A method to estimate the in-situ 4D anisotropy changes generated around producing reservoirs is discussed and demonstrated using the depleting Elgin reservoir. The method compares the offset dependency of observed 4D seismic time shifts with synthetic anisotropic ray-traced time shifts. It requires sufficient well data, or other a priori information, to calibrate 4D vertical compaction in and above a producing reservoir. As well as demonstrating a technique to estimate the average fractional change in anisotropy throughout the overburden above the reservoir, a scheme is proposed in which the lateral variation of the anisotropy parameter delta, and possibly epsilon (Thomsen, 1986), may be determined in a qualitative manner.


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