# Application of Acoustic Emission Technique in Landslide Monitoring and Early Warning: A Review
## 2 Current Landslide Monitoring Methods
### Geophysical Methods  
Geophysical techniques leverage physical field properties to characterize subsurface conditions in a non-invasive manner, offering significant advantages for landslide investigations [66]. Common applications of these methods include ERT, self-potential (SP), GPR, and downhole seismic techniques. ERT utilizes variations in electrical resistivity among geological materials to map subsurface structures and properties related to slope instability processes [67]. Resistivity distributions are inferred from surface electrode measurements, facilitated by multichannel systems and inversion modeling. Low resistivity anomalies often indicate developing shear zones due to increased fracturing and pore fluid saturation [68]. However, resistivity values can be influenced by various factors, including temperature, pressure, and the saturation levels of pore fluids. Additionally, the presence of metallic minerals or conductive fluids can adversely affect resistivity measurements. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Geotechnical Methods** & **Advantages** & **Disadvantages** \\ \hline TDR & \begin{tabular}{c} Monitors via electromagnetic pulses; tracks \\ moisture fluctuations; mature technology; \\ integrable with other systems; reliable for \\ long-term monitoring. \\ \end{tabular} & \begin{tabular}{c} Limited spatial coverage based on \\ cable length and depth; installation \\ can be challenging in rugged terrains. \\ \end{tabular} \\ \hline FOS & \begin{tabular}{c} Measures strain using optical signals; allows \\ continuous strain profile analysis; enables \\ real-time monitoring; offers high spatial and \\ temporal resolution; widely applied due to \\ multiple advantages. \\ \end{tabular} & \begin{tabular}{c} Vulnerable to moisture ingress and \\ damage from UV exposure; initial \\ setup costs can be high. \\ \end{tabular} \\ \hline Inclinometers & Effective for measuring lateral displacements. & \begin{tabular}{c} Challenging installation; limited \\ application; unable to monitor \\ internal changes underground. \\ \end{tabular} \\ \hline Settlement Gauges & \
\begin{tabular}{c} Effectively quantifies vertical settlement; \\ straightforward installation and operation. \\ \end{tabular} &
\begin{tabular}{c} Limited to measuring vertical \\ movements; unable to monitor lateral \\ or internal changes. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of geotechnical methods. SP techniques map natural subsurface potential fields generated by fluid and mineral interactions or thermal and electrokinetic coupling associated with slope deformation [69]. This method has been employed to identify preferred directions of landslide movement and areas of seepage. However, temperature variations and electromagnetic field influences can introduce contaminants, complicating the interpretation of SP responses related solely to hydrological factors. Downhole seismic measurements involve profiling compression and shear wave velocities through boreholes, allowing for a direct assessment of elastic modulus variations within unstable rock and soil masses at different depths [70]. Nonetheless, the effectiveness of this method diminishes with increasing depth due to amplitude attenuation in fractured or weathered zones, where fractures can facilitate weakening. Furthermore, limited access to potential slip zones can restrict the applicability of this methodology. GPR transmits short-pulse electromagnetic radar signals into the ground to image subsurface features through reflected wave amplitude maps [71]. GPR offers rapid, high-resolution stratigraphic profiling; however, signal penetration can be significantly impaired by water and clay content in the subsurface. The accurate determination of velocity is essential for converting signal data to depth scales, but this is not always feasible under all conditions. Integrating geophysical techniques with geotechnical and geodetic investigations enhances the characterization of landslides through a unified interpretation approach.