Abstract

Variable water depth can cause severe degradation of marine seismic data. When the water depth changes rapidly relative to the cable length, the timing variations are dynamic. To compensate for these timing variations, a ray-trace modeling program calculated the traveltimes to several depths, a second program used the model results to shift the samples on all seismic traces to the time they would have if the depth change were not present. The model is composed of: the depth of the sea floor and the velocity-depth relationships below the sea floor. The depth of the sea floor was estimated from the first arrivals on the near-offset traces of the seismic data. The subsea velocity-depth relations were also estimated from the seismic data. However, the results of applying the corrections calculated from this model show a definite enhancement of reflector continuity; velocity semblance contour plots show the same enhancement. The static corrections also improve reflector continuity, but the dynamic corrections do a better job of it. The dynamic corrections allow additional processing to produce a better final stack. Dynamic corrections allowed a much better choice of normal moveout (NMO) velocities near the valley. Also, a zone of near-surface, high-velocity material near the valley was detected by distortion of reflections on 100 percent shot records. Compensation for the zone was effected with a set of localized, static corrections. Although the final stack is greatly improved, there is still a serious degradation of the data. The valley introduces timing errors, reduces the amplitude of the reflections returned from below it and introduces coherent noise in the form of scattering off its sides and enhanced multiples. These additional problems limit the accuracy with which the model can be built to correct the timing errors. The difficulty in obtaining the corrections for the effects of highly variable water depth would be greatly reduced if accurate, closely spaced, fathometer measurements of water depth were made an integral part of marine seismic data recording.--Modified journal abstract.

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