Source Characterization of Picoseismic Events During Hydraulic Stimulation at Hectometer Scale

Abstract

From 2021 to 2023, we performed 17 hydraulic stimulations at hectometer scale in a fully controlled environment in the Bedretto tunnel. The Bedretto tunnel is a new research facility below the Swiss Alps with an overburden of 1 km. For the experiments, a multi-packer system was installed in a 400 m long stimulation borehole, dividing it into 14 intervals. Additionally, a densely spaced multi-component geophysical monitoring system was installed in six surrounding monitoring boreholes. Information on in-situ stress conditions were collected prior to the stimulations from all boreholes. A first analysis of the data showed that the dense network allowed us to resolve the seismic event locations well beyond point clouds. The seismic event locations reveal clear alignments which reflect the activation of the faults in the rock volume in space and time. In addition to the highly resolved event locations, we are able to estimate well constrained moment tensors for the picoseismic events. The moment tensors align with the known fractures and reveal differences in the mechanism between faults within short distances ( less than 10 m).