First Break - Volume 5, Issue 2, 1987

Separation of the upgoing and downgoing waves is a crucial step in the processing of Vertical Seismic Profiles (VSPs). There are many theoretical approaches to the problem but all so far published rely on a discernible separation between the two wavetrains in either the time-depth (t-z), frequency-wavenumber (w-k) or intercept time-slowness (t-p) domains. In the presence of very steep dips, for example when a well is close to a salt wall or diapir, or a near vertical fault plane presenting a high-impedance contrast, the angle of incidence of the reflected upgoing wave can be so close to the downgoing wave that both are removed by implementation of the velocity filters in current use. A practical method is described here for by-passing this failure of velocity filters in such situations. The method is an extension of the wavefield subtraction technique, described elsewhere in the VSP literature, and is illustrated by a recent case history. However the implementation of the method requires geological insight into the specification of probable models by means independent of the VSP. As such this illustrates the increasing need for synergism between the interpretation and processing teams.

Proximity surveys have been used for many years to define the shape of salt domes. A travel time is measured from a source through the salt to each of several downhole sensor locations. From prior knowledge of salt velocities, velocities of formations encountered by the well, and at least one salt tie point, distances from salt to sensor positions can be calculated; thus, the shape of the salt flank can be determined. Recent developments have entirely mechanised the interpretation process, increasing its speed. Three-axis data recording enhances the measurement of the necessary travel times, improving the accuracy of the technique. The results agree with those obtained using conventional interpretation techniques, with a considerable saving in time.