Estimation of seismic anisotropy from P-wave VSP data

摘要

Many rock formations are anisotropic. Their anisotropy, or dependence of seismic velocities on the direction of wave propagation, has been extensively studied and documented throughout the world. It is now widely accepted that understanding subsurface anisotropy yields numerous benefits for both exploration and development of hydrocarbon reservoirs. For example, seismic images produced with anisotropic velocity fields are usually crisper and geologically more meaningful than those created with traditional isotropic velocity models. In addition, relating anisotropy to its physical causes (such as lithology, fractures, stresses) enables one to obtain valuable information about them, something that cannot be done if the rocks are deemed isotropic. Making accurate measurements of anisotropy is an obvious prerequisite for all these potential improvements. To be most useful, such measurements should be done in-situ and in the seismic frequency range. Here we discuss a recently developed technique for estimating anisotropy from P-wave VSP data and present two examples of its application in different settings.