Modelling seismic event location errors at the reservoir scale: application to the geothermal site of Soultz-sous-Forêts (Alsace, France)

Abstract

The location of natural or induced earthquakes is of primary interest for geothermal fields monitoring and characterization. We investigate the impact of several parameters on earthquake absolute location errors. A two-steps approach is applied. First, body-waves travel-times between synthetic earthquake locations and seismic receiver positions are calculated. Then, the obtained travel-times are used as observed arrival times. However, several of the initial hypotheses are changed before relocation to represent the lack of knowledge or simplifications which are investigated. The method is applied to the geothermal site of Soultzsous-Forêts (France). In 2000, a massive water injection was performed at approximately 5 km depth, in the granitic basement, to enhance the GPK2 well productivity. A downhole and a surface network recorded several thousands of induced microearthquakes. In this work, we quantify the location variation due to the joint use of the downhole and surface networks, or to the use of one of those networks. Besides the description of the usual location uncertainties, the impact of the picking time precision is investigated as well as the variation of the velocity model to mimic the possible effect of a massive water injection. The downhole network is proven to be useful to decrease the location uncertainty obtained from the surface network alone. Nevertheless, the locations computed using this network may be more biased.