Performance Evaluation of AE Sensors Installed Like Hydrophones in Adaptive Monitoring Networks During a Decametre-Scale Hydraulic Stimulation Experiment.

In the framework of the STIMTEC and STIMTEC-X hydraulic stimulation experiments at the Reiche Zeche mine, Freiberg (Germany), we installed acoustic emission (AE) sensors for the recording of picoseismicity both conventionally using pneumatic coupling and experimentally like a hydrophone, i.e. the sensors were placed in the borehole without a further coupling system or cementing. We investigate performance measures of the hydrophone-like acoustic emission (HAE) sensors such as frequency bandwidth, sensitivity, first motion polarity, coupling and placement quality to assess the sensor's applicability in adaptive monitoring networks. HAE sensors can be paired with hydraulic equipment, especially with the double packer probe used for stimulation at the decametre scale because the monitored frequency content differs from injection-related noise. This offers a unique opportunity to improve the network geometry and consequently the quality of a seismic catalogue. We analyse the sensor characteristics using active ultrasonic transmission measurements from boreholes with different orientations in the rock volume, noise measurements preceding active centre punch hits in the access galleries and passive recordings of induced acoustic emission events. HAE sensors placed in water-filled boreholes show good sensitivity performance even without optimal coupling to the crystalline rock for recording distances up to 17 m. The HAE sensors record the wavefield adequately for first-arrival identification, polarity picking and amplitude characteristics but are less suitable for detecting S-waves. Due to the borehole geometry HAE sensors record waves with incidence angles from the side, resulting in opposite polarity compared to side-view AE sensors as observed in the field and lab. We discuss the advantages of adaptive monitoring networks with HAE sensors being optimally placed for each stimulation interval configuration anew to improve seismic event detection and quality of event hypocentre locations during hydraulic stimulations. We show that we are able to significantly reduce the azimuthal gap, halve the location uncertainties and improve the network coverage for the purpose of focal mechanism estimations. Highlights: An adaptive network comprising acoustic emission (AE) sensors installed both conventionally by pneumatic coupling and like hydrophones was used to significantly improve detection and localisation quality. Hydrophone-like AE-sensors placed in water-filled boreholes show good sensitivity performance for recording distances up to 17 m in crystalline rocks. Hydrophone-like AE-sensors can be combined with hydraulic equipment because the monitored frequency content differs from injection-related noises. Pairing of the hydrophone-like AE sensors with hydraulic equipment offers a simple yet effective means to improve the network geometry. [ABSTRACT FROM AUTHOR]