## 5 In Situ AE Monitoring During Hydraulic Fracturing in Mines  
A slightly different aspect of SHM concerns the monitoring of hydraulic fracturing (HF) using in situ AE monitoring. Engineered fractures generated underground by packer probes in boreholes are facilitated in a broad variety of contexts, many of which require detailed knowledge on the damage process actively initiated. HF is a common tool for underground stress determination and provides important input for designing the stope layout and for risk assessment [100; 101; 102]. HF has become a widely used engineering tool in reservoir enhancement of geothermal systems, shale gas, or conventional oil and gas extraction as it effectively increases the permeability [103; 104; 105; 106]. In addition,  
Figure 18: Foreshock activity compared to aftershock activity. (**a**) Distribution of the manually located AE events that occurred within 5 m of the aftershock surface during the period from 13 June 2007 to 150 h after the main shock. The yellow star indicates the main shock hypocenter. Black, red, blue, and green solid circles denote the events in June, September, October, and December, respectively. Orange and black thick solid lines enclose the foreshock clusters (F1, F2, and F3) and aftershock clusters (A1 and A2), respectively. Gray dots denote the aftershocks. Gray contours show the areal density of the aftershocks drawn by feeding areal densities in 5 m\(\times\) 5m cells. The thin black dashed contour indicates the aftershock area defined by [PERSON] et al. [78]. The light blue solid circle is the access tunnel along which the observation network was deployed. (**b**) Distribution of the automatically located events that occurred before the main shock and within 5 m of the aftershock surface. Their densities in 5 m \(\times\) 5 m cells are shown by gray scale. Blue, purple, and green solid circles denote events in October, November, and December, respectively. Light green contours show the areal density of the automatically located foreshocks. HF is successful in increasing the productivity in ore production e.g., HF is used in fragmenting ore bodies [107]. Several research projects have addressed HF using in situ AE monitoring or strain cells in order to increase the understanding of the rock response to HF, to study the evolution of fracture generating and predict the stimulation of existing fractures [67; 108; 109]. Two recent research projects in crystalline rock did not only record very interesting and rich data, but have pushed the limits in highly sensitive monitoring and advanced signal processing. [PERSON] et al. [43] report on HF at the hard rock underground laboratory Aspo in Sweden (Figure 19a). [PERSON] et al. [43] utilize three different monitoring networks, namely in situ AE monitoring, microseismic monitoring, and electromagnetic monitoring. Figure 19b shows a top view of the AE monitoring network, which consists of 11 AE sensors of GMuG type MA BLw-7-70-75 and four Wilcoxon 736T accelerometers. Data is recorded both in trigger mode and continuous mode (1 MHz sampling frequencies). AE sensors are installed in monitoring boreholes of 22 m to 30 m length parallel to the injection borehole and in short boreholes along the tunnels. Overall, six HF stimulations are performed using three different injection schemes (continuous, progressive and pulse pressurization), from which four produced significant AE activity outlining the fracture orientation, extension and temporal evolution. Within 20 days, about 69,400 triggers were recorded in situ, from which many correspond to noise events due to dripping water or anthropogenic activities. The strongest AE events recorded in situ with best signal-to-noise ratio and most reliable location certainty (maximum location residual 0.3 m) formed a catalog of 196 seismic events, of which all correlated in time and space directly to the HF periods.