Improving the 4D signal in the Valhall gas cloud ///

The Valhall Life-of-Field-Seismic (LoFS) project is the world’s preeminent time-lapse seismic experiment. LoFS consists of a permanent OBC system that allows frequent high-resolution data acquisition for imaging the effects of production. More than six seismic surveys have been acquired since late 2003. The data quality shows an excellent 4D response to production on the flanks of the field, but shallow gas severely attenuates the PP seismic data in the crest. Long-period multiples dominate the PP data near and in the gas-clouded crestal region. These multiples persist in the time-lapse difference volumes from LoFS surveys because of differences in their arrival times. For some time, we have known that changing tidal levels were part of the reason for these persistent, non-repeating multiples. Conventional multiple techniques can be used, but significant multiples remain. Chance played a role in uncovering another reason for the remaining multiples.

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