# Application of Acoustic Emission Technique in Landslide Monitoring and Early Warning: A Review
## 2 Current Landslide Monitoring Methods
### Geotechnical Methods  
Geotechnical methods utilize in situ instrumentation to conduct targeted monitoring in potential failure zones identified through prior investigations. Key techniques include TDR [48], fiber optic sensing (FOS) [49], inclinometers [50], and settlement gauges [51]. TDR is employed for monitoring landslides by sending electromagnetic pulses through coaxial cables and analyzing reflected signals to detect changes caused by underground movements [52]. This technology tracks landslide activity by measuring moisture fluctuations that indicate unstable conditions. The effectiveness of TDR lies in its ability to transmit and reflect electromagnetic pulses. Over the years, TDR has evolved, with improved installation methods, data interpretation techniques, and simpler data processing approaches [53]. Additionally, it has been integrated with other monitoring technologies to enhance the accuracy and early warning capabilities of landslide monitoring. However, TDR's spatial coverage is limited by the depths and lengths of the installed cables [54]. FOS technology utilizes light signals transmitted through optical fibers to measure varying axial strains resulting from soil deformation [55]. This advanced sensing method allows for continuous strain profiling along the entire length of the sensor, effectively overcoming the coverage limitations of traditional TDR technology. By strategically placing FOS in landslide-prone areas, the real-time monitoring of soil movement and strain can be achieved [56]. These sensors detect physical changes along the optical fiber line through phenomena such as Rayleigh, Brillouin, and Raman scattering, which alter the intensity, phase, and frequency of the scattered light, providing crucial data on temperature and strain [57]. Recent studies have demonstrated that distributed FOS technology is highly effective for landslide monitoring due to its exceptional spatial and temporal resolution, offering nearly instantaneous measurement results. Moreover, FOS is increasingly utilized in long-term monitoring and early warning systems due to its lightweight design, environmental sustainability, resistance to electromagnetic interference, cost-effectiveness, and rapid data transmission capabilities [58]. Nevertheless, operational challenges related to moisture ingress and exposure to sunlight remain significant concerns that need to be addressed [59]. Inclinometers are installed within vertical casing pipes and are used to measure lateral displacements by tracking changes in the inclination of the casing [60]. However, their effectiveness is limited by the difficulty of installation in pre-existing boreholes, restricting their application to known sites. Conversely, settlement gauges are utilized to quantify vertical settlement levels, providing insights into areas experiencing excessive \begin{table}
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**Geodetic Methods** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{TLS} & Active remote sensing; adaptable to diverse field conditions; capable of generating point clouds to identify terrain changes; applicable to mapping landslides at various scales. & Lack of standardized precision metrics; scanning quality affected by incidence angles, reflectivity, and ambient light. \\ \hline \multirow{4}{*}{GPS} & Provides triangulation positioning via satellite signals; measures displacements to assess stability; offers sub-centimeter-level precision; capable of autonomous data logging. & Requires unobstructed observation of at least four satellites; limited applicability in areas with dense vegetation and deep valleys. \\ \cline{1-1} \cline{2-3}  & Active microwave remote sensing; operational in all weather conditions and at all times; does not require on-site installation; suitable for both short-term and long-term monitoring; includes models to mitigate extreme interference. & Affected by environmental pollutants; signal noise complicates data interpretation; data processing is complex. \\ \hline \hline \end{tabular}
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