Estimating Azimuthal Stress-induced P-wave Anisotropy from S-wave Anisotropy Measured byVSP

Abstract

Anisotropy is sometimes hard to characterize as it requires a number of parameters which are seldom measured on the field. In this study, we present a way to infer azimuthal P-wave anisotropy from measured S-wave anisotropy in areas where anisotropy is known to be caused by the presence of differential stresses acting on the rock. This methodology involves two key steps. In the first one, we estimate the P-wave anisotropy in the dry medium from S-wave anisotropy using a rock physics model which accounts for the stress dependency of seismic velocities in dry elastic isotropic media submitted to triaxial compression. The second major step then consists in calculating the P-wave anisotropy in the saturated medium using the anisotropic Gassmann equations. This workflow is applied to data acquired off North West Australia, in an area where anisotropy is likely to be stress-induced. Such analysis could help constraining orthorhombic tomography or azimuthal AVO analysis.