Nonhyperbolic Moveout Inversion in the Presence of Velocity Lenses

Nonhyperbolic moveout analysis plays an increasingly important role in velocity model building because it provides valuable information for anisotropic parameter estimation. However, lateral heterogeneity associated with stratigraphic lenses can significantly distort the moveout parameters. We analyze the influence of a low-velocity isotropic lens on nonhyperbolic moveout inversion for horizontally layered VTI (transversely isotropic with a vertical symmetry axis) models. Synthetic tests demonstrate that a lens can cause substantial, laterally varying errors in the normal-moveout velocity (Vnmo) and the anellipticity parameter eta. To remove such errors in Vnmo and eta, we propose a correction algorithm designed for a lens embedded in an otherwise laterally homogeneous overburden. This algorithm involves estimation of the incidence angle of the ray passing through the lens for each recorded trace. With the assumption that the lens-related perturbation of the raypath is negligible, the lens-induced traveltime shifts are computed from the corresponding zero-offset time distortion (i.e., from "pull-up" or "push-down" anomalies). Synthetic tests demonstrate that this algorithm substantially reduces the errors in the effective and interval moveout parameters. The corrected traces and reconstructed "background" values of Vnmo and eta are suitable for producing a high-quality stack and performing anisotropic time imaging.