2-D Magnetotellurics and Gravity at the Geothermal Site at Soultz-sous-Forêts

Abstract

With the aim of investigating the possibilities of magnetotelluric methods and gravity for exploration of potential EGS sites in the Upper Rhine valley, a 2-D magnetotelluric and 3-D inversion of existing gravity data on the basis of a 3-D geological model have been carried out in the area of the geothermal power plant of Soultz-sous-Forêts. A magnetotelluric survey was executed on a
12 km long profile across the thermal anomaly in the winter 2007/08. Despite strong artificial noise, processing using remote referencing and Sutarno phase consistent smoothing revealed significant results from 10 out of 16 sites. Indication for 1-D structures were found in the shortest periods, 2-D effects in the periods <40 s, and 3-D effects in the long period range. Since 3-D effects were found in the longer periods, 2-D inversion was carried to periods <40 s. The results of the 2-D MT inversion are consistent with the geology of the geothermal site and distinguish well the sediments from the granitic basement including the structures given by the faults. A conductive anomaly with a resistivity of about 3 &#937; m has been found at a depth down to 2000 m in the area of the Soultz- and Kutzenhausen faults, which is attributed to circulation of geothermal brine. A corresponding density anomaly has been observed in the 3-D inversion of existing gravity data in combination with a 3-D geological model. In the area of the two major faults, a decrease of the mean density in the granitic basement of about 250 kg/m3 to 2500 kg/m3 has been attributed to an increase in porosity. A second density anomaly has been found to the North of the geothermal reservoir. In this case, an increase of the mean density has been observed, which indicates an inhomogeneity in the basement.