Seismic data processing in vertically inhomogeneous TI media

The first and most important step in processing data in transversely isotropic (TI) media for which velocities vary with depth is parameter estimation. The multilayer normal-moveout (NMO) equation for a dipping reflector provides the basis for extending the TI velocity analysis of Alkhalifah and Tsvankin to vertically inhomogeneous media. This NMO equation is based on a root-mean-square (rms) average of interval NMO velocities that correspond to a single ray parameter, that of the dipping event. Therefore, interval NMO velocities [including the normal-moveout velocity for horizontal events, V_nmo(0)] can be extracted from the stacking velocities using a Dix-type differentiation procedure. On the other hand, η, which is a key combination of Thomsen's parameters that time-related processing relies on, is extracted from the interval NMO velocities using a homogeneous inversion within each layer. Time migration, like dip moveout, depends on the same two parameters in vertically inhomogeneous media, namely V_nmo(0) and η, both of which can vary with depth. Therefore, V_nmo(0) and η estimated using the dip dependency of P-wave moveout velocity can be used for TI time migration. An application of anisotropic processing to seismic data from offshore Africa demonstrates the importance of considering anisotropy, especially as it pertains to focusing and imaging of dipping events.