# Application of Acoustic Emission Technique in Landslide Monitoring and Early Warning: A Review
## 2 Current Landslide Monitoring Methods
### Remote Sensing Methods  
Remote sensing utilizes aerial or satellite-based photogrammetric imaging platforms to delineate areas at risk of landslides [73]. Key techniques include SAR interferometry and the interpretation of optical satellite imagery. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Geophysical Methods** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{ERT} & Maps subsurface using resistivity & Influenced by temperature, pressure, differences; infers distribution with multichannel systems and modeling; identifies shear zone anomalies. & and saturation levels; measurements can be interfered with by metallic minerals and conductive fluids. \\ \hline \multirow{4}{*}{SP} & Maps natural subsurface potential fields; identifies directions of landslide & Temperature and electromagnetic fields can introduce interference; limits clear interpretation of hydrological factors. \\ \hline \multirow{4}{*}{Downhole Seismic Techniques} & Measures wave velocity in boreholes to determine elastic modulus; directly assesses subsurface stability. & Effectiveness decreases with increasing depth; limited access to potential slip zones can restrict application. \\ \hline \multirow{4}{*}{GPR} & Transmits signals for imaging, providing rapid and high-resolution profile measurements; effective for identifying subsurface features. & Signal penetration is affected by water and clay content; velocity determination may not always be achievable. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of geophysical methods. SAR is an active microwave remote sensing technology mounted on satellite platforms and capable of capturing images day and night, regardless of weather conditions [74]. It generates surface backscatter by transmitting microwave signals and measuring the distances of echoes and antenna flight bearing angles. Differential SAR interferometry (DInSAR) analyzes phase differences between interferometric SAR image pairs to map ground deformation with millimeter accuracy by comparing reference and current acquisitions [75]. However, DInSAR requires the precise co-registration of images and correction of atmospheric artifacts, which can be challenging for operational applications. Additionally, frequent imaging repeats are necessary to dynamically monitor continuously evolving slopes. Passive optical sensors acquiring multispectral data enable morphological terrain characterization through supervised classification routines assigning each pixel into distinct zones linked to terrain/slope features [76]. High-resolution commercial satellites such as IKONOS, QuickBird, and WorldView constellation facilitate detailed feature identification in stereoscopic compositions [77]. Change detection for identifying areas of significant slope gradient/stability variations through pre-/post-event comparisons is another popular application. Despite its successful applications, remote sensing faces several challenges in its role within landslide early warning systems [78]. Real-time monitoring is hindered by the intermittent revisit intervals of satellites and the availability of data acquisition, which are influenced by unpredictable factors such as weather conditions, thereby limiting the time available for preparedness [79]. Although multi-temporal data can aid in identifying past landslide occurrences, directly measuring preparatory slope deformations is often impractical. Furthermore, surface features may not accurately reflect the subsurface failure mechanisms controlled by hidden geological characteristics. In general, remote sensing enhances traditional field mapping by providing a comprehensive overview that is conducive to advanced territorial zoning, retrospective analysis of detailed landslide histories, and identification of changes following major events. However, the limitations in dynamically monitoring evolving slope movements in real-time necessitate the integration of subsurface characterization methods.