Repeated seismic reflection measurements in the Kakkonda geothermal field

Temporal variations in seismic reflection responses apparently caused by changes within a geothermal reservoir have been detected in the Kakkonda field, where production wells were shut in prior to annual power plant maintenance. Three seismic surveys were carried out during a 12-day period spanning the shut-in period. Receivers were deployed without replanting throughout the surveys, providing good data acquisition repeatability. We applied prestack time migration (PSTM) to each of the three seismic data sets and calculated the cross-correlation coefficients between PSTM sections. Our results indicate that the reservoir changes associated with shut-in are large enough to be seismically detectable. The region over which the seismic response changed corresponds to the zone of geothermal fluid flow paths inferred from reservoir temperatures and geochemical data. We have also compared our cross-correlation maps with the epicenters of micro-earthquakes, which are inferred to indicate the existence of fractured zones. Dense regions of micro-earthquake activity lie within the seismically identified zone of changes in reservoir properties. We demonstrate the feasibility of repeated seismic surveys in providing substantial improvements in temporal resolution during geothermal reservoir monitoring.