William A. Schneider

Robust, efficient upwind finite‐difference traveltime calculations in 3‐D

Current industry interest in 3D prestack Kirchhoff depth migration has provided an impetus for much recent research on methods to perform the necessary traveltime calculations to construct the kinematic component of the migration operator. Robust approaches are mandated since velocity models are usually complicated when depth migration is necessary. The generally enormous size of the depth migration problem, especially in 3D, dictates that an effcient traveltime algorithm must also be employed. While the traditional method for computing traveltimes involves ray tracing, alternatively, traveltimes may be computed by directly solving the nonlinear 3D eikonal equation (see Cerveny and Hron, 1980 for background). When the objective is to fill a 3D volume with traveltimes, the direct approach offers advantages of robustness and efficiency over the ray tracing approaches, especially when velocity fields are complex. I present a robust, efficient and stable extension to 3D of the well-known 2D method of van Trier and Symes (199 1). It performs well on the high-contrast, complex velocity models encountered by today’s 3D depth migration applications. My implementation fully vectorizes at the inner-loop level and fully parallelizes at the outer-loop level on the CRAY Y-MP, thus making it ideal for this machine, where it runs at over 180 Mflops per CPU.

A higher order correction to NM0 for improved AVO and imaging

There are many things in seismic data processing that we daily take for granted. Surely the equation must be one. In this talk we present an example in which the familiar form of the equation is inadequate. Using both synthetic and field data we first show examples of the problem and then how the use of a higher order addition to the classical equation produces significant improvement. Finally we describe a method for estimating the parameters necessary for the higher order correction.

Wide azimuth reflection response in 3-D angle gathers from OBS node data

Summary Wide azimuth seismic data illuminate the subsurface at a wide range of source-receiver azimuths. This valuable azimuthal information may be extracted from seismic data in the form of 3-D angle domain common image gathers (ADCIGs). Residual moveout (RMO) in 3-D ADCIGs generally varies with azimuth when the migration velocity is incorrect. However, to obtain accurate RMO in 3-D ADCIGs wide azimuth seismic data are necessary. Applications of 3-D ADCIGs include seismic interpretation and estimating velocity updates for wide-azimuth velocity tomography. We generate 3-D ADCIGs from wide azimuth OBS node data. OBS node data typically contain relatively fewer seafloor nodes compared to surface sources, but source coverage per node typically samples the full desired range of source-receiver offsets and azimuths. This makes OBS node data ideal for producing 3-D ADCIGs as long as the sampling of receiver nodes is sufficiently dense. Aliasing in ODCIGs caused by large node spacing is a potential problem that can degrade the quality of the ADCIGs. We address this in two ways. We image the downgoing wavefield, which offers wider subsurface illumination and overlap per node than can be obtained from the upgoing wavefield. We also perform anti-alias filtering of the ODCIGs by windowing them in the subsurface offset domain before conversion to 3-D ADCIGs. These steps allow us to obtain high quality 3-D ADCIGs from OBS node data. We demonstrate this using a deep-water OBS node field data set.

Prestack depth migration with dynamic programming traveltimes

The dynamic programming traveltime method of Schneider et al. (1992) provides an efficient means of obtaining the traveltime grids necessary for migration. Further efficiency may be obtained by performing PSDM velocity analysis from the surface downward in discrete depth zones. In this manner traveltime computations are reduced by using the previous zone for initial traveltime conditions in the current zone. With this approach PSDM can be run in an iterative velocity analysis mode on workstation type platforms.

Comparison of Sign-Bit And Conventional Seismic Recording In Eastern Colorado

Close comparison of SV and SH, PISv and SV/P, etc., will eventually provide additional information, mainly about anisotropy. This might be very useful for some field studies, because fracture direction should be involved. As of now, no examples are known. Example 1. Study in carbonate environment. On an initial experimental line P, SH and P/STwaves were recorded, using an explosive source. The sections that were obtained are good enough for automatic evaluation of the yT coefficient, using correlation techniques. On this line the yT coefficient obtained from P and SH , and the yr coefficient obtained from P and P/S” modes can be compared. They are close, and can be used in the same way (Figures 4 and 5). &phones: For recording in P and P/SV modes only, acquisition can be carried out in the same run, using the same P-wave source, merely doubling the number of recording channels. Each geophone station corresponds to two field traces: one with conventional vertical geophones, the other with horizontal X oriented geophones. For such purposes, it is convenient to use biphones, which include both vertical and horizontal detectors. Example 2. A second experimental line was recorded using a P-wave source only, with Z and X oriented detectors. Results were satisfactory, and provided an estimate of the yT coefficient.