ALBERT

Description: In-situ acoustic emission (AE) monitoring is carried out in mines, tunnels and underground laboratories in the context of structural health monitoring, in decameter-scale research projects investigating the physics of earthquake nucleation and propagation and in research projects looking into the seismo-hydro-mechanical response of the rock mass in the context of hydraulic stimulations or nuclear waste storage. In addition surface applications e.g. monitoring rock faces of large construction sites, rock fall areas and rock slopes are documented in the literature. In geomechanical investigations in-situ AE monitoring provides information regarding the stability of underground cavities, the state of stress and the integrity of the rock mass. The analysis of AE events recorded in-situ allows to bridge the observational gap between the studies of faulting processes in laboratory and studies of larger natural and induced earthquakes. This chapter provides an overview of various projects involving in-situ AE monitoring underground with a focus on recent achievements in the field. In-situ AE monitoring networks are able to record AE activity from distances up to 200 m, but the monitoring limits depend strongly on the extension of the network, geological and tectonic conditions. Very small seismic events with source sizes on approximately decimeter to millimeter scale are detected. In conclusion in-situ AE monitoring is a useful tool to observe instabilities in rock long before any damage becomes directly visible and is indispensable in high-resolution observations of rock volume deformation in decameter in-situ rock experiments.

Description: Between early 2018 and late 2019 the STIMTEC hydraulic stimulation experiment was performed at ca.~130 m below surface at the Reiche Zeche underground research laboratory in Freiberg, Saxony/Germany. The project aimed at gaining insight into the creation and growth of fractures in anisotropic and heterogeneous metamorphic gneiss, to develop and optimise hydraulic stimulation techniques and to control the associated induced seismicity under in situ conditions at the mine-scale. These aspects of failure and associated seismicity are important for the development of enhanced geothermal energy systems. A combined seismic network consisted of 12 single-component acoustic emission sensors (sensitivity 1-100 kHz) and three single-component Wilcoxon accelerometers (sensitivity 50 Hz-25 kHz) were installed in boreholes drilled into the test volume, surrounding the stimulation site (Figure 1). A stimulation borehole with 63 m length was drilled with 15° northward inclination. This data set of 314 active ultrasonic transmission (UT) measurements is supplementary to Boese et al. (2021), which introduces the STIMTEC experiment and its active measurement campaigns. This data set was used to derive an anisotropic velocity model for the STIMTEC rock volume. The active seismic data provided here are from six boreholes (BH09, BH10, BH12, BH15, BH16, BH17) as shown in Figure 1. of the associated data description. There are three tables provided as metadata that contain the STIMTEC sensor coordinates, event information of the 314 UT measurements and the UT picks. The UT measurements were recorded with a sampling rate of 1 MHz and results from an automatic stack of 1024 UT pulses generated by the ultrasonic transmitter and recorded by the STIMTEC sensors. The UT measurements are saved in binary file format (fsf file format). Fsf-files can be processed with FOCI software: https://www.induced.pl/software/foci. Each fsf file contains 32768 samples, which corresponds to 0.032768 seconds. All UT event files were manual inspected and phase arrivals identified. These are stored in the fsf-file header as well as in the table STIMTEC_UT_picks.csv.

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