- Copyright © 2002 Society of Exploration Geophysicists
Editor's note: P. Mahob is now employed by BP Exploration International Inc., Anchorage, Alaska, U.S.
An alternative approach to identifying AVO anomalies, presented by Tim Keho at SEG's 2000 Annual Meeting and subsequently published in TLE (November 2001), is to consider the AVO polarization in the intercept-gradient (A–B) plane. This method does not require deviations or separations from a background trend in traditional crossplots such as intercept-gradient (A–B) or near trace-far trace (N–F). A benefit of the hodogram or polarization method is that the wavelet is taken into consideration as it is convolved with the reflection coefficient series. Crossplotted intercept and gradient are polarized along a “background trend” for nonanomalous events and at angles different from the “background trend” for anomalous events. This allows recognition of anomalous behavior otherwise buried in a background.
New attributes resulting from this methodology include (1) the polarization angle, (2) the polarization angle difference, (3) the AVO strength, (4) the linear-correlation coefficient, and (5) the product of strength and polarization angle difference. These different attributes can be used to enhance AVO interpretation.
In this paper, we show synthetic results for a succession of gas and brine layers encased in shale units and a case study for the NW Shelf of Australia.
At any given interface, AVO intercept and gradient for a time-windowed reflection have a preferred orientation in the A–B plane or intercept-gradient space. The angle defining any preferred orientation in the intercept-gradient space is called the polarization angle. Nonanomalous events related to shales and brine sands can exhibit a well-defined orientation (or background angle).
Hence, events at angles different from the background angle can be considered anomalous. Therefore, the angle of polarization can help identify AVO anomalies.
A main benefit of this approach is enhancing seismic anomalies that are small or embedded in the …