## 5 In Situ Evaluation Methods  
### Leakage Evaluation Based on Resistive Chromatography Imaging Technique ERT  
Traditional detection methods for vertical cut-off wall leakage, such as excavation and drilling, are time-consuming and costly. As an alternative, the non-destructive testing technique known as electrical resistivity tomography (ERT) is often employed. ERT provides 2D and 3D images of resistivity changes (which are inversely proportional to conductivity) based on electrodes placed on the ground, allowing for the assessment of the resistance or conductivity distribution within the cut-off wall [121]. However, while ERT can detect wall seepage and predict necessary repairs, it may not accurately locate seepage points on its own and often needs to be used in conjunction with other geophysical methods.  
[PERSON] et al. [11] utilized ERT alongside surface geophysical surveys (SRT) to compare subsoil stratigraphic imagery with an initial geological model, thereby enhancing the effectiveness of seepage repairs in a dam's wall. Similarly, [PERSON] et al. [122] innovatively combined ERT with SRT to improve seepage repair effectiveness in dams. Additionally, [PERSON] et al. [123] integrated ERT with induced polarization (IP) methods to detect cut-off wall leakage  
Figure 13: Influence of environmental parameters on expansion index at different numbers of freeze–thaw cycles [120]. (**a**) Variation in expansion index with salt concentration. (**b**) Variation in expansion index with temperature. (**c**) Variation in expansion index with PH.  
and internal erosion. [PERSON]'s team [124] employed ground-penetrating radar (GPR), ERT, thermal infrared (TIR) imaging, and geotechnical in situ measurements to diagnose dams, finding that this combined approach was effective in identifying seepage issues.  
Despite the widespread application of geophysical detection techniques for landfill structures and leakage problems, accurately determining the location of leakage in vertical containment walls remains challenging [124]. [PERSON] et al. [125] investigated specific subsurface zones to locate seepage using ERT. [PERSON] et al. [126] combined geophysical methods with traditional water chemistry and hydrological techniques, utilizing surface and water ERT to scan reservoirs and identify seepage areas [127; 128]. Additionally, drilling process monitoring systems, borehole resistivity tomography, borehole sonic logging, and water injection testing are suitable for detecting potential leakage locations in grouted cut-off wall walls in karst areas. While ERT is commonly used to detect general seepage issues, a combination of ERT, time-domain induced polarization (TDIP), very low frequency (VLF), frequency-domain electromagnetic (FDEM), time-domain electromagnetic (TDEM), seismic refraction (SR), and seismic tomography (ST) methods are required for detecting and locating seepage of heavy metal-contaminated fluids [129; 130]. The resistive tomography imaging technique ERT is primarily applied to detect localized leakage points, and further investigation is needed to understand the correlation between conductivity and the service life of the cut-off wall.