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Abstract

Geothermal energy is an attractive renewable energy; however, its exploitation requires a sound knowledge of the nature, quality and boundaries of the reservoir to evaluate the resource potential and reduce the risk of induced seismicity. The purpose of this project is to use broad-band magnetotelluric (MT) data to investigate a medium-enthalpy geothermal zone in the region of Annecy, France. In this pre-alpine area, karstic limestones constitute a deep aquifer (up to ~2 km) intersected by the Vuache fault, an active strike-slipe fault where a M5.3 earthquake occurred in 1996.A data set of 27 MT sites was acquired, with generally all 5 components measured. Data quality was mostly satisfactory up to 3 Hz and sometimes up to 1 s. Major problems were related to the presence of electromagnetic noise in this urbanized region, mitigated by the careful use of advanced processing methods (FFMT). For the 3-D inversion required by the data (phase tensors, WAL, topography), we have chosen a joint inversion of induction vectors, phase tensors and off-diagonal impedances (previously corrected for static shift with help of phase tensor inversion and few TDEM measurements). This allowed us to obtain a suitable 3D model using the ModEM inversion code, explaining all three data types reasonably well. We aim at the integration of the obtained resistivity models with already existing results (seismic reflection, borehole information) into a geological model, which will be used within re-inversion of the MT data by turning off the smoothing across interfaces detected by seismic reflection.