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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
1 - 50 of 69 results
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A rule based system for the determination of stacking velocities
By F. CoppensOne of the most commonly used tool for obtaining stacking velocities is the velocity spectrum. An expert system for aid in the interpretation of velocity spectra on a profile is presented. The raw velocity spectra computed by conventional seismic data processing software and a simplified description of the geological and geophysical context of the survey are input data for the system. In the expert system, the reasoning of the geophysicist picking a velocity spectrum is expressed in the form of rules using the objects of the velocity spectrum (spectrum extrema) and the information about the survey context. The generating of the search space and its filtering are carried out with heuristics specific to geophysical criteria and adapted to the velocity analysis problem.
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Accurate interactive velocity analysis using localised semblance analysis
More LessEstimation of stacking velocities from surface seismic data remains an interpretative procedure. Although various methods for automatic analysis and picking have been proposed, none of them are completely reliable under a range of differing conditions.
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Use of horizon oriented velocity analysis on 3D seismic
By M. SewardIn mid-1989, Shell Canada acquired a 3D seismic survey, Ram 3D, in the foreland basin of the Canadian Rocky Mountains. The surveyed area includes steeply dipping layers from surface outcrop to Precambrian basement. The azimuths of these layers vary greatly throughout.
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Velocity analysis by iterative delay time modeling of plane wave seismograms
Authors W. T. Wood and P. L. StoffaVelocity analysis in the domain of intercept time and ray parameter can be performed exactly for shot and CMP (common midpoint) gathers for the case of dipping isovelocity layers.
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The use of common offset migration for velocity model building and verification
Authors B. Barley, S. M. Deregowski, J. Longbottom, B. Raynaud and T. RedshawFor some time the seismic processing industry has possessed the algorithmic capability to accurately migrate seismic data from areas of structural complexity. What has stopped this ability being used in practice is the fact that the required depth migrations are themselves very sensitive to the supplied velocity depth model.
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Velocity gradient measurement from time migration analysis
Authors A. L. Downie and M. O. MarcouxTime migration velocity analysis is discussed for velocity gradients of sufficient magnitude to require nonhyperbolic diffraction time surface definition. Since multiparameter semblance analysis is costly and can be ambiguous, we developed a method of common-offset time migration that uses linear gradient theory to control the expansion coefficients defining the diffraction surface. Synthetic and field data are used to demonstrate our approach to migration parameter analysis and time imaging.
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Estimation of macro P and S velocity models
Authors H. L. H. Cox, G. Blacquièri, C. P. A. Wapenaar and A. J. BerkhoutIn this paper a macro model estimation technique is presented based on non-recursive wave field extrapolation. By shot record redatuming, using an initial macro model, true Common Depth Point (CDP-) gathers are generated at grid points along one or more lateral positions (such as potential boreholes).
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Resolving short-wavelength lateral variations in interval velocity using prestack depth migration - A Case Study
Authors D. E. Eggers and T. S. LathamLateral variations in structure which are short relative to the receiver array cause distortions in conventional CDPstacking parameters. These effects are easy to recognize over complexstructures, when the data will nat stack using any self-consistent, physically-reasonable NMO velocities.
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Analysis of a Depth Model - Common Offset Depth Migration as a Velocity Analysis Tool
Authors S. Wang, D. Hanson, T. Cavanaugh and R. BaumelTime domain processing has been extensively developed during the eighties to the stage that, with DMO, good quality images can be formed for most situations. However, in structurally complex areas, converting the time image to a correct depth image has been a serious problem. Recently, prestack depth migration has attracted more and more attention because of its capability of placing the image at the correct location, laterally and vertically.
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Interval velocity estimation using iterative pre-stack, depth migration in the constant angle domain
Authors N. D. Whitmore and J. D. GaringPre-stack imaging can be applied in a variety of domains to achieve accurate subsurface images and subsurface velocity information. In particular, depth migration in the constant angle (Tau-P) domain has proven to be a very useful tool for helping to unravel interval velocity models and complex structure.
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Velocity estimation using the prestack split-step fourier migration algorithm for plane-wave decomposed CMP data
Authors R. Tatalovic, P. L. Stoffa, J. T. Fokkema and M. SenThere are basically two approaches to the dipping layers problem: the combination of normal move-out (NMO) and dip move-out (DMO) followed by post-stack migration and depth conversion, and full pre-stack depth migration.
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Velocity analysis for prestack depth migration: Examples using synthetic and real subsalt data
Authors G. W. Purnell, D. v. Sukup and W. W. KaminerIn seismic prospecting under salt, poststack depth migration often provides unsatisfactory results. A common contributor to the problem is the occurrence of large lateral velocity variation associated with salt (high velocity) of nonuniform thickness encased in sediments (lower velocity).
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Invariant-Convolution PSI
Authors E. J. Isom and G. H. F. GardnerSeveral methods for dip moveout (DMO) and prestack Imaging (PSI) have reduced the operations to invariant convolutlons by resampling the data.
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Estimation of detailed P- and S-velocity models
Authors J. C. de Haas and A. J. BerkhoutThe angle dependent reflectivity (ADR), as present in multi-offset seismic data, contains information about the elastic parameters of the reflecting medium.
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Limitations on the determination of interval velocity from reflection seismic data
Authors C. J. Sicking and T. K. KanInterval velocity is often computed from Normal Moveout (NMO) or migration velocities. Examples of using interval velocity from these sources are shown for lithology discrimination and geopressure prediction.
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Limits on the velocity model
More LessIn order to obtain from seismic data a good image of complex geologic structures we must meet several demands. First among these is a correct velocity model whose detail is related to the complexity of the structures and which is geologically plausible. In order to limit our search for the velocity model to part of the model space we should incorporate in our model construction a priori geophysical and geological constraints on the model. My presentation deals with these limits on the velocity model.
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PS-wave stacking velocities
Authors G. Tessmer, P. Krajewski, J. Fertig and A. BehleNumerical tests show that the traveltime of PS-reflections can be approximated with sufficient accuracy for a certain offset range by a two-term series truncation. Therefore NMO-corrections can be calculated by standard routines which use the hyperbolic approximation of the reflection traveltime curves.
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Model-based interval velocity analysis
Authors S. Maher, G. Mellman and D. HadleyA velocity analysis process, SIVA, is described which combines the information in CMP gathered trace data with interpreted seismic data (eg, time maps) to create a model of structure and P wave interval velocity.
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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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Petrophysical interpretation of P- and S-wave velocities
By M. KriefIt has been observed that, in the case of a clean, homogeneous formation, points on a cross-plot of the squares of the P and S-wave sonic velocities fall along a straight line when the porosity is changing.
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Laboratory measurements and applications of seismic velocities
By Z. WangLaboratory measurements of seismic velocities play an important role in understanding seismic wave propagations in reservoir rocks. These seismic velocities are widely used in seismic data processing, correlations of seismic stratigraphy, seismic and weIl logging interpretations, lithology identification, and most recently in reservoir description and surveillance. In order to make fuIl use of the seismic velocity data, we must understand relationships between seismic velocity and a variety of reservoir and physical parameters.
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Estimates of velocity dispersion between seismic and ultrasonic frequencies
More LessVelocity dispersion in rocks is a pervasive, yet poorly understood, aspect of exploration geophysics.
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Velocity and absorption in sedimentary rocks at seismic and ultrasonic frequencies
Authors R. Mörig, R. Schütt and H. BürkhardtVelocity and attenuation of seismic waves are of great potential use in deducing lithological parameters from seismic data if we understand the physical laws governing dissipation of seismic energy and the accompanying velocity dispersion.
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Effects of grain-scale pore fluid flow on velocity dispersion in rocks
More LessDispersion in fluid saturated rocks appears to be dominated by two mechanisms: large scale average motion of the fluid phase relative to the solid phase, analyzed by Biot, and local grain scale relative motion.
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Static and dynamic properties of sandstones
More LessUsing laboratory measurements, we investigate the relationship between static and dynamic bulk moduli in 43 tight gas sandstones with porosity ranging from 2 to 14 percent and clay content ranging from 0 to 66 percent. Dynamic moduli are computed using compressional and shear velocities measured on the sandstones in air-dry condition.
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Pore-filling material and its effect on velocity in rocks
More LessA method is proposed to estimate the dependence of P and S velocities in rocks on elastic properties of the pore-filling material. The method goes beyond Gassmann 's relations in that it can account for saturating materials with nonzero shear stiffness.
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Theoretical modelling of the impact of free gas on seismic velocities
By E. FjaerSediments at the sea bottom of ten contain gas - normally methan, nitrogen or carbon dioxide (see f.i. Wheeler, 1986, Boden 1987). The presence of gas in a sediment affects its mechanical properties, and may thus have an impact on the stability of oil platforms resting on the sea bottom. Also, pockets of gas may cause dangerous situations during oil drilling, especially if the driller is not prepared for it. Seismic data often represent the only available information from the sediment prior to planning of platform sites, and drilling. Thus, it is important to be able to decide from the seismic data - with largest possible certainty - whether a sediment contains gas or not.
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Inversion of seismic velocities for rock- and pore parameters
Authors G. J. M. Lörtzer and A. J. BerkhoutThe ultimate result of the inversion of (surface) seismic data is in terms of rock and pore parameters, together describing the lithology.
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Pore structure mapping and permeability prediction prediction from velocity measurements
Authors S. Xu and M. S. KingKnowledge of perrneability of in situ rocks is of great importance in studies of nuclear waste disposal, underground water movement, petroleum and geothermal energy exploitation from reservoirs and in civil engineering applications such as dam constructions. Great difficulties have been experienced in predicting permeability of in situ rocks because of the limited information on discontinuities of all scales and of the pores of rocks, which together contribute to the permeability.
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