Apparent azimuthal shear anisotropy from normal-moveout differentials

A simple normal-moveout (NMO) correction can move common-depth-point (CDP) reflection times up or down a few milliseconds when the stacking velocity is varied a small fraction. Different stacking velocities on two orthogonal shear components are often required, such as in the cases of azimuthal anisotropic media (Thomsen, 1986) or transversely isotropic media (Levin, 1979). The difference between these two cases is that CDP reflection times are different on S1 and S2 in the case of azimuthal anisotropy, while CDP reflection times are equal for transverse isotropic media. The similarity is that both cases require different stacking velocities on the S1 and S2 components.