## 3 In Situ AE Monitoring in Salt Mines  
### Salt Mine Asse in Germany  
The first example we present in this review is the study of [PERSON] et al. [38], which demonstrates structural monitoring in cavity roofs of the salt mine Asse II. In the Asse II mine in Lower Saxony there is the risk of permanent brine inflows due to failure processes in pillars and tunnels in the southern flank and in the adjacent overburden [90; 91]. In order to monitor these failure processes, in situ AE measurements are carried out to detect microcracking in the roof of two cavities (that are subject to work operations and noise). It is expected, that AE events outline weakening in rock, structural damage due to dilatation and other dynamic processes long before significant damage is visible and in areas that are not accessible. Figure 2(a) displays a top view onto the two chambers, which are monitored. The contours of the chambers are indicated by rectangles. North is on top. The monitoring system consists of two networks with 16 AE sensors, which are installed in short borehole of 1 m to 3 m length in the chamber's roof. The positions of the AE sensors of the western and eastern galleries are marked by grey and black dots, respectively. The dashed lines in Figure 2(a) indicate the positions of Crosscut 1 to 4 with a width of 12 m. Figure 2(b) shows a perspective view of the two test sites. The vertical short lines mark the boreholes for the bottom-view AE sensors, which are especially sensitive in the frequency range 1 kHz to 150 kHz. Data is recorded in trigger-mode (1 MHz sampling rate) and automatically processed i.e., that events recorded are localized in near-real time. For this P- and S-wave onset picking is performed using a picking algorithm based on the Hilbert transform. The network geometry of both networks (network dimension 37 m \(\times\) 31 m \(\times\) 5 m and 46 m \(\times\) 39 m \(\times\) 4.5 m) differs owing to the actual usage of the two chambers that define the accessibility of the roof for sensor installation e.g., the roof could not be accessed above three large brine ponds. The monitoring project has been ongoing for several years, but the study considers data of a 10-month period, namely the time period 4 February to 31 November 2013. In this time period more than 100,000 AE events were recorded that populate a rock volume of approximately 250 m \(\times\) 250 m \(\times\) 160 m outlining dynamic processes not only in the chambers roof, but also in the salt dome flank and in the upper salt dome. Figure 4 displays a geological cross section (sketch) through the Asse salt dome. The sketch shows that the southern flank (left-hand side) has a steep slip. In this cross section, the chambers in Figure 3 are marked by a rectangle at the left-hand side in Figure 4. This figure gives a good overview of the approximate location of the four main clusters C1, C2, C3a, and C3b of AE events, which are marked by framed zones. More than 70% of the located AE events occur in the Cluster C1 along the so-called southern flank of the salt mine Asse. In addition to these events, high AE activity occurs in zones above the roof of the chambers. At the roof of the western and eastern chamber (Cluster C2) more than 5000 and 10,000 AE events are located, respectively. In addition, in the upper part of the salt dome and the northern flank (Clusters 3a  
Figure 4: Geologic cross section (sketch) of the Asse salt mine with the four AE clusters C1, C2, C3a, and C3b (framed zones) [38]. Figure 3: (a). Top view onto the monitored chambers. The contours of the chambers are indicated by squares. North is on top. The positions of the AE sensors are marked by grey and black dots. (b) Perspective view of the two test sites. The vertical short lines mark the boreholes for the bottom-view AE sensors (modified from [38]). and 3b) two clusters of AE events are located. Because of the low wave attenuation of salt rock, the AE networks are very sensitive, so that source-receiver distances greater than 150 m are possible.