Theory of Wave-equation migration velocity analysis by cascaded image perturbations

Wave equation migration velocity analyis (WEMVA) is based on wavefield linearization using the Born approximation which allows image perturbations to be related to the corresponding slowness perturbations. The problem with this linearization is the divergence of the inversion scheme if the image perturbations are large. This imposes severe restrictions on the amount of slowness anomaly that can be estimated using WEMVA. In this paper we suggest the use of cascaded image perturbations to overcome the small slowness perturbation restriction. The idea is to proceed towards the correct image in small steps or cascades so that the image perturbations at each cascade never violate the Born approximation. The large slowness anomaly is then obtained by solving two linear systems at each of these cascades and adding them up.