Application of turning-ray tomography to the offshore Mississippi Delta

Abstract

The area immediately offshore of the Mississippi Delta is well known for its poor seismic data. Investigation by others has shown the cause to be gas-charged mudflows than can lower the velocity to less than 300 m/sec. The dimensions of the anomalies are typically less than 100 m deep and 300 m wide, but laterally very abrupt. Induced statics can range from 0 to 300 ms within one or two group intervals. Contributing to the problem is an additional long period static due to deeper Holocene prodelta sediments. The magnitude of this problem is generally beyond the capabilities of conventional automatic reflection statics alone, and the methods of refraction and turning-ray tomographic statics were tested for their applicability to the situation. Due to interference of the direct water arrival, only first arrival times beyond 3000 m in offset were used for either analyses. The results from refraction analysis were helpful in places, but the deep velocity gradient and the strong variations in the shallow velocity layer are severe departures from the model assumed in refraction statics. A significantly better result was obtained using turning-ray tomography. Starting with an initial background model, velocity grid cells were ray traced to obtain travel times. The differences from the observations were inverted to obtain velocity perturbations, and a final velocity model was iteratively calculated. The resulting statics were good enough to continue the processing flow through velocity analyses and reflection statics with a vastly improved final section.