Imaging Salt Dome Flank and Dipping Sediments Using Time Reversed Acoustics

Abstract

In this paper we define the theory and basic principles to move (redatum) the surface shots from a walk away VSP to be as if they had been located in the borehole. We will refer to this theory using several of the terms used in the literature including Time Reverse Acoustics (TRA), Seismic Interferometry (SI) and Virtual Source (VS) technology. Regardless of the name, the theory is built upon reciprocity and the time symmetry of the wave equation. We apply these TRA principles, together with prestack depth migration, to produce images of a modeled salt dome flank. We create a set of synthetic traces representing a multi-level, walk away VSP for a model composed of a simplified Gulf of Mexico vertical velocity gradient and an embedded overhanging salt dome. The sediment reflectors in the model dip up towards the salt dome flank. The energy from the surface shots is bent into turning rays by the linear v(z) gradient which illuminate the steeply dipping sediments and overhanging salt edges. The illuminating energy is reflected and scattered from these surfaces and then captured by the downhole VSP receivers. To simplify the processing of these data, we move (redatum) the surface shots into the borehole using our TRA or seismic interferometry principles. This removes from the seismic traces the entire, potentially complicated, path from the surface shot location to the borehole without having to perform any velocity analyses or moveout corrections. Each of these new (redatummed) traces mimics the output of a down-hole source and down-hole receiver pair. We apply prestack depth migration to these new traces to produce the final image of the beds and the salt dome flank which agree very well with the original model structure.