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53rd EAEG Meeting
- Conference date: 26 May 1991 - 30 May 1991
- Location: Florence, Italy
- ISBN: 978-90-73781-03-0
- Published: 28 May 1991
201 - 300 of 315 results
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Considerations on the meaning of bouguer anomalies in areas with complex morphology and density
Authors M. Bernabini, P. Favoro and L. OrlandoBouguer anomaly is usually worked out considering the zones above zero level as masses with constant density or with a density varying according to the outeropping rocks' density. Although subsequent elaborations are made considering Bouguer anomalies at station level, the masses between the topographic surface and zero level are no longer taken into account.
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Inversion of 2D gravity data by a relative entropy algorithm
By A. BassreiA stochastic approach, called MRE (minimum relative entropy) principle, is used here to invert 2-D gravity data. The main idea of this work lies on the concept of relative entropy, first defined by KuIlback and Leibler (1951). Among several applications of this principle - minimum relative entropy, we quote the interesting application of the MRE in the field of speetral analysis, as a inverse procedure (Shore, 1981), who also have studied the principle in a axiomatic manner (Shore and Johnson, 1980). This approach was already applied in several geophysical inverse problems, both in fundamental and exploration geophysics: inversion of interval velocities, seismic transmission tomography, potential data (gravity and magnetics), correction of the effect of aliasing, band limited extrapolation, and the recovering of the density profile within the earth (Bassrei, 1990a; Bassrei, 1990b; Ulrych et alii, 1990).
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Two applications of space-variable convolution filters reducing to the pole within a large survey area and continuation from or to an irregular surface
Authors P. Andrieux, A. Galdeano and A. TheraThe purpose of this presentation is to demonstrate that variable filters can be used economically, when required, in the space domain. Two applications are considered.
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Radon transform application to 2D processing of potential field data
Authors Y. Zhou, C. V. Reeves and P. ZeilWe have been trying to make use of the Radon transform, a tool for plane-wave decomposition in reflection seismology, in evaluating anomaly trend information from parallel airborne geophysical profile maps, in order to improve the quality of line-to-line interpolation. During our research exercise, we find that this transform has a great application potential in 2D potential field data processing and image processing. In this paper we show how the Radon transform can be generalized and used as a new approach for directional filtering on a gridded data set.
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Extensions to Euler's method for three-dimensional potential field interpretation
Authors K. A. Whaler, C. Neil and A. B. ReidReid et al.'s (1990) method for interpreting magnetic survey data in grid form to give source locations and depths uses Euler's homogeneity relation. The relation is formulated in terms of a 'structural index' by which geological constraints are imposed - the index represents the rate of fall-off of the field with distance, which can be calculated for a given source type (thin dyke, finite step, etc.). Reid et al. (1990) used a least squares technique to solve for the source position (lateral coordinates and depth) and regional field value at each grid point, using a window of surrounding values at which the field gradients were known, and the structural index specified. Solutions were accepted or rejected using a criterion which compared the calculated source depth to its standard deviation.
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A method to estimate the total magnetization direction from a distortion analysis of magnetic anomalies
Authors M. Fedi, G. Floria and A. RapollaThe knowledge of the declinations and inclinations of the total and induced magnetization vectors is normally required, in the interpretation of magnetic anomalies. While the direction of the last (i.e. : its declination, AF, and inclination, IF) can be likely recovered by some appropriate I.G.R.F. model, the declination and the inclination of the total magnetization vector (AT and IT) are more difficult to be determined, depending on the loc al direction of the remanent magnetization vector too.
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Norway offshore - Lista and Kristiansand volcanic areas controlled by 3D interactive magnetic modelling
More LessFollowing the regional high sensitivity aeromagnetic surveys performed by NORGES GEOLOGISKE UNDERSOEKELSE (NGU) in 1974-75 for oil exploration and re-processed by AMAROK in 1989-90, it has been possible to identify and reconstruct the most important anorogenic volcanoes of the Lower Tertiary (Paleocene) in the passive margin of the southern Norwegian offshore, covering an area of approximately 27,000 Km2•
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CQSO: Marine pulse source quality control
By A. RegnaultThe use of various marine source synchronizers has yielded a number of ideas which have now been applied to developing new technology, introducing parameters that had not previously been utilized, and thus opening the way to more sophisticated quality control. The new source synchronizer discussed here is an open system, allowing for further development.
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A Drill-bit source experiment using a 2D array
More LessA number of authors have used the drill bit as a seismic source. In one type of study (Rector et al, 1989), an accelerometer is placed on the drill string, giving an estimate of the signature of the drill bit source. Crosscorrelating this estimate with the data recorded by one or more geophones enhances drill-bit energy in the recorded traces and gives results whose quality is comparable to VSP data.
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Seismic study of the dynamic response of rock to cylindrical charges fired in a half and a quarter space geometry
Authors S. K. Reamer and K. -G. HinzenA series of controlled seismic experiments was performed a limestone quarry in southem Germany to study seismic effects of cylindrical charges fired in both a half space (HS) (burden 63 m) and quarter space (QS) (burdens of 3 and 3.5 m) geometry. Charge columns contained from 20--32 kg of explosives and initiated frorn the bottom. The explosives formed a cylinder 4 rn (20 kg) and 5 rn (32 kg) high and 90 rnrn in diameter at depths from 3-8 rn below ground level. Prior to each of the larger explosions, srnall charges of 0.12 kg seisrnogelit were fired at 4 m and 8 m depths in the same boreholes.
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The shallow reflection profiling (SRP) method for field statics
Authors R. D. Tinsley and M. G. BarbierThere are many areas where simple LVL refraction statics are invalidated by complex near-surface geological conditions, for example, velocity inversions. Uphole drilling is often carried out in these circumstances, but it is a costly business and yields only a single control point with no information about the geological structure around the hole.
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Complex geophysical prospecting of a terrigenous oil field
Authors G. G. Tenchov, W. Hubanov and G. WassilevThe present paper offers the geophysical part of the case history of a terrigenous oil field. The investigated area is situated in the Northem Central part of Africa. The geological and geophysical investigations include seismic and well logging methods and drilling information. The seismic common - depth - point reflection method was used with a profile density of about 3 line km/sq.km..
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Biharkeresztes lower-pannonian natural gas field, Hungary - Interpretational case-study
Authors E. Loboczki and J. ViszkokBiharkeresztes natural gas field is located in east-Hungary, by the Romanian border. Natural gas was first found in the metamorphic rocks and was then detected in lower-Pannonian beds. The sequence at Biharkeresztes consists of Precambrian metamorphite, , Miocene fine-grained sandstone and Pannonian deposits of Paratethys. The reservoir rock is fine-grained, light-grey sandstone, varying from 0.2 to several metres in thickness. The overlying bed is siltstone. The reservoir contains many small gas lenses.
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Objectivised interpretation of seismic sections by tools of artificial intelligence
Authors B. Rugenstein and V. WittkopfThe "Knowledge Based Interpretation System" KnowBIS is our successful first step to computerise the geological interpretation of migrated seismic sections by some rule-based tools of the so-called AI.
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Automated horizon picking by multiple target tracking
Authors E. Harrigan and T. S. DurraniGeophysical horizons are of interest to the interpreter as an indicator of geological boundaries and structures. As such, they are also important for accurately obtaining the earth's velocity model. Horizon picking is still commonly done by hand, a process which is error-prone and time consuming. Attempts to automate the process are hindered by the absence of a clear, robust and universal picking algorithm.
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Computer aided fault pattern analysis (CAFPA): A prototype of an expert system for seismic data contouring
Authors C. Chiaruitini, P. Galuppo, M. Pipan and V. RobertoDrawing structural maps of seismic horizons is a task that requires the efficient management of a large amount of input data. Artificial Intelligence techniques can help the geophysical interpreter in the analysis and synthesis of the information available. CAFPA aims at providing an intelligent assistance to the interpreter by means of Knowledge Based System (KBS).
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Integration of 3D subsurface models of different scales
Authors I. L. Ritsema, P. N. Nobili and J. L. MalletThe integration of 3D subsurface rnodels derived from geophysical, petrophysical and geological data on computers bas always been a problem in Exploration and Production of Oil and Gas. Especially the discrepancy between the various geometrical scales used for the various geophysical and geological properties has prohibited the availability of a fully integrated computerized facility to build, rnanipulate and view the 3D subsurface model geometry with their properties at all scales. An extension has been constructed to the GOCAD interactive 3D modelling facility to handle models at different scales describing different properties in an integrated way.
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2D Synthetics and interpretation of deep seismic profiling data from Skagerrak
Authors O. A. Sandvin, E. S. Husebye, B. Rosland and J. E. LieThe evolutionary history of a basin is in general attempted reconstructed on the basis of the relative distribution of sedimentary strata as a function of the geological history. Associated basin extension models, say of the McKenzie type, presume a homogeneous crust subject to uniform stretching. The validity of such a hypothesis can be examined on the basis of deep (16-40 sTWT) seismic reflection data and is the topic of this presentation.
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Non-standard P-and S-wave experiments at the KTB deep drilling site: Nature of reflections and seismic anisotropy of crystalline rocks
Authors E. Lüschen, W. Söllner, A. Hohrath and W. RabbelThe interpretation of standard 2D-reflection profiling data collected in crystalline areas often suffers from the lack of direct calibration. Deeper crustal reflections differ significantly from those known from industrial sedimentary basin surveys. Although they are often very bright, reflections from crystalline rocks appear discontinuous and of chaotic or, in the lower crust, of laminated character.
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Shear wave investigations at the KTB deep drilling site
Authors W. Rabbel, E. Lüschen, W. Söllner and A. HohrathIn 1989, after the pilot hole of the Continent al Deep Drilling Project (KTB, Oberpfalz, F.R.G.) had beed finished, an extensive seismic survey was carried out at the KTB site by the German DEKORP group. Besides a 3D-reflection survey a large number of special experiments were realized in order to get a comprehensive petrophysical description of the rocks in the KTB surrounding. Parts of the data basis of this so - called Integrated Seismics Oberpfalz (ISO 89) had been presented during the last EAEG meeting in Copenhagen, Denmark. Meanwhile first interpretations are available.
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A high-density 3-component reflection-refraction survey across an exposed lower crust in Southern Italy
Authors E. Lüschen, M. Romanelli, J. Feddersen, R. Nicolich and L. CernoboriThe outcropping crystalline basement of the Serre Mountains in Calabria (Southern Italy) presents a tilted block containing a nearly complete section of the continentallower crust (granulites, metabasites). This has been regarded as key area to study the structure, composition and evolution of the Hercynian lower crust in a multidisciplinary approach. Petrological, petrophysical and structural data are available from surface mapping and laboratory experiments at varying P-T conditions on rock samples. Using these data as direct input for modelling techniques, the seismic in-situ characteristics, e.g. the reflectivity and the P- and S-wavevelocities can be interpreted. Since direct observations of the actual lower crust are not possible, this calibration of a fossil lower crust is expected to provide a better understanding of deep seismic results.
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Shot gathers amplitude and frequency processing and fast prestack depth migration. Application to ECORS deep reflection seismic data
Authors A. Bitri, D. Rappin and J. M. MarthelotStacked sections of ECORS deep reflection seismic profiles are characterized by: - an important variability in the lateral continuity of the reflections, - the presence of transparent zones, - the occurrence of highly dipping reflections, - the often strong and complex reflectivity of the lower crost. The CDP stacking procedure may not be well adapted because: - the quality of the shot gathers is quite variable, - the reflection events are assumed to have continuous hyperbolic moveout, - the lateral smearing of the CDP on dipping reflectors is large since the spreads used are long, - normal moveout is a dip selective procedure.
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The application of shear waves to studying the upper part of section with view to detecting the karst zones
Authors T. I. Oblogina and D. A. KouchenevThe shear SR-waves were used for studying the upper part of the geological section in the Moscow region. The field observations were carried out by the refraction and reflection surveys.
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Algebraic reconstruction for 3D seismic quality control
More LessDerived from medical applications, algebraic reconstruction techniques (ART) were first implemented by geophysicists for crosswell tomography (Bois et al, 1972 ; Bishop et al, 1985 ; Paulson et al, 1985; lvansson, 1985). 2-D refraction cases were recently studied (krajewski et al, 1989 ; Ruhl and Luschen, 1989), using a particular ART variant known as Simultaneous Iterative Reconstruction Technique (or SIRT), whereby each model bin slowness is modified when all raypaths pertaining to that bin have been taken into account .
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Two case histories of diffraction tomography applied in engineering geophysics
More LessThis paper presents the application of diffraction tomography to real data in engineering geophysics. In the past many papers developed the theory of diffraction tomography and discussed possible applications (Devaney and Beylkin, 1984; Wu and Toksöz, 1987). This new inversion technique is based on the acoustic wave equation. It tries to reconstruct the velocity perturbation against a constant background (constant density assumed) from a scattered, diffracted wavefield. In contrast to traveltime tomography where only picked times of some phases are inverted diffraction tomography uses the full wavefield.
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Deep and shallow aquifer modelling from gravity and reflection seismic data in Southwestern Denmark
Authors S. Thomsen, H. Lyke-Andersen, B. H. Jacobsen and R. FribordGravimetry is efficient and cost effective as a means for revealing the horizontal geometry of shaIlow as weIl as deep aquifers. Depth resolution of this method however is poor.
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Integration of mineral exploration data using fuzzy set theory
Authors W. M. Moon and P. AnThe traditional geophysical information processing theory is founded on Boolean logic and in Euclidean space. However, accurate assessment of given sets of multiple layer exploration data requires that one must define preciseness of one's knowledge as precisely as possible. Fuzzy logic provides a theoretical means of quantifying imprecise information, which was first utilized as early as 1965 and has been extensively applied since in engineering and related sciences. Fuzzy set theory is tested and applied to integrate geophysical, geological and remote sensing data sets in this research.
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A combined square array resistivity and magnetic survey in the archaelogical site of Noville, Coruna, Spain
Authors M. J. S. Matias and F. E. R. AlmeidaGeophysical exploration techniques have been adapted to archaeological exploration successfully. The use of Exploration Geophysics in Archeology has proved to be a fast and economical tooI in excavation planning of archaeological sites.
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Bedrock fractures as seismic reflectors
By O. OkkoThis paper presents the interpretation of reflections from same very shallow fracture zones located at the depth of less than 250m and recorded in soundings carried out on bedrock outcrops and in boreholes. The resolution of the seismic reflection method is compared to the corresponding acoustic logs and other geophysical and geological investigations.
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Conventional and modern seismic investigations for elastic properties determination in gneiss-schist. A case history
Authors J. Louis, T. Papadopoulos, G. Drakatos and P. PantzartzisRefraction seismics and modern tomographic surveys were employed at Platanovrissi damsite on Nestos river in Northern Greece in an effort to investigate the dynamic elastic properties and rockmass condition of gneiss-schist which outcorps in the area and it is prone for a hydroelectric dam construction.
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3-component colored 'attribute seismograms'
Authors H. Steentoft and S. WendeAn integrated system for recording, processing and interpretation of seismic data has been developed in order to increase the resolution in near - surface measurements and to perform first analysis during the field measurements. An extensive 3 - component - processing software - package was installed on a 68000 CPU micro - computer for completion of this processing system.
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Test park for environmental geophysical measurements
More LessIn the last decade the role of geophysics has been increasing in prospecting and prevention of environmental damages in Hungary. The reason is that geophysical methods are the most effective, quick and economical tools and what is similarly important, they can be used without causing damages to the environment. Inspite of these favourable features these methods have not spread in the expected degree, because theory for application of geophysical methods in environmental protection has not developed sufficiently and application has not been proved with practical measurements.
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Geoel: An IBM PC algorithm for the automatic processing and graphic presentation of geoelectric data
Authors G. -A. Tselentis and G. DelisThis paper addresses the problem of automatic processing and interpretation of geoelectric data with personal computers. The main algorithm is divided into two processing stages. During the first processing stage, the field curve is aproximated by a group of layers which are determined automatically by the program in such a way that the resulting theoreticaI curve describes as close as possible the field curve. This operation is performed in the resistivity - space domain and the optimum number of layers needed to describe the resistivity distribution versus depth is determined from the algorithm.
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Model curves for vertical electrical sounding over 2D structures
Authors J. Pous and P. QueraltVertical electrical sounding is one of the most used electrical methods in Geophysical Prospecting and its application is restricted to those problems according to structures that do not differ strongly from stratified media. Even in these situations the simplicity of a near one dimensional earth approach is sometimes not reached and some important lateral resistivity variations may appear.
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Borehole-to-surface D.C. resistivity and time domain electromagnetics monitoring of contaminant plumes. A model study
Authors A. Zerilli and B. A. JamesContamination of ground water and soils is one of today's most pressing problem. It has its origin in a number of causes, such as improperly buried wastes, poor effluent disposal and improper use of agricultural and industrial chemicals and spillages. The waste migrate along paths determined by subsurface soil types, stratigraphy and presence of faults, shear zones and fracture zones.
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Transient airborne EM applied to geological mapping
By A. P. AnnanGEOTEM is a high performance time domain airborne EM (AEM) system developed for mineral exploration and geological mapping applications. The historical focus for transient AEM has been on direct detection of massive sulfide type targets or other mineralized zones which have intrinsically high electrical conductivity. A bi-product of airborne EM surveys, particularly transient airborne EM surveys, has been a wealth of data which has considerable potential for mapping geology but which has seen limited use. With the advent of high quality digital GEOTEM data, new applications are being addressed.
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Ground penetration radar survey design criteria
Authors A. P. Annan and S. W. CoswayGround penetrating radar (GPR) is now becoming a weIl accepted geophysical technique. Basic criteria for the design and execution of surveys as weIl as the processing of data subsequent to these surveys have yet to be established in any satisfactory form. Experience from many sites over the past 15 years have provided many useful guidelines for survey design and data presentation. In this paper, criteria which are helpful to the new user in this field are presented.
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The TRH procedure for transient soundings. Practical results and some new ideas
By K. L. KakasOn the Oostende meeting of EAEG in 1986, a simple depth inversion of transient (time-domain EM) soundings was suggested by the author for the CIL (coincident loop) layout. This procecure (called TRH, because the transformation goes from the time to depth via resistivity) has been proved serviceable for preliminary interpretation of sounding curves, and combined with master curves and/or interactive curve fitting, the results seem to be stables and geologically acceptable.
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Magnetic interpretation using the 3D analytic signal
Authors W. R. Roest, M. Pilkington and J. VerhoefWith the availability of large digital datasets of magnetic data (e.g. Committee for the Magnetic Anomaly Map of North America, 1988) the need for automated techniques for the analysis of magnetic anomalies is growing. Several methods have been developed that have been successful in predicting source characteristics, using certain assumptions.
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Apparent density mapping from gravity data using the walsh transform
Authors P. B. Keating and M. PilkingtonThe purpose of apparent density mapping is the identification of uniform density areas that can be related to geological units. Apparent density maps are usually calculated by linear filtering in the Fourier domain (Gupta and Grant, 1985). The Earth model is then a set of right rectangular blocks extending to depth d, below which significant contributions to the residual gravity anomaly do not arise.
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Depth determination from gravity gradients
Authors M. M. Gobashy, E. M. Abdelrahman and A. I. BayoumiEstimation of the depth to a buried structure from the horizontal or vertical gravity gradients (Hammer and Anzoleaga, 1975; and Stanley and Green, 1976) has drawn considerable attention. The simplest and most widely used method to determine the depth of a buried structure from gravity gradients is the Xe-rule (Piek et al., 1973).
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An efficient 2D resistivity inversion technique and its application
By H. ShimaFor two-dimensional (2-D) resistivity analysis, a number of inversion techniques have already been proposed (e.g., Shima,1990). However when underground is close to a horizontally layered structure, an one-dimensional (1-D) assumption is considered to be more effective in arriving a further accurate solution. To make the 1-D assumption more useful, I propose a filtering procedure that improves the sensitivity characteristics of the potenrial data obtained by a pole-pole array.
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Regional tectonics derived from gravity gradients
Authors E. Kilenyi and Z. SzaboTwo-dimensional filtering of Bouguer anomaly maps enhances different 'wavelengths' according to the applied parameters. The meaning of wavelength in gravity can be interpreted in a more familiar way: long waves - regional anomalies, short waves residual anomalies, very short waves - derivatives.
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Magnetic-anomaly map of the Republic of Yemen
More LessA standardised composite magnetic-anomaly map of the Republic of Yemen is presented. This has been compiled from 23 airborne surveys of widely differing specifications undertaken between 1975 and 1985. The airborne magnetic surveys flown on behalf of the Natural Resources Project of the Ministry of Oil and Mineral Resources over the central parts of the Republic of Yemen in 1985, which included a high altitude regional coarse grid survey, completed the magnetic coverage of the Republic. The magnetic-anomaly map of the Republic of Yemen has been produced by The Robertson Group plc for the Natural Resorces Project. The surveying and compilation was primarily financed by loans from the Arab Fund for Economic and Social Development with technical assistance from the United Nations Department of Technical Co-operation and Development (UN/DTCD).
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A new view of the gravity and aeromagnetic fields in the UK
Authors I. F. Smith, M. K. Lee, C. A. Green, A. J. W. McDonald and K. E. RollinPublished maps showing the regional coverage of both gravity and aeromagnetic data for the land and parts of the sea areas of the UK have been available for a number of years. These data are now available in digital form, which allow the geophysicist to take advantage of advanced analytical and imaging techniques. Horizontal and vertical derivatives, inversions, grey-scale and colour shaded relief plots are startlingly effective in defining structural trends and characteristic anomaly pattems.
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Integrated interpretation of borehole investigations for structural modelling of crystalline bedrock
By P. PitkanenThe poster presents an integrated approach to use various borehole investigation methods in order to analyze structural and hydrological properties of crystalline bedrock. Geological and geophysical borehole data are interpreted to obtain engineering geological structures of the rock mass. The results are applied for the creation of three-dimensional bedrock models. The structural interpretations are also used as a basis for geohydrological modelling.
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Integrated presentation of all data from well
More LessThe goal of this paper is to illustrate what can be achieved by using all available information in integrated form with the help of composite log, for correlation, formation evaluation and documentation.
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Multiple offset VSP and multiple component VSP data processing to improve structural definition
Authors E. del Pino and E. RamirezA flower structure with two prospective blocks was drilled in the northem central part of Lake Maracaibo. In order to define the structural position of the fault plane separating the two blocks and its distance from a well driIled into the upper block, a multi offset 3 component VSP was acquired immediately upon penetration of the top of the structure.
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VSP and steep reflectors: A case history of data processing and interpretative modelling
Authors F. Ogliani and J. MullerIn the presence of dipping reflectors, a zero-offset VSP allows a lateral investigation far from the well. In data processing, such property is routinely exploited when reflectors are gently dipping or involved in salt tectonics. Some aspects, however, may become not familiar or problematic, when structural dips exceed 45 degrees.
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Evaluation of the VSP technique in the northern part of the Oseberg Field - A case history
Authors H. Rutledal, S. E. Johnstad and V. DanielsenThe Oseberg Field was discovered in 1979 and is located in the North Sea Norwegian Blocks 30/6 and 30/9 about 140 km west-northwest of Bergen. The field is a fault trap reservoir with about 600 m of structural relief at top reservoir level, and consists of three major eastwardly rotated fault blocks: Alpha, Gamma and Alpha North.
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Use of walkaway VSP in imaging a pre-zechstein reservoir in the UK Southern Gas Basin
Authors J. M. Smidt, P. Conn and M. LappinThe results of many VSPs are disappointing because they have not been planned in detail, and processing has not been optimised to achieve the prime objectives due to lack of communication between the specialists involved in planning, acquiring, processing and interpreting the data. In the case study reported here the close co-operation between interpreter, technical advisor and VSP processing geophysicist resulted in a carefully designed survey and an optimised processing sequence which fulfilled the planned objective and yielded further information.
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Some characteristics of coupled wave propagation
More LessCoupled wave propagation describes propagating wavefields of different physical quantities; hereby it is possible that all those fields interact with each other. In exploration geophysics coupling effects are generally believed to be small enough to be neglegable. Comparing different material relevant to exploration, however, shows that coupling can contribute to noticable propagation effects.
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Effects of processing on three-component shear-wave data
Authors P. K. Schruth and I. BushRecent developments in seismology have led to an increase recording of three-component (3-C) data, in particular for the study of seismic anisotropy where shear-waves have vector polarizations in three dimensions. By carrying information about the symmetry structure along the raypath the polarization of shear-waves can be used to evaluate the parameters of the anisotropy and hence provide information about the internal structure of the rock (S. Crampin, 1985). Crampin has shown in numerous papers that shear-waves are particularly sensitive to changes in crack- and pore-geometry caused by changes in pore-pressure, pore-fluid properties and the orientation of the stress field that aligned the cracks.
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Seismic velocities of marine sediments in box cores with concentration on the relationship between shear strenght and shear wavevelocity
Authors I. A. Pecher, Fr. Theilen and S. NebenThe determination of geotechnical parameters at the sea floor is of growing importance for engineering purposes. The measurement of compressional (P-) and especially shear (S-) wave velocities seems to he an appropriate method for the estimation of other physical properties in marine sediments.
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P-S waveseparation using parametric inversion: An offset VSP case study
Authors D. Belaud and W. S. LeaneyThe separation of interfering compressional and shear wavefields is an important problem in the processing of three-component offset VSPs,where, because of the raypath obliquity, the amount of converted shear energy may be significant compared to the P energy.
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S-Wave velocities and shallow layering in a Campi Flegrei site (Naples) through R-wave spectral analysis
Authors F. M. Guadagno and C. NunziataThe Spectral Analysis of Surface Waves (SASW) method is a nondistructive seismic testing aimed to determine shear wave velocities and layering from surface measurements of artificially generated Rayleigh waves (R-Waves). The SASW method has been mostly employed in the evaluation of the elastic properties of pavements (Heysey et al. 1982; Nazarian and Stokoe, 1983) and was also experimented for determining shear wave velocity profiles in a liquefaction susceptible site.
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Images of rock properties estimated from a crosswell seismic velocity tomogram
More LessMethods for obtaining an accurate image of the upper crust in terms of seismic velocities have evolved rapidly during the last decade, especially with well-to-well tomography techniques: well-to- well tomography allows us to constrain velocities more than with classical reftection seismology methods. At the same time, laboratory experiments have increased our knowledge of the influence of rock properties, such as porosity, saturation or clay content, on acoustic measurements. Our goal is to use results obtained in the laboratory for the interpretation in terms of porosity of an actual cross-well survey. Three different approaches are used: applying (1) geostatistical techniques, (2) rock physics relations obtained in the laboratory, and (3) a combination of geostatistics and rock physics.
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Direct estimation of lithology or compaction from seismic and well log data
Authors O. Sylta and J. HelgesenTraditionally, compaction has been determined from seismic stacking velocities or sonic logs. The use of stacking velocities requires a considerable human effort, since consistent velocity picks must be obtained. We propose to fit a depth dependant velocity function to seismic travel time information using the semblance criterion, and thereby obtain estimates of important parameters describing lithology and compaction. The need for human interaction is thus reduced, and more reliable estimates are obtained. Furthermore, the method uses all available travel time information in the depth interval to be analyzed, including zones of weak reflections.
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A comparison of petrophysical equations for extrapolation of lithilogy beyond well locations using seismic data
Authors S. H. Archer and R. D. MartinezSeveral alternative petrophysical models exist for relating the seismic properties of rocks to the elastic parameters of the rock constituents. This paper shows the effect of some alternative petrophysical models on the extrapolation of lithology using seismic data.
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Rock samples microcracks produced by temperature, hydrostatic pressure and fluid pressure stresses: electrical responses and acoustic emissions
Authors G. Losito, M. Muschietti, A. Trova and E. BiagiDuring the last years, many laboratory experiments have been carried out to study electrical properties of rock samples subjected to temperature, hydrostatic pressure and internal pore fluid pressure cycles. For the experiments an "ad hoc" apparatus has been designed at Applied Geophysical Laboratory of University of Florence, that can condition temperature (up to 300 ·C), hydrostatic pressure (up to 40 MPa) and internal pore fluid pressure (up to 40 MPa).
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Surface waves: Nothing but noise?
Authors C. Schneider, A. Virtlmayer, R. Misiek and L. DresenIn exploration geophysics, seismic surface techniques have been developed to a high degree. Information about the underground is solely based on body waves. Dispersive surface waves are believed to be undesired noise. Thus, field layouts try to avoid surface waves, and data processing metbods are used to attack Love- and Rayleigh-waves. Methods to generate and process dispersive surface waves to search tbe underground, as well known from earthquake observations or in-seam seismic techniques, are most unusual in exploration geophysics. However, ignoring surface waves means not seeing valuable signaIs, at least from the near surface underground.
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Determination of elastic constants pertaining to a symmetry plane of an orthorhombic medium under arbitrary overburden from three-component offset VSP measurements
By K. HelbigThe propagation of elastic waves in the 1-3 plane of an orthorhombic medium (3-axis vertically downwards) is controlled by six elastic stiffnesses: c11, c13, c33, c44, c55, and c66. For propagation in the 2-3 plane c1 and c13 are replaced by c22 and c23. The remaining elastic stiffness c12 influences propagation in the horizontal (symmetry) plane. Propagation in oblique directions involves all nine elastic stiffnesses.
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A new approach to studying the anisotropy of seismic waves velocities
More LessThe traditional approach to studying seismic anisotropy is based on using elastic parameters Cij according to generalised Hook's law. The geological medium is approximated by a homogeneous anisotropic model in which seismic rays are straight lines.
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The wave packages change in the wave propagation
More LessThe investigation of the non-linear effects during the propagation of the seismic waves like behaviour of the structure of the wave field from the intensivity, non-linear interaction of the seismic waves, soliton property etc. is one way to receive additional information for nonlinearity of the media.
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Observations of seismic anisotropy on the Norwegian continental shelf
Authors T. Stordal and R. KanestrømWe have investigated the P wave anisotropy at three different locations on the Norwegian Continental Shelf. Two of the locations are in the North Sea and one location is in the Mid- Norwegian Shelf. In one of the locations the seismic data consisted of zero-offset VSP data and reflection and refraction data along two perpendicular lines crossing through the well. In the other two locations the seismic data consisted of zero-offset VSP data and walk-away VSP-data along perpendicular profiles symmetrical around the well. For all the wells sonic logs and density logs were available.
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Acoustic anisotropy of synthetics with controlled crack geometries
Authors J. S. Rathore, E. Fjaer, R. M. Holt and L. RenlieThe knowledge of crack distributions is of vital importance for the withdrawal of fluids from reservoirs in which cracks exist as penetrating fractures, serving as primary channels for fluid flow. The presence rif cracks affects the acoustic wave velocities giving rise to anisotropy. It is to be able to predict theoretically whether the observed anisotropy is caused by a large number of small, isolated microcracks, or by a few similarly oriented, larger and possibly connected fractures. Several theoretical models for the acoustic behaviour in cracked and jointed media exist (Hudson, 1981, Thomsen, 1986), yet none of them have been confirmed in a controlled experiment, one in which sizes, shapes, amounts and orientational distributions of cracks are known. In geomaterials these are generally unknown parameters. Hence we have undertaken the task of producing a synthetic material in which the above crack parameters are known.
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Anisotropic scalar imaging
Authors M. Karrenbach and F. MuirWith increased interest in anisotropic propagation effects and integration of different acquisition geometries and scales into one data set, one would like to be able to conveniently use existing well-known scalar algorithms. Many generic seismic processing algorithms are based in one way or another on an isotropic assumption. Here we show a practical way, how to extend existing scalar migration or modeling algorithms to simple anisotropic symmetry systems. We compare exact anisotropic dispersion relations to their double elliptic approximation. These schemes apply only to scalar wave fields. A scalar Eigenfield is a scalar representation (not necessarily complete) of a vector Eigen field, such as reflected p-vector wave field, s-vector wave fields, Means for obtaining a scalar Eigenfield include operations like: rotations or sophisticated removal of free surface effects.
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Comparison of theoretical formulations for deriving the elastic constants of finely-layered cracked media
By I. BushThe main causes of seismic anisotropy in sedimentary rocks are the layer-induced anisotropy of finely stratified isotropic rock (fine-layer anisotropy) and the anisotropy due to distributions of stress-aligned fluid-filled micro-cracks (extensive-dilatancy anisotropy). Shear-wave propagation in finely-layered cracked media is characterized by the presence of point singularities, which occur at different directions (incidences and azimuths) of shearwave propagation.
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Anisotropic physical modelling of multioffset profiles over an orthorhombic medium
Authors R. J. Brown, D. C. Lawton and S. P. CheadleIn reflection seismic surveys involving an anisotropic medium, the apparent velocity variation is no longer hyperbolic because the NMO (normal-moveout or stacking) velocity is, in general, no longer invariant with direction of propagation. This variation of velocity with direction in symmetry planes has been studied by many (Postma 1955; Crampin and Kirkwood 1981; among others) and recently Thomsen (1986) presented approximate equations for qP, qSV and SH velocities for transversely isotropic media (valid also in symmetry planes of any anisotropic symmetry system) correct to the fust order in quantities that are small for weak anisotropy. Thomsen's (1986) expressions are for phase velocity but he reaffirms that the difference between group and phase velocity is a second-order quantity. We examine this relationship to enable comparison of experimentally observed NMO (group) velocities with phase velocities from Thomsen's approximate relations.
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Information content of amplitude versus offset of shear-waves in anisotropic media
Authors G. S. Yardley, G. Graham and S. CrampinMany recent publications (Alford 1986, Martin and Davis 1987, Squires et al 1989) have analyzed anisotropy in shear-wave reflection surveys. Such studies yield values of the percentage anisotropy over wide depth ranges in the shear-wave section. Percentage anisotropy is usually calculated from the time delays between the arrivals of the fast and slow split shear-waves and gives information about the crack density and porosity in the section. The polarization direction of the fast split shear-wave is determined by the crack and stress geometries in the rock and is Iikely to yield information about preferential directions of fluid flow within a reservoir.
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Inversion of VSP field data for anisotropy with singularities
By C. MacBethInversion of the parameters characterizing shear-wave splitting, provide a way of obtaining a good starting model for full-wave modelling, improving the convergence to a final waveform match. This is necessary as the shear-waves behave in a highly non-linear fashion with respect to anisotropy. To arrive at such a model for the subsurface. a two stage process is involved. Firstly, estimates for the parameters which specify the morphology of the split shear-waves are obtained, and secondly, these values are interpreted in terms a range of physically plausible anisotropic models. The estimation stage in this work is still in its infancy, however MacBeth (1990) demonstrated that it is possible using current rudimentary algorithms, to achieve a well resolved inversion result using the polarization of the leading split shear-waves, provided VSP data for several azimuths of direction are recorded. The subject of this present work is to show that these results may be used to facilitate the final fit using full-wave modelling.
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Processing anisotropy with the shear-wave analysis package (SWAP)
By P. WildOver the three years that the Edinburgh Anisotropy Project has been running, a significant number of computer programs have been written with a view to analyzing the effects of seismic anisotropy from multi-component data sets. The computer programs contain a number of important new tools for understanding and evaluating the behaviour of split shear-waves.
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Implementing pre-stack rotation for four-component shear-wave reflection data
Authors X. -Y. Li and S. CrampinSource-geophone rotation has now become a conventional procedure for processing four-component (two-horizontal sources and two-horizontal receivers) shear-wave reflection data, as demonstrated by Alford (1986), and Squires et al. (1989) and among others. To apply source-geophone rotation, a post-stack rotation analysis (Alford 1986; Squires et al. 1989), or a least square fitting procedure (Murtha 1989) is required to determine the optimum rotation angle. Since it is computing intensive and time consuming, pre-stack rotation is an unwelcome additional procedure (Sriram et al. 1990).
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Interpretation of seismic anisotropy parameters
Authors F. M. Lyakhovitsky and L. Y. BrodovIn accordance with experimental data, in many cases sedimentary deposits are found to be anisotropic media for long seismic waves. With sufficient approximation the model of these rocks is transversely isotropic media (TIM) with horizontal or vertical plane of isotropy, and they are called polar or azimuthal anisotropy, respectively.
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Modelling anisotropic waves in three dimensions
Authors H. Igel, P. Mora and D. RodriguesA finite difference approximation to the elastic wave equation has been implemented on the parallel connection machine (CM). Realistic three dimensional Earth models can be created by a joint surface modeling package (GOCAD). To each homogeneous domain anisotropic stiffnesses may be attributed using Hudson's theory for cracks [1] or the group-theoretical formulations from Schoenberg and Muir [2]. The resulting anisotropic symmetry system can have any spatial orientation. Wavefronts in three dimensions and seismograms for a saltdome structure including anisotropic layers are shown.
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Wavefield decomposition for anisotropic media
By D. NicholsMany authors have proposed processing multicomponent data by treating each wavetype as a separate scalar wavefield. While this is not valid in general media it is a very convenient approximation that has proven to be useful in practice. The first stage in using this method is to decompose the displacement fields measured at the surface into fields corresponding to the amplitudes of the different wavetypes.
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Migration velocity analysis: A comparison of two approaches
Authors K. L. Schleicher, D. J. Grygier and M. A. BrzostowksiWave equation processing algorithms are available to image very complex structures. In most cases these processes are limited by our ability to estimate the correct velocity field. There are two types of migration Velocity Analysis (MVA) that use migration as a tooI to estimate velocity. The first type, velocity scanning MVA, prestack migates the data with a suite of velocities.
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Processing of reflection data with the CRE method
Authors H. Steentoft and W. RabbelDiviations from horizontal layering are often observed and lead to imperfectly stacked sections. In order to compensate shortcomings of the Common - Midpoint (CMP) - method additional processing steps such as Dip Moveout (DMO) are usually performed.
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Orthogonalized noise subtraction
Authors F. Poletto and A. CragliettoExperience gained processing data recorded in a seismic experiment with a drilling-bit source suggests that the acquisition could be improved by integrating it with a monitoring of the principal sources of the noise due to engines and yard activity.
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Removal of water multiples - A new F-K approach
Authors D. E. Lokshtanov, O. A. Sandvin and E. S. HusebyeHere we propose a frequency-discrete wavenumber (F-K) method for removing water multiples from shot-record reflection data. The method is based on the acoustic wave equation in 2-D media. modelling the seismic reflection response from a stack of sub surface interfaces. which can have irregular boundaries. However, no knowledge of the subsurface geology below the water bottom is required in order to remove water multiples.
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The F-X dip barrier removed with a pie slice
By M. GalbraithThe method of noise reduction known as FX-Deconvolution or FX-Prediction was fust introduced by Canales (1984). Gulunay (1986) pointed out that FX is incapable of correctly predicting coherent energy when more than one dip is present. Examples show that the effect of this in the real world is not as drastic as the mathematics might indicate and FX is now widely used as an effective after stack process. (or pre-stack on fully corrected gathers).
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An introduction to FKK techniques
Authors L. G. Peardon and C. W. M. BaconIn the early days of seismic data analysis, much emphasis was placed on single trace, or 1D, processing. Eventually, we became accustomed to the idea of treating the data as a two-dimensional image and 2D processing became commonplace. Today, of course, with the advent of more and more 3D surveys, there is a requirement for 3D processing techniques. In this paper we discuss a particular 3D analysis tool, namely the FKK transform, and its applications.
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A new environment for land 3D processing
By P. DixonThe traditional medium of magnetic tapes for seismic data storage and processing is being superceded by an increasing use of discs. As these plummet in cost, they are, and will increasingly be, a much more attractive medium for the seismic industry. As well as rendering significant improvements in data handling, the benefits of disc to disc processing also extend to substantial improvements in the quality of processing due to the rapid access to data in various domains.
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Complexity analysis in the double transformed radon domain
Authors M. Vissinga and J. T. FokkemaLet Dr,s(xy;xs;w) represent a data set of a splitspread seismic experiment in the freqency domain, organized for different shot and receiver coordinates xs and xy, respectively.
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On estimation of minimum-phase wavelet
By V. BardanAn important goal of seismic processing is to shape reflection events so that the final seismic section can be interpreted easily and unambiguously. The manipulation of seismic data to achieve this goal is generally called wavelet processing. Typically, some form of deconvolution is used to perform the desired shaping of the wavelets in a seismic dataset. Many different deconvolution algorithms have been developed for processing of seismic data. They all have one thing in common: their derivation are based on mathematical assumptions about the seismic trace. The most important assumptions are: the autocorrelation of the trace is a scaled version of the seismic wavelet and the seismic wavelet is minimum-phase, In these conditions we want to estimate the minimum-phase wavelet.
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Frontier exploration techniques applied to multivintage 2D seismic
Authors E. Rieser and J. D. KerrThe Laa Area lies in the North East of Austria close to the Czechoslovakian border. It is situated in the Undisturbed Molasse of Lower Austria at the edge of the Washbergzone Overthrust. The exploration targets are within the Mesozoic Malm and are changing from structural to stratigraphic traps within the Oncophora Sandstone. Multifold seismic exploration began in 1977-79 with vibroseis data being acquired.
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A Regional water-gun profile across the quaternary basin of the Southern North Sea
Authors J. Bulat, D. Cameron and C. MesdagIn 1989, a European-Community funded project was set up to study the Quatemary deposits of the southem North Sea, involving collaboration between the geological surveys and universities in Denmark, Germany, Netherlands, Belgium and the United Kingdom. The funding has enabled the drilling of nine stratigraphic boreholes and the acquistion of 1000km of digitally recorded 10cu. in air gun data.
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A case history of high resolution seismic methods for exploration of shallow oil deposits in Suriname
Authors P. Goerdajal, J. A. C. Meekes and M. H. MulderIn the coastal region of the Republic of Suriname oil is found in Paleocene sand deposits at depths of about 300 m. Although drilling costs are relatively low for these shallow depths, the structure of the sand bodies is so complex that it was decided to carry out high resolution seismic measurements with the objective to increase the succes ration of producing wells.
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Shallow seismic and gravimetric exploration of brown coal - A review
Authors H. Gaertner, R. Seitz, H. Petzold and H. SchubertFor many years coal was the main resource of primary energy in East Germany. The production of more than 300 mio. tons a year implemented a great volume of geophysical exploration activities including shallow seismic surveys which started 15 years ago. Depending on the actual tasks and seismogeological conditions several shallow seismic techniques were developed which will be presented in this paper together with gravimetric results.
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Optimum acquisition and processing for seismic reflection techniques for extractive industries
Authors I. Hill and L. VannesteSeismic reflection is a highly desirable tool for exploration for the extractive industries due to the combination of good vertical resolution with the ability to image complex structures. The disadvantages are equally apparent, high cost, complexity and delay in receiving processed sections. In addition, the technique is not weIl understood in the extractive industries and has areputation for uncertain results. Our work at Leicester has been directed towards investigating the applicability of the reflection method to a variety of geological environments, and of clearly defining the optimum acquisition and processing techniques for this type of data. With this knowledge we can develop a total system which optimises field acquisition and provides in-field processing and section display.
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Estimation of reflection co-efficients from zero-offset field data
Authors B. Arnsten, B. Ursin and M. LandrøThe classical one-dimensional inverse problem consists of predicting reflection coefficients from surface seismic data using the one-dimensional wave equation. In one spatial dimension only plane waves propagating along one axis can be described. Spherical waves originating from a point source can not be adequately described. Many authors have considered this problem, and given examples of use on field data. An early example is Bamberger et.al. (1982). This work considers the inverse problem of predicting reflection coefficients from vertically travelling spherical waves in a one-dimensional horizontally layered medium. It is a more realistic problem than the classical one in the sense that point sources are commonly used in seismic exploration.
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Calibration of seismic data for inversion
Authors A. Ziolkowski and K. -J. KosterWe present a method, based on the critical reflection theorem, for determining the system sensitivity of a seismic recording system, using only the recorded data and the known source signature.
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A massively parallel implementation of prestack Kirchhoff depth migration
By J. van TierIntegral or Kirchhoff migration methods compute the integral solution of the wave equation, and have been used extensively in seismic imaging (Schneider, 1978; Berryhill, 1979; Berkhout, 1980). For each image point in the subsurface, these methods sum amplitudes of the recorded wavefield along a diffraction curve. The shape of the diffraction curve is described by the impulse response of the image point, the Green function, which depends on the velocity field and the shot-geophone geometry.
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Sub-surface structural images from pre-stack time migrated data at an optimum velocity using a cost effective approach
Authors G. Hodgkiss, G. E. Lane and G. HorganIt has long been recognised that tbe application of dip-moveout (DMO) does not necessarilyresolve all stacking velocity conflicts in areas where: - there is a velocity gradient - where there are steep dips Such circumstances may require pre-stack time migration. A problem with full pre-stack time migration is knowledge of the velocities with which to drive the migration. In principle finding out could be a very expensive exercise. The following case-history demonstrates how to obtain high quality from an efficient approach to gaining a good image from pre-stack time migrated data at an optimum velocity.
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The modified elastic wave equation: Applied to modelling and migration
By O. SelviIn this study a modified elastic wave equation is described which simultaneously permits both upgoing and downgoing propagation. This equation can be expressed as a nonreflecting elastic wave equation Which gives reduced reflection coefficients for transrnission across material boundaries. For horrogeneous region, it becomes the full elastic wave equation.
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An interactive approach to macro model estimation
Authors B. C. Scheffers, J. W. de Bruijn and A. J. BerkhoutAdvanced seismic processing techniques are elastic and depth-oriented. With elastic, depth-oriented processing a more accurate image of the subsurface can be obtained than with acoustic time-oriented processing. Properties of the subsurface can he divided into properties that mainly influence the propagation of seismic waves (macro properties) and properties accounting for reflection of these waves (detail). If the macro properties of the sub surface are known, the propagation effects can be removed from the data, yielding a depth image of the detail. Since the elimination of propagation effects is a major processing goal, a clear understanding of the macro properties of the subsurface is necessary.
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Prestack depth migration in the double transformed radon domain
Authors R. Tatalovic and J. T. FokkemaThe proposed migration procedure is performed in the double Radon transfonned frequency domain. The main advantages of the procedure are that it does not require a detailed input velocity model, does not need decomposition to up- and downgoing waves, and does not assume laterally invariant geology.
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High-dip time migration
Authors J. L. Black, M. A. Brzostowski and H. CrookWe introduce a steep-dip time migration algorithm which exploits the strenghts of current time migration algorithms and minimizes their weaknesses. This new time migration scheme uses Stolt and phase-shift migration as a base and finite-difference migration as a residual. The data is first redatumed using a phase-shift operation and then Stolt migrated using a constant velocity over some time interval.
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Parallel implicit finite-differences prestack migration
Authors B. Moorhead and B. BiondiPrestack finite-difference depth migration is a widely used method for imaging complex geological structures (Schultz and Sherwood, 1980). This migration method not only can be used for imaging seismic data in presence of lateral variations in velocity, but its results can be also used for improving the velocity model by applying focusing analysis to the migrated wavefield (Faye and Jeannot, 1986).
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Smoothing constraints in cross-hole seismic tomography
Authors M. Pilkington and J. P. TodoeschuckThe cross-hole seismic tomography problem is ill-posed in the sense that small changes in travel times may cause excessive variations in the calculated slowness image. These artifacts can have amplitudes large enough to mask the true velocity structure and a spatial extent small enough than the resolving power of the data. Consequently, the image should be smoothed to pro duce results that are geologically meaningful.
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