The first update for the SEG/EAEG 3-D Modeling Project appeared in the February issue of TLE and the March issue of First Break. Our goal is to design salt and overthrust 3-D models and then simulate realistic 3-D surveys based on those models. Given the project's significance and scope, we plan frequent progress reports. In conjunction with this goal and to solicit input, we have made recent presentations at SEG Cairo, the SEG/EAEG Summer Research Workshop in The Netherlands, the Stanford Exploration Project, and the Center for Wave-Phenomena at the Colorado School of Mines.The committee expects to freeze the models based on feedback from the SEG and EAEG membership. They will then be available to the general public, most probably via CD-ROM.We are soliciting commercial entities to be considered as the repository of the models and simulation results (perhaps one in Europe and one in North America). Progress reports of the project working groups (salt model, overthrust model, and computations and algorithms) follow. Salt ModelThe first version has been constructed and placed in the public domain, accessible via Internet, in GOCAD format. This version, primarily describing the model's structural components, measures 90 000 ft in x,y and 24 000 ft in z. Figure 1 shows a 3-D visualization of the major components.The velocities surrounding the salt body are typical of Gulf of Mexico sedseismic response and the computational iments and described by compaction cost of the synthetic generation. Prelimgradients based on K-V, curves (K, the inary results on a 2-D cross section gradient, varies spatially and V, is the using the spike technique have been obinitial velocity) and a geopressure surtained. Figure 3a shows the velocity face. However, a constant density conprofile and Figure 3b the resulting 2-D straint has been placed due to the finite zero offset simulation. These prelimidifference software employed and nary results seem to indicate that the available computational resources. This spike technique is adequate; however, constant density assumption and simple more testing is required. velocity gradient results in no seismicIn the meantime, a 3-D velocity grid, reflectivity in the sediments surroundemploying the spike technique, has ing the salt. Two techniques (Figure 2) been generated and provided to US Naare being investigated via 2-D finite diftional Laboratories for testing of the 3-D ference modeling. The first ("spike") finite-difference software. increases the velocity of the finite dif-A second area of considerable invesference grid cell nearest each layer tigation is how to perform 3-D finite boundary by a certain percentage. The difference simulation on the salt model second ("block") alternately increases within the current resources offered by and decreases the velocity of the K-V, the US Department of Energy. A first function for each successive layer. estimate on the required computing reSince these techniques can vary the seissources far exceeded the existing budmic response of the model, two-dimeng...
An original method is presented that allows us to measure the local shear-wave birefringence properties over any depth interval. It requires the acquisition of two shear-wave vertical seismic profiles (VSPs), each with different initial polarizations of the shear wave. The method is based on the estimation of a two by two matrix (called the propagator matrix) that represents a linear operator between two states of polarization. No information is required about layering above the zone of interest (in particular, about the weathering zone). If these two states of polarization correspond to the direct downgoing shear wave at two different depths z 1 and z2, the operator represents the transmission properties between the two depths. Under the previous hypothesis, this operator is independent of the source polarization and can be accurately estimated by a least-squares method in the frequency domain. Physically, this operator is a multicomponent deconvolution, whose column vectors represent the state of polarizations at a depth Z2 for two linear and INTRODUCTION
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