Reflection moveout of pure modes recorded on conventional-length spreads is described by a normal-moveout (NMO) velocity that depends on the orientation of the common-midpoint (CMP) line. Here, we introduce the concept of NMO-velocity surfaces, which are obtained by plotting the NMO velocity as the radius-vector along all possible directions in 3-D space, and use it to develop Dix-type averaging and differentiation algorithms in anisotropic heterogeneous media.
The intersection of the NMO-velocity surface with the horizontal plane represents the NMO ellipse that can be estimated from wide-azimuth reflection data. We demonstrate that the NMO ellipse and conventional-spread moveout as a whole can be modeled by Dix-type averaging of specifically oriented cross-sections of the NMO-velocity surfaces along the zero-offset reflection raypath. This formalism is particularly simple to implement for a stack of homogeneous anisotropic layers separated by plane dipping boundaries. Since our method involves computing just a single (zero-offset) ray for a given reflection event, it can be efficiently used in anisotropic stacking-velocity tomography.
Application of the Dix-type averaging to layered transversely isotropic media with a vertical symmetry axis (VTI) shows that the presence of dipping interfaces above the reflector makes the P-wave NMO ellipse dependent on the vertical velocity and anisotropic coefficients ∊ and δ. In contrast, P-wave moveout in VTI models with a horizontally layered overburden is fully controlled by the NMO velocity of horizontal events and the Alkhalifah-Tsvankin coefficient η ≈ ∊ − δ. Hence, in some laterally heterogeneous, layered VTI models P-wave reflection data may provide enough information for anisotropic depth processing.