*3.1. Salt Mine Asse in Germany*

The first example we present in this review is the study of Philipp 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 3a 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 3a indicate the positions of Crosscut 1 to 4 with a width of 12 m. Figure 3b 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 × 31 m × 5 m and 46 m × 39 m × 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 × 250 m × 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.

**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]).

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.

**Figure 4.** Geologic cross section (sketch) of the Asse salt mine with the four AE clusters C1, C2, C3a, and C3b (framed zones) [38].

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

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. Figure 5 displays AE activity above the roof of the chambers and in the southern flank above the two chambers in a rotated coordinate system. The location of the AE events are marked as black dots in the four crosscuts as shown in Figure 3. In Crosscut 1 and 2 (left-hand side) and Crosscut 3 and 4 (right-hand side), the AE events were recorded by the AE system in the western and eastern chamber, respectively. The width of the crosscut is about 12 m.

**Figure 5.** Location of AE events (black dots) in projection onto four vertical crosscuts in a rotated coordinate system above the chamber and along the anhydrite-host rock boundary at the southern flank [38]. All dimensions are given in meter.

The observed AE activity is not homogenously distributed but clusters clearly, in this way outlining planes of activity. In the chamber's roof most events outline planes oriented east–west and dipping to the south according to the salt's layering. The events extend up to 15 m from the roof into the salt rock, but at greater depth no activity is recorded, which demonstrates that currently no active damage process is occurring. In the roof of the chambers, all events occur in a homogenous part of the younger salt rock (Leine formation) and most likely correspond to damage processes owing to stress re-distribution. A geomechanical survey confirmed an increased permeability in the area of AE clusters, but did not show macroscopic damage.

The strongest AE activity is observed on the southern flank of the salt dome. The events outline a plane of activity that is oriented roughly east–west and dipping to the south. The AE events seem to follow the rock salt–anhydrite-sandstone rock boundary, which is subject to significant geomechanical processes including the loss of integrity owing to a barrier thickness of only 15 m in the upper mine. Events are observed as far as 70 m above the network. Within this plane, AE events cluster on vertical structures.

Overall, the authors conclude that although a singular AE event is too small to have a damage potential, the AE events are clearly able to outline in high-resolution areas and, even more precisely, the exact position, extension, and orientation of potential damage zones.

#### *3.2. Salt Mine Merkers in Germany*

From another salt mine in Germany, SHM during a borehole loading experiment is reported by Manthei et al. [42], Popp et al. [81], and Plenkers 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 × 25 m × 27 m monitoring a rock volume of approximately 40 m × 40 m × 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). Here color-coded lines mark the most outward extension of AE activity with time, which showed a migration of approximately 0.6 m per day [82]. In Figure 7b the cemented plug is shown in grey and the positions of AE sensors are shown by red triangles.

**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].

The experiment shows that quite different processes that influence the strength and the permeability of rock are successfully monitored using in situ AE monitoring technique. Not only is the influence of mechanical penetration (drilling) mapped by AE events (formation of the so-called excavation disturbed zone (EDZ)), but also the influence of environmental influences (humidity and temperature during cementation). Last but not least, the aseismical opening of a pathway for gas-brine percolation is accompanied by AE events.

#### *3.3. 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

150 m × 100 m × 120 m. This network was enlarged to 48 channels and covers a rock volume of about 250 m × 200 m × 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].

**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.

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 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.

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. 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) [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].

**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.

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 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. The peaks of the curve are caused by the fact that the fresh concrete cools down the surface of the concrete body during the working week. At the weekend backfilling is interrupted and temperature rises quickly until the beginning of the next week. The quick temperature increase is accompanied by a very sharp and high increase in the location rate of AE events characterizing the intensity of microcrack processes taking place. When temperatures decreased after the weekend, the location rates also decreased instantaneously indicating that the microcrack activity was generated by thermoelastic effects due to the heating of the rock mass by the setting of the concrete.

**Figure 12.** AE location rate and temperature in a room in the central part of the salt mine Bartensleben during backfilling [38].

Becker et al. [72] compare in a study the spatio-temporal evolution of the AE event distribution (Figure 12) with results from a 2D-finite element study of the evolving stress field well constrained by the known geometry of the structure and the material properties. They found out that for the first five thermal loading cycles a pronounced Kaiser effect can be observed. The so called Felicity Ratio is the ratio of the AE onset stress value to the previous peak stress [93]. It is a measure of the quality of the Kaiser effect with a value of 1 indicating a perfect stress memory effect. The observed Felicity Ratio of greater than 0.96 for the first five loading cycles indicates a very pronounced Kaiser effect. The deviation from the Kaiser effect during later loading cycles seems to be caused by the initiation of a

planar macroscopic crack, which is subsequently reactivated. AE activity tends to concentrate along this macrocrack.
