| Abstract |
Large velocity contrasts are regularly encountered in geothermal fields due to poorly consolidated and hydro-thermally altered rocks. The appropriate processing of seismic data is therefore crucial to delineate the geological structure. To assess the benefits of surface seismic surveys in such settings, we applied different migration procedures to image a synthetic reservoir model and seismic data from the Coso Geothermal Field. We have shown that the two-dimensional migration of synthetic seismic data from a typical reservoir model resolves the geological structure very well despite the extremely strong and sharp velocity contrasts. In addition, small fracture zones can be inferred from the inspection of the common image gathers. After carefully preprocessing seismic data the 2 D and 2.5 D pre-stack depth migration of line 109 in the Coso Geothermal Field shows a well defined reflector at about 16'000 ft depth. Compared to the 2 D pre-stack migrated image the 2.5 D pre-stack migrated image resolves the deep reflector better, which indicates that the subsurface shows significant three-dimensional structures. The 3-D pre-stack depth migration at the intersection of line 109 and line 110 shows that the deep reflector lies horizontally and recedes in the cross-line direction of line 109. The results demonstrate that three-dimensional surveys greatly improve the image of the subsurface, where geological structures and seismic velocities vary both vertically and laterally. From a variety of seismic attributes the offset stacks, the energy and the frequency prove to be the most powerful in the Coso Geothermal Field. The offset stacks show that the deep reflections are strongest and most continuous for 15'000 ft offset to 20'000 ft offset. The far-offset stack combined with the energy yields an almost perfect indication for the deep reflector. The frequency power spectrum shows distinct peaks, which may be related to the reflector thickness. |