Modeling shear‐wave splitting in VSPs in South Casper Creek, Wyoming

Abstract

Multi-component multi-offset VSPs in South Casper Creek, Wyoming are analyzed and modelled to examine the mechanism of anisotropy and to understand the characteristic of shear-wave propagation in anisotropic media. The data set contained nine-component VSPs (three-component source and three-component receiver), and was recorded at one near-offset and two orthogonal far-offsets. The data display typical features diagnostic of anisotropy with orthorhombic symmetry forming from a combination of crack-induced and layering-induced anisotropy. These include a 10-15’ change in the orientation of the polarization ellipses in one of the far offset VSPs as the depth varies, and changes in the time delays of the split shear-waves between the near-offset and the far offset VSPs. Comparison of hexagonal models containing only vertical cracks with orthorhombic models containing both vertical cracks and horizontal thin layers shows that both kinds of models can yield good tit to the shear-wave polarizations and the time delays in the near offset VSP, but only orthorhombic models can yield good fit to those in the far offset VSPs.