## 3 In Situ AE Monitoring in Salt Mines
### Salt Mine Morsleben in Germany
It can be stated, that most of the activity took place near the cavities with especially high AE activity at the roof of the cavities [92]. Figure 10: Perspective view of the central part of the salt mine Bartensleben (Levels L1 to L3) with location of AE events (blue dots) and AE borehole sensors (red dots). The area shown has an extension of about 300 m in the N–S direction and 240 m in height. The AE events were located within 2 days in May 2010 (see [[https://www.bgr.bund.de](https://www.bgr.bund.de)]([https://www.bgr.bund.de](https://www.bgr.bund.de))) [92]. Reproduced with permission. Figure 9: Development of the location rate in the time period from August 1995 to December 2012 in the area of the AE network in the central part of the salt mine Morsleben [92]. Reproduced with permission. Almost a year after backfilling of the cavities from 2003, AE events are distributed with distinctive stripe shapes above cavities at different depth levels (see Figure 11). The physical forces driving the creation of these stripes are still unknown. One possible explanation might be that these spatial patterns of the AE activity originated from the extensional stress developing in the cavities roofs. This strip-shaped pattern runs transversely to the longitudinal axis of the excavation in the ridge area of the room [92]. In situ AE monitoring in the salt mine Morsleben provide a very large and unique dataset of approximately 100 million located AE events and it offers a wide range of options for evaluating fracture processes in a salt mine. In conclusion, the AE activity in salt rock is detected around open cavities and at the boundaries between different rock types. Creep processes cause high AE activity due to high deviatoric stresses at the walls of the cavities in the EDZ. This kind of AE activity is interpreted as ongoing deformation (convergence) in the vicinity of the open cavities and it is always present until convergence has been stopped, e.g., by backfilling of the open cavities. Apart from seasonal fluctuations due to the variation of humidity, the AE activity does not vary with time. Because of its ductile behavior, rock salt is usually capable of performing creep deformation without occurrence of microcracking at stresses below the so-called dilatancy boundary. Above the dilatancy boundary, microcracking occurs. Most of the microcracks occur on grain boundaries and form no continuous macroscopic fractures. Deviatoric stresses above the dilatancy boundary result in the growth and opening of these microcracks, which are mainly responsible for dilatancy and the increase of permeability for fluids [37]. Although these microcracks have small dimensions in the order of the grain size (millimeter or centimeter) of the rock salt, in the course of time in dilatant zones microcracks may join and form  
Figure 11: Patterns of approximately parallel stripes of AE activity over the ridges of backfilled excavation chambers. The view is downwards in diagonal direction [92]. Reproduced with permission. macroscopic fractures especially in pillars between open cavities and in locations of high stress due to the edges of rooms, which are superposed in different levels. In such highly stressed zones, local instabilities may arise as spalling from the walls and roof falls. In this case, the EDZ migrates into the intact rock salt. AE activity in the EDZ will be stopped not before the open cavities are closed by convergence or backfilling. Closed cavities stabilize the environmental rock and microcrack formation will be stopped [92]. Figure 12 displays the evolution of AE location rates per hour (average of one day) and the evolution of temperature in the open space beneath the roof of a room. Backfilling started in October 2003 and the room was filled completely by the end of March 2004. Clearly, a very good correlation of AE location rates and temperature could be stated in the first three month. The general increase of temperature resulted from the heating due to the concrete setting of hydration.