*2.2. Applications and Characteristics of In Situ AE Monitoring*

Many examples of application of in situ AE monitoring in mines as found in the literature are given in Table 2. This table shows the test site of the project, the resulting publications, the type of AE networks with source–receiver distance and mode of recording, and the rock type with number of recorded events. Above the frequencies of microseismic measurements, the lower frequency of the in situ AE method begins at 1 kHz and ranges up to approximately 200 kHz. Due to the high sensitivity of the AE sensors, the in situ AE method allows to monitor fractures in the millimeter scale to decimeter scale (see Figure 2). This is important information because such small events indicate weakening in the rock long before macroscopic fractures occur [32–34]. Due to the high frequencies, the signals of AE events attenuate more than signals of microseismic events [35,36]. Thus, in situ AE monitoring is limited to a few tens of meters in hard rock or a few meters only in soft rock like clay rock or sedimentary rock. However, if the rock is very homogeneous and the attenuation of the seismic waves is low, larger rock volumes over 150 m in length can be monitored [12,33–38]. The applications in Table 2 can be roughly divided into three groups. The first group shows applications from underground laboratories in Switzerland and Sweden. The second group deals with the application of in situ AE monitoring in gold mines in South Africa. The third and biggest group shows applications in salt mines in Germany. These in situ measurements in rock salt often focused on monitoring AE activity caused by work like backfilling [39,40], excavation [41] or gas and fluid injection [42]. Early projects in in situ AE monitoring were limited to small rock volumes.

#### *2.3. Method of In Situ AE Monitoring*

With the in situ AE method, a network of AE sensors records very small events with low seismic energy in the kilohertz range. The frequency range of in situ AE monitoring is 1 to approximately 200 kHz. In contrast to seismic and micro-seismic sensors, AE sensors do not measure ground movement based on the principle of spring-mass or a pendulum, but detect stress changes purely based on the piezoelectric effect. AE sensors are accordingly piezoelectric-based sensors, which are much more sensitive in the kHz frequency range than spring-mass based accelerometers or pendulum-based geophones/seismometers as shown in Plenkers et al. [36] and Zang et al. [43]. But, in situ AE measurements are often missing absolute calibration of the AE sensors (see Box 2). The differences between those sensor types are discussed in more detail in Box 3.

#### **Box 2.** Annotations about characterization of AE sensors.

An important issue in today's works on in situ AE measurements is the often missing absolute calibration of the AE sensors. These piezoelectric sensors (not to be confused with piezoelectric accelerometers, see Box 3) work mostly in resonant mode and do not have a flat sensor response. For this reason, the AE sensors are very sensitive at the resonant frequencies, but the exact sensor response necessary for magnitude estimations (especially based on waveform amplitudes) or source type analysis is normally unknown. By contrast, AE sensors used at frequencies *f* much higher than 100 kHz and used in the laboratory are successfully calibrated [44–51]. The calibration of AE sensors used in situ remains difficult due to the longer wavelength [45]. In addition, in situ coupling of AE sensors can have a severe influence on the sensor recording and need to be taken into account. Today, mostly two characterization methods are used in situ. sensor characterization by signal deconvolution [16,52,53] or by regression analysis [27,54,55]. An absolute calibration technique for reliable in situ calibration is still missing.

For this reason, in the case of in situ AE measurements in rock, magnitudes are often listed nominal magnitudes [56] or relative magnitudes [54]. These magnitudes are useful to gain insights into the relative event size, but may not be compared *directly* to other seismic magnitudes or in between events from different regions, different source types or different source–receiver distances as the effect of the sensors resonant response is not corrected for. Few studies exist of in situ AE events, where reliable source parameters (seismic moments) were estimated after careful sensor characterization and correction for the sensor response in situ [16,27,53,55].

Many authors applied the in situ AE method in order to detect AE events in mines. This method was applied to most applications as shown in Table 2. Due to the high measurement frequencies and the very high number of AE events, fast data acquisition systems are required. A multi-channel transient recorder (often 16 or 32 channels) running in trigger mode does the digitization. This means, a limited time window (e.g., 32 ms) is stored, when the threshold was passed. In this time window, all signals modes like P wave and S wave should be included. In order to detect the complete waveforms during the whole measurements, some measuring system also allows continuous recording of the measuring signals, where the waveform is recorded without interruption.

#### **Box 3.** Comparison of accelerometer and piezoelectric AE sensor.

A combination of AE sensors and triaxial accelerometers were used in the gold mines in South Africa, salt mines in Germany, and underground hard rock laboratories in Switzerland and Sweden. These AE sensors manufactured by Gesellschaft für Materialprüfung und Geophysik (GMuG) uses a piezoelectric disk of PZT ceramic, which is sensitive in the frequency range from approximately 1 kHz to approximately 200 kHz. On the other hand, the commercial triaxial piezoelectric accelerometer composed of Wilcoxon 736T has a flat frequency response between 100 Hz and 25 kHz with a sensitivity of 100 mV/g and a resonance frequency at 60 kHz. Zang et al. [43] concluded, "that the Wilcoxon accelerometers were not able to record AE events detected with the in situ AE sensors, despite the fact that AE events were present in the frequency range of the accelerometer". Note that the in piezoelectric accelerometers the piezoelectric disk is solely used to measure the movement of the seismic mass, whereas for AE sensors the piezoelectric effect is exploited to measure stress changes.

AE sensors are piezoelectric sensors that are much more sensitive in the kilohertz frequency range than a spring-mass based accelerometer. In the case of a constant acceleration, the acceleration force is in equilibrium with the restoring force of the spring deflected by *x* with the spring constant *c*. *F* = *m*·*a* = *c*·*x*. The measuring sensitivity *S* of the system results in *S* = *x*/*a* = *m*/*c*, if *x* is proportional to the output voltage. Accordingly, a large mass and a low spring stiffness lead to a high measuring sensitivity. In the dynamic case, damping forces and inertial forces have to be taken into account in addition to the spring force. The essential damping force is proportional to the velocity . *x* and is described with the attenuation coefficient *p*. The inertial force is proportional to the acceleration. The resulting equation describes a resonant system: *F* = *m*·*a* = *c*·*x* + *p*· . *x* + *c*· .. *x*. Starting from a negligible damping (*<sup>p</sup>* <sup>≈</sup> 0), it has a resonance frequency *<sup>ω</sup>*<sup>0</sup> <sup>=</sup> <sup>√</sup>*c*/*m*. Thus, according to the above equation, the measurement sensitivity S is firmly linked to the resonance frequency *ω*<sup>0</sup> in the following way: *ω*<sup>2</sup> <sup>0</sup>·*S* = 1. This means that a twice as high resonance frequency must be paid for with sensitivity reduced to the factor 1/4. Such spring-mass systems show only sufficiently below their resonance frequency a sufficiently constant proportionality between the measured variable and the deflection.


CharacteristicsandapplicationsofinsituAEmonitoringinmines.


**Table 2.** *Cont*.

Often real-time processing (P- and S-wave onset picking and localization) of events is implemented in trigger mode recording. As data needs to be processed and stored on computer hard drives some dead time in recording occurs in between different windows. In the trigger mode approximately 10 AE events per second can be recorded. The signals of AE events are recorded when a specified threshold is crossed on one or more channels. As mentioned above, continuous recording of the waveform is possible in the last few years, as large computer storage is now available. Typical sampling rates are 500 kHz to 1 MHz. The P-wave and S-wave onsets are automatically picked after band-pass filtering of the traces by applying an adapted, speed-optimized short-term-average to long-term-average (STA-to-LTA) trigger algorithm [89]. After picking of the onsets, a least-square algorithm based on a gradient method is used to determine the location of the AE events. A location is valid if a sufficient number of P-wave and S-wave onsets are used for source location and the time residuals are small enough. With this location procedure working, noise without discernible onsets can be eliminated. For post-analysis, the digitized waveforms and location results are stored on hard disk. With fast Internet access, the AE measuring system can be remotely controlled. Transient noise of anthropogenic or electronic origin on the other hand is often localized and can pollute the seismic catalog, especially in active mines [36,38,87].
