In reflection seismic surveys involving an anisotropic medium, the apparent velocity variation is no longer hyperbolic because the NMO (normal-moveout or stacking) velocity is, in general, no longer invariant with direction of propagation. This variation of velocity with direction in symmetry planes has been studied by many (Postma 1955; Crampin and Kirkwood 1981; among others) and recently Thomsen (1986) presented approximate equations for qP, qSV and SH velocities for transversely isotropic media (valid also in symmetry planes of any anisotropic symmetry system) correct to the fust order in quantities that are small for weak anisotropy. Thomsen's (1986) expressions are for phase velocity but he reaffirms that the difference between group and phase velocity is a second-order quantity. We examine this relationship to enable comparison of experimentally observed NMO (group) velocities with phase velocities from Thomsen's approximate relations.


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