Elastodynamic Redatumming for VSP Salt Dome Flank Imaging

Abstract

In this paper we construct an application methodology for the theoretical elastodynamic redatumming work by Wapenaar (2006) to be able to approximate its use in the problem of imaging a salt dome flank using a walk away VSP (WVSP). Different from acoustic redatumming where only one quantity, the pressure, is involved, full elastodynamic redatumming requires both P-wave source and S-wave sources with different polarizations. We approximate these P-wave and S-wave sources by implementing a shooting pattern for each shot position of an ordinary WVSP acquisition. We create a synthetic WVSP dataset with modified shooting pattern for a model composed of a simplified Gulf of Mexico vertical-velocity gradient, an embedded overhanging salt dome together with a nearby salt canopy. To process these data, we first compile the raw dataset due to the shooting pattern into a new dataset which approximates common receiver gathers due to P-wave source and S-wave sources, respectively. We then apply elastodynamic redatumming to this new dataset to mimic the response between different components for downhole source and receiver pairs. The reconstructed traces show a good match to the traces in the benchmark case, which we create by firing actual downhole sources and recording the responses at all downhole receivers. Instead of a more proper, full elastic migration, we apply simple acoustic migration on individual components of the full elastic Green’s functions to produce pre-stack depth migrated images of the salt dome flank and the dipping sediments. By performing four migrations of these components, we obtain very good separate images of the subsurface using predominately either P or S wave energy. The horizontal sediments and vertical salt flank decouple and appear on separate images.