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

Abstract. In 2018 and 2019, we performed STIMulation tests with characterising periodic pumping high-resolution seismic monitoring for improving prognosis models real-time TEChnologies the creation of hydraulic conduits in crystalline rocks (STIMTEC). The STIMTEC underground research laboratory is located at 130 m depth Reiche Zeche mine Freiberg, Germany. experiment was designed to investigate rock damage resulting from stimulation link activity enhancement properties strongly foliated metamorphic gneiss. We present results active passive prior during stimulations. characterise structural anisotropy heterogeneity reservoir site induced high-frequency (>1 kHz) acoustic emission (AE) activity, associated brittle deformation centimetre-to-decimetre scale. derived best velocity model per recording station over 300 ultrasonic transmission measurements high-accuracy AE event location. average P-wave 12 %, agreement values on core material. use a 16-station network comprising sensors, accelerometers, one broadband sensor hydrophone. All instrumentation removable, providing us flexibility existing boreholes multiple purposes. This approach also allowed optimising (near)-real-time system experiment. To locate events, tested effect different inferred their location accuracy. Based known measurement points, obtained an relocation error 0.26±0.06 where events occurred using transverse isotropic station. uncertainty simplified increased 0.5–2.6 m, depending whether considered or not. Structural overprints host has significant influence attenuation, up 4 % 50 decrease wave amplitude, respectively. Significant variations responses were observed ranging abundant (several thousand stimulated interval) no breakdown pressure between 6.4 15.6 MPa. Low-frequency signals varying amplitudes all intervals that are more correlated injection flow rate rather than curve. discuss observations context similar experiments facilities highlight small-scale rock, stress and/or decametre complexity this scale supports our conclusion field-scale benefit high-sensitivity, wide-bandwidth flexible approaches adapt unexpected challenges stages