Seismic 3D imaging at the Bedretto Underground Laboratory (Switzerland): active seismic cross-hole tomography using fat rays

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab), operated by ETH Zurich, is a unique research facility providing optimal conditions for conducting experimental research on understanding the responses of the deep underground when stimulating it. Our experiments were performed in a Geothermal Testbed in the BedrettoLab. It includes six monitoring boreholes, ranging from 250 to 400 m length. They are equipped with multiple instruments including seismic sensors (geophones, accelerometers and acoustic emission) and active seismic sources (piezoelectric transducers). In addition, two stimulation boreholes are used to access the underground. A fault zone is crossing the boreholes in the volume of interest, which is one of the main targets of our investigations. Advanced knowledge of the spatial distribution of the seismic velocities (i.e. elastic properties) is essential for several purposes, including, for example, geological and geotechnical characterizations of the rock volume, locating microseismicity caused by the hydraulic stimulations, and performing active seismic monitoring experiments. For that purpose, we have compiled a comprehensive active seismic travel time data set. As seismic sources we considered borehole sparker shots and the permanently installed piezoelectric transducers. The seismic waves were recorded with hydrophone streamers and the permanently installed seismic sensors. This resulted in roughly 45’000 travel time picks. Here, we present first results of a 3D P-wave velocity tomography. Even with this relatively large data set, the ray coverage within the volume of interest is still relatively incomplete, when using classical (infinitesimally thin) rays. Therefore, we considered a fat ray approach, with which the finite bandwidth of seismic waves can be approximated more realistically. We will compare the classical ray-based tomography (high frequency approximation) with results from the fat ray tomography (frequency dependent). The resulting tomograms can be compared with borehole image logs.