Potential of the AVO Inversion at Post-critical Offsets for HTI Media

AVO inversion is an essential and powerful interpretational tool widely used in the industry. However, linearized reflection coefficients are valid only for small offset and thus are limited in estimating many anisotropy parameters needed to define, for example, fractures. Exploiting larger offsets, specifically, post-critical ones improve our chances in recovering of more anisotropic parameters. However, dealing with long offsets requires accounting for such phenomena as wavefront curvature and interference between reflected and head waves. We use effective reflection coefficients extended for HTI media, which account for such phenomena and provide sensitivity analysis in order to understand how post-critical offsets can help the inversion results for HTI media. We show that pre-critical offsets are sensitive to the vertical P- and S-wave velocities and parameters epsilon and delta, whereas post-critical offsets can help in resolving the vertical P-wave velocity, anisotropy parameter epsilon and most importantly, direction of symmetry axis. Additionally, we show that the resolution of the parameters depend on a healthy azimuth coverage with optimal results coming from a 90 degrees azimuth range.