156 Geophysics in Geothermal Exploration During the development of the Rittershoffen project, again vintage 2D seismic lines were used for targeting the first vertical well GRT-1 drilled at 2560 m in the granite. However, those reflection seismic data were mainly acquired for imaging oil embedded in Cenozoic sedimentary reservoirs. Thus, the deepest sediments and the related fault network were not imaged properly. It is the reason why a new reflection seismic campaign was achieved at Rittershoffen as well as a three-component VSP for better targeting the second well, GRT-2, drilled to 3196 m. Then a new 3D geological model was built, and the second deviated well was targeted to the North with the same technical design as the first well (Baujard et al., 2017). This well was drilled parallel to the main local fault and about six permeable fractures crosscut the GRT-2 well in the sandstone and in the granite. This highly deviated well became the production well in which a down-hole line shaft pump was set. Consequently, the first well, GRT-1, became the reinjection well which is mainly crosscut by a branch of the local fault in the granitic section. From 2016, geothermal exploitation is running properly by using this doublet. After this success story in Rittershoffen, the Illkirch project started in 2010. From the lessons learned at SsF and Rittershoffen, two main sets of geophysical exploration methods were carried out. First, vintage 2D seismic lines were reprocessed and additional new 2D seismic lines were acquired. It aimed to better define the geological and structural properties in this part of the Rhine Graben where the sedimentary layers are rather thick. In this part of the Rhine graben, this sub-basin could reach more than 4000 m thick. In parallel to this new seismic acquisition campaign, two gravity data acquisitions were made in 2013 and 2016 for refining the existing datasets. Moreover, a high-resolution aeromagnetic survey was conducted in 2015 in this area. Both geophysical methods aimed to determine the nature of the deep basement (granite, schist, metamorphic rocks, …), which was only known from outcrops lying in the Vosges mountains. Therefore, a better image of the local faults and layers was achieved by combining in a geomodeller, both vintage and recent 2D seismic lines. Then, a combined interpretation of gravimetric and magnetic data correlated with a structural interpretation of 2D seismic lines made it possible to highlight nature and the structure of the deep basement (Edel et al., 2018). Moreover, the drilling of the deepest part of the GIL-1 is well confirmed that the deepest basement is characterized by brittle crystalline formation. The drilling of this deviated well perpendicular to a local fault at Illkirch between 2018 and 2019 demonstrated that the faulted interface between the Triassic sandstone and the granite was tight, that the basement was made of granite as predicted by aeromagnetic results and that permeable fractures took place in the crystalline basement. Because of felt induced seismicity triggered by another competitor at Vendenheim in the northern part of Strasbourg during this period, the Illkirch geothermal project was temporarily suspended by local mining authorities in 2021 till now. In Northern Alsace, a 3D seismic reflection campaign was carried out in 2018 over a surface of about 180 km² in superimposition with the geothermal licenses and concessions hold by Électricité de Strasbourg. It was the first 3D seismic survey done in France for deep geothermal energy. The large surface of the 3D seismic acquisition allowed having an underground image at a regional scale rather than at
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