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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
101 - 200 of 315 results
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Anisotropic tomography: Part I, Background
Authors F. Muir, R. Michelena and J. HarrisHydrocarbon reservoirs may often be found in rocks which have quite variable degrees of velocity anisotropy. For example, marine shales may be much more anisotropic than the porous sands associated with them. It follows that anisotropic inversion methods present opportunities for adding useful information, and for preventing image mispositioning due to an overly simplified velocity model.
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A complete waveform inversion
Authors S. C. Singh, M. Dietrich and F. ChapelSeismic waveform inversion consists of minimizing misfit between observed and calculated wavefields. Since this misfit function could be highly non-linear, the only way to be sure of arriving at the global solution is to use a Monte-Carlo search. This is feasible if the number of unknown parameters is not too large. If it is large, as is the case with the seismic inverse problems, it is extremely expensive to use the Monte-Carlo search to optimize for a single misfit function over the whole model space. However, if one uses a number of misfit functions depending up on the sensitivity of the model parameters on the data, one can implement the Monte-Carlo method efficiently. Fortunately, this is possible as the different wavelengths of the model parameters influence the different parts of the data.
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Nonlinear wave form inversion: Application to real data
Authors N. Roxix and A. TarantolaIn principle, nonlinear inversion that finds the distribution of elastic parameters in the Earth whose synthetic data best matches the observed data is an ideal inversion. All seismic wave phenomena such as reflected waves, mode-conversions, refracted waves, transmitted waves, Rayleigh waves etc. are used to obtain the inversion result.
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Petrophysical characterisation of the tertiary for seismic AVO investigations, UK Central North Sea
By J. M. ReillyThe investigation of the variation in seismic Amplitude Versus Offset (AVO) is an increasingly important geophysical technique in the exploration for, and exploitation of, hydrocarbon bearing reservoirs in the Eocene of the U.K. North Sea.
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Simulated annealing based 2-D inversion of reflection seismic data from a carbonate evaporite sequence
Authors J. M. Pedersen, P. D. Vestergaard, K. G. Maver and J. J. MøllerA recently developed inversion method based on a global model optimization method called simulated annealing has been applied to seismic reflection data from a carbonate evaporite sequence onshore in Denmark. The inversion resulted in detailed information about the acoustic impedance profile of a target zone.
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Null spaces in AVO inversion of single interfaces
Authors A. Mazzotti and G. DrufacaThe AVO Inversion and Interpretation are research topics of great interest since they may yield precious information on the elastic parameters of sub-surface targets. This paper is focused on the problem of AVO Inversion of reflections from a single interface. A real data example shows that, even in the simple case of single-interface reflections, a non-unique estimate of the unknown parameters results from a limited-offset data set. The real data are reflections from a low-velocity gas-sand embedded in shales at about 0.7 s traveltime. They produce a classic bright spot that clearly stands out in the stack section.
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Statistical analysis on AVO data
Authors F. Hindlet, Y. Z. Ma and A. G. HaasAmplitude versus offset (avo) interpretations involve a large volume of data. In order to carry out lithologic predictions, the avo data is reduced and simplified by various methods: avo gradients, zero offset amplitudes, even common mid point (cmp) stacks. All these displays are simplifications of a data set that may be influenced by several amplitude factors, such as lithology, tuning effects, correlated noise etc.
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Transformations of volume reflectivity into seismic classifiers
Authors L. Sonneland, O. Barkved, M. Olsen and J. -H. NittebergThe conventional description of the seismic reflection process is an interaction between the incident wavefield. the reflected and transmitted wavefield at a surface boundary. The surface-boundary is defined by the discontinuity surface of some elastic parameter. Dependent on the wavefield and the contrast in elastic parameters a reflection coefficient can be associated with this discontinuity surface. We will refer to this description of the reflection process as "surface reflectivity".
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Evaluation of the ambiguities of seismic inversion
More LessThe problem of acoustic and elastic inversion has been studied by several authors; however, there is still the need to understand completely the ambiguities of the inversion, i.e. to determine the reliability of the solution. The wavefield obtained insonifying the medium that results from the inversion will match the data, but there is no guarantee that the solution proposed is the realone. Besides, another interesting problem, not yet completely solved, is the determination of the resolution of velocity analyses, in presence of an unknown AVO.
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A study of the orientational variation of induced polarization time domain data
Authors F. E. R. Almeida and M. J. S. MatiasThe Induced Polarization Exploration Method has been used intensively in areas of complex geology. In such areas pronounced orientational effects have been observed in resistivity measurements. However the orientational behaviour of induced polarization data has been overlooked.
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Two-modal cole-cole modelling of lP time domain relaxations
Authors R. di Maio and D. PatellaThe dominant method for detecting and delineating native pyrite mineralization is Induced Polarization (lP). In presence of hydrocarbon reservoirs the lP anomalies are thought to he caused by epigenetic pyritization located in a plume over oil fields in a geochemically active environment. In geothermal areas the observed lP anomalies can be associated with pyrite concentrations deposited where sulfur-rich thermal waters interrnix with meteoric waters.
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Transformation of transient decays to continuous apparent resistivity curves, a generalized apparent resistivity definition
By L. SöresIn the interpretation of electromagnetic methods a very simple way of getting rid of the geometric and primary field effects is transforming measured data to apparent resistivities. The main principle for this transformation is generally to determine the resistivity of the substituting homogeneous halfspace.
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Sampo, a new wide-band electromagnetic system
Authors H. Soininen and T. JokinenThe SAMPO(Gefinex 400 S) wide-band electromagnetic (EM) system was designed and constructed during 1986-1988 by Outokumpu Electronics in cooperation with the Geological Survey of Finland.
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Application of an electromagnetic borehole induction logger to groundwater contamination mapping
Authors J. D. McNeill and M. BosnarAlthough electromagnetic induction loggers have been used by the oil industry for many years their sondes are generally unsuitable for groundwater applications, for which the requirements are 1 . Slim probes (monitoring Wells are often only 5 cm in diameter) 2 . Good vertical resolution, so as to resolve thin contaminant plumes, combined with a moderate radial depth of investigation (of the order of 1m) 3 . Low noise and good stability so as to measure background conductivities as low as 1 mS/m (1000 ohmm ) 4 . Linear response with conductivity to about 3000 mS/m (0 .3 ohmm ) 5 . Modest depth of exploration, seldom in excess of 250 m, and lastly 6 . The overall system (including winch and console) must be compact, lightweight and relatively inexpensive .
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Geoelectrical methods in geological mapping - Case histories from the Balaton Highland, Hungary
Authors I. Szilagyi, B. Csatho, G. Csillag and L. KoloszarVarious geoelectrical measurements have been widely used for bauxite and coal prospecting in the Hungarian Transdanubian Central Range for decades. These mountain form a syncline structure, bauxite deposits can be found in their central part in tectonically preformed karstic depressions of resistive Mesozoic -mainly Upper Triassic- carbonates. Conductive Cenozoic formations including coal beds cover the bauxite lenses and the basement. This resistivity contrast and the approximately 1-D model allow to determine the depth to the basement and to distinguish the main structural units in the study area using geoelectrical methods.
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Application of a surface-to-borehole frequency EM method for the detection of deep conductive orebodies: A test study in the Iberian Pyrite belt
Authors B. Bourgeois and J. BernardThrough recent advances in instrumentation, procedures and interpretation, borehole electromagnetic (EM) methods have become of great interest in the exploration of conductive orebodies lying at several hundreds of meters below the surface. In particular, when a borehole has missed a suspected mineralised target, these methods can indicate whether or not the expected conductive body does in fact exist in the vicinity of the borehole, and in certain cases, in which direction it lies. Consequently, these techniques are valuable for mining companies, as they can save a great deal of expensive drilling.
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Fixed source EM two-dimensional modelling using finite elements with quadrature base functions
By P. VallaAlthough 2-D modelling has been widely applied to the magnetotelluric method, few results have been published for frequency EM controlled source methods, besides the work of J. Doherty (1988) using surface integral equations. Because it allows to deal with more complex modeis, the finite element techniques (J.H. Coggon, 1971, L. Rijo, 1977) is the most popular, but its application to controlled source EM requires specific improvements of the algorithm, as shown in the following.
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Two dimensional forward seismic modelling: A case study of lithology prediction of the Zechstein interval, Southern Jylland, Denmark
By K. G. MaverFrom a number of wells and a closely spaced grid of seismic sections, it is possible by two dimensional forward seismic modelling, to predict the lithological framework below what is normally thought of as the seismic resolution limit. It is thus possible to build a geological model with sequence thicknesses of for instance 5 meters at a depth of 1600 meters. The modelling is executed by generation of a synthetic section imaging the seismic section and results in a geological model with a high degree of detail.
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An integrated approach to characterization of Pennsylvanian carbonates
Authors B. J. Rafison and G. A. LivesayA multi-disciplinary integration of seismic techniques was used in this work to characterize carbonate mounds in the Midland Basin. Conventional seismic data and computed seismic attributes were studied for four lines from a 3D seismic survey. The analyses were carried out in six phases (Figure 1): I) Pre-stack AVO Analysis; 2) Geological Analysis ; 3) Petrophysical Analysis; 4) Post-stack Attribute Analysis; 5) Seismic Lithologic/Stratigraphic Modeling ; and 6) Interpretation and documentation of the results.
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Integrated study of an unconsolidated reservoir
Authors J. P. Blangy and S. StrandenesThe following two objectives were sought in order to improve volumetric analyses and to enhance the production of a field from the North Sea: map the gas/oil contact, and its evolution through time as the field is produced, and identify areas of high permeability. By integrating petrophysical data such as empirically derived porosity-permeability relationships for each facies, an attempt is made at characterizing the seismic response of high permeability sands and to identify them seismically.
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Minor fault geometries and their effect on seismic data in the West African Salt Basin
Authors P. M. Trayner, G. G. Grant and D. StaughtonSeismic data recently acquired by GECO in the offshore West Africa Salt Basin image numerous minor faults in the post-Miocene section. The faults have normal throw and the majority dip shoreward, ie opposite to the present bathymetric slope. This direction is contrary to that which would be expected in a model of gradual downslope creep and their consistency on dip lines has led to the images being dismissed as spurious features introduced during data processing.
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Improved reservoir description by use of seismic amplitude maps at Veslefrikk Field, North Sea
Authors L. Pedersen, S. Befrin, E. Berg and E. HegreThe Veslefrikk field was declared commercial based on observations from only two exploratlon wells and a 3-D seismic survey. The field has been in production for one year now.
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Reservoir characterization: A case history of the Sarir Field, Sirte Basin, Lybia
Authors R. O. Lindseth and N. A. BerruienThe elevation of the oil/water contact within the massive Sarir field, located in the Sirte basin of Libya, is relatively uniform, suggesting continuous communication of the reservoir over the entire field. Yet, preliminary injectivity tests and drilling of peripheral injectors suggested otherwise. In some cases injection of water was found to he poor and distribution of fluid did not appear uniform. Also, the location of some peripheral injectors was well within the productive limit because the water/oil contact was poorly defined.
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An integrated approach to reservoir petrophysical parameters evaluation
Authors E. Poggiagliolmi and D. LowdenPetrophysical parameters necessary for reservoir characterisation are normally derived from borehole data. Such information can be very accurate in the depth direction but has a small lateral penetration. Surface seismic, on the other hand, has low vertical resolution but is laterally continuous. Calibration of the seismic data to borehole information through integrated processing permits reservoir petrophysical parameters to be accurately mapped from seismic data. The relationship between seismic and petrophysical properties is addressed with reference to a reservoir evaluation study. The relationships are used to calibrate seismic data and to map heterogeneities in reservoir description parameters.
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G.I. Gun sea trials
More LessThe search continues for the ideal marine seismic source. Each of the currently available sources has its good and bad points. Air guns suffer from long-period bubble oscillation, and so have to be used in large arrays. Water guns have good short signatures, but these are non minimum phase. A 'bubble-free' airgun would be a very attractive source, and the G.I Gun is an attempt to attain this (Pascouet 1989). This paper presents results from trials of the G.I. gun, and a comparison between G.I. guns and water guns.
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Modelling of G.I. Gun signatures
By M. LandrøTwo years ago A. Pascouet (1989) presented a new airgun called the GI gun. The main characteristic of this gun is its ability to control (i.e. damp) its own bubble oscillation. The removal of the bubble osciallion is achieved through a tuning of air injection within the bubble. In order to achieve optimal bubble suppression the amount of injected air must be of the order of two times the original amount of air used to create the primary pressure pulse (see Pascouet, 1989). It has been shown however (SSI-brochure), that even if the volume ratio between injected air and original air is reduced to 1.0 (i.e. the gun is operated in the harmonic mode), farfield signatures with a very good primary to bubble ratio can be achieved.
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The low frequency response of high resolution airgun arrays
Authors D. J. Painter, R. H. Seymour and P. J. SmithDuring the past ten years the development and deployment of sleeve airguns and water guns have significantly improved the high frequency content on the final seismic section with a consequent gain in resolution. A critical issue is to what extent the industry's quest for improved high frequency has resulted in loss of low frequency energy and a reduction in penetration. This paper uses a simple theoretical model to examine the compromises, and illustrates the conclusion using results from seismic surveys.
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Modelling air gun array performance specifications
Authors W. R. Cotton and D. J. MonkAir gun arrays have been the preferred source for hydrocarbon exploration for nearly 25 years. One of the challenges over that period has been the computer simulation of the array signatures. The acoustic signal from an array of many air guns is sufficiently complex to thwart many early attempts at accurate modelling. As a result, it was not possible to predict the performance of an array under deteriorating conditions of operation such as the failure of specific guns. operating specifications were deduced from sparse information obtained from far field signature recordings and they carried the risk of being conservatively described without being comprehensive in coverage.
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Comparison of airgun clusters
Authors S. Strandenes, S. Vaage, G. Zaalberg-Metselaar and A. SodalLast year, Vaage, Strandenes & Metselaar [1] reported results from measurements of several symmetrical two gun clusters with varying separation distance, depth, firing pressure and source type. The source-types involved were Bolt 600 C airguns with volumes 10, 20 and 40 cu. inch, Sleeve airguns with volumes 10, 40, 70 and 150 cu. inch and Bolt Combo airguns with volumes 10, 40 and 90 cu. inch. It was found that the optimal gun separation in asymmetrical two gun cluster, based on this database, was 2.4 times the equilibrium radius, that the improvement in primary/bubble ratio on the average was a factor of 2.4 times the ratio for two corresponding non interacting guns and that the loss in primary peak amplitude at the optimal distance was 15 %.
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Signature measurements concerning the exploitation of interaction in the design of compact airgun arrays
Authors Mo. H. Safar, V. B. Hewett and P. HaskeyIn a paper presented at the 37th EAEG Annual Meeting held in Bergen, June 1975, Safar proposed a method (UK Patent No. 1542261) for efficient design of compact airgun array. The proposed method (Safar, 1976) involves the damping of the bubble pressure pulses by exploiting the interaction between a number of properly spaeed identical airguns.
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A New marine vibrator based on Terfenol-D
Authors R. Fritsvold, B. Ursin-Holm and R. TenghamnThe Vibroseis technique was introduced to the geophysical industry in the early 1960'5 and has been used successfully in land seismic acquisition world-wide. In the early 1980'5, steps were taken to convert the technology from land to the marine environment. The Hydraulic based Marine Vibrators developed 50 far, are mainly constructed as large and complex mechanical systems that are intricate to operate from a vessel.
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Synthetic seismograms from extended ray tracing
Authors A. Hanyga and H. B. HelleIn [1,2,3] a method of numerical computation of wave fields is developed. It is an extension of Asymptotic Ray Theory (ART) applicable in regions containing caustics. The essential features of the new method. called Extended Ray Tracing (ERT), consist in application of expressions which are well-defined at caustics in combination with complex ray tracing in caustic shadows.
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Extrapolation operators by beam tracing
Authors S. R. G. Kremer, J. T. Fokkema and C. P. A. WapenaarThe key process in seismic imaging is given by migration. The accuracy and efficiency of the migration process is directly determined by the involved downward extrapolation operators. Taking into account that a downward extrapolation operator equals the response at the surface due to a point source in a subsurface grid point of a macro model, the computation of point source responses in macro models is of vital importance for the seismic method.
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2D Modelling of thin layers
Authors G. Bernasconi and G. DrufacaForward modelling of seismic response of thin layers is not perforrned satisfactorily by conventional techniques. Finite-Difference methods stumble upon small grids, Ray-Tracing becomes cumbersome with conversions, multiples and post critical effects, and Reflectivity works only with horizontal layers. Hybrid schemes that join ray methods with reflectivity might offer a useful low-cost alternative [1,2]. Our scheme uses rays to transport the reflectivity of thin-layered small-scale discontinuities across large scale homogeneities. Conversely transmissivity - that takes into account interbed reverberation and conversions - correctly energizes transmitted rays, even in post critical situations.
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Reflectivity revisited
Authors A. J. Berkhout, C. P. A. Wapenaar and C. G. Mde BruinThe seismic method is based on the fundamental property that downward travelIing seismic source wave fields are returned to the surface by reflection (and/or refraction). Hence the reflectivity property of the subsurface plays a key role in seismic exploration. Structural information is gene rally derived from 'average' reflectivity. Lithologic information can be derived as weIl, provided angle-dependent reflectivity is taken into account.
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Generalized born scattering and Quasi-Shear ray coupling in anisotropic media
Authors C. H. Chapman and R. T. CoatesThe propagation of high-frequency seismic waves through inhomogeneous media can often be described well by asymptotic ray theory. Nevertheless, situations in which ray theory breaks down are widespread and of interest: rays may be focused at caustics causing amplitude singularities; wavefronts may contain amplitude discontinuities causing diffracted signals, e.g. at critical points or shadows; signals may be scattered by inhomogeneities causing low-frequency reflections; and rays may couple due to rapid variations in the ray solutions. In this paper, we concentrate on extensions of ray theory that solve the last two problems - scattering and coupling between ray solutions due to inhomogeneities in the model and ray solution.
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Computation of ray-born synthetic seismograms for anisotropic, inhomogeneous media
Authors R. L. Gibson and M. N. ToksözScattering of seismic waves by inhomogeneity in the earth leads to significant effects on observations of seismic energy. We apply an algorithm similar to that proposed by Beydoun and Mendes (1989) to model these scattered wavefields by combining the Born approximation and ray theory. Scattered displacements generated by a perturbation to a known background earth model are described by the Born approximation, whereby the inhomogeneity is treated as a secondary source radlating energy as it interacts with the incident waves from the true source. Variations in density act as single force sources, while perturbations to any of the 21 independent elastic constants lead to moment tensor secondary sources.
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Seismic traveltime equations for Azimuthally Anisotropic and Isotropic media
By A. G. SenaThe interpretation of traveltime curves over layered anisotropic media would give more physical insight if analytical (although approximate) formulas for such traveltime curves in terms of the subsurface anisotropy were available. In this paper, we derive the analytical expressions for traveltime-offset curves over multilayered weakly azimuthally anisotropic and isotropic media in terms of the elastic properties of each layer. This method is based upon an approximate skewed hyperbolic moveout formula involving three measurement bulk velocities for each reflector: the average vertical velocity and horizontal and skew moveout velocities. This technique allows for fast traveltime computation and extremely rapid estimation of the interval elastic parameters.
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Electrical conductivity of the earth crust in Europe
By L. VanyanFar from relatively narrow conducting anomalies (graphite shist belts, fracture zones, etc.) the background electrical resistivity of the upper part of crystaline Earth crost reaches 105 -106 Ohm.m. It coincides with laboratory measurements if taken into account porosity value and water in pores. One could expect an increase of the resistivity with depth due to closing of pores. But unexpectedly resistivity is decreasing at dep th of 10-20 km practically for all investigated areas of Europe. Magnetotelluric soundings of the last decade showed that the conductivity value of the lower crost is similar to that of the sediment cover. This fact is in a sharp disagreement with high resistivity of the dry lower crost rocks derived from laboratory measurements.
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Integrated gravity, seismic and geological modelling in the Rhine Graben
More LessA combined geophysical and geological interpretation of the Rhine Graben and vicinity in SW Germany is presented in the form of three 2-D, 250km profiles, starting from the S edge of the Rhenish shield and adjacent Lorraine and Saar-Nahe basins, crossing the Rhine Graben in three separate sections, and extending across the Mesozoic platform to the fore-alpine Molasse Basin.
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2D and 3D seismic modelling of crustal structural heterogeneities
Authors S. Hestholm, B. O. Rosland, E. S. Husebye and B. O. RuudSeismic surveys are based on analysis of complex short wavelength signaIs, while the resolved model elements are often simple and of much longer wavelength. Furthermore, attribute analysis of three component seismic records testifies towarels medium complexities not commonly incorporated in the structural modeIs. To provide better understanding of high frequency wave propagation within the heterogeneous crust, we use finite difference (FD) modeling in 2D and 3D in combination with more conventional analysis of seismological data.
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Shear-waves from 3-C ocean bottom seismographs indicative of anisotropy in the lower crust
Authors R. Mjelde, M. A. Sellevoll and E. W. BergThis refraction study was performed in August 1988 by the Institute of Solid Earth Physics, University of Bergen, Norway in cooperation with the Hokkaido and Tokyo Universities, Japan. The purpose of the experiment was to map the crustal structure off Lofoten, northern Norway, from the outer part of the continental shelf to the oceanic crust, and to make use of all three components of the Ocean Bottom Seismographs (OBS) in a detailed shear-wave study. Twenty one 3-C OBSs were used along 8 profiles in a 3D grid.
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Amplitude and time-frequency analysis of ecors deep reflection seismic data
Authors D. Rappin, J. M. Marthelot and E. de BazelaireThe discontinuous aspect of deep crustal reflections on stacked seismic sections has led us to a study of the original records, the true amplitude shot gathers. We look at the amplitudes, the lateral continuity of reflections and the frequency content of the signal in order to obtain more precise informations about the physical properties and the geometrical structures in the crust. These studies are done on ECORS deep reflection seismic profiles and can also be done on seismic exploration data.
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Geophysical study of the Ross Sea (Antarctica)
Authors M. Pipan, J. Finetti, A. D. Ben and R. GelettiThe interpretation of a large amount of multichannel seismic reflection sections recorded during the first Italian geophysical cruises to Antarctica (1978-88, 1988-89), performed by OGS-Trieste, has revealed new and interesting elements about the structure, the stratigraphy and the evolution of the Pacific sector of the Antarctic Platform.
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Statistical investigations of the density-velocity relation using the database DABACOS
More LessEmpirical correlations between seismic velocities and bulk densities of rocks are of particularly interest for practical purposes like combining results of seismic and gravity measurements. Relations like the Nafe-Drake-curve or the Gardner-formula are used to get a priori data for the estimation of a density model by seismic data. Beyond that, they are frequently employed for theoretical penetration studies of seismic waves and for the calculation of synthetical seismograms. Combined with deep drilling programs relations for crystalline rocks become even more important in the near future.
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Application of fuzzy pattern recognition to seismic stratigraphy
Authors B. Lashgari and R. E. EstillA new method for seismic stratigraphic analysis is proposed and its effectiveness on seismic data is discussed. The method utilizes seismic attributes and the concept of fuzzy pattern recognition to automatically track horizons and identify termination points. In sequence stratigraphy this is an effective approach in providing valuable information to interpreters in relation to sequence boundaries, seismic facies, and horizon mapping.
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Logical object model and derived exchange format for 3D subsurface model data
Authors I. L. Ritsema, S. Pen and Th. KemmeIn the Oil and Gas Exploration and Production Industry many data are being exchanged or traded. For many object classes commonly agreed data exchange formats exist (e.g. SEGY. LIS. UKOOA. OLG. etc.). An important object class without any standard exchange format is the class of 3D geological or geophysical data. In the Geoscience Research Programme funded in part by the EC (OG XII) a logical model has been developed for objects describing 3D subsurface modeis. From the logical object model three types of implementations and a exchange format can be derived.
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Interpretation and geomodelling of diapiric structures using the SlIS workstation
Authors D. Jackson, A. Carter and J. FarnsworthThe value of workstations for exploration and development has long been appreciated, but recently in addition to the interpretation and general mapping capabilities, interactive geomodelling techniques carried out wholly within the workstation environment are becoming available. using examples from recently drilled diapiric plays in the Central North Sea, we illustrate how interactive interpretation and geomodelling have been applied during a series of pre-drill/ post-drill phases.
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Practical use of advance software in an interactive stratigraphic and structural analysis of roar field: A case history
Authors I. Abatzis and J. D. KerrThe Roar structure is situated in the Danish Central Graben and is surrounded by the Bo, Boje, Adda, Tyra and Jens structures. It is a low relief porous chalk closure generated by structural inversion during the Late Cretaceous and Tertiary periods. In addition, the general increase in subsidense toward the center of the Central Graben in Tertiary times has also influenced the structural evolution.
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Visualization of map and seismic attributes
Authors H. R. Nelson Jr., S. Mastoris and C. HuxohlNew map and seismic attributes have been developed for use on interactive seismic workstations. These techniques were applied to the interpretation of both geologic structure and stratigraphy from 3d seismic surveys from Canada and the Gulf of Mexico. In both of these geological domains the new attributes provided new infonnation leading to a more complete and accurate interpretation. We will first describe the seismic attributes used to assist identifieation of reservoir reflectors, and then describe the map attributes and how they aided the interpretation. Specifically, we show how these map and seismic attributes explained why wells in one part of a project area were more productive than those in other places in the project, and how this interpretation fonnulated new leads for additional well locations.
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Interactive volume modelling
Authors P. A. Irwin and A. ParadisDynamic Graphics, Inc. has developed a tool that enables the display and analysis of three-dimensional data. Users are able to rapidly gain a much clearer understanding of internal geometric and property relationships within three-dimensional singel of multiple layer frameworks.
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Seismic attributes in borehole full waveform sonic logging
By S. KnizeApplication of surface seismic data processing and display techniques to borehole sonic data provides a means for formation description near the borehole, as well as for the integration of sonic information with surface seismic and cross-borehole tomography sections. The seismic trace attributes analysis and the colorcoded displays which are adapted for Investigation of full waveform sonic (FWS) data reveal additional FWS characteristics helpful in describing rock properties such as dipping boundaries, fractures, and permeability. In this appllcatlon the seismic attributes have distinct geophysical meaning which are specifically pertinent to FWS logging and are called Instantaneous Waveform Characteristics (IWC). The color-coded displays are called colorgrams. The IWC displayed in the colorgrams are also quantified, plotted as log curves, and interrelated to give geophysical parameters such as the quality factor (a) of the formation.
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Cross-well continuity logging using stoneley waves at the Whitchester borehole test site
More LessThe Imperial College borehole test site is situated in the the county of Northumberland, England. It consists of four vertical 250 metre boreholes positioned as in figure 1. The holes intersect an Upper Carboniferous cyclical sequence of sandstones, shales and limestones. (see Figure 2a) Holes 1 and 2 have been fully cored and all holes have been extensively logged. Although the bedding is approximately horizont al over the area of the site, minor faulting has been identified. This is very clearly indicated in the FMS data which has been acquired in all four holes.
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Fracture detection using reverse vertical seismic profiles and cross-hole surveys
Authors E. Liu, S. Crampin and J. H. QueenPrevious observations have suggested that the effects of fractured-related seismic anisotropy is observabIe, particularly in vertical seismic profiles (VSPs) (Crampin et al. 1986; Johnston 1986, and among many others). Fractures can be a significant feature of hydrocarbon reserviors where their presence (either natural or .induced) may increase the productive capacity. Many reserviors with a low matrix permeability would not be commercially attractive without a natural or induced fracture system. Fractures at the shallow depths are also important for the fluid flow in engineering and hydrological applications.
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Seismic imaging at the Whitchester test site using cross-hole data
Authors Q. Li and P. R. WilliamsonThe Imperial College test site at Whitchester consists of a group of four near-vertical boreholes of approximately 250 metres depth. These have been extensively logged and are now cased. A crosshole seismic dataset has be collected between boreholes 1 and 2 over a depth range of 50 to 120 metres. The holes are 25 metres apart. The sources were detonator caps fired at 2 metre intervals, and the signals were recorded with a twelve-component hydrophone streamer. The data contain frequencies in excess of 1.5kHz, with peak energy around 800Hz in the early arrivals. Despite severe contamination by tube waves the data are suitable for use in a variety of imaging algorithms.
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Tube-wave-VSP in cracked crystalline rocks: some new aspects
More LessIn vertical seismic profiling tube waves are generalty classified as noise, They have, however, one useful application: Tube waves can be generated by compressional waves at fluid filted cracks crossing the walt of the borehole. In this case the permeability of the crack can be estimated from the amplitude ratio of the incident p - wave and the generated tube wave.
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Borehole stoneley wave propagatoin across heterogeneous and permeable structures
Authors X. M. Tang and C. H. ChengFractures or permeable structures in reservoirs are of great importance in the exploration and production of hydrocarbons. Heterogeneous layers in the formation are also of major significance. A very good example of such heterogeneous and permeable structures are the sand-shale sequence found in sedimentary fonnations. Full wavefone acoustic logging offers an effective tooI for characterizing these structures.
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Fault mapping using scattered, guided arrivals on a single-level walkaway seismic profile
Authors G. M. Jackson, I. M. Mason and D. LeeField recordings of a 2-D grid of surface shotlines by a single triaxial down hole geophone (single-level Walkaway Seismic Profile) showed guided arrivals scattered from faults. Downgoing P-wave energy scattered from breaks in a strong reflector became trapped in a low velocity layer immediately beneath the reflector.
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Reservoir sonics: A north sea case study
Authors B. E. Hornby, W. F. Murphy III, H. -L. Liu and K. HsuFull waveform sonic data, acquired with a research sonic tooI in a borehole penetrating a North Sea reservoir, were used to explore potentially new applications for analysis and interpretation of full-waveform sonic data. The reservoir is a high porosity, unconsolidated sandstone with Darcy permeability and is inter-bedded with calcite and coal stringers. The wellbore is deviated approximatly 50 degress through low-angle bedding and intersects a region of strong normal faulting.
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Field studies of the anisotropic properties of sedimentary rocks
Authors C. K. Jituboh, M. S. Sams, M. S. King and M. H. WorthingtonExperiments have been perforrned at the Imperial College test site, Whitchester, Northumberland (Worthington, 1991) to determine the anisotropic properties of a variety of sedimentary rock types over a wide range of frequencies. The four boreholes at the site have been extensively logged and cored, so that detailed information is available on many properties of the rocks under investigation.
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Optimal estimation of crack strike
Authors C. MacBeth and G. S. YardleyIt has been suggested (Crampin et al. 1989) that the study of shear-wave splitting may yield valuable infonnation about the stress- and crack-geometries within reservoirs for hydrocarbon production. Shear-waves upon entering an anisotropic medium split into (usually) two components, with the faster component polarized parallel to the strike of vertical parallel cracks in the rockmass. Detennining the polarization of this leading split shear-wave for sub-vertical raypaths, will give infonnation about the strike of aligned vertical cracks, and hence the preferential directions of fluid flow within a reservoir. The time-delay between the arrival of the first and second split shear-waves gives the percentage anisotropy of the medium along the particular raypath, and from this crack density and porosity may be inferred.
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Orthorhombic media: Elastic waves in a vertically fractured earth
Authors M. Schoenberg and K. HelbigAnisotropy in the Earth's crust seems to be well modeled by orthorhombic symmetry (three mutually orthogonal planes of symmetry. In many places rocks can be regarded as a transversely isotropie (t.i.) background with a vertical symmetry axis (due mainly to fine horizontal layering ) in which is embedded a system of vertical parallel fractures (equivalent to aligned microcracks ), which themselves are assumed to be orthorhombic with respect to the natural coordinate system, with x3 positive downward and x1 perpendicular to the fractures (Schoenberg and Douma, Geophysical Prospecting 36, 571-.590, 1988).
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Observations of shear-wave splitting from mining induced stress-failures
Authors G. Graham and S. CrampinLocal networks operated in the gold mines of South Africa record mining-induced events with local magnitude as low as -4 (Legge and Spottiswoode 1987) and occasionally exceeding 5 on the Richter scale. Nearly all events of magnitude 2 and above are also recorded and located by the National Seismological Network of the South African Geological Survey (Femandez and Du PIessis 1988).
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Shear-wave point singularities in sedimentary basins
By S. CrampinShear waves may not behave uniformly for all directions of propagation in sedimentary basins. Splitting of seismic shear-waves (bi-refringence) is observed in many sedimentary basins, and appears to be caused by propagation through a combination of two phenomena: 1) the azimuthal isotropy of fine horizontal layering or Iithology, leading to transverse isotropy with a vertical axis of symmetry, and velocity anisotropy of anything up to 30 or 40%; and 2) the azimuthal anisotropy of stress-aligned fluid-fiIled cracks, microcracks, and preferentially oriented pore-space, leading to transverse isotropy with a horizontal axis of symmetry, and much weaker velocity anisotropy (often less than 5 %). The combination of the two transverse isotropies with orthogonal axes of summetry leads to an orthorhombic symmetry system.
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Anisotropic and velocity dispersion of finely layered media
Authors J. M. Carcione and B. RommelFine layering is one of the geological systerns that more frequently contribute to the formation of sedimentary basins. Moreover, the constituents of a fine layered medium are among the cornponents of reservoir rocks: sandstones and Iirnestones, for instance, which are the recipient rocks, and shales which form the seal rocks. By fine layering we mean that the dominant wavelength of the seismic putse is much longer than the thicknesses of the individual Iayers. When this occurs, effective anisotropy takes place. Moreover, in the presence of hydrocarbons, the media may show substantial attenuation properties and velocity dispersion. This is the case with poreus or cracked rocks like sandstones and limestones, respectively, and even shale formations with considerable fluid content.
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Coupled wave propagation
More LessWave propagation in exploration geophysics is usually modeled elastically or viscoelastically, Phenomena, which include non-seismic quantities are conventionally neglected, when determining elastic constarits of rocks. Coupled wave propagation has been observed in laboratory measurements for a long time; however in field experiments observations have been less convincing.
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An alternative approach to the Cagniard de Hoop method
Authors N. Bleistein and J. K. CohenThe Cagniard-de Hoop method is ideally suited to the arialysis of wave propagation problems in stratified media.
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Selected 3D LOTEM interpretation case histories in exploration situations
Authors A. Hördt, G. Karlik, K-M. Strack and P. WolfgramIn the past few years the activities of TEM measurements in oil exploration have significantly increased. One reason why these methods could not yet reach a breakthrough is that the interpretation is usually based on layered earth models. However, many cases which are interesting for hydrocarbon exploration include 3D structures, such as overthrusts or anticlines.
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An introduction to transient EM imaging via interpretation of current distributions
Authors B. A. James and A. ZerilliA method of subsurface resistivity imaging with transient electromagnetics (TEM) has been created for generaI mapping of geoelectric structure. The method first maps the distributed current flow in the earth as a function of time. The geoelectric structure is then imaged from the interpreted current networks.
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Experimental electromagnetic prospecting for hydrocarbon exploration in the Venezuelan mountain fronts
Authors H. Passalacqua, A. S. Orange, C. Marquez, J. Copley and R. B. FurgersonINTEVEP, S.A. has been involved in the evaluation of alternative techniques to the seismic reflection since 1983. The first activity of this project was related with design and construction of a digital multifonetional data acquisition system, TEM and MT, and related software, which was used for an initial evaluation of these techniques in the Eastem Basin of Venezuela
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Airborne bathymetry in ice covered waters
Authors P. Walker and S. HolladayThe Through-Ice Bathymetry System, or TIBS, is a helicopter electromagnetic system currently under development by the Canadian Hydrographic Service. The aim of the system is to provide reliable bathymetric coverage in areas where bathymetric data would otherwise be difficult to acquire. Development is currently focused on assessing the performance of the system in ice covered Arctic waters with depths up to 50 meters, but TIBS could also be used to map areas where the presence of shoals or other hazards renders conventional ship-borne bathymetric sounding dangerous or impractical.
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Magnetotelluric analysis in the wave number domain
Authors C. Torres-Verdin and H. F. MorrisonOur study is an attempt to a scertain practical vertical and lateral resolution bounds with which geoelectric features in the subsurface can be resolved from lateral measurements of magnetotelluric (MT) fields. For this purpose, we have linearized the relationship between a perturbation in subsurface resistivity and the ensuing perturbation in the surface MT fields (Torres-Verdin and Bostick, 1990). Even though the linearized MT field solutions apply only to low-rontrast models, they hold for any geometrical complexity in the subsurface. In fact, the linear-system analysis yields a set of response equations than can be easily particularized to the cases of one-, two-, and three-dimensional (1-D, 2-D, and 3-D, respectively) subsurface resistivity distributions.
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Two level magnetovariational measurements for the determination of the underground resistivity distribution
Authors D. Patella and A. SiniscalchiThe natural electromagnetic field suggests some different types of measurement to determinate the electrical conductivity of the Earth's crust and upper mantle. In particular, if a measurement at two levels is possible, we can obtain the impedance from purely electric or purely magnetic data (Berdichevskii et al., 1969). In this note we study this second case which can have interesting developments in some applicative fields, as e.g. offshore or in presence of tunnels.
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Development and improving of georadar system as a geophysical method for mining exploration
By M. PiccoloThe scope of this report is to summarize all the experiences arose during the project "DEVELOPMENT AND IMPROVING OF GEORADAR SYSTEM AS A GEOPHYSICAL METHOD FOR MINING EXPLORATION", supported by EEC funds.
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Downward continuation of some stationary geophysical fields
More LessThis paper offers a new method for downward continuation of some stationary geophysical (geothermal or geo-electrical) fields in 3D gradient media.
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On the application of statics in marine 3D data processing
Authors G. P. Lourantos, F. Kirchheimer and H. KoitkaIn the process of of land seismic data static corrections play a firmly established role and a wide spectrum of rnethods for the estimation of statics has been evolved. This is quite in contrast with the situation for offshore surveys, Where statics have been largely neglected. Little is published about the specific problems of estimating and correcting the effects of near-seabottom velocity anomalies.
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Recent advances in seismic processing provide the key to evaluating the commerciality of the Wealden oil play in the Celtic Sea Basin
Authors J. Pender, L. Godfrey and S. CoperudeEsso Exploration discovered oil in the Cretaceous Wealden of the Celtic Sea Basin in the early 1970'5. Since then, other operators have found oil in the Wealden but have been unable to justify the development of these finds. The main obstacle to commerciality has been the inability to predict the extent and distribution of oil- or gas-bearing reservoir sand using seismic data.
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A Comparative study of pre-salt imaging techniques in the pre-Caspian Basin
Authors K. Schleicher, M. A. Brzostowski, P. V. Zahharov and G. F. MillsWe compare different techniques for imaging pre-salt reflectors In the Pre-Caspian Basin of the Sovlet Union. This basin is the site of the newly discovered Tenglz Field. Until recently only shallow fields could be imaged In this basin. Modern seismic processing techniques have allowed the imaging of Devonlan events below Permian salt.
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Seismic data processing and interpretation problems encountered beneath rapidly changing water depths: A case history from the carbonate banks of the Seychelles
Authors A. H. Roberts and S. A. ThornThe Seychelles Archipelago consists of a series of steep-sided carbonate banks that were formed by deposition of a thick, high velocity , shallow marine carbonate sequence. Deposition of these carbonates has been concentrated on local highs and has kept pace with the Tertiary thermal subsidence which followed the rifting between India and the Seychelles.
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Model based depth conversion and processing - A case study offshore mid-Norway
Authors K. Willersrud, A. I. Evensen, K. Mathisen and I. MagnusAccurate depth maps and correctly depth converted cross sections are primary products of geologic and geophysical investigations. Because the time structures on our seismic sections do not necessarily mirror the true depth structures, depth conversion may become a demanding task. Despite the obvious importance of reliable depth models, approximations in the depth conversion process often reduce the quality of the final products.
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Depth migration of diapirs from the Central Graben of the North Sea
Authors M. J. Sugrue and D. C. RockliffA key factor in BP's recent diapir related discoveries in the Central Graben has been the successful use of depth migration. This is an attempt to distil to a few stages the lessons learnt from this work.
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3D Pre-stack depth migration in the Gulf of Sueza. A case history
Authors P. G. Western and G. BallDuring 1989 and 1990 Unocal developed the capabilty to apply true 3D pre-stack depth migration to 3D seismic reflection data. The technique was applied to a 3D data set from the Gulf of Suez, Egypt, over a sub-salt prospect initially identified from 3D post-stack time migrated data. The prospect was subsequently drilled by Unocal Egypt as operator for a group including LASMO Oil (Egypt) Ltd and Nedlloyd Energy (Suez) BV. The paper describes, in the form of a case history, the processing and interpretation of the data and the drilling results proving the effectiveness of the technique.
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Rockall through: New images from deep seismic surveys
Authors J. K. Neish and F. MalcolmThe Rockall Trough presents many problems for geophysical analysis, including the generation of deep water bottom and complicated interbed multiple series, severe attenuation of incident energy, and disruption by pervasive Tertiary intrusives. The acquisition and processing parameters of a composite seismic survey conducted in 1989 were based on conclusions drawn during an intersecting survey carried out in 1987. Pre-stack depth migration was applied to the 1989 data set to improve imaging. Velocity control was provided by the integration of coincident expanding spread and sonobuoy data. The pre-stack depth migration is then further expanded to encompass the deepest limits of the composite deep seismic data sets.
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Case history of a six streamer seismic survey in the North Sea
Authors J. V. Kingston, G. Hampson, T. Hansen and H. JakubowiczThis paper describes the design and operation of a 3D survey in an area containing a number of novel and interesting geophysical problems. Special data were acquired to enable the selection of a suitable configuration of source, streamer and positioning equipment to meet stringent spatial and temporal sampling requirements. Low signal to noise ratios in the area prohibited the loss of stack fold inherent in multiple source systems. The survey was performed using a single source array and six 600 m streamers. To provide high positioning accuracy, the navigation system included two radio positioning systems, differential GPS and a network of acoustic ranging devices.
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Developments streamer tail positioning
By A. ThevenotAn ever increasing degree of accuracy is required in positioning for 3D marine seismic surveys. Accuracy between the vessel and the streamer tail is significantly impaired when the only means of positioning are compasses placed along the streamer. This problem has been partially solved by the use of radiopositioned (Syledis) tail buoys. However, there remains some uncertainty as to the position of the streamer tail because the end of the streamer is connected to the tail buoy by a tail rope of significant length (to prevent noise on the hydrophones) and for which the bearing is unknown (owing to currents different from those affecting the streamer).
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Goal-oriented binning for the initialisation of marine 3D geometry
Authors I. A. Henderson, R. C. Atkinson, P. R. A. Swar and D. G. WrightIn recent years 3D surveys have become progressively larger and more complex. In marine environments, multi-line acquisition has become the norm, with up to sixteen subsurface lines being collected simultaneously. On land, 3D is used in harsher areas requiring irregular geometries and mixed source and receiver types. In both cases, careful attention is required during the initialisation of 3D geometry data to ensure the optimum performance of processes such as CMS, OMO and multiple attenuation.
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Cable positioning using compasses, tailbuoys, and acoustic devices
More LessThe combination of acoustic front end measurements, cable compass readings, active tailbuoy positions, and inter-cable acoustic ranges has been exploited in a sparse-net, least-squares solution to decrease the overall uncertainty in cable receiver position estimates. Comparisons among solutions derived from various combinations of sensors demonstrate that acoustic and tailbuoy measurements effectively supplement the compass data. We estimate that the maximum uncertainty in cable receiver positions for a system employing all of the above sensors in a sparse-net solution with good Syledis control is about 12 m.
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A Digital telemetry system implementation of the dual-sensor, bottom-cable method
Authors F. J. Barr, J. I. Sanders and D. A. ChamberlainThe bottorn-cable rnethod of seismic reflection data acquisition has been used for many years in shallow water and in are as densely populated by obstacles, where towed streamer marine vessel access is rendered irnpossible or risky (Rigsby, et al, 1987). And as production and drilling platform obstacles have rnoved to deeper and deeper water, the application of the bottom-cable method has moved with them.
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Monitoring of an underground flow by sea bottom seismic instruments - A case study
Authors K. Kolbjørnsen, D. O. Larsen, P. Kristiansen and K. A. BerteussenIn January 1989, when drilling a deep Jurrasic well in the Southern North Sea, offshore Norway, Saga Petroleum a.s had to shut in the well by activating the BOP. The semisubmersible rig was moved off the location and a relief well was spudded. When a jack-up rig was reconnected to the well three months later, the pressure in the well had dropped considerably, indicating internal underground flow.
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Case history vertical cable 3D acquisition
By P. M. KrailA twelve channel vertical seismic cable has been constructed, tested, and a full scale 3-D acquisition program in the Gulf of Mexico has been completed. The basic vertical cable concept, in the marine environment, is to deploy the receivers vertically instead of horizontally as is the usual case. The vertical cable had its genesis in walkaway Vertical Seismic Profile (VSP) with the important difference that it constitutes VSP without, the borehole, the tube waves or expensive standby rig time. Construction of a vertical cable requires a different set of ocean engineering requirements than does astreamer. The oil filled jacket is replaced with a high-strength Kevlar stress member surrounding the conductors. The hydrophones are attached to the outside of the Kevlar cable and fairing is attached to the cable to reduce the drag of ocean currents.
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Search for better seismic data - A case study from Troms III
Authors M. Brink, K. Kolbjørnsen and J. StærkebyeThis paper documents experimental seismic exploration performed by Saga Petroleum in the Troms-S area along the Norwegian coast. The objective was to establish techniques to improve the seismic reflection data in order enable an assessment of the hydrocarbon potential. Results of two testlines, shot late 1988 in cooperation with GECO, are discussed. The preferred acquisition and processing parameters have been applied in a seismie survey in autumn 1990 which was undertaken in cooperation with Statoil and Norsk Hydro.
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Implementation of anelasticity to an integral equation method for seismic modelling
Authors J. Schrader, H. Freter and A. BehleThe introduction of attenuation to direct methods for seismic modelling such as finite differences or the Fourier method is still quite difficult. Recently an integral equation method has been presented which allows an easy incorporation of attenuation mechanisms (Freter and Kummer, 1989, Freter, 1990). This method is based on the calculation of the Green's function associated to a heterogenous medium by solving a Fredholm integral equation of the second kind. It is sufficiently general as it allows the implementation of various models of viscoacoustic media with an arbitrary dependence of the quality factor on both space and frequency (Freter, Schrader and Behle, 1991).
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Wavefield synthesis using the boundary element method
Authors P. Gerstoft, H. Schmidt, W. Seong and O. VilmannA hybrid forward modelling method combining the boundary element method with the direct global matrix (DGM) method for propagation in stratified media [1] has recently been developed [2,3]. The method allows for simulation of scattering and reverberation from elastic facets in a stratified environment. Assuming both the interior of the facet and the exterior domain are plane stratified, the exterior and interior fields can be expressed in terms of a boundary integral only along the surface of the facet, provided the Green's functions are chosen to be the ones satisfying all boundary conditions in the stratifications.
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High-order spectral element method (SPEM) for acoustic wave propagation
Authors G. Seriani and E. PrioloThe most widespread of the discrete numerical methods for modelling seismic wave propagation, are the finite difference (FDM), the Fourier or pseudo-spectral (FSM), and the finite clement (FEM) methods (Boore, 1972; Kelly et al., 1976; Lysmer, 1972; Gazdag, 1981; Kosloff, 1982; Kosloff et al., 1984; Marfurt, 1984), Even though based on different mathematical approaches, all three methods rely on the space discretization of the geological structure to be modelled. In particular, the pseudo-spectral rnethod can he seen as a limiting case of the finito difference methods of increasing orders and accuracies (Fornberg, 1987).
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Large scale elastodynamics: A cost-effective, accurate and non-dispersive approach
Authors F. Dellagiacoma, A. Kamel, P. Sguazzero and A. VesnaverHigh-order finite-difference (PD) schemes for the numerical integration of elastodynamic equations offer a viabIe answer to the challenges of large scale geophysical modeling. Several authors have proposed non-dispersive numerical spatial differentiators: such operators are designed by maximizing the spectral frequency band for which a peak relative error in the numerical group velocity is bounded by a specified threshold. In previous publications, the authors of this paper have explicitly addressed the cost-effectiveness issue in non-dispersive numerica! wave propagation.
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Domains in the 3D modelling tool GOCAD
Authors J. -L. Mallet and P. N. NobiliIn the frame of the GOCAD project, it has been decided to model surface (e.g. geophysical or geological interfaces) by connected triangles (J.L. Mallet, 1988, 1989). This very powerful approach allows for the modelling of any kind of 3D shapes, including complex shapes such as overhanging salt domes. It is also compulsory for a lot of applications to deal with volumes and not only with surfaces. Due to this fact, we have developed an extension to the Geometrical database of GOCAD, allowing the partition the 3D space into volumes or domains.
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A moveout equation for media with lateral velocity gradients
By B. PayneAccurate estimates of velocity are sometimes compromised where the propagation velocity changes in the horizontal direction. Extreme lateral velocity gradients can impair the coherency of a stacked reflection image. To address these problems I extend the standard model used to predict traveltime of reflected waves by allowing the propagation velocity to change in the horizontal direction. The initial model developed here uses a constant thickness layer in which the velocity varies linearly in the horizontal direction. The traveltime equation for a wave reflected off the base of the layer is derived, assuming a source and receiver on the surface. The new traveltime equation has been integrated into a seismic processing system and by reprocessing selected seismic data we have improved reflector continuity and achieved more reasonable velocity distributions. The flat layer model is discussed by Payne (1990a,b). In the present paper the traveltime model has been further extended to allow aplanar, dipping base to the layer, with a velocity gradient parallel to the base of the layer.
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Scattering mechanisms in finely layered acoustic and elastic media
Authors J. Prüssmann and A. BehleThe transmission of compressional waves through a stack of acoustic or elastic layers is investigated with respect to the influence and behaviour of certain scattering mechanisms at offset. At the chosen model depth of 5 km the transmitted signals are nearly identical for corresponding plane and spherical waves. This result facilitates the modelling and allows the description of the scattering properties of an interface by a fixed set of reflection and transmission coefficients.
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Parameterisation of detail in macro models
Authors F. J. Herrmann, C. P. A. Wapenaar and A. J. BerkhoutInspection of both well and seismic measurements reveals a subdivision of the earth 's subsurface into a few straugraphic sections. The trend in and throughout these intervals gives information on the depth dependent compaction properties of the subsurface. The remaining detail gives inforrnation on the different rock and pore properties of the individual geologic layers (within the resolution of the measurements).
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Interpretation of borehole gravity data in crystalline environment
Authors P. A. Wolfgram, K. -M. Strack, U. Casten, H. Gatto and J. E. T. RichterBorehole gravity meter (BHGM) surveys are mainly used in sedimentary environments. Only a limited number of case histories for crystalline rocks is known (Richter 1989). The pilot well of the Continental Deep Drilling Project (KTB) of the FRG has penetrated amphibolites of densities 2.81 to 2.95 g/cm³ embedded within gneisses of lower density (2.12 to 2.74 g/cm³). The apparent densities calculated from the BHGM data are combined with independent information from other sources for the interpretation of dipping structures. The result is an interpretation procedure which is also valid for the sedimentary environment of hydrocarbon exploration.
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