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Understanding subsalt illumination through ray-trace modeling, Part 3

Salt ridges and furrows, and the impact of acquisition orientation

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

Davis Ratcliff

Diamond Geophysical, Houston, Texas, U.S.

Corresponding author: davem@dgrc.com

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

The complex structure and high velocity of salt in the Gulf of Mexico create seismic imaging difficulties. 3-D prestack depth migration (PreSDM) of seismic data allows imaging of reflectors under the salt sheets and of detached bodies of irregular shape. But 3-D PreSDM cannot fill in shadow zones below salt where little data were recorded. Additionally, amplitude variations caused by salt structures focusing or dispersing seismic energy are usually poorly handled by 3-D PreSDM. Ray-trace modeling can clarify subsalt imaging problems, and the modeling results should be incorporated into exploration and development plans.

The modeling process in this paper involves creating computer models that include salt shapes and velocity variations, ray tracing an entire 3-D seismic survey, and sorting the simulated data into CRP gathers. Care must be taken in building and ray tracing the model to produce amplitude results that can be compared with seismic amplitudes on subsalt reflectors.

This is the third and final part of the subsalt illumination paper. In the first two parts, simple 2-D models were ray traced to investigate the subsalt illuminations beneath flat salt slabs, salt lenses, salt edges of various shapes, dipping salt bodies, and salt peaks. In this third part, salt ridges and troughs or furrows are studied, including the effect of the shooting direction angle to the structural dip. Comparisons are made with similar salt shapes in real seismic data taken from 3-D prestack depth migration surveys in the Gulf of Mexico. Although this study provides insights into possible imaging anomalies, these simple models may not exhibit the illumination produced by complex interaction of real-world shapes and velocity variations. Ray tracing or other modeling techniques should be used to ascertain the illumination under these more complex structures.

As in the previous parts of this paper, the selection of features to . . . [Full Text of this Article]

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