Quasi-shear waves in inhomogeneous weakly anisotropic media by the quasi-isotropic approach: A model study

In inhomogeneous isotropic regions, S-waves can be modeled using the ray me thod for isotropic media. In inhomogeneous strongly anisotropic regions, th e independently propagating qS1- and qS2-waves can similarly be modeled usi ng the ray method for anisotropic media. The latter method does not work pr operly in inhomogenous weakly anisotropic regions, however, where the split qS-waves couple. The zeroth-order approximation of the quasi-isotropic (QI ) approach was designed for just such inhomogeneous weakly anisotropic medi a, for which neither the ray method for isotropic nor anisotropic media app lies. We test the ranges of validity of these three methods using two simple synt hetic models. Our results show that the QI approach more than spans the gap between the ray methods: it can be used in isotropic regions (where it red uces to the ray method for isotropic media), in regions of weal; anisotropy (where the ray method for anisotropic media does not work properly), and e ven in regions of moderately strong anisotropy tin which the qS-waves decou ple and thus could be modeled using the ray method for anisotropic media). A modeling program that switches between these three methods as necessary s hould be valid for arbitrary-strength anisotropy.