Inversion of multi-component, multi-azimuth, walkaway VSP data for the stiffness tensor

Vertical seismic profiling (VSP) is an established method for estimating in-situ anisotropy that might provide<br>valuable information for characterization of reservoir lithology, fractures, and fluids. The P-wave slowness components,<br>conventionally measured in multi-azimuth, walkaway VSP surveys, allow one to reconstruct some portion of<br>the corresponding slowness surface. A major limitation of this technique is that the P-wave slowness surface alone<br>does not constrain a number of stiffness coefficients that may be crucial for inferring certain rock properties. Those<br>stiffnesses can be obtained only by combining the measurements of P-waves with those of S (or PS) modes.<br>We show that, when polar and azimuthal coverage of the data is good, the polarizations and slownesses of P<br>and two split shear (S1 and S2) waves are sufficient for estimating all 21 local elastic stiffness coefficients cij that<br>characterize the most general triclinic anisotropy. The inverted stiffnesses themselves indicate whether or not the data<br>can be described by a higher-symmetry model. We discuss three different scenarios for inverting noise-contaminated<br>data and then apply our methodology to a multi-azimuth, multi-component VSP data set acquired in the Vacuum<br>Field, New Mexico, USA. Our inversion indicates that the medium at the receiver level can be approximated by an<br>orthorhombic model.