Conventional time-lapse seismic has been less successful for stiff-rock reservoir monitoring, such as carbonates. This is mainly because of the negligible time-lapse changes in the seismic properties. Therefore, we propose to use long-offset time-lapse seismic as an alternative method to estimate small velocity changes. More specifically, we monitor the maximum amplitude offset that is beyond critical offset. The properties of the maximum amplitude offset are similar to critical offset, except that they appear for longer offsets and are frequency dependent. Increased frequency reduces the gap between this offset and the critical offset. We find that the maximum amplitude offset is a function of overburden and reservoir velocity and practically independent of density. This method requires a velocity increase across the interface that is to be analyzed. This criterion usually is satisfied for stiff-rock reservoirs. Also, by long-offset acquisition, we mean typically 1 to 2 kilometers beyond the critical offset for typical depths. The method is tested on the Valhall chalk field in the North Sea. The predicted velocity change using this method is in-line with an independent acoustic impedance study. The velocity changes quantitatively match reasonably well with the synthetic data.

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