High-resolution analysis of seismicity induced at Berlín geothermal field, El Salvador

Abstract

We investigate induced microseismic activity during a hydraulic stimulation monitored at the Berlín Geothermal Field (BGF), El Salvador. The site was monitored for a time period of 17 months using 13 3-component seismic stations deployed in shallow boreholes. Three stimulations were performed in the well TR8A with a maximum injection rate and well head pressure of 140l/s and 13MPa, respectively. For the entire time period of our analysis, the acquisition system recorded 581 events with moment magnitudes ranging between −0.5 and 3.7. In this study an initial seismic catalog provided by the operator has been substantially improved: (1) We re-picked P- and S-wave onsets and relocated the seismic events using the double-difference relocation algorithm based on cross-correlation derived differential arrival time data. Forward modeling was performed using a local 1D velocity model. (2) We recalculated source parameters using the spectral fitting method and refined the results applying the spectral ratio method. We investigated the source parameters and spatial and temporal changes of the seismic activity based on the refined dataset and studied the correlation between seismic activity and production. The achieved hypocentral precision allowed resolving the spatiotemporal changes in seismic activity down to a scale of a few meters. We observe clustering of the seismicity around the injection well as well as the migration of seismicity outside of the injection point along pre-existing faults. The migration of seismicity is determined by increasing injection rate and “crustal memory” (Kaiser) effect. We observe larger magnitude events after the shut in of the injection well. We finally observe a decrease of the static stress drop with increasing distance from the injection point. Of special interest is the largest event (MW 3.7) and its nucleation process. The event occurred in the center of the BGF about two weeks after the termination of the second injection in TR8A. We investigate whether the refined seismic data display any signatures that the largest event is triggered by the shut-in of the well. These characteristics would be in accordance with the occurrence of induced “larger magnitude events” in a number of other geothermal sites.