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EAEG/SEG Summer Workshop - Construction of 3-D Macro Velocity-Depth Models
- Conference date: 24 Jul 1994 - 27 Jul 1994
- Location: Noordwijkerhout, Netherlands
- ISBN: 978-94-6282-131-6
- Published: 24 July 1994
34 results
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Applied Seismic Inversion for Estimating Velocity-Depth Models
By O. YilmazProcessing and inversion of seismic data differ in one fundamental respect -- the output from processing often is displayed in time, and the output from inversion is intended to be displayed in depth. The main goal in inversion is to estimate a geologically plausable subsurface velocity-depth model, which comprises two sets of parameters -- layer velocities and reflector geometries. To resolve the well-known velocity-depth ambiguity in inversion, these parameters need to be estimated, independently.
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3D Procedure for Layer Velocity Analysis - Coupled Map Migration/Coherency Measuring
More LessA technique of layer velocity estimation tbrough semblance analysis was reported in Hadleyet al. (1988) and Landa et al. (1989). lts basic premise is that a coherency measure on unstacked seismograms reaches a maximum along reflection time trajectories that correspond to the correctly estimated velocity-depth model.
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Example of Strategies in 3D Stacking Velocities Inversion
Authors E. Robein, R. Cognot, B. Raynaud and P. SextonSeveral software tools have been developed by the industry in the last few years to build up the depth- or time-migration velocity model using travel times or stacking velocities.
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Interactive 3D Velocity Analysis and Model Building for Stacking and Time Migration
Authors C. T. Dale, M. Griscom, J. B. Hartman and G. KissingerAn interactive method has been developed for carrying out seismic velocity analysis that results in a three dimensionally tied and consistent velocity field for either stacking or time migrating 3-D seismic data (also grids of 2-D data). This method uses interactive horizon-based velocity picking and statistical data fitting as the primary model building tools. The motivation for this development comes from difficulties we experience in picking seismic velocities from data which is either poor in its signal to noise ratio characteristics or is structurally complex and requires complicated velocities to image the subsurface.
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Estimation of Layer Velocities Using 3D Pre-Stack Traveltime Inversion
Authors O. Sandvin, C. Kostov and P. FarmerA velocity-depth model is defined by layer velocities and reflector geometries. We present a method of estimating layer velocities using reflection traveltimes from 3-D prestack data. The method honors ray bending at layer boundaries, hence provides interval velocity estimation beyond the Dix approximation.
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Velocity and Structure Determination from Multi 2D Pre-Stack Traveltime Inversion - Mis-Tie Analysis and Consistency Control
More LessPractical application of traveltime inversion on multi 2D seismic surveys is limited by difficulties to be confident of the final velocity-depth model reliability due to a lot of mis-ties. An integrated approach combining seismic interpretation techniques and seismic processing like traveltime inversion and depth migration is developed in order to improve the geological and geophysical consistency of results. This approach is characterized by the use of coherence criteria firstly applied to each 2D seismic profile and secondly extended to 3D structural analysis. Consistency control procedures are described using examples deduced from multi 2D dataset study recorded in the Paris Basin. The objective is then to obtain accurate layer velocity estimates and reliable time to depth conversion of seismic sections.
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3D Model-Based Time Gather Velocity Analysis
Authors S. W. Fagin, A. Litvin and A. BrownHighly accurate earth models are required before one can experience the benefits of depth imaging. As structure and prospect become increasingly complex and subtle, Dix-based methods become ineffective as velocity estimation tools. Model-based methods are more effective 'because they bring the interpreter's evolving understanding of structure to the velocity estimation process. We describe the use of two types of model-based methods employed in three dimensions; stacking value matching and coherency inversion. These methods are used to define interval velocities on a land 3D survey.
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Iterative 3D Depth Model Building via One-Pass 3D Depth Migration
By I. F. JonesWith the advent of steep-dip non-dispersive Isotropic 3D time and depth migration algorithms, the ability to image complex structures has entered a new phase. However, the most important issue in subsurface imaging is nor the migration algorithm, but the elaboration of the subsurface velocity model.
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3D Velocity Model Building - an Accurate and Efficient Interpretive Workflow
Authors M. Roth, D. Kessler and D. M. SibleyThe workflow for creating accurate 3D depth images varies widely in cost and effort, depending largely on data complexity. Much of the effort associated with depth imaging is associated with the construction of an accurate macro velocity model. Depth imaging is an integral component of velocity model building, providing both velocity and depth horizon information. Both the cost and effort of performing 3D depth imaging and, hence, velocity model building increases as data complexity rises from simple to moderate and on to complex geology. In this paper, we will describe an accurate and efficient interpretive workflow for 3D velocity model building for data with simple-to-moderate levels of geologie complexity.
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Delineation of 3D Reflector Geometries Using 3D Post-Stack Depth Migration
Authors J. Rutledge and A. PieprzakExploration targets in structurally complicated areas with strong velocity contrasts require interpretation of seismic data imaged in depth. In areas where conventional processing which includes dip-moveout correction yields CMP-stacked data with good signal strength and continuity, 3-D poststack depth migration offers an attractive solution to the imaging problem.
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Demigration - a Better Way to Derive an Interpretation of Unmigrated Reflections
By S. W. FaginOne key to model-based depth imaging procedures is the ability to work flexibly in the three principle seismic domains; migrated time, unmigrated time and depth. The observations and analyses performed in each of these domains play a distinct role in creating an earth model accurate and robust enough to yield a suitable depth image tor exploring in the complex and subtie structure environment.
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Velocity Model Building Using Pre-Stack Depth Migration
Authors C. Koeninger, N. Jones and S. DeregowskiTime migration algorithms prove to be inadequate when trying to correctly image seismic sections in areas of complex geology. In such media, velocities vary both vertically and laterally giving rise to non-hyperbolicity in the observed moveout of the seismic reflections. To deal with this situation, a depth migration must be employed. However, a dilemma arises in that the performance of the pre-stack depth migration is sensitive to errors in the velocity model supplied by the user, that is the more accurate the underlying velocity model, the better the migrated image. This sensitivity provides an excellent tool for the iterative building of depth models (Figures 1 & 2). We present a method that combines pre-stack depth migration with velocity analysis and mapmigration.
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The Use of 3D Pre-Stack Depth Imaging to Estimate Layer Velocities and Reflector Positions
By M. ReshefIn the case where subsurface structures are complex, it is recognized that correct depth imaging should be carried out with prestack data. When 3-D data are available, the implementation of a 3-D prestack depth migration is required. The quality of depth images heavily depends on the accuracy of the interval velocity function. Taking into account the computational effort associated with the migration, the problem of defining the interval velocity model must be solved prior to the execution of the migration task. Experience gained from 2-D work shows that structural imaging can be effectively used as a tool for interval velocity analysis in complex areas. This study tries to imp lement similar procedure in the 3-D case.
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Using Two-Pass 3D Pre-Stack Migration to Build Velocity-Depth Models in 3D
Authors A. Canning, M. Ramaswamy and G. H. F. GardnerPrestack migration is a successful tool for imaging complex structures and has recently become quite popular. Experience with prestack migration indicates that the quality of the image is very sensitive to the velocity model, and therefore prestack migration is also used as a velocity analysis tooI. The migrated image is usually obtained in an iterative procedure and the velocity model is updated between iterations. In other words, prestack migration and velocity estimation are related procedures. The velocity model is required so a good depth image can be obtained while the migration process itself is the tool for deriving the velocity models.
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Issues in Implementing the Kirchhoff Integral for 3D Pre-Stack Depth Migration
Authors B. Godfrey and K. BergThe Kirchhoff approach to depth migration requires computation of travel times. We use dynamic ray tracing to shoot a bundle of rays upward from each subsurface point to be imaged. The number of rays varies between a few to several thousand rays per bundle and is dependent on the local complexity of the velocity-depth model. For instance, in sedimentary sections. 1500 rays may be sufficient to accurately determine travel times. However, in the presence of salt structures, up to 7000 rays may be required. For large-scale problems associated with complex structures, implementation of 3-D prestack depth migration requires efficient construction oftravel time tables and their use in Kirchhoff summation.
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3D Velocity Analysis Using Limited Aperture Migration
By J. R. KrebsMigration velocities can be determined by performing several prestack migrations with a series of test velocity models. The velocity model that produces a consistent image for all source-receiver offsets is taken to be the correct migration velocity.
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Building Complex Velocity Models Using an Adaptive Tetrahedral Mesh
Authors U. Albertin and W. WigginsWe use an irregular tetrahedral mesh to represent 3-D velocity models. The mesh is generated by individually inserting data points into the model with refinement of the mesh to include the newly added point. When isolated data points are to be interpolated, a Delauney tessellation is often a good choice since it partitions the model into tetrahedra connecting nearest-neighbor data points by its empty-circumsphere criterion. We use Delauney tessellation to import isolated velocity picks to form a 3-D velocity field and to import isolated surface picks to form a simply connected 3-D surface (Wiggins et. al, 1993). Velocity data comes to the explorationist in small parts: first, velocity picks for a shallow layer; then, a shallow horizon; then a deeper layer velocities and surfaces. A modeling system must represent these parts and combine them lnto a single model.
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Representation and Visualization of 3D Velocity-Depths Models
Authors F. Daube, O. Hagenes, H. Bernth and P. FarmerBuilding velocity-depth models for structural imaging requires combining information about layer boundaries and interval velocities in a geologically consistent manner. This requires working with seismic data and modelobjects in a single environment and is best done on an interactive workstation. The complexity of a software system designed to support such an activity increases dramatically when moving from 2-D to 3-D velocity-depth models. We present a system for 3-D interactive velocity-depth model building and visualization.
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Generating a 3D Velocity Model - the GOCAD Approach
Authors R. Cognot, P. Lavest, F. Bosquet and J. L. MalletBoth of the Overthrust and the Salt Dome (SEG/EAEG) models have been built synthetically from a set of cross sections, into a set of surfaces, then converted into 3D velocity grids. In both cases, the GOCAD software has been intensively used for these operations, and the goal of this presentation is to focuse mainly on the grid generation step, more specifically taking the example of the Overthrust model. It must be said that the methods that will be discussed here are very general, and has also been with success to the salt dome model.
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Building the SEG/EAEG Overthrust Velocity Macro Model
Authors J-C. Lecomte, E. Campbell and J. LetouzeyThrust zones are areas in the crust that give extreme imaging and interpretation problems to reflection seismologists. This article presents the development of a 3D overthrust model that will be used by geophysicists to construct synthetic seismograms in order to test and improve the validity of the seismic imaging processes.
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Velocity Model Building for 3D Pre-Stack Depth Migration - a Case Study
Authors D. Hinkley, R. Ho and S. LeeIn this case study we used three different velocity estimation methods, SIVA/RAYMAP (Sierrat tm) velocity analysis package, focusing analysis, and residual velocity analysis, to build velocity models for 3D prestack depth migration. The first two methods are more interpretation oriented than the last one. The imaging results from 3D Kirchhoff prestack depth migration from these three different models are discussed in this paper.
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Construction of 3D Velocity-Depth Models
Authors N. R. Hill and J. L. ToldiThree-dimensional velocity-depth models are playing an increasingly important role in our seismic data processing and interpretation. In addition to being critical input for 3D depth migrations, the models are becoming an integral part of our structural interpretations of complex prospects. Even when working with 2D seismic data, a 3D model enforces consistency across interpretations of individual lines. This talk presents our methods of constructing and refining velocity models for a series of examples ranging in complexity from a simple four-Iayer model for West Africa, to a more intricate faulted model for West Africa, to a complex Thrust Belt model. All of these examples begin with 2D analysis and then advance to fully 3D model building.
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3D Structural Inversion via GOCAD
Authors S. Kapotas and P. GuillaumeIssues of accurate structural interpretation along with more accurate reservoir characterization have led the oil industry to acquire more and more 3D data over more complex geological regions. This type of strategy imposes the need of a method that wiIl allow us to estimate velocity and depth variations for these types of environments which in turn can provide a model for seismic imaging.
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A 3D Velocity Modelling Strategy that Includes Structural Geology
By A. DownieThe structural features of a 3D subsurface velocity model must be defined not only to support seismic modeling, seismic processing and geologic interpretation but also to support the kinematic history of the subsurface structure. A strategy for implementing a 3D velocity model is presented. The key feature of our strategy is that, in addition to being topologically correct, the model is also supported by the principles of structural geology.
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Efficient Generation and Verification of 3D Velocity Models - UK Southern Gas Basin
Authors I. G. Mitchell, J. M. Reilly and D. L. HinkleyThe need for robust 3D model-building technology and methodology to support 3D prestack imaging is weIl recognised within the seismic processing industry. What has received relatively less attention is a) development of techniques which maximise the value of the conventional 3D processing sequence; and b) critical evaluation of the performance of the ray trace algorithms which generate the traveltime tables for the prestack migration "engines". Enhancements in these two areas are considered essential components in the creation of an economically viabIe 3D pre-stack processing product. In this paper we present a case history which specifically examines these issues.
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Estimation of Velocity-Depth Model for Structural Targets - a Case History from the North Sea
Authors O. Yilmaz, P. Farmer, A. Pieprzak and B. GodfreyThree-dimensional seismic data from the North Sea were analyzed to remove the deleterious effect of the Zechstein diapiric formation on imaging the underlying Permian sands of Rotliegendes. This Required accurrate imaging of the overburden and delineating the geometry of the halite and anhydride-dolomite units in the Zechstein formation.
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Performance and Validation of the 3D Acoustic Modelling Code for the EAEG & SEG 3D Modelling Project
Authors J. Brac, L. Anne, A. Bamberger and P. DuclosThe EAEG&SEG 3D Modeling Committee decided (Progress Report in Leading Edge and First Break of February, 1994) the main features of the 3D modeling code for the project. A finite difference method was selected in order to model complex structures and a one-parameter acoustic wave equation was accepted
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Easy-to-Use Modelling - 3D Ray Field Propagation in Open Ray Models
By K. AstebølRay modelling has proven useful in a wide range of seismic applications, including survey planning, velocity inversion. map migration. synthetic seismogram generation, model-based stacking, and pre- and post-stack depth migration. Still, the spreading of modelling-based techniques relies very much on how easy modelling is to use. If modelling requires lots of special interpretation and preparation and very specialized skill to make useful results, the application is limited to the rare, special cases. On the other hand, if modelling in large can do with data made in standard processing and interpretation, and easy-to-use, efficient, and robust ray calculation is available, ray-based modelling applications may flourish. Simple ray-based applications, like map migration, may even become part of the standard, routine tools.
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Seismic Evaluation of the Overthrust Model
Authors J. Brac, C. Naville, S. Serbutoviez and L. AnneAs mentioned in the companion paper "3D Seismic Modeling on the Overthrust Model", it is essential to check the relevance of the overthrust 3D model with respect to its geophysical objectives before starting intensive computations. The critical decisions concern the choice of the source frequency (15 Hz main frequency), the mesh size (25m), the velocity range & distribution, and the acquisition parameters, which determine the computational cost. These choices take into account the geological constraints, the geophysical objectives and the current limits of computer technology (hardware and software). The choice of the numerical modeling parameters has been carefully studied and is presented in another paper concerning the validation of the 3D acoustic modeling software.
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The 3D SEG Salt Model - Dreams and Reality
More LessWave-equation modeling of 3-D seismic data in non-trivial geologic models is a non-trivial computational problem. While ray-tracing and other simple methods offer advantages in speed and/or simplicity, we are drawn toward using as complete a physical description of wave phenomena as we can aspire to. Typically this means full wave equation finite-difference or finite-element methods. If we are interested in P-wave or "acoustic" data, we have the choice of modeling a zero-offset section directly, or modeling shot records which would simulate a typical field experiment; one can also model other experiments, hypothetical or real, such as plane wave or conical wave sources. From the point of view ofthe numerical simulation, the goal is to get as many wavelengths of some specified frequency into the computational model as possible.
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