Dense Seismic Recordings at Hengill Geothermal

A temporary array composed of 21 seismic stations was deployed in the Hengill geothermal field in SW Iceland during the months of July and August 2005. The Hengill experiment was designed to closely monitor seismic activity related to cold water injection in two nearby wells (HE-8 and HE-16) within the geothermal area. Nineteen of the seismic stations installed were L-28 4.5-Hz sensors while the remaining two were Guralp CMG-40T broadband seismometers. The seismic array recorded an average of approximately 7 events (microearthquakes) per day at a sampling rate of 500 samples per second. The high sampling rate is critical for shear-wave splitting analysis, which requires accurate measurements of fast shear-wave polarization directions and time delays between the arrivals of the fast and the slow shear-wave. Based on the high-resolution seismic data produced, it is possible to track temporal as well as raypath-dependent variations in polarizations and time delays. Such changes are used to model the response of the geothermal reservoir (in terms of fracture geometry and fracture density) to cold water injection activities. More specifically, we are interested in studying the response of subsurface cracks to fluid pressure changes as injection is started, halted, and then resumed again.