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Shear- and compressional-wave seismic tests using the Vibroseis system were performed near a well in south-central Louisiana to study acquisition, processing, and interpretation problems typically encountered in low-velocity, relatively uncompacted Gulf Coast sediments. The primary objectives envisioned for these tests were to improve S-wave data quality by studying surface noise patterns to optimize source and receiver arrays, provide a direct correlation of P- and S-wave seismic data by using vertical seismic profiles (VSP), and measure the decay of P- and S-wave seismic energy with depth by using a downhole geophone.
To achieve these objectives, an expanding reflection profile (ERP), a walkaway noise analysis, and a VSP were recorded with both S- and P-wave sources. The S-wave ERP shows reasonable data quality although it was very band-limited (5-12 Hz). In contrast, the P-wave data quality is excellent. The difference in data quality is primarily due to strong, source-generated
noise from the S-wave vibrators. Time-depth data available from the S- and P-wave VSPs were used to convert the time sections to depth, and hence provide the best visual tie of common reflecting horizons. The accuracy of the ties is mainly limited by the poor signal-to-noise ratio and narrow bandwidth of the S-wave data.
Finally, energy decay measurements from first breaks on VSP data show that S waves have a higher loss than P waves in the near surface. However, below 3,000 ft (915 m), the slopes of the energy decay curves are similar, thus implying S-wave data quality will not deteriorate faster than P-wave data quality at greater depths.
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