Correction for the influence of velocity lenses on nonhyperbolic moveout inversion for VTI media

Abstract

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 such as channels and reefs can significantly distort the moveout parameters, even when the structure is relatively simple. Here, we discuss nonhyperbolic moveout inversion for 2D models that include a low‐velocity isotropic lens embedded in a VTI (transversely isotropic with a vertical symmetry axis) medium. Synthetic tests demonstrate that a lens can cause substantial, laterally varying errors in the normal‐moveout velocity (Vnmo) and the anellipticity parameter η. The area influenced by the lens can be identified using the residual moveout after the nonhyperbolic moveout correction and the dependence of errors in Vnmo and η on spreadlength. To remove lens‐induced traveltime distortions from prestack data, we propose an algorithm that involves estimation of the incidence angle of the ray passing through the lens for each recorded trace. Using the velocity‐independent layer‐stripping method of Dewangan and Tsvankin, we compute the lens‐induced traveltime shift from the 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 parameters Vnmo and η. The corrected traces and reconstructed “background” values of Vnmo and η are suitable for anisotropic time imaging and producing a high‐quality stack.