## 2 In Situ Acoustic Emission (AE) Monitoring--Fundamentals
### 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 [PERSON] et al. [36] and [PERSON] 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.  
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; 45; 46; 47; 48; 49; 50; 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.