SEISMIC IMAGING OF THE GEOTHERMAL FIELD AT KRAFLA, ICELAND

摘要

During the summer of 2004 we recorded thernseismicity at the Krafla geothermal field for fortyrndays with an array of twenty PASSCAL L-28 4.5-Hzrnsensors. The Krafla field is located approximately 60rnkm East of Akureyri in northern Iceland. The arrayrncovered an area approximately 5 km N-S by 4 km EW.rnThe field area is located on Holocene lava flowsrnon the Mid-Atlantic Ridge. The array recordedrnapproximately 5 micro-earthquakes per day at arnsampling rate of 500 Hz. This high sampling rate isrnrequired to exploit newly developed theories on thernfrequency-dependence of shear-wave splittingrn(SWS). During the experiment, the injection well wasrnstopped for ten days to study the response of thernsubsurface crack system to changes in water pressure.rnSWS is an exploration method based on the analysesrnof polarizations and time delays of shear waves thatrnhave been distorted by the anisotropy of the mediumrnthrough which the seismic waves have propagated.rnEpicenters roughly align along the E-W direction,rnwhile hypocenters are shallow around the injectionrnwell and appear to be related to the on-goingrninjection. Observations of SWS at Krafla havernprovided evidence for at least two major crackrnsystems oriented approximately N-S and E-W. Thisrnlast, rather unexpected direction is consistent withrnresults from a simultaneous MT (magneto-telluric)rnsurvey. Further SWS study will lead to a morerndetailed understanding of the fracture locations, sizes,rnand orientations in the geothermal field.