Amplitude Vs. Offset Analysis in Finely Layered Media

Herbert W. Swan

In 1987, it was shown that amplitude vs. offset analysis (AVO) can be sensitive to small residual velocity errors. However, even when the velocity is perfectly determined, serious AVO distortions remain due to normal moveout stretch, thin-bed tuning, differential tuning as a function of offset, and event time misestimation. This presentation shows that all of these effects are closely related, and can be analyzed in a unified framework. If the analytic form of the observed AVO slope distortion is expanded in a Taylor series, the first term represents the residual velocity error term found by Spratt, and the second term represents the remaining distortion effects mentioned. In practice, either term can be larger than the underlying lithologic AVO slope term being estimated. For example, errors caused by wavelet stretch lead to a peak slope distortion which is 61% of the peak zero-offset reflectivity, even though the velocity field is uniform and the data are moveout corrected with the perfect velocity. These errors become particularly troublesome in a finely layered target zone, and must be considered during the processing and i terpretation of AVO data.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.