## 2 Materials and Methods
### Methods
#### 2.2.1 Electrical Resistivity Tomography Method  
Electrical resistivity tomography (ERT) is a geophysical exploration technique method widely used to image the shallow subsurface by measuring differences in the electrical conductivity of the subsurface. Evolved from the traditional direct current (DC) resistivity method, ERT addresses engineering and geological challenges by mapping the distribution of the subsurface electric field. During a survey, a linear array of electrodes is placed along the ground in the area of interest. A typical measuring unit consists of four equidistant electrodes labelled A, M, N, and B. Electrodes A and B serve as current injectors, while electrodes M and N measure the resulting voltage to calculate apparent resistivity. The electrodes are systematically moved along the survey line to collect data across the entire profile until the measurement is complete.  
As shown in Figure 3, this survey arranged 2 groups of electrical resistivity tomography survey lines, a total of 5 sections, 1 of which was nearly parallel to G1 and G2 arranged on the side of the road by the pond, with a distance of about 10 m between the 2 survey lines, to detect possible water diversion channels, and the other set of survey lines G3-G5 were laid along the alley along the inside of Tianmeixin village, and the extension direction of the subway was obliquely intersected to detect the overall distribution of karst in the profile. The detection equipment uses the EDGMD-60 electrical resistivity tomography measurement system developed by Chongqing Dingfeng Geological Exploration Instrument Co., Ltd. (Chongqing, China), and the basic composition of the equipment is shown in Figure 4A. Five measuring lines use 96 measuring electrodes, the electrode spacing is 2 m, the measuring line length is 192 m; due to the limitation of the surface cement pavement, detection using clay-wrapped non-polarized electrodes is used instead of copper electrodes. Before the measurement of the whole circuit of each electrode on and off and grounding resistance tests, the electrode that does not meet the standard is poured with salt water and undergoes electric drilling to increase the buried depth and other treatment to reduce noise. The power supply voltage is 400 V during detection, and the Wenner measuring device is used for data acquisition, with a minimum isolation factor of 1, a maximum isolation factor of 31, a maximum measurement depth of 62 m, and 1584 data points for each profile.  
For the collected apparent resistivity data, after data import, format conversion, and pre-processing, relevant software is used to calculate and process Zodi inversion, the true resistivity distribution at a certain depth underground is obtained, and the resistivity profile contour map is made. Finally, the geological and geophysical interpretation is carried out, and the karst cave anomaly area and the possible water conduction structure area are circle-drawn.  
Figure 4: (**A**) Electrical resistivity tomography data acquisition. (**B**) Cross-well seismic computed tomography data acquisition.  
Figure 3: The electrical resistivity tomography method and cross-well seismic computed tomography method survey line layout in the study area.