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Understanding subsalt illumination through ray-trace modeling, Part 2

Dipping salt bodies, salt peaks, and nonreciprocity of subsalt amplitude response

Diamond Geophysical Research, Redmond, Washington, U.S.

Mike Kelly

Diamond Geophysical Research, Dayton, Montana, U.S.

Davis Ratcliff

Diamond Geophysical, Houston, Texas, U.S.

Corresponding author: D. Muerdter, davem@dgrc.com

The first 20% of the full text of this article appears below.

The interest in subsalt imaging has been fueled by numerous discoveries beneath salt in the Gulf of Mexico in the last decade. A most effective tool in making these discoveries is 3-D prestack depth migration (PreSDM), which allows imaging of reflectors under the salt sheets and of detached bodies of irregular shape. Ray-trace modeling can simulate seismic illumination problems such as shadow zones or seismic focusing and allow better interpretation of subsalt seismic data.

The modeling process involves building a computer model that includes salt shapes and velocity variations, simulating an entire 3-D seismic survey with ray-trace modeling, and sorting the data into common reflection point (CRP) gathers. Plots and maps of the fold and especially the amplitude on the reflecting horizons are compared with the real seismic data to better understand the illumination complexities.

In the first part of this three-part article (TLE, June 2001), the theory and methods of ray-trace modeling were discussed and simple 2-D models were ray-traced to investigate the effects of salt edges. In this paper illumination beneath dipping salt bodies is examined, lack of amplitude reciprocity beneath dipping salt is reported, and the effects of peaks and pits in salt bodies are studied. The Hickory discovery area in Grand Isle South block 116 is shown to have an amplitude anomaly produced by focusing through a salt peak. The third and final part of this series examines the effects of salt ridges and salt furrows, including the effects of acquisition direction relative to the structural orientation.

By limiting the number of variables, ray tracing of simple seismic models can isolate and investigate individual structural and velocity variables. Ray-trace results from simple models are then analyzed to ascertain the subsalt illumination produced by various salt structures. Analogous examples from the Gulf of Mexico show . . . [Full Text of this Article]