[1] An experiment on the uses of shear-wave splitting as an imaging tool in fracture-controlled geothermal reservoirs was conducted at Krafla, Iceland. Fifteen days after the beginning of the seismic recording the injection was stopped for eleven days and then restarted, a sequence designed to determine whether shear-wave splitting measurements can detect the transient response of the subsurface crack system to changes in fluid pressure. It was observed that time delays between the fast and slow split shear waves changed significantly and promptly with the stoppage and resumption of injection. Large time delays occurred only during injection, decreased substantially during the stoppage phase, and increased again as injection restarted. Comparisons of these results with similar observations at the Coso geothermal field in California strongly suggest that the time delay of split shear waves can be a useful proxy to monitor fluid pressure in the cracks and changes in crack density. Citation: Tang, C., J. A. Rial, and J. M. Lees (2005), Shear-wave splitting: A diagnostic tool to monitor fluid pressure in geothermal fields,

During the summer of 2004 we recorded the seismicity at the Krafla geothermal field for forty days with an array of twenty PASSCAL L-28 4.5-Hz sensors. The Krafla field is located approximately 60 km East of Akureyri in northern Iceland. The array covered an area approximately 5 km N-S by 4 km E-W. The field area is located on Holocene lava flows on the Mid-Atlantic Ridge. The array recorded approximately 5 micro-earthquakes per day at a sampling rate of 500 Hz. This high sampling rate is required to exploit newly developed theories on the frequency-dependence of shear-wave splitting (SWS). During the experiment, the injection well was stopped for ten days to study the response of the subsurface crack system to changes in water pressure. SWS is an exploration method based on the analyses of polarizations and time delays of shear waves that have been distorted by the anisotropy of the medium through which the seismic waves have propagated. Epicenters roughly align along the E-W direction, while hypocenters are shallow around the injection well and appear to be related to the on-going injection. Observations of SWS at Krafla have provided evidence for at least two major crack systems oriented approximately N-S and E-W. This last, rather unexpected direction is consistent with results from a simultaneous MT (magneto-telluric) survey. Further SWS study will lead to a more detailed understanding of the fracture locations, sizes, and orientations in the geothermal field.

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