# Application of Acoustic Emission Technique in Landslide Monitoring and Early Warning: A Review
## 2 Current Landslide Monitoring Methods  
### Geodetic Methods  
Geodetic methods are surface movement monitoring techniques that can be applied over extensive areas, making them particularly useful for landslide investigations. These methods offer the advantage of being able to map large zones and have the potential for satellite-based applications [33]. Three prevalent geodetic techniques include TLS, global positioning systems (GPS), and SAR.  
TLS is an active remote sensing method that captures surface topographic data across diverse field conditions [34]. By generating point clouds that represent the geometry of the entire scene, TLS enables the differentiation of terrain changes over time. However, the lack of standardized metrics for precision and accuracy can limit the reliability of the results [35]. Additionally, factors such as incidence angles, surface reflectivity, and ambient light variations can significantly influence scanning quality [36]. Despite these limitations, TLS has proven effective for mapping landslide-induced terrain modifications across various scales, from local to regional levels [37; 38].  
GPS provides spatial point positioning by triangulating signals from orbiting satellites. This technique measures displacements to assess slope stability by periodically determining the coordinates of reference markers placed within the hazard zone [39; 40]. Key strengths of GPS include accuracy levels down to the sub-centimeter range and the capability for autonomous data logging [41]. However, the requirement for an unobstructed view of at least four satellites poses challenges in areas with dense vegetation or deep valleys, which can limit its applicability [42].  
SAR functions as an active microwave remote sensing tool that captures terrain echoes in terms of sensor-target distances and azimuth directions during platform flight [43]. This non-contact imaging radar operates effectively under all weather conditions and at any time of day, making it ideal for both short- and long-term slope monitoring without the need for on-site instrumentation [44]. To address atmospheric phase screen effects encountered during monitoring in steep mountainous regions, [PERSON] and [PERSON] [45] proposed a semi-empirical model. However, environmental contaminants, such as atmospheric heterogeneities, can introduce additional noise into the signal [46]. This complicates data processing and interpretation, necessitating advanced techniques to enhance the reliability of SAR data for accurate slope monitoring [47].  
In summary, while geodetic methods are effective for mapping landslide shapes and identifying surface movements, they are not suitable for the continuous, real-time monitoring required for effective early warning systems. In complex landslides where failures extend deep underground, these methods provide incomplete information. Furthermore, the high costs and operational requirements associated with establishing geodetic networks in remote or mountainous areas present additional challenges. These methods are also limited in their ability to detect small deformations that may indicate potential slope issues before they escalate, as shown in Table 1.