Of particular interest is the case of reservoirs where the pressure configuration is such that gas is at the limit of release in solution (). Any drop in pressure, likely to occur during production, leads to gas release which results in substantial change in velocity. In these cases, the 4-D effects are mainly driven by the changes in acoustic velocity. For these reservoirs, 4-D seismic monitoring can be seen as a velocity analysis problem.
() introduce a method of migration velocity analysis based on wave-equation techniques (WEMVA) which uses the changes visible in the entire seismic image to infer velocity information. Such a technique is ideally suited to deal with velocity-related 4-D changes observed over entire images, including the case of multi-layer reservoirs.
Traveltime-based MVA methods cannot be easily used to solve this problem for several reasons: the traveltime changes that occur over time are too small to be picked with enough accuracy; amplitude information, although very important, cannot be used and is, therefore, ignored.
WEMVA applied to 4-D problems has limitations as well. First, WEMVA can only handle the image changes due to perturbations of the acoustic velocity since our current implementation is based on the acoustic wave-equation. Second, WEMVA can only handle small velocity anomalies, due to the inherent Born approximation. This, however, is unlikely to be a problem for 4-D analysis since the image changes are smaller than a fraction of the seismic wavelet.