Moshe Reshef

Migration of common‐shot gathers

Three depth migration methods which operate on common‐shot data are presented. The first migration method maps digitized horizons from the X-T domain to the X-Z domain. Because this method is based on ray tracing, its computation time is short; it is suggested for iterative velocity analysis. The other two migration methods map the entire common‐shot gather into a depth section. The common‐shot migration requires calculation of the arrival time of the direct wave from the source to all the depth points, and is done through a direct solution of the eikonal equation. All three methods are suitable for areas with both lateral and vertical velocity variation.

Depth migration from irregular surfaces with depth extrapolation methods

Nonflat surface topography introduces a numerical problem for migration algorithms that are based on depth extrapolation. Since the numerically efficient migration schemes start at a flat interface, wave-equation datuming is required (Berryhill, 1979) prior to the migration. The computationally expensive datuming procedure is often replaced by a simple time shift for the elevation to datum correction. For nonvertically traveling energy this correction is inaccurate. Subsequent migration wrongly positions the reflectors in depth.

Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa

Widespread occurrence of bottom-simulating reflectors (BSRs) has been detected in multichannel seismic profiles on the upper continental slope in the southern periphery of the Orange River delta, probably indicating the presence of large quantities of gas hydrate in this area. This report is the first to show the presence of BSRs on seismic records on the southwest African continental margin south of the Walvis Ridge. Another remarkable feature in the area is the occurrence of a large number of mud volcanoes. The distribution of the BSRs and the location of the mud volcanoes are controlled by the locations of active faults. The gas hydrate in this region may consist of a mixture of microbial and thermogenic gas, whereas much of the gas flowing through the mud volcanoes probably originated from deep-seated Aptian source shales.

Influence of structural dip angles on interval velocity analysis

Common scattering-angle and traditional common-offset gathers can be of limited use for interval velocity analysis in regions with complex geologic structures. In the summation process, which occurs when generating each trace in the common-image gather, vital information about structural dip is lost during prestack depth migration. This inadvertently lost data can provide important input to moveout-based velocity-updating algorithms. Maintaining this crucial dip information can improve the quality of the velocity analysis and imaging processes.

Some aspects of interval velocity analysis using 3-D depth-migrated gathers

Analysis of depth‐migrated gathers is the basis of most interval velocity estimation techniques. These gathers, known as common image gathers (CIGs), should consist of flat events when the velocity used for the migration is correct (Faye and Jeannot, 1986; Al‐Yahya, 1989). As a result of this assumption, interval velocity analysis has become a procedure of flattening events in CIGs.

The Eastern Mediterranean Messinian salt-depth imaging and velocity analysis considerations

Newly acquired 3D seismic datasets over the Eastern Mediterranean Basin are used to image the massive Messinian salt body. Accurate imaging of this salt body is critical to the precise definition of the prospective pre-salt geological section. The availability of recent well logs, which are the only ones to date to have penetrated the entire Messinian salt sequence, enables a clear definition of the highly deformed clastic units within the salt. This study shows that these highly reflective clay units cause a significant reduction in the overall seismic velocity of the salt. It also demonstrates why 3D pre-stack depth migration, including a tomographical velocity update inside the salt body, is recommended as the preferred imaging technique.

3D prediction of surface-related and interbed multiples

Multiple attenuation during data processing does not guarantee a multiple-free final section. Multiple identification plays an important role in seismic interpretation. A target-oriented method for predicting 3D multiples on stacked or migrated cubes in the time domain is presented. The method does not require detailed knowledge of the subsurface geological model or access to prestack data and is valid for both surface-related and interbed multiples. The computational procedure is based on kinematic properties of the data and uses Fermats principle to define the multiples. Since no prestack data are required, the method can calculate 3D multiples even when only multi-2D survey data are available. The accuracy and possible use of the method are demonstrated on synthetic and real data examples.

Interval velocity analysis in the dip-angle domain

When interval velocity analysis is conducted over complex geologic regions, scattering-angle gathers may cause significant inaccuracies. These inaccuracies are related to the loss of structural dip information when generating common-image gathers (CIGs). In this study, the idea of performing interval velocity analysis in the dip-angle domain was examined and demonstrated with synthetic and real data examples. The effects of migration velocity errors and their identification in this domain were analyzed in detail. Carrying the analysis directly on dip-angle gathers is practically impossible. The ability to perform a standard analysis based on flattening the events in the CIGs is achieved by replacing the dip-angle measure with an equivalent offset measure. This equivalent offset provides higher sensitivity to velocity errors and may improve the accuracy of the resultant velocity model.

VTI anisotropic corrections and effective parameter estimation after isotropic prestack depth migration

In the method for applying anisotropic corrections after isotropic prestack depth migration (PSDM), the correction, which is calculated and implemented in the depth domain, is defined as a time difference between isotropic and anisotropic traveltimes, under the assumption that the vertical velocity is known. The definition of this correction uses a special postmigration common-image-gather (CIG) ordering, which collects the migrated data according to the input-traces source and receiver distance from the surface CIG location. In this postmigration domain, the dip of the events can be directly related to their horizontal position in the CIG, called the imaging offset, and the separation of flat and dipping reflectors becomes easy to perform. The dependency of the seismic anisotropic effect on the subsurface dip angle is well pronounced in these CIGs. After application of an isotropic PSDM, effective anisotropic-parameter estimation is performed at selected CIG locations by using a simple two-parameter scan procedure. The optimal anisotropic parameters can be used to perform a final anisotropic PSDM or to apply a residual correction to the isotropically migrated data. We demonstrate the method for P-wave data in 2D media with vertical transverse isotropy (VTI) symmetry by using both synthetic and real data. We also present a strategy for handling the ambiguity between the vertical velocity and the anisotropic parameters.

Image enhancement by multi-parameter characterization of common image gathers

We present an innovative approach for seismic image enhancement using multiparameter angle-domain characterization of common image gathers. A special subsurface angle-domain imaging system is used to generate the multi-parameter common image gathers in a summation-free image space. The imaged data associated with each common image gathers depth point contain direction-dependent opening-angle image contributions from all the available incident and scattered wave-pairs at this point. Each direction-dependent opening-angle data can be differently weighted according to its coherency measure. Once the optimal migration velocity is used, it is assumed that in the actual specular direction, the coherency measure (semblance) along reflection events, from all available opening angles and opening azimuths, is larger than that along non-specular directions. The computed direction-dependent semblance attribute is designed to operate as an imaging filter which enhances specular migration contributions and suppresses all others in the final migration image. The ability to analyse the structural properties of the image points by the multi-parameter common image gather allows us to better handle cases of complicated wave propagation and to improve the image quality at poorly illuminated regions or near complex structures. The proposed method and some of its practical benefits are demonstrated through detailed analysis of synthetic and real data examples.