We have monitored the mine-scale, hydraulic stimulation experiment STIMTEC-X in real-time using an adaptive, high-resolution seismic monitoring network. We installed AE hydrophones in addition to regular AE sensors and accelerometers used during the prior STIMTEC experiment. Location accuracy is significantly improved by reducing location uncertainty by approximately half. Hydrophones allow for best possible monitoring of selected stimulation intervals, can be combined with hydraulic equipment for installation, leading to a better 3-D coverage and density of sensors in the volume of interest, and allow for magnitude determination for all incidence angles. This comes at a cost of reduced detection ranges (15 m versus 30 m for common AE sensors), an inability to record S-waves and some polarity complications due to the operating mode of the hydrophone.
In 2018–2019, the STIMTEC hydraulic stimulation experiment was conducted at the Reiche Zeche underground laboratory in Freiberg, Saxony/Germany, at a depth of about 130 m below surface in metamorphic gneiss (Renner and STIMTEC-Team, 2021). This experiment was designed to investigate the role of hydro-mechanical processes for the often required enhancement of hydraulic properties in deep geothermal projects. It involved stimulation and repeated hydraulic testing of ten 0.7 m-long intervals of the 63 m-long injection borehole and five local stress measurements in the vertical validation borehole (Figure 1). We applied the same injection protocol to each of the intervals, yet, we observed significant small-scale variability in the seismic and hydraulic responses to stimulation and in parallel stress field heterogeneity on the meter scale. While acoustic emission (AE) activity was high in the upper part of the injection borehole, no AE events were detected in its deepest part, ending in a high-permeability damage zone (Boese et al., 2020). These observations raised questions concerning the seismic detectability throughout the stimulated volume, because average station distances to the deepest stimulation intervals are increased (27 m compared to 18 m on average) and attenuation estimates as determined from lab experiments on core samples showed higher values in the vertical compared to the horizontal direction due to the strong sub-horizontal foliation. To place sensors closer to the stimulation intervals in the follow-on experiment STIMTEC-X, we installed AE-type hydrophones in down-going, water-filled boreholes. Up to six hydrophones were optimally placed for each measurement configuration anew with at least one deployed in the direct vicinity (~3-4 m) of the stimulated interval. This was to ensure best possible seismic monitoring of selected borehole intervals and resulted in an adaptive monitoring network.