## 2 Current Status of Development of Geophysical Exploration Techniques
### Exploration Techniques Based on Apparent Resistivity Parameters
#### 2.1.2 Electrical Resistivity Tomography Method  
The electrical resistivity tomography method can be categorized under resistivity profiling methods. It is a geophysical detection method based on the difference in the electrical conductivity of a subsurface medium, used to identify subsurface stratification and geological structures and search for subsurface electrical inhomogeneities by observing and studying the distribution pattern of the artificial electric field related to these differences [21]. In this method, the location of underground anomalies and hidden dangers can be visualized by laying several electrodes and continuously debugging the measuring electrodes and power supply to obtain resistivity data at different locations and then mapping these data to the imaging results. [PERSON] et al. [22] summarized the applicable conditions of the electrical resistivity tomography method and the distribution law of the apparent resistivity through engineering examples and tests. [PERSON] et al. [2] applied the electrical resistivity tomography method to detect seepage hazards in earth and rock dams, and the results showed that the leakage location detected by the electrical resistivity tomography method was largely the same as the saturation location of the field survey, proving the accuracy and efficiency of the electrical resistivity tomography method in the leakage detection of earth and rock dams. [PERSON] et al. [23] selected different devices used in the electrical resistivity tomography method to survey the same geological profile and analyzed the advantages and disadvantages of these devices. [PERSON] et al. [24] stated that in the leakage detection of trapezoidal embankments, the electrode distance should not be greater than 1.0 m to ensure the resolution of the electrical-based detection. Although the electrical resistivity tomography method is widely used, it requires a longer survey line because of the poor grounding effect of media, such as concrete and stone, in embankment leakage detection. Figure 2 shows the inversion results of the electrical resistivity tomography method for an earth and rock dam. It can be seen that there is a low-resistance zone at the burial depth of 4-10 m, which can be inferred as a leakage hazard. Figure 3 shows the electrical resistivity tomography method probe line layout for an earth and rock dam. The blue lines in the figure represent the layout of the detection profiles for the electrical resistivity tomography method. The H1 detection profile is the dam axis profile. The H2 and H3 detection profiles are the berm routes of the earth and rock dams. Figure 3: Line layout of an earth and rock dam using electrical resistivity tomography. Figure 2: Inversion results of an earth and rock dam using electrical resistivity tomography. 2.1.3. Diffusion Method Logging Exploration  
In the diffusion method logging [25], artificially salted well fluids are used to induce resistivity differences between the well fluids and groundwater, and the temporal changes in the well fluid resistivity are measured to determine the movement of groundwater and delineate the location of aquifers within the borehole and the hydraulic connection between multiple aquifers [26]. The basic principle is to utilize the diffusion phenomenon of groundwater in special solutions with different resistivities. The resistivity curve of the well fluid was first measured with a resistivity meter. The change in resistivity of the well fluid was measured at intervals after the well fluid was uniformly salted. The temporal changes in the resistivity of these special solutions are monitored to determine the movement of groundwater, and the location of aquifers and the hydraulic connections between multiple aquifers are delineated. An artificial salinization fluid is used in most of the measurement methods to replace the original well fluid. Salted well fluid and groundwater have evident differences in their resistivity.