Monitoring of microearthquakes and acoustic emissions within the Hengill-Hellisheidi geothermal reservoirs, June-August 2007

Abstract

With increased emphasis on geothermal development, new exploration methods are needed in order to improve the general understanding of geothermal reservoirs, characterize their extent and assess the potential for sustainable utilization. Monitoring of microearthquakes and acoustic emissions within geothermal areas may provide a new tool for evaluating the spatial extent of geothermal fields, their dynamics and model rock-fluid interactions. In this project, data was collected within the Hengill-Hellisheidi geothermal area. A temporary seismic network was in operation from June 13 to August 13, 2007 and the area covered was 220km2. In this period more than 60 events were recorded during the field campaign, which were located using the program Hypoinverse 2000. Seismicity in the Hengill-Hellisheidi area during the observation period was low. Most of the events followed a NW-SE trend, from Reykjadalur, across Ölkelduháls into Hengill, correlating with surface geothermal manifestations. A few events were also located within the fissure swarm north and south of Hengill. Earthquakes at Ölkelduháls clustered at 4-6 km depth whereas events within the Hengill fissure swarm have a broader depth range, from 0 to 9 km. These earthquakes are most likely associated with tectonic movements or hydraulic fracturing and fluid flow within the geothermal systems. A preliminary study of the noise distribution within the Hengill-Hellisheidi geothermal fields has revealed 4-6 Hz spectral peaks at seismic stations located in the vicinity of surface geothermal manifestations. Further investigation is needed to determine their relationship to the Hengill-Hellisheidi geothermal field. Spectral peaks within the range of 8-30 Hz have been observed at most of the stations and are probably related to various sources of noise such as wind and overpressurized boreholes.