Subsalt velocity analysis by combining wave equation based redatuming and Kirchhoff based migration velocity analysis

Abstract

Due to the geometrical complexity of the typical Gulf of Mexico (GOM) velocity models, with embedded salt bodies of any shapes, wave equation migration is used preferentially over Kirchhoff methods for subsalt velocity model building. This preference is based on the ability of wave-equation based migrations to overcome the need for tracing complex ray paths through the salt bodies and for a better handling of multi-path arrivals via wavefield reconstruction. Subsalt velocity analysis uses prestack wave equation migration scans that are created from perturbed velocity models: this is an accurate albeit expensive and time consuming approach that requires multiple runs of prestack migrations. To make this process more efficient, we present in this paper an alternative methodology to perform subsalt velocity analysis. For those cases where sediment velocity structure is relatively simple, we perform a single one-time redatuming to the base of salt (BOS), using existing prestack wave equation tools. By redatuming, we remove the complexity of the wavefield caused by the salt bodies. Once having obtained a simplified wavefield by stripping off the effects of the complex overburden, we can employ less expensive Kirchhoff imaging algorithms for performing subsalt velocity model building.