Processing for time-lapse seismic
As illustrated by the foregoing material, data from two or more time-lapse surveys are quite likely to have repeatability problems. One question that therefore arises is: how successfully or justifiably can we "force" a match between them? After all, 99% of the earth volume covered by the survey will not have changed perceptibly between the two surveys. In this section we investigate a few guiding examples of attempts to do this.
Fig. 5.1 shows a dip line from two co-positioned 3D data volumes from 1985 and 1995 surveys–a typical "legacy" data situation. The oil-water contact is initially at 1.9 s on the 1985 data, and the column has moved about 50 m upwards during the 10-year interval. It also shows the difference section, first that obtained with just a single "global" amplitude matching scalar, and without any trace "matching" of any kind, although the two data sets have been processed with time-lapse intentions. Clearly, the subtraction has not been especially successful, although there is a strong event in the region of the oil-water contact.