## 3 Methodology  
Electrical Resistivity Tomography (ERT) technology is based on the conductive theory of porous media. This method involves injecting direct current (DC) into the ground through two electrodes (current electrodes) and measuring the potential drop between two other electrodes (potential electrodes) at different locations to calculate the subsurface resistivity distribution [46]. The apparent resistivity (\(\rho_{\text{a}}\)) is then calculated using the aforementioned measured values (I, \(\Delta\)V) and a geometric factor (K) related to the electrode configuration (Equation (1)).  
\[\rho_{\text{a}}=\frac{\Delta\text{V}}{\text{I}}\bullet\text{K} \tag{1}\]  
The apparent resistivity (\(\rho_{\text{a}}\), measured in ohm-meters, \(\Omega\cdot\)m) is determined by the transmitted current (I, measured in amperes, A), the measured potential difference caused by the current (\(\Delta\)V, measured in volts, V), and the geometric factor (K, measured in meters, m).  
Measurements can be conducted along a line (two-dimensional subsurface geometry) or a grid (three-dimensional subsurface geometry). Several electrode arrays are widely used for various purposes, such as Wenner, Schlumberger, dipole-dipole, and multiple gradient pole-dipole configurations. The Schlumberger array offers higher spatial resolution for both horizontal and vertical variations in resistivity. For the dipole-dipole array, the Depth of Investigation (DOI) depends on the distance between the current dipole and the potential dipole, whereas, for the Schlumberger array, it depends solely on the length of the current dipole. For the dipole-dipole and Schlumberger arrays (where L is the profile length), the DOI is approximately 0.2 L and 0.12 L [47], respectively. The DOI is also influenced by the level of ambient electrical noise. High noise levels can reduce the signal-to-noise ratio. In this experiment, a Wenner electrode array is selected for detection because a Wenner array can better depict horizontal characteristics and is more conducive to identifying the scale and characteristics of underground cavities.  
Based on a comprehensive analysis of the on-site geological conditions and previous geological survey data, this study designed five ERT survey lines, oriented from NNE to SSW, with the lines labeled L1, L2, L3, L4, and L5 sequentially from west to east (Figure 1b). The directions of all five lines are consistent with that of the expressway and are parallel to each other, with a spacing of 10 m between each line. The initial design length of each measurement line was 900 m; however, due to site construction conditions and the influence of the road along the Yuanjiang River, there were some deviations from the design length. Specifically, Line 1 has a length of 867 m, Line 2 is 855 m, Line 3 is 867 m, Line 4 is 873 m, and Line 5 is 858 m. The survey utilized the WDJD-2 DC resistivity and induced a polarization system developed by the Pentium Numerical Control Research Institute, employing the Wenner electrode array for field data acquisition. The electrode spacing was set at 3 m. After collecting the measured data from the five survey lines, the Res2 dinv software (version 3.54.44) was used to perform an inversion and interpretation of the data, resulting in five high-resolution two-dimensional cross-sectional profiles.