## 4 In Situ AE Monitoring in Gold Mines  
### Gold Mine Cook 4 in South Africa  
Performing in situ AE monitoring inside the deep mines in South Africa is challenging, because these mines are active working mines that come with an especially harsh environment. What is beneficial for AE monitoring is the hard rock environment that allows to observe seismic waves with frequencies far above 25 kHz from significant distances from more than 100 m [36]. For SHM, in situ AE monitoring was shown to be a valuable tool, because damage zones prone to failure can be identified beforehand and rock bursts can be analyzed in great detail after. The SATREPS project [33], that took place in Cooke 4 Mine, near Westonaria in South Africa at about 1 km depth demonstrated how to monitor a mining front in quartzite from afar, using a network of 24 field AE sensors and 6 triaxial accelerometers installed in development tunnels approximately 20 m to 50 m below the slope (network dimension 95 m \(\times\) 50 m \(\times\) 30 m, monitoring area 100 m \(\times\) 180 m \(\times\) 50 m). Figure 13 shows configuration of the in situ AE monitoring system installed 1 km beneath the ground in the Cooke 4 shaft. This figure shows the top view of the stope and the positions of sensors. The black squares denote the positions of the borehole sensors. The hatched area represents the excavated area. The mining faces are typically 30 m across and 1 to 2 m high, and the stope is sub-horizontal orientated. By daily blasting, the mining face advances to the north by about 10 m per month (red arrows). The mining face was located about 20 m above the AE network. Over a time period from 11 July 2011 to 24 August 2011 (approximately 50 days), about 290,000 AE events were recorded in trigger mode recording and localized using the joint hypocenter determination  
Figure 13: Top view of the stope and the positions of the borehole sensors (black squares). The shaded area represents the excavated area. The mining faces are typically 30 m across and 1 to 2 m high and the stope is sub-horizontal orientated. By daily blasting, the mining face advances to the north by about 10 m per month (red arrows) [34]. (JHD) approach and double-difference re-localization. The moment magnitudes M\({}_{\rm W}\) of events ranged from \(-\)3.7 to 1.0 [53]. Figure 14 shows the JHD result of 289,015 events, that had RMS residuals smaller than 1.0 ms in a top view (a) at an elevation of 690 m above sea level (about 1 km beneath ground), and (b) NNE-SSW vertical projection viewed from the west. Figure 14: Source locations of AE events determined by JHD in a top view (**a**) at an elevation of 690 m above sea level (about 1 km beneath ground), and (**b**) NNE-SSW vertical projection viewed from the west. Black squares indicate AE sensor locations. Thick cyan and red lines represent positions of the mining face on 21 July and 24 August 2011, respectively. Blue lines are outlines of the mining infrastructure. Box B outlines the area that is shown in Figure 15 in detail. A total of 289,015 AE events were located, with their dates of occurrence indicated by the color scale [34]. Black squares indicate AE sensor locations. Thick cyan and red lines represent positions of the mining face on 21 st July and 24 th August 2011 respectively. Blue lines are outlines of the mining infrastructure. The 289,015 AE events were located, with their dates of occurrence indicated by the color scale. Nearly all (90%) of the AE events occurred in a zone extending about 20 m ahead (north) of the active mining face, forming what is referred to as the stope-front cloud. AE events clearly clustered on structures' outlining planes of localized damage, which is why the authors referred to the monitoring as an advanced AE mapping technique. [PERSON] et al. [84-86] relocated clustered AE events by using the double difference technique [94]. After a pre-selection of events, the cross-correlation technique for all event pairs whose inter-event distance was smaller than 4 m was applied.