Abstract

In an effort to understand better the amplitude variation with offset for reflections from an oil sand and the sensitivity of the AVO response to shear-wave velocity variations, I studied synthetic and field gathers collected from an onshore field in the Gulf of Mexico basin. A wave-equation-based modeling program generated the synthetic seismic gathers using both measured and estimated shear-wave velocities. The measured shear-wave velocities came from a quadrupole sonic tool. The estimated shear-wave velocities were obtained by applying published empirical and theoretical equations which relate shear-wave velocities to measured compressional-wave velocities. I carefully processed the recorded seismic data with a controlled-amplitude processing stream.Comparison of the synthetic gathers with the processed field data leads to the conclusion that the model containing the measured shear-wave velocities matches the field data much better than the model containing the estimated shear-wave velocities. Therefore, existing equations which relate shear-wave velocities to compressional-wave velocities yield estimates which are not sufficiently accurate for making quantitative comparisons of synthetic and field gathers. Even small errors in the shear-wave velocities can have a large impact on the output. Such errors can lead to an incomplete and perhaps inaccurate understanding of the amplitude-versus-offset response. This situation can be remedied by collecting shear-wave data for use in amplitude-versus-offset modeling, and for building databases to generate better shear-wave velocity estimator equations.

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