Seismic shear‐wave observations in a physical model experiment

Abstract

The observation and common‐depth‐point (CDP) processing of mode‐converted shear waves is demonstrated for real data collected in a physical model experiment. The model, submerged in water, represented water depth scaled to 250 ft, the first subsea reflector at 4000 ft, and the last reflector at 7000 ft below the sea floor with a structural wedge at the center. Very efficient mode conversion, from P to SV and back to P, is anticipated for angles of incidence at the liquid‐solid interface (sea floor) between 35 and 80 degrees. The model, constructed of Plexiglas and 3180 resin, will support elastic shear‐wave propagation. One anticipated problem, internal reflections from the sides of the model, was solved by tapering the sides of the model to 45 degrees off vertical. The P wave reflection coefficient at an interface between Plexiglas and water is 35 percent for vertical incidence, but it diminishes to very nearly zero between 43 and 75 degrees. Thus, by tapering the sides of the model, any undesired internal P wave reflections had to undergo at least two reflections at angles of incidence in the low reflection coefficient range for P waves. Data were collected in both an end‐on CDP mode, with offsets from 1000 ft to 20,000 ft, and a variety of walkaway experiments with scaled ranges from 1000 ft to 31,000 ft. Processing and analysis of the data confirm the existence of mode‐converted shear‐wave reflections in a modeled marine environment. In particular, the S wave reflections from all interfaces are identified on both the 100 percent gathered records and the final stacked records. These SV wave reflections were isolated for stacking by considering those portions of the gathered records, both offset and arrival time, that correspond to optimum angles of incidence. In addition, τ-p processing isolated particular angles of incidence, further confirming the incidence angle‐range criterion. Thus, the desired events are unambiguously identified as mode‐converted shear waves.