Abstract

While cross-well traveltime tomography can be used to image the subsurface between well pairs, the use of cross-well reflections is necessary to image at or below the base of wells, where the reservoir unit is often located. One approach to imaging cross-well reflections is to treat each cross-well gather as an offset VSP and perform wavefield separation of direct and reflected arrivals prior to stacking or migration. Wavefield separation of direct and reflected arrivals in VSP is accomplished by separating the total wavefield into up and downgoing components. Since reflectors can exist both above and below the borehole wavefield, separation of cross-well data into up- and downgoing components does not achieve separation of direct and reflected arrivals. In our technique, we use moveout filters applied in the domain of common vertical source/receiver offset to extract reflected arrivals from the complex total wavefield of a cross-well seismic data set. The multiple domains available for filtering and analysis make cross-well data more akin to multifold surface seismic data, which can also be filtered in multiple domains, rather than typical VSP data, where there is only one domain (common source) in which to filter. Wavefield separation of cross-well data is shown to be particularly effective against multiples when moveout filters are applied in common-offset space.

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