Seismic Methods for Observing Geothermal Field Evolution

Abstract

Seismic monitoring of a geothermal reservoir is feasible, but it is necessary to design the seismic experiment carefully in order to be able to observe changes in the field. Travel time differences due to changes in the geothermal field with time are estimated to be small ñ on the order of 10-20 milliseconds. Changes in P velocity and Poissonís ratio over time are large, but occur over small regions of less than a few hundred meters extent leading to the predicted small changes in travel times. Seismic experiments should therefore concentrate on the areas where these differences would be observed. Such changes are likely to be observable in cross-hole surveys, particularly near the regions of injection and the edge of the expanding two-phase zone. The largest effect is due to the presence of steam, which causes a sharp drop in P-velocity, an even sharper drop in Poissonís ratio, strong P-wave attenuation, and seismic reflections from the water/steam boundary. It should, therefore, be possible to map the location and size of the two-phase zone in the producing region of a geothermal reservoir with a well-designed seismic study.