157 4. Towards a revisited geothermal conceptual model in the Upper Rhine Graben the level of a single project, which helped in terms of interpretation of the continuity of the structures and of the horizons. The target is to obtain a better geological and structural image of the sedimentary layers and the fault system by focusing on the deep formations in the basin, e.g. the Lower Permo-Triassic sandstone and the top crystalline basement. Therefore, special attention has been paid on the faults intersecting the top basement for evaluating how they rooted into it (Figure 4.4). The main striking parameters of this 3D acquisition correspond to broadband seismic ranging from 2 Hz up to 96 Hz delivered by 62000 lbs vibro-trucks and 27000 vibrated points. Various former old petroleum wells, used for the seismic interpretation, are clustered around the SsF and Rittershoffen geothermal wells (Figure 4.4). The two operational EGS sites of SsF and Rittershoffen were comprised within the 3D acquisition area. It represents a tangible advantage for better calibrate the 3D results from seismic in terms of geology by comparison with the geological layers observed in the boreholes of SsF and Rittershoffen. In order to assess the imaging quality of the deep faulting, the pre-processing flow chart was validated by a 3D migration of the full cube. Iteratively built from picked faults and key horizons on delivered volumes, the 3D structural model was built according to the results of an Advanced Fault Enhanced volume. To refine the fault definition and location, a fault-oriented velocity model was carried out (Toubiana et al., 2020). Figure 4.4 Local 3D model of the geological layers and the main steeply dipping faults in Northern Alsace derived from 3D seismic interpretation. The green arrow indicates the North, and X and Y axes the geographic coordinates. On surface, geothermal and former oil well names from Rittershoffen (left) and SsF (right) geothermal sites are represented. Geological legend for the layers from surface to the deep basement: (brown) Eocene with top of the Dolomitic Zone, (purple) Upper Trias with top of the Keuper, (green) Middle Trias with top of the Muschelkalk, (pink) Lower Trias with top of the Buntsandstein, (red) Carboniferous with top of the crystalline basement.
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