## 3 In Situ AE Monitoring in Salt Mines
### Salt Mine Morsleben in Germany  
The following example of in situ AE monitoring originated from the central section of the underground repository of Morsleben in Germany. Mining in these areas continued until the 1960s, but most of the rooms in the rock salt were mined more than 80 years ago. In this section, in situ AE monitoring has been performed since 1994. In the central section of the underground repository, the borehole sensors are distributed at three excavation levels and installed in 3 to 20 m deep boreholes. Originally, a network of 24 AE sensors monitored this section and covered an area of  
Figure 7: AE swarm activity during gas-brine break-through in map view (**a**) and side view (**b**). For orientation, the neighboring pillars are shown in dark grey in (**a**). Here color-coded lines mark the most outward extension of AE seismicity with time, which shows a migration of approximately 0.6 m per day. In (**b**) the cemented plug is shown in grey and the positions of AE sensors are shown by red triangles [82]. 150 m \(\times\) 100 m \(\times\) 120 m. This network was enlarged to 48 channels and covers a rock volume of about 250 m \(\times\) 200 m \(\times\) 120 m [37; 39; 69]. The average depth of the monitored volume is 400 m. This in situ AE monitoring provides a dataset of currently approximately 15 million located events per year [92]. For most events no waveforms are stored, but only the results of real-time processing. The aim of in situ AE monitoring is to investigate micro- and macrocracking, which are important for the evaluation of the stability of cavities and the hydraulic integrity of the rock, which is of special interest in the case of an underground disposal of hazardous waste in salt rock [39]. Figure 8 shows a cross section (sketch) through the central part perpendicular to the average direction of strike, where cavities in rock salt were mined beneath thick anhydrite blocks. The actual geological situation and the arrangement of cavities is more complicated and strongly varying along the strike. Stress redistribution causes high AE activity at the walls of the cavities (in the EDZ) particularly, where cavities are close to one another and at the boundary between rock salt and anhydrite [39]. However, the temporal and spatial occurrence of the events differs. Apart from seasonal fluctuations that may be explained by variations in humidity, the AE activity along the walls of the cavities does not vary with time. Outside the cavities near the anhydrite border, the AE events occurred in clusters. In some cases, such clusters were repeatedly located in the same volume; in other cases, significant emission occurred only within a limited time period [37]. To maintain the integrity of the barrier to the top of the salt deposit and the stability of the rooms for a long time, the rooms in the central part were backfilled with salt concrete from September 2003 to January 2011. During and after backfilling, the rock in the vicinity of these rooms were additionally loaded due to thermally induced stresses by released heat during hydration of the salted concrete for a period of several months to several years. Figure 9 shows the development of the located AE events per hour over a very long period, including the time before backfilling. One recognizes the strong increase of the microcracking activity up to approximately 1200 events per hour with beginning of the backfilling starting from September 2003 [92]. Figure 8: Sketch of geology and geometry of rooms in the central mine segment, where the AE network was installed (levels L1 to L3) [39]. Reproduced with permission. Figure 10 shows a perspective view of the central part of the salt mine Bartensleben with the rooms at Levels L1 to L3. The located AE events and the AE borehole sensors are marked by blue and red dots, respectively. The AE events were located during backfilling within a time period of two days in May 2010.