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EAGE/SEG Research Workshop 1990
- Conference date: 15 Jul 1990 - 18 Jul 1990
- Location: Cambridge, UK
- ISBN: 978-94-6282-136-1
- Published: 15 July 1990
21 - 40 of 69 results
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Application of efficient ray tracing algorithms within schemes for interval velocity estimation in two and three dimensions
Authors E. Iversen and H. GjøystdalIn this presentation we consider methods for estimation of seismic interval velocities using ray tracing as a key element. The methods under consideration shall be based on travel time observations, and they shall have the objective to produce a 2D or 3D subsurface 'macro model', that means, a subsurface representation for which the fine details have been omitted, whereas the main structural variations have been retained. Without doubt, such macro models show an interesting potential, f.ex., in ray-theoretical time-to-depth conversion and in model-dependent processing schemes (e.g., depth migration).
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Accurate determination of seismic velocities from non-hyperbolic move-out using ray-tracing
Authors I. J. Totterdell, R. W. Hobbs and M. R. WarnerThe accurate determination of seismic velocities requires a new approach to velocity analysis. In many areas of geological interest the assumptions underlying conventional velocity analysis fail; the raypaths cannot be taken as being straight, and lateral variation of the velocity structure becomes significant. Seismic reflection datasets collected in such areas often feature moveout with offset curves which are not hyperbolae. This leads to ambiguity in the determined stacking velocities and hence in the derived interval velocities.
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Seismic depthing by travel-time inversion in areas with low structural dip
Authors A. Cook, S. H. Archer, J. Empoliti and C. R. SireyThe use of seismic data to estimate geological depths and seismic interval velocities is of fundamental importance in oil exploration. Several different methods are widely used for deriving depth and interval velocities from seismic data. The choice of method depends on the geological environment, the available information and data quality. This paper describes a method that is very effective for low-dip environments where low-relief structure may be masked by lateral variations in interval velocities.
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Seismic traveltime inversion
By B. UrsinIn most seismic applications, the traveltimes of the primary reflections constitute the main information about the subsurface. The traveltimes may be picked and used directly. or the parameters in a traveltime approximation may be estimated from the seismic data. For a given geological model the traveltimes may be computed by raytracing methods. Traveltime inversion consists of computing the geological structure and the velocity functions from the observed traveltimes.
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Traveltime inversion in laterally varying media with a priori information: A versatile tool for the structural interpreter
Authors M. Léger, L. N. Luc, A. Carlini and S. CorniniDepth-domain images of structures can be obtained by depth migration, but this requires an accurate velocity model, especially in case of laterally varying velocities. Tomography, viewed as the estimation of the velocity model using prestack picked reflection traveltimes and geological a priori information, is a good means to fulfill this requirement. We study a gas prospect in which gas accumulations located in an anticline induce lateral variations in the velocity. From the arrival-times of 6 reflections, tomographic inversion gives a velocity model showing negative velocity anomalies at the top of the anticline. Checking the quality of the model by depth migration leads to refine it by constraining the bright-spots to be horizont al. This is easily done using a priori information.
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Strategies in non-Iinear inversion for seismic velocities
Authors M. S. Sambridge and P. R. WilliamsonThe retrieval of 2- and 3-D seismic velocity fields from surface, or borehole observations may be formulated as a non-linear inverse problem.
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Non-linear AVO inversion for a stack of anelastic layers
More LessParameters in a stack of homogeneous anelastic layers are estimated from seismic data, using the offset variations of the amplitudes and the traveltimes.
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A reliability study of tomographic inversion of stacking velocity
Authors A. Vesnaver, G. Seriani and G. BoehmThe anomalous spatial oscillations of stacking velocity due to local inhomogeneities have been studied over the last decade (e.g. Rocca and Toldi, 1982; Loinger, 1983), but only recently have they been exploited as a quantitative tool for measuring the variations of seismic wave propagation velocity in the Earth. Harlan (1989) introduced an algorithm for tomographic inversion of stacking velocity anomalies using reflected rays, which is tested in this paper for information on its advantages and lirnits.
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Interpretation of velocity estimates from coherency inversion
More LessThe natural output of most velocity inversion schemes is an estimation of a macro-model describing the real medium as a series of layers separated by interfaces across which the velocity can vary discontinuously.
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In situ fracture zone velocity and dip inferred from VSP apparent velocities and three-component VSP P-wave first motions
More LessTwo deep fracture low velocity zones below 2600 meters in continental crystalline rock were drilled during the US National Science Foundation Cajon Pass scientific deep drilling project near the San Andreas fault in southern California.
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Emerging technology in borehole geophysics with multiple applications in drilling, production, exploration and enhanced recovery
By A. KlavenessIn this study, it is demonstrated that sonic pulses can be emitted from a borehole energy source concurrently while drilling oil and gas welIs, and that these pulses are of sufficient strength and suitable frequencies to be very useful.
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VSP velocities (P and SV) and reflection seismic analysis
More LessTraveltime inversion of direct aniving energy in the VSP and crosswell seismic tomography (XST) cases can provide a high resolution and useful interval velocity. This velocity can be used in further reflection processing and petrophysical interpretation. Two case histories (one VSP and one XST) are discussed here where traveltime inversion and mapping were used to create interpretable images.
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Reliability of walkaway VSP imaging given inaccurate velocity estimates
More LessData acquired using Walkaway VSP techniques are used to image subsurface structure. The migration techniques involved require some prior knowledge of the structure and velocity distribution to generate a background model. Although VSP surveys provide accurate interval velocities along the borehole we cannot always assume to have a good estimate of layer velocities away from the well, even in relatively smoothly varying structure. Velocity inaccuracies can arise in the presence of transverse isotropy, laterally varying velocities or incorrect estimates of overburden dip. Velocity perturbations are applied to a background model in order to examine the sensitivity of the imaging of a fault structure. Using a case study we show that inaccurate estimates of velocities produce significant time shifts but do not have much effect on the lateral positioning of a fault edge.
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Velocity modelling using arrival times from multi-offset VSP: Convergence of constrained models
By R. HiggsTravel time tomography has received much academic and commercial interest in recent years, mainly as a stand-alone technique for delineating velocity anomalies, but also as a means of refining the velocity model used for the migration of conventional reflection data.
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Tomographic velocity imaging of geological structures in gulf coast sediments
Authors M. Harris, H. Tan, L. Lines, C. Pearson, S. Treitel, G. Mavko, D. Moos and R. Nolen-HoeksemaIn the following case study, cross-well seismic tomography was used to image both structural and stratigraphic features in Gulf Coast Miocene sediments. A piezoelectric downhole source was used to produce seismic waves which were recorded by hydrophones between wells 250 apart. Traveltime tomography was then applied to a set of over 5000 picks. In addition to the targeted fault delineation, the tomography successfully imaged a number of porous sandstone layers previously identified from type logs for the Miocene. Two different methods of traveltime picking and tomographic inversion were used. The two resulting velocity tomograms show similar velocity variations that lead to a consistent geological interpretation. Borehole gravity meter (BHGM) data, obtained in both wells following the tomographic survey, were used to associate densityvariations with the velocity features imaged by the tomography. The lD BHGM-derived low density zones were found to coincide with the 2D seismically imaged low velocity zones, thus supporting the identification of porous sandstone layers.
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Velocities from crosshole surveys: Some causes of image distortion
More LessDespite impressive progress in the development of migration and inversion algorithms over the past decade, the theme of this conference is a clear admission that the weak link in surface seismic reflection imaging is still our limited knowledge of velocity.
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Travel time tomography in anisotropic media
Authors R. G. Pratt and C. H. ChapmanA useful method of estimating the velocities of geological structures between boreholes is traveltime tomography. Measurements of the traveltimes along different paths can be used to form an image of the velocity distribution. This approach is valid when seismic velocities are the same regardless of the direction in which waves propagate. Thus, the target media is required to be isotropic for the method to work correctly. However, in many environments, velocities to depend on the direction of wavepropagation. This effect, known as seismic anisotropy, can lead to distortions in tomographic images. The distortions may be serious enough to cause the images to be misinterpreted.
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Shear-wave velocities derived from VSPs and full-waveform acoustic logs
Authors F. Leveuvre, R. Turpening, C. B. Caravana and A. BornThis paper describes a case history from a hard rock (limestones, dolomites and anhydrites) region of Michigan. Shear wave velocities have been derived from a nine component zero offset VSP and compared to shear wave velocities from two full waveform acoustic logs (Welex and Elf). In order to investigate the shear wave birefringence (azimuthal anisotropy) which effects the shear wave's vertical propagation, we used the "propagator matrix" technique as described in Nicoletis et al. (1988) and Lefeuvre (1990). Then the picked times obtained by correlation have been corrected in the birefringent areas to compute the fast and the slow velocities. Although there exists some differences between the three velocity sets, there is a good fit between the velocities from the shear wave VSP and those from the two logs.
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Estimation of shear-wave velocity logs from conventional well logs
Authors M. L. Greenberg, M. V. Popa, R. L. Phillips and J. P. CastagnaShear-wave velocity logs are useful for a variety of seismic interpretation applications, including bright spot analyses, amplitude-versus-offset analyses and multicomponent seismic interpretations. Measured shear wave velocity logs are, however, often unavailable.
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