ESTIMATING SV-WAVE STACKING VELOCITIES FOR TRANSVERSELY ISOTROPIC SOLIDS

Conventional seismic experiments can record converted shear waves in anisotropic media, but the shear-wave stacking velocities pose a problem when processing and interpreting the data. Methods used to find shear-wave stacking velocities in isotropic media will not always provide good estimates in anisotropic media. Although isotropic methods often can be used to estimate shear-wave stacking velocities in transversely isotropic media with vertical symmetry axes, the estimations fail for some transversely isotropic media even though the anisotropy is weak. For a given anisotropic medium, the shear-wave stacking velocity can be estimated using isotropic methods if the isotropic Snell's law approximates the anisotropic Snell's law and if the shear wavefront is smooth enough near the vertical axis to be fit with an ellipse. Most of the 15 transversely isotropic media examined in this paper met these conditions for short reflection spreads and small ray angles. Any transversely isotropic medium will meet these conditions if the transverse isotropy is weak and caused by thin subhorizontal layering. For three of the media examined, the anisotropy was weak but the shear wavefronts were not even approximately elliptical near the vertical axis. Thus, isotropic methods provided poor estimates of the shear-wave stacking velocities. These results confirm that for any given transversely isotropic medium, it is possible to determine whether or not shear-wave stacking velocities can be estimated using isotropic velocity analysis.