Seismic monitoring and its application as an exploration tool in the Berlín geothermal field, El Salvador

Abstract

Seismic monitoring of the Berlín geothermal field has been carried out since 1996. The initial objectives were to identify major seismic areas in and around the geothermal system, to monitor natural and possible induced seismicity due to exploitation, to identify active faults and fluid circulation paths in the reservoir and to gain information on the heat source. The seismic network has detected important shallow and low-magnitude seismic activity in the geothermal area, mainly in the south and in the central part of the exploited area. This activity is related to the heat source and fluid-bearing fractures. A cluster of seismic events was detected close to the reinjection zone which is thought to have been induced by increasing pore pressure and fluid motion. There is no clear correlation between seismicity and the rate of mass extraction and injection, and pressure change. A large scale exploitation of the Berlin field started in February 1999 and since then, seismicity has reduced. The b-value calculated for the geothermal area is 1.51, which is common in a volcanic environment and reflects a low stress state of the source. The Vp/Vs and Poisson´s ratios indicate higher steam fraction in the volcanic zone south of the production area, but both areas are still liquid-dominated. The seismic activity is relatively low and with low magnitude and therefore, does not pose any threat to the environment and surrounding communities.