Redatumming through a salt canopy — Another salt‐flank imaging strategy

Abstract

We describe a new short cut strategy to image the sediments and salt edge around a salt flank through an overburden salt canopy. We demonstrate its performance and capabilities on a synthetic acoustic seismic data from a Gulf of Mexico (GOM) style model. In this strategy, we first redatum the surface shots from a walk away Vertical Seismic Profile (WVSP) survey to be as if the source and receiver pairs had been located in the borehole at the positions of the receivers. This process creates effective downhole shot gathers by completely moving the surface shots through the salt canopy without any knowledge of the overburden velocity structure. After redatumming, we apply reverse time prestack depth migration to the effective downhole shot records using a simple linear v(z) gradient velocity model. This first pass of migration reveals the salt dome edge quite well. Once the salt dome edge is defined, a second pass of reverse time prestack depth migration is performed with an updated velocity model that now consists of the v(z) gradient and the salt dome. The second pass migration brings out the dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank forming prismatic reflections. The full, two‐way wave equation migration algorithm images these double bounces correctly. In the new strategy, we do not need to perform the traditional complex process of velocity estimation, model building, and iterative depth migration to remove the effects of the salt canopy and surrounding overburden. The redatumming process eliminates all of that. In terms of computational effort, it takes less than 10% of total computation time and requires virtually no human effort. The reverse time depth migrations are target‐oriented and, for this example, involve only 1/3 of the model. The new strategy provides a simple, target oriented method to image a salt flank and nearby sediments with minimal velocity estimation and model building effort.