## 5 In Situ AE Monitoring During Hydraulic Fracturing in Mines
All relocated AE events are shown in a perspective view (Figure 20a) and a lateral view in a rotated coordinate system (Figure 20b) for seismically active HF stimulations (HF1 to HF4 and HF6). AE events recorded during the different fracture experiments clearly delineate the fractures and display differences between the different hydraulic fractures generated. A faster and further expansion of AE events away from the stimulation point is observed with each subsequent re-stimulation stage outlining the damage extension. In-depth source analysis of the  
Figure 19: (**a**) Test site of hydraulic fracturing tests in the underground Aspó Hard Rock Laboratory in Sweden (see [[http://www.skb.se/upload/publications/pdf/Aspo_Laboratory.pdf](http://www.skb.se/upload/publications/pdf/Aspo_Laboratory.pdf)]([http://www.skb.se/upload/publications/pdf/Aspo_Laboratory.pdf](http://www.skb.se/upload/publications/pdf/Aspo_Laboratory.pdf))). (**b**) Location of the AE borehole sensors together with the central injection well (blue line). The blue star identifies the fluid injection segment corresponding to the hydraulic fracturing (HF2) experiment [43; 87]. largest AE events including energy estimation, moment tensor inversion, source parameter estimation, and stress inversion by [PERSON] et al. [27] estimated the moment magnitude \(M_{W}\) of the AE events ranges from \(-4.2\) to \(-3.5\). The source analysis clearly reveals that most events correspond to shear slip events on pre-existing fractures. AE event activity starts as soon as a certain pressure level is reached. First optimum oriented fault planes fail but, overall, differently oriented fault planes are observed. Stress inversion reveals stress rotation during and after the stimulation. Using the continuous data recordings from the Aspo experiment, [PERSON] et al. [87] demonstrated that using automated full waveform detection algorithm during post-processing could significantly increase the amount of triggered AE events. No seismic fracture event was recorded by the microseismic monitoring network or the Wilcoxon accelerometers, although the latter is capable of measuring the frequency range of the observed AE events. Owing to the small nature of all AE events recorded, not only seismic monitoring in the kHz range was essential, but also the significantly higher sensitivity of the in situ AE sensors. A different experiment was conducted at the Grimsel Test Site (GTS) in Switzerland (Figure 21a,b) operated by the Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra). The GTS is located at 1,733 m above the sea level and has an overburden of 400 to 500 m. [PERSON] et al. [54] implement in situ AE monitoring for stress determination by HF in the Grimsel site. The so-called in situ stimulation and circulation (ISC) [110] was performed between two tunnels i.e., the VE and the AU tunnel (see Figure 21c), and the injection and monitoring boreholes were mostly drilled from the AU cavern at the southern end of the AU tunnel (Figure 21c). The host rock is the so-called Grimsel granodiorite, which changes into the Central Aar granite about 50 m north of the experiment volume [111]. The rock mass in the experiment volume is exceptionally intact. The Grimsel test site was monitored using 28 AE sensors (type GMuG MA-Bls-7-70) and four Wilcoxon accelerometers. Most AE sensors were installed on the tunnel wall on polished rock face, while eight AE sensors were installed in a water-filled vertical borehole (Borehole SBH1 in Figure 21c). For stress determination, a series of hydraulic fracturing tests and overcoming were performed. During hydraulic fracturing, nearly 2000 AE events were recorded with a source-receiver distances smaller than 30 m that outlined, similar to the Aspo experiment, clearly the fracture plane that extended  
Figure 20: Location of AE events of six HF stimulations (HF1 to HF4 and HF6) presented in a top view (**a**) and a lateral view (**b**). For lateral view the coordinate system was rotated in that way to indicate the preferred fracture traces.