Abstract

Marine shear-wave reflection methods using the conventional data acquisition system (i.e., source and receiver in water) rely on two mode conversions at the water bottom to produce shear reflections such as PSSP. Some theoretical considerations and the results of a marine check shot survey conducted in the Gulf of Mexico demonstrate that the difficulty in observing PSSP events is attributable to weak P-S and S-P conversion at the bottom in regions with very low shear velocity (a few hundred ft/s or less) sediments at the bottom. For a simple water bottom with a low shear-wave velocity, water over a uniform half space, the PS conversion factor is proportional to V s , and the SP conversion factor is proportional to V (sub s 2 ) , where V s is the bottom shear velocity. For V s nearly equal 1500 ft/s their product gives PSSP reflections that can be comparable in amplitude to typical PPPP events. For V s < or = 500 ft/s, the PSSP events should be about 30 dB weaker and probably not visible. For typical Gulf of Mexico sediments with a shear velocity transition zone several tens of feet thick at the bottom, the situation is even worse, since the velocities start near zero and may not reach 500 ft/s. This condition is common in many areas of recent sedimentations.

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