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Abstract. In 2018 and 2019, we performed STIMulation tests with characterising periodic pumping tests and high-resolution seismic monitoring for improving prognosis models and real-time monitoring TEChnologies for the creation of hydraulic conduits in crystalline rocks (STIMTEC). The STIMTEC underground research laboratory is located at 130 m depth in the Reiche Zeche mine in Freiberg, Germany. The experiment was designed toinvestigate the rock damage resulting from hydraulic stimulation and to linkseismic activity and enhancement of hydraulic properties in stronglyfoliated metamorphic gneiss. We present results from active and passiveseismic monitoring prior to and during hydraulic stimulations. Wecharacterise the structural anisotropy and heterogeneity of the reservoirrocks at the STIMTEC site and the induced high-frequency (&gt;1 kHz) acoustic emission (AE) activity, associated with brittle deformation atthe centimetre-to-decimetre scale. We derived the best velocity model per recording stationfrom over 300 active ultrasonic transmission measurements for high-accuracyAE event location. The average P-wave anisotropy is 12 %, in agreementwith values derived from laboratory tests on core material. We use a16-station seismic monitoring network comprising AE sensors,accelerometers, one broadband sensor and one AE hydrophone. Allinstrumentation was removable, providing us with the flexibility to useexisting boreholes for multiple purposes. This approach also allowed foroptimising the (near)-real-time passive monitoring system during theexperiment. To locate AE events, we tested the effect of different velocitymodels and inferred their location accuracy. Based on the known activeultrasonic transmission measurement points, we obtained an averagerelocation error of 0.26±0.06 m where the AE events occurred using a transverse isotropic velocitymodel per station. The uncertainty resulting from using a simplifiedvelocity model increased to 0.5–2.6 m, depending on whether anisotropy wasconsidered or not. Structural heterogeneity overprints anisotropy of thehost rock and has a significant influence on velocity and attenuation, withup to 4 % and up to 50 % decrease on velocity and wave amplitude,respectively. Significant variations in seismic responses to stimulationwere observed ranging from abundant AE events (several thousand perstimulated interval) to no activity with breakdown pressure values rangingbetween 6.4 and 15.6 MPa. Low-frequency seismic signals with varyingamplitudes were observed for all stimulated intervals that are morecorrelated with the injection flow rate rather than the pressure curve. Wediscuss the observations from STIMTEC in context of similar experimentsperformed in underground research facilities to highlight the effect ofsmall-scale rock, stress and structural heterogeneity and/or anisotropyobserved at the decametre scale. The reservoir complexity at this scalesupports our conclusion that field-scale experiments benefit fromhigh-sensitivity, wide-bandwidth instrumentation and flexible monitoringapproaches to adapt to unexpected challenges during all stages of theexperiment.

Seismic monitoring of the STIMTEC hydraulic stimulation experiment in anisotropic metamorphic gneiss