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Understanding subsalt illumination through ray-trace modeling, Part 1

Simple 2-D salt models

Diamond Geoscience Research Corporation, Redmond, Washington, U.S.

Davis Ratcliff

Diamond Geophysical Service Corporation, Houston, Texas, U.S.

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

Editor's note: The other two parts of this study—titled "Dipping salt bodies, asymmetry of amplitude response and salt peaks" and "Salt ridges and furrows, shooting orientation of acquisition"—will be published in the July and August issues of TLE.

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

The complex structure and high velocity contrast of salt in the Gulf of Mexico create a difficult seismic imaging problem. 3-D prestack depth migration (PreSDM) of seismic data has allowed 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 energy is reflected. 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 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 CRP gathers. Care must be taken in building and ray tracing the model to produce amplitude results that can be compared to seismic amplitudes on subsalt reflectors. Because of space limitations, this study will appear as a three-part article. This initial paper contains an introduction to ray-trace modeling, ray-trace methods, and examples of simple 2-D salt models. In the second part, more complex models in 2-D and 3-D will be investigated to determine effects of more complicated structures. In the final part, results of ray-traced 3-D models will indicate the effects of shooting direction relative to the structural orientation. In all cases, comparisons are made to similar salt shapes in real seismic data taken from 3-D prestack depth migration surveys in the Gulf of Mexico. This study provides insights into possible imaging anomalies based on simple shapes that can be extrapolated to "real world" situations. However, the complex interplay of real world structure and velocity variations may mean that ray-trace or other modeling is needed to ascertain the specific illumination under these more complex . . . [Full Text of this Article]