Green's functions are obtained for depth migration in heterogeneous media through estimation of subsurface scattering-potential V . Multiple vertical-seismic profiles (VSPs) are used to compute V , and elastic variation and density variation estimated independently are used to perturb V to represent all surface / subsurface scattering potentials. Then, V for all points in the image space are converted to Green's functions for use in depth imaging.
In the absence of prior knowledge of elastic variation and density variation, perturbation of V is computed for common-offset, common-azimuth gathers under the assumption of smooth variation in velocity where density is constant. These assumptions are consistent with conventional moveout velocity, so perturbation of V from moveout analysis is developed.
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