Separation of S‐wave and P‐wave reflections offshore western Florida

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Hard water‐bottom marine environments, such as offshore western Florida, have presented particular problems in the acquisition and processing of seismic reflection data. One problem has been the limited angle of incidence (less than critical) available to P‐wave penetration into the subsurface. Mode conversion from P‐wave to S‐waves (SV), however, is quite efficient over a broad range of angles of incidence. After the success of a previously reported physical model experiment, an experimental line was acquired offshore western Florida. The 19 mile line, located approximately 100 miles west of Key West, Florida, was shot and processed. Three key factors have contributed to the successful recording of mode‐converted S‐wave reflections: (1) recognition of the effect of the group length on attenuation of energy arriving at large angles of incidence; (2) tau‐p processing techniques that allow separation of energy by angle of incidence; and (3) velocity filtering over a range of hyperbolic normal‐moveout (NMO) velocities as part of the forward tau‐p transform. These three factors, two of them data processing techniques, have allowed separation of P‐ and S‐wave energy in the marine environment. Overall, S‐wave reflections have been unambiguously identified to a reflection time of 2 sec and may be interpreted to a reflection time of 2 sec. Integrating an S‐wave section with P‐wave interpretations of offshore Florida data allows an independent confirmation of structural events. This independent confirmation may be more significant than improvements in the P‐wave data quality alone. Lateraly stable [Formula: see text] values are computed in intervals 1500 to 5000 ft thick and to S‐wave reflection times as great as 3 sec. The opportunity of [Formula: see text], interpretations for lithologic identification, gas thickness estimates, and general stratigraphic trap exploration makes mode‐converted shear waves a new tool in this area.