Case History Time-lapse tomography

In time-lapse analysis, we have to distinguish the seis- mic response changes due to oil and gas production at a reservoir over the years from several other causes, such as the recording signature and random noise. In this pa- per, we focus our attention on the velocity macromodel provided by seismic tomography, which is a basic tool for the data regularization, its depth or time migration, and a possible final subtraction among different vintages. We show first that we cannot use just a single velocity model for all data sets, because of seasonal variations of the overburden velocity (which is mainly due to sea- water temperature in marine cases and to the water ta- ble depth in land cases). However, we can exploit the basic assumption of time-lapse analysis for constraining reflection/refraction tomography, i.e., by imposing the constraint that the layer structure and the local veloci- ties do not change outside the reservoir (and in the shal- lowest part) over time. We thus get coupled models that are physically consistent, with a better spatial coverage and higher information redundancy. The new method is illustrated by a marine case history from the North Sea.