Characterising induced acoustic emission activity observed during a mine-scale hydraulic-fracturing experiment in anisotropic crystalline rock

Between early 2018 and late 2019 the STIMTEC hydraulic stimulation experiment was performed at ca.~130 m below surface at the Reiche Zeche research mine in Freiberg, Saxony/Germany. The project aims at gaining insight into the creation and growth of fractures in anisotropic and heterogeneous crystalline rock units, to develop and optimise hydraulic stimulation techniques for EGS applications and to control the associated induced seismicity under in situ conditions. A series of ten hydro-frac experiments were performed in a 63 m-long, 15°-inclined injection borehole and five mini-fracs for stress measurements in a sub-vertical borehole. These were monitored using a seismic monitoring system of twelve high-sensitivity Acoustic emission (AE) sensors, three accelerometers and one broadband sensor. More than 11,000 high-frequency AE events with source sizes on the cm-to-dm scale accompanied the hydraulic stimulation in five of ten stimulated intervals in the injection borehole. Several hundred AE events were recorded during the mini-fracs in the vertical borehole. We investigate the characteristics of induced AE events by combining information obtained from high-accuracy event locations using a transversely isotropic P-wave velocity model per station with station corrections, relative hypocentre locations, and focal mechanism solutions of selected events. The AE event clouds extend ca. 5 m radially from the injection points and show variying orientations and dips. The ca. 150 focal mechanism solutions obtained using P-wave polarisations display mixed-mode failure with a significant portion of them showing compaction. The orientation of the maximum principal stress inferred from the hydro-fracs in the injection and vertical boreholes has a trend of N348°E and a plunge of 20°, as typical for southeast Germany. However, discrepancies in the magnitudes of the principal stresses were measured between these boreholes ca. 15 m apart, resulting in different faulting regimes. We present stress orientations obtained from inverting focal mechanism solutions to provide additional information for interpreting stress-characterisation measurements.