We applied fat ray travel time tomography to image the geothermal testbed at the BedrettoLab (Switzerland). An active seismic crosshole survey provided a dataset of 41’881 manually picked first breaks. The complex major fault zone was identified by a 3D velocity model and validated with wireline logs and geological observations. Induced seismicity from hydraulic stimulation experiments preferentially occurs in intermediate-velocity regions, possibly due to the presence of stress gradients.