geological Institute of Romania
23-25 Dimitrie Onciu St., Bucharest, Romania

Forty years after the results published by Constantinescu and Milea we succeed in settling a new and modern digital station for recording the telluric field at the Surlari Geophysical Observatory (IAGA code USA). The observatory, established in 1943, is well known as the reference station for the normal geomagnetic field and secular variations.
The characteristics of the station are:
- up to six telluric components with a resolution of ± 20 V;
- a frequency range of 1 Hz ÷ DC;
- a sampling rate programmable between 2 Hz and 1/60 Hz (120 samp./min. ÷ 1 samp/min).
The data are recorded on a 3.5" floppy disk which has to contain the operating system MS-DOS and the program for data acquisition and control.
In order to avoid the frequent breaks in power supply, the station has a double alimentation: 220 V and 12 V.
The station is fully compatible with an IBM-PC/AT computer.
The electrodes are placed at the ends of two orthogonal lines, having a 100-m length. The old lead electrodes have been replaced by the impolarizables Pb + Pb Cl2 electrodes, buried in a salted mud.
The electrodes have 180 mm length, 32 mm diameter and 235 g weight. They have been buried with the porous part downward in order to ensure a good contact with the soil.
For the purpose of studying the telluric field in the period range of 1 min ÷ 36 hours, the electrodes have been buried at 2 m depth.
During the first 40 days the electric data have been analyzed only for the NS and EW components. Starting with March we have recorded the electric field on 6 channels: NS, EW, NE, NW, SE and SW.
We are planning to use and analyze this information for two different reasons: editing a telluric yearbook and monitoring precursory features generated by earthquakes.
Key words: telluric field, recording station, Surlari, Romania.

COMPUTATION OF GRAVITY GRADIENT TENSOR
IN A RECTANGULAR SYSTEM OF PRISMS
FOR VRANCEA ZONE, GALATI-FOCSANI ALIGNMENT

DORINA DANCHIV, D. STIOPOL, V. NACU, A. BALA
National Institute for Earth Physics
P.O.Box MG-2, Magurele-Bucharest, Romania

The paper presents a method of computing the gravity gradient tensor resulting from a system of right rectangular prisms into 3-D method. The investigated area, located on Galati-Focsani alignment is characterized by a strong subduction developed on SE-NW direction and responsible for the tectonic accidents in the crust. Eastward to Focsani, the upper crustal layer has a thickness of about 8-10 km which near Galati obviously increases, 22 km. The thickness of the lower crustal layer is 15 km eastward to Focsani, and it reaches up to about 24 km in Galati area. This thickness is due to Moho discontinuity. Gravimetric studies carried out in the Focsani-Galati subduction area have evidenced the presence of an important negative anomaly, its values increasing towards the West. Bouguer anomaly reflects the effect of crystalline basement surface rising.
The basic method of computing the gravity gradient tensor consists in dividing, which may vary from one prism to another. Theoretically, the computation method starts with the equation of the gravitational anomaly generated by a rectangular prism into an observation point "P". The computer program is in FORTRAN 77 language and, since it does not require a large memory, it can be run on a PC. The computer program consists of a main routine of 3 subroutines. The main routine calls the INPUT, GRGRD, and OUTPUT subroutines. INPUT subroutine reads the input parameters such as density, distance (km), and number of prisms. The axes of coordinates are in the Cartesian system with Z position downwards. If Z = 0, the density gradient tensor calculation is made for a 2-D model. Density gradient tensor is calculated by GRGRD subroutine. OUTPUT subroutine is the subroutine for writing, and displays the input data and the results of computations. It is designed so as to display the dimensions of the rectangular prism sides (in km) on three directions (x, y, z), the number of rectangular prisms on each axis of coordinates, the density of each axis of coordinates, the density of each prism (in g/cm3), and the components of the gravity gradient tensor, in mgal unit, for first-order derivatives and in "Eotvos" unit for second order derivatives. The computer program has been tested on a synthetic model consisting of 4 rectangular symmetrical prisms of 1-km sides and 2.65 g/cm3 density. The results are graphically shown and they are similar to the theoretic results obtained by other methods.
Key words: gravity gradient tensor, software, Vrancea area, Romania.

ACCELEROGRAPHIC NETWORK FOR STRONG-MOTION DATA ACQUISITION EXISTING ON THE ROMANIAN TERRITORY

GAVRIL DANCI
National Institute for Earth Physics, P.O. Box MG-2, Magurele-Bucharest, Romania

It is known that Romania, throughout her geological history, has been subjected to many great earthquakes.
Earthquakes represent an important source of information for the technical and scientific activity.
In order to understand the seismic motions, directly instrumental data are necessary.
Seismic activity control in Romania is based both on the recordings supplied by the seismic network of the National Institute for Earth Physics (NIEP) and on the accelerographic network coordinated by NIEP, the Building Research Institute (INCERC), and the Institute of Geotechnical and Geophysical Studies (GEOTEC).
We have developed a methodology that incorporates the fundamental elements of decision theory for distribution of the recording instruments throughout the country.
This paper presents the recent stage of Romanian accelerographic network, referring particularly to the strong motion recording network coordinated by NIEP and INCERC.
The intensity estimation has been performed on the basis of a new methodology.
Given the high randomness of instantaneous values of ground acceleration, the instrumental criteria should not be directly based on the values of such parameters, but rather on some significant functions of them. The parameters of such functions should account, first of all, for some spectral characteristics of ground motions which are considered to be the most appropriate start point for the instrumental intensity evaluation of any earthquake.
Key words: accelerographic network, Romania.

CONTRIBUTIONS TO THE ESTIMATION OF SEISMIC HAZARD IN THE BANAT REGION (ROMANIA)

CORNELIUS RADU, TRAIAN MOLDOVEANU
Institute of Geotechnical and Geophysical Studies (GEOTEC)
5-7 Vasile Lascar St. 70211 Bucharest, Romania

The Banat region located in the south-western Romania is characterized by a relevant seismic activity. So, in this century, 28 earthquakes with M4.1 (IoVI) have been identified; the strongest one (MS = 5.7; 5.6; 5.6) occurred in the second part of 1991, the year of the highest observed seismicity. The events have a shallow depth (maximum -25 km; minimum -4 km) and are associated with aftershocks/foreshocks. The correlation of earthquake face with local tectonics is remarkable. For the estimation of seismic hazard in the Banat region, the theory of largest values (Gumbel distribution – I and III) and Cornell method have been used. The characteristic parameters of Gumbel distribution for magnitude (MS) and epicentral intensity (Io), for two time intervals (1901-1993; 1781-1993; M4.1/IoVI), are computed.
The application of Cornell method allows us to draw the isoacceleration contours for different return periods (TR): 50, 100 and 200 years. The quantitative study of the seismicity in the Banat region confirms its highly seismic potential and the necessity to adopt real measures for the antiseismic protection in the area.
Key words: earthquake, seismic hazard, magnitude, iso-acceleration maps.

A LATE TRIASSIC FOLDED BELT BENEATH THE MOESIAN PLATFORM: IMPLICATIONS FOR THE CIMMERIDE OROGENIC SYSTEM

GABOR C. TARI1, GEORGI GEORGIEV2, MIHAI STEFANESCU3, GARY WEIR1
1 AMOCO, 501 Westlake Park Boulevard, WL I, 2.372, Houston, Texas 77079-2696, USA,
2 Sofia University, Bulgaria
3 AMOCO Romania

Widespread folding of the Pre-Jurassic sequence underneath the Moesian Platform of Romania and Bulgaria is traditionally attributed to the Late Triassic Cimmeride phase (e.g. Georgiev and Atanasov, 1993). This Carnian-Rhaetian Cimmerian compressional stage replaced an Anisian-Ladinian aborted rifting period in the Moesian Platform (e.g. Pene, 1994). Although the Cimmeride folds were undoubtedly formed due to compression, detailed structural analysis of seismic reflection and well data permitted a more specific interpretation of the deformation in terms of a northvergent thin-skinned thrust-fold belt.
In our interpretation the Cimmeride folds are actually 10-30 km wide ramp anticlines eroded to highly variable stratigraphic levels (Middle Triassic-Silurian). Between the anticlines a 0-2000 m thick Upper Triassic classic sequence, the Segarcea Formation in Romania (Paraschiv, 1981) or the Moesian Group in Bulgaria (Vaptsarova et al., 1984), corresponds to the syntectonic growth of the structures. Using the recent modelling results of Hardy and Poblet (1995) the very characteristic internal geometry of the growth sequence (i.e. onlap on the forelimb and thinning and truncation on the backlimb) suggests the fault-bend folding origin (in the sense of Suppe, 1985) of the Cimmeride folds. Given the thick (>8 km) Triassic/Paleozoic succession in the Moesian Platform (e.g., Paraschiv, 1982), the upper decoupling level seems to be localized in a Permian halite and anhydrite horizon, whereas the lower detachment probably runs close to the base of the Paleozoic. Since the observed Cimmeride anticlines show a consistent northerly vergence the lower detachment is not associated with an evaporite and/or salt horizon based on the findings of Davis and Engelder (1985). On the basis of the fault-bend fold geometry cross section balancing suggests a relatively minor shortening (5-20%) associated with the Cimmeride deformation.
To the South, within the Balkanides, the relics of the Cimmeride orogen were classified by Boyanov et al., (1989) as single folds, fold chains and folds displaying the involvement of the basement as well. The spatial pattern of these deformational styles clearly indicates the gradual increase of shortening in a southerly direction. Indeed, the Cimmeride nappes in the Strandzha region (e.g., Chatalov, 1988; Gochev, 1991) represent the more internal part of the orogen. Thus, in a wider paleotectonic scenario, the northvergent anticlines beneath the Moesian Platform are interpreted as the frontal, foreland thrust-fold belt of the Mediterranean Cimmerides of Sengör (1984). The recognition of a Cimmeride foreland thrust-fold belt in the Moesian Platform area has an important impact for the reconstruction of the western Paleo-Tethys (cf. Sengör, 1984; Dercourt et al., 1993).
Key words: Moesian Platform, folding, Late Triassic, Cimmeride folds.

FAULTS IN THE SEISMOLOGICAL SELECTION FORMATION OF THE DNIEPER-DONETSK DEPRESSION (DDD)

V.V. MUZYKA, E. DVORIANIN, M. VERPOVSKIY
Ukrgeofizika, 5 Zapadynska St., 254123 Kiev, Ukraine

The subvertical faults with relative block displacements running along them played an essential part in the formation of the DDD inner structure. Such displacements result in the overburden layers deformation and, in particular, in the formation favourable for hydrocarbon accumulation structures. The faults serve as channels for the salt and hydrocarbon fluids migration.
The potentials of seismic methods for the faults location are demonstrated on two regional seismogeological traverses, crossing DDD from the Ukrainian shield to the Voronezh crystalline massif. The kinematic as well as dynamic characteristics of the seismic records show that, in particular, in the lower parts of the cross-sections many faults were clearly determined as some specific subvertical zones of the loss of correlation on seismic recording. The boundaries of such zones, having a breadth from 1 to several kilometers, correspond, as a rule, to the displacements and breaks of seismic reflections. The identification of such zones with the faults is effectively confirmed by the spatial coincidence with boundaries of the Pre-Cambrian basement blocks, a fact which is of a special interest because of the latest hydrocarbon discoveries in this region basement.
Using as an example the tectonic and geodynamic peculiarities of a definite sector of Northern Board Zone of DDD, connected to the inner basement depression, the authors present the concepts of second-order tectonic structures (reverse-staged blocks) forming mechanism, based on the data of modern geology and geophysical exploration.
It is achieved the prognosia of the said favourable for hydrocarbon deposits exploration blocks within the limits of Northern Edging fault of DDD.
Key words: faults, seismics, Dnieper-Donetsk Depression, Ukraine.

SALT FLANK IMAGING USING 3C VSP DATA. A CASE HISTORY

TH.K. TER BURG
Schlumberger Petroleum Services C.V., Laan van Meerdervoort 55, 2517 AG The Hague, The Netherlands

An offshore borehole seismic survey was carried out for Clyde Petroleum Company B.V. in the North Sea, North off the Dutch coast. The objectives comprised: the resolving of the well structure, an accurate definition of the flank of a salt dome located east of the well, and the confirmation of a possible fault near the salt dome at approximately 2500 m depth, in a cost-effective manner. The pre-survey interactive modelling showed that all the objectives could be met by utilizing a simple rig source VSP survey.
The source used for the survey was a tuned sleeve gun array (3x150 cubic in) fired with a pressure of 120 bar. Downhole data were recorded with the CSI® (Combinable Seismic Imager) three component decoupled downhole geophone assembly. The CSI® is a new generation three-axis borehole seismic tool. It is designed for a superior performance and combinableness with other CSI® tools, to form an array of sensors, or with other tools.
In order to achieve a reliable model, the initial velocity-depth model is updated by using an iterative VSP inversion technique in conjunction with VSP-derived velocities. As it is a VSP property to record reflections of dipping layers in their maximum dip direction, the VSP aided reconstructed model will represent a 2-D projected subsurface plane in that maximum dip direction.
As the VSP data are processed by using 3 components rather than only a standard vertical (Z) component it becomes possible to effectively attenuate distorting shear wave energy by means of parametric wavefield decomposition techniques. This allows migration of a practically noise free reflected compressional wavefield. The iteratively updated velocity-depth model will be input to GRT® migration resulting in a most accurate image of the subsurface in time and depth of the VSP illuminated plane.
The method used to transform VSP data coordinate systems into CDP-offset vs time is based on a type of Kirchoff migration, called GRT® migration (Generalized Radon Transform). Conceptually, the method is based on Huygens' principle, where the subsurface can be thought of as a grid of diffraction points. Here the subsurface is assumed to consist of a dense grid of tiny reflectors, each with a ±3° dip range aperture defined by the iteratively updated velocity-depth-background model.
The three component VSP-processed product of migration shows improvements in reflector identification and continuity over a single component VSP. In particular, the top Keuper salt reflector can be traced eastwards, into an area of poor data on surface seismics.
The improvements are due to the parametric decomposition, allowing better separation of the individual wavefields in the three-component dataset. In the vertical component only, S wave attenuation is attempted through velocity filtering, which leads to smearing and blurring of the target reflectors. Three-component processing allows a recombination of P reflections from Z (true vertical) and HMX (maximized horizontal) components, improving the imaging.
The three-component VSP has allowed a structural reinterpretation of horizons adjacent to a salt wall by introducing information not available to a surface seismic survey.
Key words: seismics, offshore, borehole, salt dome, North Sea.

DETECTING COAL SEAM BY USING SHALLOW SEISMIC REFLECTION

B. ZDRAVKOVIC, D. BROTINA, M. BERIC
Geophysical Institute, Belgrade, Yugoslavia

The majority of companies use very often the drilling method for coal exploration and evaluation of reserve on a certain area. This kind of evaluation is expensive and gives data in one point while seismic survey gives continual vertical cross section, information between wells and cost is very low.
This survey is performed in Kuvin Depression, the south-western part of Pannonic Basin, where there are two coal fields.
In this paper, the target of seismic survey in coal exploration is definition of space extension, folding and thickness of coal layers, an impression of general nature of tectonics and structures and information on vertical movement.
Shallow reflection seismic, high resolution technique, allowed us to follow coal seams and to define their extent. Choice of adequate field parameters – charge size, geometry, time and space sampling and use of modern processing programs with optimal numeric treatment – allowed us to obtain good quality seismic sections.
Key words: seismics, reflection, coal exploration, Kuwin Depression, Yugoslavia.

SEISMIC REFLECTION TESTS FOR COAL

BOGDAN STANCHEVICI, STELIAN CIOBOTARU, PAUL CRISTEA
Geological Institute of Romania
1, Caransebes St., 78344 Bucharest, Romania

Recent progress concerning reflection seismic exploration techniques has allowed the approach of higher resolution seismic studies in order to investigate the location of profitable coal seams (more than 2 m thickness). Within the framework of these investigations, we performed our experiments in some regions of Romania.
The strong petrophysical contrast between lignite coal and host sediments, especially due to its remarkably lower density, results in highly enough reflection coefficients of the seismic waves.
The target horizons of the reflection seismic exploration for coal are located at shallow depths of about 300 m. For this reason, it is used the term "shallow seismic" and we applied the corresponding modifications for recording parameters.
Experimental works were carried out in a few coal areas – south Brasov, north Targoviste, and west Covasna.
The field arrays and the seismic recording equipment parameters were selected for our experiments by considering the mathematical reduction of NMO stretch and errors, enhancing accuracy of RMS velocities, attenuating multiple waves improving signal/noise ratio, and also by tests in order to determine a suitable shot-depth as well as the best relationship between the charge size/seismic amplitude and the recorded seismic signal frequencies. The amplitude spectrum moves to high frequencies as the charge size decreases.
Our experimental recordings were performed under different generating and recording conditions, weathering zone thicknesses varying between 2–4 m and 35-50 m. Recording parameters were as follows: 48 channels IFP-SD 16 field equipment; 1 ms sampling rate; no filtering; Anstey array with four 8 Hz geophones at a 3-m spacing; 6 m geophone station spacing; 1s recording time; 10-30 m end-on offset; 2-4 m shot depth; 100 gr charge size. Field data processing was similar to the oil exploration one.
We performed analyses of the synthetic trace processing based on the convolution between the reflectivity series, corresponding to the coal seam locations and Ricker impulse.
The comparison of synthetic traces and seismic section shows a good agreement with respect to favourable shooting conditions (2-3 m weathering zone thicknesses) and higher frequencies of seismic waves (over 60-80 Hz).
The accomplished research proves a good capability of the seismic reflection method for the investigation of thin coal seams (1-2 m thicknesses) and, also, the utility of comparative analyses of synthetic traces and time section in order to improve the seismic exploration data interpretation for coal.
Key words: seismic reflection, coal seams, high resolution, synthetic trace.

POST-STACK MIGRATION METHODS IN PRACTICE

VIRGIL BARDAN, CRISTIAN CAPRA, ELENA CHISCAN, IOAN GHITA
" Prospecþiuni" S.A., 20 Coralilor St., 78449 Bucharest, Romania

A conventional stacked (zero-offset) seismic section is the response of a subsurface on a two-dimensional plane of profile. It can be considered as the image of a passing wave field recorded at selected points on the Earth's surface. In regions of complex geology, this image may bear little resemblance to the image of subsurface reflectors. Migration is the process used to transform the wave field of a seismic section into another, which represents a reflectivity subsurface image. For this reason, any seismic migration method should be related to a solution of the wave equation. Such a solution can be derived from either an integral (summation) or differential (finite difference) form of this equation. The problem of migration can be approached either in the space-time or in the wavenumber-frequency (or in wavenumber-time, or in space-frequency) domain. The output of a migration method can be a time section (in the case of time migration) or a depth section (in the case of depth migration). Certainly, the approach for a migration method yields an approximate solution of the same wave equation. Although these approximations are not identical, the different migration methods work approximately in the same manner on stacked seismic sections, that is, moving dipping events (reflections) into their assumed true subsurface positions and collapse diffractions. But every migration method has limited performances as compared to an ideal migration method (the ability to handle fully variable interval velocity fields in time or depth, the maximum dip which can be properly migrated, the possibility to attenuate the evanescent energy, the preserving of reflection amplitudes, the ability to image overturned reflectors when waves that turn beyond 90 degrees have been recorded, the robustness in presence of ray caustics, the sensitivity to velocity errors, etc.).
In this paper after the discussion of basic principles, many practical aspects of some well-known industrial post-stack migration methods are considered. To begin with, the migration requires the true medium velocity. If we use a velocity model that is significantly different from the medium velocity, then the migrated section can be misleading. A procedure for estimating migration velocities is presented.
Key words: post-stack migration, wave equation, time and depth migration, practical aspects, medium velocity.

A PROPER ACCOMPLISHMENT OF STATIC CORRECTION
IN AN AREA WITH DEVELOPED TECTONICS AND ELEVATION

ALI MEMA
Albseis, Lagja "15 Tetori" Pallati 90, Fier, Albania

Squepur-Osmanzeze region extends in the Berati tectonic belt of External Albanides. From the tectonic point of view it is presented more complicatly. There are plenty of tectonic faults besides the gravity block of Gorica, which has been a part of Shpiragu carbonatic structure.
This area has at all time drawn the attention for hydrocarbon exploration because it is between two discovered oil fields (Kuçova and Marinza).
The main difficulties of seismic data acquisition and geological-geophysical interpretation of this region are related to carbonate section at outcrop (Gorica and Kullesi) and ex-marsh Konisbalta, which is between them.
The tectonic allochthonous block of Gorica is verified through well data. We underline that it is necessary to know the geometric form of Gorica, especially its bottom, performing the static correction.
In the paper there are shown methods of mapping the bottom of Gorica, which is not regular.
The reason of getting velocities is explained, with first break arrival method over Gorica which are smaller than their analogous over Kulles and Shprigu. It is also explained the reason why they are greater than those which we got through traditional methods weathering.
It is shown the best method for excluding the relief corrections and shoot point at the zanies with developed elevation.
The better ways are given for increasing the signal/noise ratio in field registration, at the ex-marsh Konisbalta.
It is shown the author's experience in selecting the datum plane for zones with a very high relief.
Key words: seismics, data acquisition, static correction, Albania.

PROGRAMS OF COMPUTER PROCESSING
IN REFRACTION SEISMIC MEASUREMENT

TUDOREL ORBAN
Bucharest University, 6 Traian Vuia St., P.O. Box 37,
70139 Bucharest, Romania

In this paper, there are presented two programs for processing and interpreting the refraction seismic data on a PC compatible microcomputer for the most used methods, namely the intercept time method and the reciprocal method.
The intercept time method, the best-known technique of processing and interpretation in the refraction seismics, has the advantage of being used for any number of seismic layers. The presented program is designed for the processing of geological models with four seismic layers, separated by the plane and dipping interface. Furthermore, the thickness of each layer must be sufficient so that each layer be recognized in the travel time data, preferably as a first arrival. The program allows the optional choice to processing seismic data for two, three, or four layers. After the introduction of intercept times – obtained by projecting the observed segment of the travel time curve back to the shot point – and of velocities – determined on the basis of the curve slope –, the program will allow calculation of the refraction interface depth under the shooting points. The intercept method is recommended when the seismic interface is of a small depth – down to 25 m – and has a nearly plane shape.
The reciprocal method implies a geological model consisting of two homogeneous seismic layers, separated by a discontinuity of the physical properties. The reciprocal method can be considered to be an improved intercept time method, whereby the computations are extended from shot points to each geophone station. The data acquisition is carried out according to a particular field technology by means of which any geophone will record refraction waves generated by two shooting points, located on one side and on the other side of the geophone, at a distance that must be bigger than the critical distance. Like in the case of the intercept time method, processing of field data can be separated into two distinct stages: first, recovered refractor velocities, and second, computed depth seismic interface. The processing consists of the representation of the arrival times of the refracted waves by travel time curve slopes. The program has the property to reveal accurately enough the value of the travel velocity, the linear regression properties being applied for its determination. Characterization of the determination of regression line is made by calculating and displaying the correlation coefficient, whose value should be as near as the maximum value: +1. After the seismic interface is determined both perpendicularly and vertically, the seismic surface can be built on the basis of the wave front methods. The reciprocal method is recommended in the specialized reference for the seismic interface, whose depth exceeds 25 m.
The reciprocal method and the intercept time method represent an excellent combination for defining the shallow target.
Key words: seismic refraction, processing, computers.

SEISMIC INFORMATION ON THE EAST EDGE
OF THE PANNONIAN BASIN

ANGELA LUNGU1, MIHAI STEFANESCU2, CONSTANTIN PETRE1
1"Prospecþiuni" - S.A., 20 Coralilor St., 78449 Bucharest, Romania
2 AMOCO Romania

On a large number of seismic lines, recorded between Oradea, to the north, and Chisinau Cris to the south, under the unconformity surface belonging to the basement of the Pannonian Basin, it develops a seismic sequence, with chaotic signature. It is delimited at the bottom by strong reflectors, with good areal continuity, which can be mapped and need higher velocity in order to be NMO corrected.
East from the Chisinau Cris area, these reflectors develop more than 5,000 m depth, with westward and northward updeeping tendency, being signaled between 2,000 m – 1,500 m depth at Socodor, Miersig and Giris.
On the basis of well data, from Miersig and Giris, this seismic facies unit is made of crystalline rocks.
Conformably to the geometrical configuration of the seismic reflectors, the unit belonging to the crystalline type thrusts another one made of strong reflectors corresponding to the Mesozoic formations of the Bihor Unit, as the wells from Oradea-Felix and Bors have shown.
The two seismic facies units have been interpreted as the westward prolongation of the North Apuseni structure beneath the Pannonian Basin, i.e., the low part of Codru Nappes System and Bihor Unit, respectively.
The most important new data pointed out by the complex interpretation are:
— the presence of two seismic facies units depending on thrusting relationships;
— the northern thrust front of Codru Nappes System has been mapped at several kilometers south-southwest from Oradea;
— the presence of Codru Nappes System patch at 2 km north-east from Oradea.
Key words: seismic line, seismic facies unit, geometrical configuration, well data, thrusting relationship.