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55th EAEG Meeting
- Conference date: 07 Jun 1993 - 11 Jun 1993
- Location: Stavanger, Norway
- ISBN: 978-94-6282-135-4
- Published: 08 June 1993
81 - 100 of 500 results
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Study of traveltime using 3D ray tracing through triangulated surfaces
By P. ThisseThe modelling of traveltimes picked on seismic sections can be performed using a ray tracing method. In the case of a complex medium, traditional ray tracing algorithms (e.g., a shooting method) do not work well. The Sistre project developed on top of the GOCAD's modeller (Guiziou et al., 1992) is able to handle complex 3D geological structures. It is based on the discretization of the surfaces via triangles Forward modelling in Sistre is based on a three-step ray tracing algorithm which takes full advantage of the decomposition of interfaces via triangles (Guiziou et al., 1991). The first step constrains the ray to pass at triangle vertices only, while the next two steps locate the impact points inside the triangles.
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Estimation of multivalued arrivals in 3D models using wavefront construction
Authors V. Vinje, E. Iversen, H. Gjoystdal and K. AstebolWe have developed a ray tracing based wavefront (WF) construction method that finds arrivals in general 3D models consisting of blocks with smooth velocity and density variation separated by interfaces where the velocity and density change discontinuously. The method is based on the ideas in Vinje et. al. (1992) and it mimics the true wave propagation in the lense that entire WFs are propagated time step by time step creating a moving surface travelling through the model. The WF may stretch, twist and fold in any way as it moves through the medium and the spatial sampling rate of the WF is kept above a pre-defined limit. The receivers are given value as the WFs pass them, one arrival for each sheet of WF that passes. Multi-arrival travel times, amplitudes, geometric spreading etc. are thus found in an efficient way in all areas of the model that are reachable by conventional rays.
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3D Interpolation of geophysical parameters - The GOCAD approach
Authors J. L. Mallet, R. Cognot and S. CasesThere are many applications in geology which require to know the distribution of physical parameters (velocities, porosities,...) in a region of the 3D space corresponding to the subsurface. Most of the time, these applications require to know the values of these parameters at the nodes of a regular 3D grid which must be interpolated from scattered data points. Classical methods, like for example Krigging, have the following drawbacks - they are numerically unstable as soon as data are clustered, and this is often the case in geophysics - they are mathematically unable to account for discontinuities generated by complex surfaces corresponding to horizons or faults, and it is necessary to use "programming tricks" ; - they are often too slow for initializing huge grids having for example a size of 500³ nodes like it is often the case in seismic migration methods!
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3D Geologic modelling based on topological objects
Authors W. Wiggins, U. Albertini and G. StankovicThe lack of adequate methods for representing velocity fields and other spatially indexed geologic parameters has limited the sophistication, economic viability, and accuracy of many seismic processes, especially 3-D seismic depth imaging. Recently, techniques borrowed from research in computeraided geometric design have been reported by Sword (1991), Ritsema, Nobili, and Mallet (1991) and Wyatt, et al (1992). We describe a similar computer representation based on topological data structures as developed by Weiler (1988).
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A 3D cellular, smooth boundary representation modelling system for geological structures
Authors L. A. Froyland, A. Laksa, K. Strom and J. PajchelThe modelling system, Galled GeoCell, is a 3D boundary representation modelling system to build and manipulate a geometric model of geological structures. Physical properties of the geology such as density and porosity are represented in the model by functions defined over the region of interest. These property functions are smooth within each geological block or cell in the geological structure. The purpose of the modelling system is to perform or prepare for: - Visualization of the geometry and physical properties of geological structures - Seismic modelling using e.g. ray-tracing and finite differences, - Geological simulation done by incremental change of geometry, - Reservoir simulation The topological representation of GeoCell is of a non-manifold, cellular form. Therefore the system can model the anomalies of a reservoir model such as faults. Also the interfaces between geological cells are modeled as smooth faces. This is in accordance with the low frequent seismic image of geological structures, and is necessary in order to use ray tracing techniques in seismic simulations.
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Source signatures in finite difference schemes
More LessFor a marine seismic air gun array each gun location does not necessarily coincide with the nodes in a finite difference grid. Especially for coarse grid finite difference modeling this problem must be handled with care since there might be up to three or four air guns between two grid points. Most examples of coarse grid finite differente modeling of seismic data have up to now used single point sources. A conventional marine source array consists of a number (from 10 to 70) of individual air guns, typically separated by about 2 - 4 meters. For coarse grid finite difference schemes, however, the distante between each grid point is typically 10m or more. This means that effective point sources (i.e. notional sources) estimated from measurements can not directly be implemented in a coarse grid finite difference scheme. We will circumvent this problem by introducing several different source functions at the same node in the finite difference grid. The set of grid source functions at each node is a multipole expansion, and we will estimate how many terms in the multipole expansion, that are sufficient in order to achieve a reasonable representation of the directive source wavefield.
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Viscoelastic rheologies via eigenstrains
Authors J. Carcione and F. CavalliniAs Lord Kelvin quoted in his Encyclopedia Britannica article on Elasticity (Thomson 1878, p. 823): A single system of six mutually orthogonal (strains) types may be determined for any homogeneous elastic solid, so that its potential energy when homogeneously strained in any way is expressed by the sum of the products of the squares of the components of the strain, according to those types, respectively multiplied by six determinate coefficients. The six strain-types thus determined are called the Six Principal Strain-types of the body
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Integrated approach to multiple elimination and wavelet estimation
Authors D. J. Verschuur and A. J. BerkhoutSurface-related multiple elimination is a method that remover all surface-related multiplee from the seismic data without using any knowledge of the subsurface. It is assumed that all information about the surface reflectivity and the source and receiver behavior is known. However, by applying the method adaptively, the surface and source/receiver properties can be estimated from the data itself. Simultaneously, the multiples are eliminated. So far, we considered seismic data as point source responses, with an equal source signature for all shot records, see Verschuur et al (1992). In this paper we show how we can improve on source signature estimations, especially with respect to the shot to shot variations.
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Efficient multiple elimination using generalized radon transform with pseudo-hyperbolic stacking surface
More LessThis paper presents a new procedure for suppressing multiples by using a pseudo-hyperbolic transform, which is equivalent to the parabolic transform with an analytical solution of time variable p-parameter. Examples show that the new procedure is more efficient in the multiple elimination than conventional methods, so that it benefits velocity analysis (VA) and amplitude versus offset (AVO).
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Surface multiple attenuation - Theory, practical Issues, Examples
More LessSurface multiple attenuation (SMA) is a prestack inversion of a surface-recorded, 2-D; wavefield that removes all orders of all surface multiples present within the wavefield. In addition, the process determines the average acquisition wavelet imbedded in the wavefield. Surface multiple attenuation requires no assumptions or modeling regarding the positions, shapes, or reflection coefficients of the multiple-causing reflectors. Instead, SMA relies on the internat physical consistency between primary and multiple events that must be present in any properly recorded marine data set.
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North Sinai of Egypt - Re-Processing leads to a prospectivity re-appraisal
Authors A. El-Emam and M. NessimThe El-Arish area of North Sinai is characterized by NE-SW elongated anticlines which are partially breached at surface by erosion. Exploration activity over the last decade, including several Wells, has been without success, and prospectivity had been downgraded. An attempt to understand the reasons for this lack of succes through reprocessing of seismic data, together with a review of well results, has shown a serious and previously unidentified seismic multiple problem. Careful reprocessing with f-x deconvolution, together with multiple modelling & elimination has allowed a solution, and the results of the follow-up well are encouraging.
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Pre-stack multiple suppression analysis of marine seismic data - Norwegian North Sea
Authors R. V. Schneider, K. J. Marfurt and P. G. GarossinoSeismic data from the North Sea, oftentimes contain poor S/N values. Sources of noise range from random bursts to coherent patterns observed throughout the dataset. In particular, coherent noise in the North Sea is produced by geologic features including a hard water bottom, chalk, out of plane faults, and in the dataset to be shown, scattering of the seismic signal due to dewatering of the Hordaland shale.
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A Methodological comparison of several multiple suppression techniques in two real cases
Authors P. Julien, P. Desegaulx and P. M. HallThe aim of this paper is to compare several long period multiple suppression techniques and to draw practical conclusions from their results on real data. As the results from such algorithms tend to be very data - dependant, two sets of seismic data are presented: the first is from an area of quite poor data quality, the second from an area of quite good data quality.
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Industrial and seismic noise removal in marine processing
Authors M. Manin and J. N. Bonnot3D marine surveys often take place in regions cluttered with oil rigs and sometimes other seismic vessels. The acoustic environment is therefore not very conducive to the acquisition of accurate seismic data. The oil rig may be in the process of drilling and there fore creating a great deal of vibration. It may have engines or noisy pumps. It may even read to the seismic arrival by diffracting part of the energy. The other seismic vessels may also be shooting. Numerous attempts have been made to demonstrate that these kinds of noise can be attenuated efficiently by methods based on the levels of amplitudes or on randomization of this noise within certain groups of traces. Apparently the concerns of the industry have not been allayed by these method sand, as a result, drilling and seismic acquisition must be planned on a regional scale (time-sparing), so that each activity respects the silence required by the other. In certain cases, this time-sparing is uneconomic and therefore impracticable.
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Biangular decomposition of marine seismic data
Authors P. A. Vercruijsse, P. J. K. ter Doest and J. T. FokkemaThe goal of seismic imaging is to produce the reflection operator at target depth. This reflection operator is independent of the acquisition parameters and is an intrinsic property of the medium parameters (Berkhout, 1982). In the so-called Angle-versus-Angle (AVA) analysis one is concerned with the behaviour of the reflection operator as a function of the strike angle of the incident wavefield at a particular interface. In order to enhance the AVA analysis we propose a biangular decomposition of the seismic data. In the resulting domain the axes are related to the strike angle of the incident wavefield with respect to the normal at the interface and the local dip angle of that interface. Operating in this way we go from from acquisition dependent parameters to intrinsic medium parameters.
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The application of common offset and common angle pre-stack depth migration in the North Sea
Authors N. D. Whitmore Jr., W. F. Felinski, G. E. Murphy and S. H. GrayPre-stack depth migration procedures have been used in many areas for seismic imaging, interval velocity analysis, and depth conversion. Most migration procedures assume isotropic wave propagation models. However, in areas like the North Sea, the isotropic assumption is not correct, and this can produce incorrect depth imaging if not properly accounted for.
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Pre-stack turning ray imaging with a 60° operator
More LessSeismic energy traveling in an inhomogeneous medium may move along a curving ray path causing wavefronts which were originally going downward to turn through 90 degrees and move back toward the surface. Special algorithms are required to image reflectors which are illuminated by turning rays if the imaging is done poststack, since the only path between the surface and the reflector seen by the migration is a turning ray path.
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Residual depth moveout after common offset depth migration
Authors R. G. Williams and J. CowleyCowley et al (1992) have shows that common offset depth migration can be used in a layerstripping mode to perform depth migration velocity analysis (DMVA). The DMVA procedure allows direct picking of interval veldcity and depth, thus enabling a velocity depth model to be built and used for common offset depth migration. After the migration, the data are sorted info true common depth point gathers and stacked. It is common to find small residual moveout on these CDP gathers. This may happen either at CDP locations between the original DMVA locations or at intermediate depths between picks. In the first case, the lateral variation of the model has not been sufficiently defined whilst in the second case the vertical variation has not been sufficiently defined.
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Imaging salt with pre-stack depth migration
By J. R. GranliMost seismic data are processed using conventional methods onder the assumption that the earth is close to horizontal layering. This assumption is rarely fulfilled. Nevertheless, the seismic image quality obtained using these methods, is reasonably good, even in areas with structures of moderate complexity. Areas with saltdomes are beyond moderate complexity. By experience it is very difficult to obtain reliable seismic images using conventional methode here. Altemative processing methode are therefore needed. The purpose of our study is to investigate whether Pre-Stack Depth Migration can improve the seismic image when large saltdomes are present.
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Subsalt imaging via target-oriented 3D pre-stack migration
Authors C. A. Jacewitz, D. W. Ratcliff and S. H. GraySubsalt imaging and subsalt trap identification have been recognized for years as important but difficult exploration objectives. By using subsalt imaging technology the explorationist hopes to determine the depth of the subsalt structures and the presence or absence of structural closure associated with the subsalt prospect, thereby obtaining a realistic assessment of the risks involved with drilling a subsalt well. Target-oriented 3-D prestack depth migration is the most powerfuI tool available to assist the explorationist in the quest for hydrocarbon exploration below salt. In this paper we present a geophysical case study describing the steps taken to produce a nseful subsalt image.
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