Summary
P-wave local slowness data from a walkaround VSP can provide a measure of azimuthal velocity anisotropy to characterize fractured rocks or stress fields around a well. When the acquisition is constrained to have irregular shot points, shot-offset-dependent corrections are required. For unconventional plays, a correction for background polar anisotropy might also be necessary to effectively extract the azimuthal anisotropy around the well. This study introduces a modified total slowness and a two-step correction technique to remove the background VTI and minimize the residual offset bias in the walkaround VSP. Field data tests demonstrate that this new technique can provide a quick and quantitative measure for the orientation and magnitude of fracture- or stress-induced azimuthal anisotropy.
Introduction
A walkaround vertical seismic profile (VSP) can provide various seismic attributes to characterize fractured rocks or stress fields around a well. In particular, because direct Pwave arrival time and particle motion polarization are extracted from three-component (3-C) data recorded by geophones without much processing, they can offer a quick look for fracture- or stress-induced azimuthal velocity anisotropy in a depth interval over the receiver array. Consider an ideal situation where a studied area is a horizontally layered medium, the well is vertical, and shots are placed on a circle with a constant offset. The measured travel time or the travel time difference over the receiver array, that is, local slowness, could be used directly to detect and estimate the azimuthal anisotropy.
In practice, a land walkaround survey can be constrained by road and permit availability to have shot points off the ideal circle. In addition, an area of interest can have vertical transverse isotropy (VTI or polar anisotropy), which could be greater than the azimuthal anisotropy. As both types of anisotropy are dependent on shot offsets (or polar angles), irregular shot points can make it difficult to reveal and quantify the azimuthal anisotropy from the P-wave data.
A challenging question is how to compensate effectively for the shot-offset effects to extract seismic azimuthal anisotropy around the well. This paper introduces a technique of P-wave local slowness correction for a walkaround VSP to minimize offset-dependent biases in azimuthal anisotropy. The new correction method is demonstrated using field data to explore fracture- or stressinduced azimuthal anisotropy in an unconventional shale play.
