Seismic experiment at Krafla Geothermal Field, Northern Iceland

Jonathan Lees and Jose Rial

University of North Carolina, Chapel Hill

We deployed an array of 20 PASSCAL L-28 4.5-Hz sensors for 39 days during the summer of 2004 at the Krafla Geothermal field. The Krafla Geothermal field is located approximately 60 km East of Akureyri in northern Iceland. We registered approximately 5 micro-earthquakes per day at a sampling rate of 500 Hz. The high sample rate is required to exploit newly developed methods using the frequency-dependence of shear-wave splitting (SWS). The array covered an area approximately 5 km North/South by 4 km East/West. During the experiment, hydrothermal injection of water into the geothermal reservoir was halted for ten days during which seismic activity decreased. When injection resumed, seismic activity returned to its previous intensity. SWS measurements exhibit a strong correlation to fracture locations, sizes, and orientations in the geothermal field. These fractures control the directions of fluid migration in the subsurface. We show examples of crack density estimation from Coso, where they commenced injection is the converse of density estimates at Krafla, where injection was halted.