## 3 In Situ AE Monitoring in Salt Mines
### Salt Mine Merkers in Germany  
From another salt mine in Germany, SHM during a borehole loading experiment is reported by [PERSON] et al. [42], [PERSON] et al. [81], and [PERSON] et al. [82]. In the first experiment that took place from January 2010 to January 2012, the rock response of four different stages were monitored using in situ AE monitoring: (1) pre-excavation; (2) drilling of wide-diameter (1.3 m diameter, 60 m extension) borehole; (3) partial backfill with MgO concrete to create gas tight seal; and (4) during borehole loading with compressed air. In the second experiment, that took place October 2017 to Summer 2018, in the same wide-diameter borehole, the rock response to brine loading was monitored [82]. The monitoring system of the first experiment consisted of 12 AE sensors installed in four monitoring boreholes in 12 m to 15 m distance to the large injection borehole. The sensors were equally spaced along the borehole and therefore, monitor the whole borehole with a similar recording sensitivity. The AE events are recorded in trigger mode. The dimension of the network is 28 m \(\times\) 25 m \(\times\) 27 m monitoring a rock volume of approximately 40 m \(\times\) 40 m \(\times\) 90 m.  
Figure 6 shows at the left-hand side and in the middle results of the located AE events during excavation and cementation in side view (projection onto the x-z plane). The right-hand side of Figure 6 shows the number of located events per day during start of excavation (blue vertical arrow) and during stepwise cementation (red vertical arrows). In total, more than six million AE events were recorded and localized. Highest event rates are found during excavation and cementation, when more than 170,000 events are recorded and located per day (blue vertical arrow). It is shown that the activity starts in formerly inactive homogenous salt rock as soon as excavation is starting (left-hand side of Figure 6). After excavation and also after cementation is finished, the activity rate dropped nearly to the level of background seismicity rates (approximately 100 to 300 events/day). During cementation, the AE activity was limited to a zone approximately 0.5 m from the newly created borehole outlining the excavation damage zone [82]. The AE activity was extending outwards with time, but was also limited within a zone approximately 2 m from the borehole boundary. During excavation, AE events outlined the migration of humidity and temperature from the cementation into the rock. Figure 6: Temporal and spatial distribution of AE events in Merkers 1 experiment. The location of AE events during excavation (**a**) and cementation (**b**) are given in side view projection. The number of events per day of the whole monitoring period from March 2010 to December 2011 is shown in (**c**). The blue and red vertical arrows indicate the start of excavation and cementation, respectively [82]. It should be noted that from June 2011 on the wide-diameter borehole was loaded stepwise with compressed air. The pressurization took place in several steps. Each time, when pressure was increased, the daily activity rate increased by a few hundred events. In January 2012 at a pressure of 60 bars for the first time AE events were observed that migrate as far as 20.2 m from the wide-diameter borehole. Those events were concentrated on a layer with minor vertical expansion and outline the migration of gas and brine. On 24 January 2012a gas and brine breakthrough was observed at two monitoring boreholes in combination with a pressure drop from 68 bars down to 56 bars. It was discussed that the AE events represent the break down of grain boundaries (source radius of a few centimeters) during percolation of the gas and brine mixture. Figure 7 shows AE swarm activity during gas-brine break-through in top view (Figure 7a) and a lateral view (Figure 7b). For orientation, the neighboring pillars are shown in dark grey areas in (Figure 7a).