Geophysical joint inversion applied to deep geothermal exploration

Abstract

A geophysical joint inversion approach of magnetotelluric, gravity and seismic noise tomography is developed to attempt imaging deep geothermal reservoirs targets in the Massif-Central, France. These techniques allow us to image complex geology, with water filled fractures, down to several kilometers. We carried out magnetotelluric, gravity surveys and seismic noise tomography in the Sioule valley, West of the Chaîne des Puys volcanoes. We obtained a 3-D 30km deep conductivity model from the 3-D inversion of the full tensors from all MT sites . The conductivity structure is highly heterogeneous in depth with a north-south trend. The full Bouguer anomaly shows a regional West/East gradient. A large negative anomaly is correlated with deep resistive structures, while positive anomalies are correlated with conductive structures. Surface wave dispersion curves were obtained from the seismic noise processing , then were inverted to obtain a shear wave velocity model down to 12-15 km depth. In spite of the cross shaped layout of the seismic network, the location of the sources of the seismic noise allowed only a final 2-D velocity model across the studied area. Shear velocity increases from 2.9 km/s at the surface to 4km/s in depth. A low velocity region 12 km in depth is located beneath the volcanoes of the Chaîne des Puys then the deep velocity increases westward. The preliminary results of the joint inversion of these three data sets are presented and discussed.