Velocity analysis for transversely isotropic media

摘要

The main difficulty in extending seismic processing to anisotropic media is the recovery of anisotropic velocity fields from surface reflection data. Velocity analysis for transversely isotropic (TI) media can be done by inverting the dependence of P-wave moveout velocities on the ray parameter. P-wave NMO velocity in homogeneous TI media with a vertical symmetry axis depends just on the zero-dip value V(sub nmo) and a new effective parameter (eta) that reduces to the difference between Thomsen parameters (epsilon) and (delta) in the limit of weak anisotropy. It is possible to obtain (eta) and reconstruct the NMO velocity as a function of ray parameter using moveout velocities for two different dips. Moreover, V(sub nmo)(0) and (eta) determine not only the NMO velocity, but also also long-spread (nonhyperbollic) P-wave moveout for horizontal reflectors and time-migration impulse response. Inversion of dip-moveout information allows performance of all time-processing steps in TI media using only surface P-wave data. Isotropic time-processing methods remain entirely valid for elliptical anisotropy ((epsilon) = (delta)). Accurate time-to-depth conversion, however, requires the vertical velocity V(sub P0) be resolved independently. If I-P0 is known, then allisotropies (epsilon) and (delta) can be found by inverting two P-wave NMO velocities corresponding to a horizontal and a dipping reflector. If no information is available, all three parameters (V (sub P0), (epsilon), and (delta)) can be obtained by combining inversion results with shear-wave information. such as the P-SV or SV-SV wave NMO velocities for a horizontal reflector. Generalization of Tsvankin's single-layer NMO equation for layered anisotropic media with a dipping reflector provides a basis for extending anisotropic velocity analysis to vertically inhomogeneous media. The influence of a stratified overburden on moveout velocity can be stripped through a Dix-type differentiation procedure.