Picoseismic response of hectometer-scale fracture systems to stimulation under the Swiss Alps, in the Bedretto Underground Laboratory

We performed a series of hydraulic stimulations at 1.1 km depth in the Bedretto underground laboratory in the Swiss Alps. The goal was to achieve an unprecedented detailed and profound understanding of hydromechanical and seismic processes during hydraulic reservoir development with a dense multi-sensor monitoring network. With our seismic network that includes various sensor types with different sensitivities, we succeeded in characterizing induced seismicity down to the pico-seismicity level (Mw<-4), thus illuminating details of a complex fracture network more than 100 m from the injection locations. Here, we present the experiments and seismic catalogs as well as a comparative analysis of event number per injection, magnitudes, b-values, seismogenic index and reactivation pressures. During a first-order data analysis, we could make the following observations: - We find that the ultra-high frequency seismic network with custom-made AE sensors, allows us to observe seismicity over 3 orders of magnitude scale. Thanks to collocated accelerometers and acoustic emission sensors, AE sensors could be calibrated in-situ and adjusted moment magnitudes could be implemented into the seismic catalog. - The volume impacted by the stimulations in different intervals differs significantly with a lateral extent from a few meters to more than 150 m. Most intervals activated multiple fractures. Only during the stimulation of an interval located next to a dominant shear zone, an extended single fracture was activated, which is likely attributed to the dominant shear zone in this area. The seismic clouds typically propagate upwards towards more permeable, shallow depth on parallel dipping planes that are consistent with the stress field and seem to a large extent associated with preexisting open fractures. - It is worth noting the strong correlation between the propagation patterns observed in the seismic events and the hydromechanical observations, specifically in terms of the strain and pressure data obtained from Distributed Strain Sensing (DSS), the Fiber Bragg Grating (FBG) and the pore pressure sensors that form part of the multi-component borehole monitoring system. - This experiment confirms the diversity in seismic behavior independent of the injection protocol. Some intervals showed rapidly increasing seismicity that is spatially restricted to the volume in direct vicinity of the injection point, while others have seismicity extending as far as 150 m away from the injection point. - The reactivation pressures hint at hydraulic shearing as the dominant process, since the elastic fracture opening appears to be mostly aseismic. - The seismicity shows no distinct deviation from normal behavior with regard to Gutenberg Richter or McGarr. We have the opportunity to analyze the seismic data jointly with a multitude of other geophysical observables, such as strain and pressure, to allow more insights into the correlation of slow fracture opening and aseismic deformation processes. In future studies, the existence of these multi-disciplinary observations will allow us to put more constraints on the processes responsible for the diversity observed in seismicity.