Fracture Mapping in the Soultz-sous-Forêts geothermal field from Microearthquake relocation

In 2003, a massive hydraulic fracturing experiment was carried out at the European Geothermal Hot Dry Rock site at Soultz-sous-Forêts, France. The two week injection of water generated a high level of microseismic activity. About 90,000 microearthquakes were triggered during and after this fluid injection. Of these, 21,000 events, detected at all stations, were located individually with a grid search algorithm to characterize the extent of the seismic zones and, ultimately, of the fracture network. The accuracy of these initial locations was around 70 meters, not sufficient to map detailed fracture patterns. We undertook a relocation effort using two different techniques: Joint Hypocenter Determination (JHD) and Multiplet analysis. The JHD technique allows for the simultaneous location of a group of events and the determination of a common set of station corrections. We added the “collapsing” method to the JHD results to further consolidate the hypocenters. This was followed by a multiplet analysis for identifying microearthquakes with similar waveforms. We found 7463 events whose seismograms correlated with a correlation coefficient of 0.8 or higher, most of which were doublets. For the relative location of the correlated events, we computed the delay in travel-time by wavelet analysis. We found that multiplets were located on small planes with lengths of a hundred to several hundred meters striking mostly along N150°E. Comparing the distribution of seismicity with fluid outflows during the fluid injection, we observed that majority of events were concentrated along the newly created fractures.