# Application of Acoustic Emission Technique in Landslide Monitoring and Early Warning: A Review
## 3 Fundamentals of Acoustic Emission Technique
### Parameters of Acoustic Emission Signals  
AE signals consist of transient elastic waves that propagate through materials as a localized response to rapid stress and strain energy releases associated with irreversible damage events occurring internally. As non-stationary stochastic signals, AE signals are primarily analyzed through the extraction of parameters in the time, frequency, and time-frequency domains for empirical quantification [98]. In the time domain, essential features of AE are characterized by the detected waveforms and envelopes, as illustrated in Figure 5 and summarized in Table 6.  
Moreover, advanced time-frequency domain analyses using wavelet transforms allow for the simultaneous extraction of signal components across a broad frequency range, capturing changes in dominant frequency contributions over time [99]. This technique effectively tracks the dynamic evolution of the dominant frequency in the time dimension, presenting a comprehensive and accurate portrayal of the signal's frequency characteristics at different moments. This provides crucial data support for studying the developmental patterns and internal mechanisms of the signal. [PERSON] et al. have validated the effectiveness and reliability of this method in their research on acoustic emission signals generated during the melting and crystallization of granites [100]. Compared to Fourier analysis alone, this approach is particularly adept at revealing temporal variations in signals \begin{table}
\begin{tabular}{c c c} \hline
**Parameter Name** & **Definition** & **Function or Significance** \\ \hline Amplitude & \begin{tabular}{c} The intensity of the peak signal, measured \\ in decibels relative to a reference value or \\ in volts, depending on sensor \\ characteristics. \\ \end{tabular} & \begin{tabular}{c} Represents the energy released by the seismic \\ source and the degree of attenuation during \\ the propagation process. \\ \end{tabular} \\ \hline Rise Time & \begin{tabular}{c} The time interval from when the signal \\ first exceeds the initial threshold to when \\ it reaches peak amplitude. \\ \end{tabular} & \begin{tabular}{c} Reflects the rate of deformation energy \\ release; a short rise time indicates a sudden \\ release of energy (e.g., brittle intergranular \\ fracture), while a longer rise time suggests a \\ more gradual process, such as ductile \\ yielding. \\ \end{tabular} \\ \hline Count & \begin{tabular}{c} The number of times the signal amplitude \\ exceeds a preset threshold level within a \\ user-defined time window. \\ \end{tabular} & \begin{tabular}{c} Affected by the amplitude and frequency of \\ the wave; serves as a proxy for the amount of \\ energy released. \\ \end{tabular} \\ \hline Duration & \begin{tabular}{c} The time interval from the first instance \\ the signal exceeds the threshold to the last \\ instance it does. \\ \end{tabular} & \begin{tabular}{c} Represents the total duration of the signal \\ packet when a discrete event is detected; a \\ longer duration indicates contributions from \\ multiple sources during the event. \\ \end{tabular} \\ \hline Energy & \
\begin{tabular}{c} A parameter obtained by integrating the \\ square of the instantaneous voltage signal \\ over the entire duration of the event. \\ \end{tabular} &
\begin{tabular}{c} Quantitatively represents the total magnitude \\ of energy released during the event. \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 6: Parameters of acoustic emission signals. Figure 5: Parameters defined for a typical AE waveform. with non-stationary characteristics. Parameters such as wavelet energy, cross-correlation coefficients, and standard deviation are valuable for quantitatively comparing signals and serve as references for qualitative interpretations [101]. Empirical observations have demonstrated distinct correlations and sensitivities among various AE parameters in relation to different failure mechanisms in soils and rocks [102].