New insights on the fault structure of Bedretto geothermal testbed and the associated seismicity based on active seismic crosshole tomography

Abstract. Fat ray travel time tomography was used to obtain reliable high-resolution subsurface images in the geothermal testbed of the Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab). The aim of the research was to better understand the relationship between structural features and the seismicity induced by hydraulic stimulation tests. Eight boreholes were used to provide a large data set comprising 41 881 manually picked first breaks. Our results demonstrate that the fat ray approach offers improved image quality compared to traditional ray-based methods. The 3D model was further validated using ground-truth information from wireline logs and geological observations. We successfully imaged a major fault zone (MFZ) that exhibits a complex structure including considerable heterogeneity. Relocation of passive seismic events generated during hydraulic stimulations indicates that the 3D velocity model has only a minor influence on hypocentral parameters. However, comparing a selection of well-constrained seismic events with the velocity structures revealed a remarkable spatial correlation. Most events occurred in regions of intermediate and slightly decreased seismic velocities, thereby avoiding both high- and very low-velocity zones. Based on small-scale laboratory studies, we speculate that these observations can be explained by the presence of stress gradients in the intermediate-velocity zones.