## 5 Methodology and Field Survey  
In this study, the 2D electrical resistivity imaging method is used. The essence of the electrical resistivity imaging instrument is that different frequency signals of special waveforms are sent and received simultaneously to investigate the electrical properties of the subsurface by measuring the resistivity distribution of the materials. The electrical resistivity of the materials depends on a combination of ohmic and dielectric effects related to the lithology of the subsurface [26]. The apparent resistivity, \(\rho_{\alpha}\), is calculated based on Equation (1).  
\[\rho_{\alpha}=k\frac{V}{I} \tag{1}\]  
where  
\(V=\) voltage  
\(I=\) current  
\(k=\) geometric factor  
The depth of penetration depends on the current injected subsurface materials and is also dependent on the \(k\) value for the distance between the current electrodes. In this survey, the electrical resistivity imaging measuring instrument employed is the SQ-3C portable, and its work frequency is in the range of 4-10 Hz. The distance between the electrical resistivity sounding points is 20, 30, and 40 m, and an infinity electrode distance from the nearest measuring point is greater than 800 m. A total of 6 survey lines were laid out; the survey lines were numbered L1 to L6. All physical points are measured using RTK dynamic real-time kinematics; the survey lines layout is shown in (Figure 4). The electrodes used for measurement consist of a single copper electrode. The powered electrodes consist of four or five parallel groups of aluminum foils. A 1-kilowatt (kW) small power generator, along with a rectifier source, was employed to provide power to the equipment. Figure 5 displays the topographic variations in the study area.  
Figure 4: Survey lines layout map of the study area.  
The selection of array configuration, electrode separation, and frequency allocation plays a crucial role in accurate data acquisition. The forward and inverse pole-dipole array was employed in our survey (Figure 6), a widely utilized method for conducting measurements in both profile and area exploration. In this setup, the forward measurement configuration involves the current electrode at location C\({}_{1}\), the near-offset distance na between C\({}_{1}\) and the first potential electrode P\({}_{1}\), and the potential electrodes P\({}_{1}\) and P\({}_{2}\) where the voltage is measured. The inverse measurement configuration has the potential electrodes P\({}_{1}\) and P\({}_{2}\), the dipole spacing a between them, and the near-offset distance na between the second potential electrode P\({}_{2}\) and the current electrode C\({}_{1}\). The dipole spacing a refers to the distance between the two potential electrodes P\({}_{1}\) and P\({}_{2}\). This forward and inverse measurement approach helps gather comprehensive data to better understand the subsurface resistivity and identify groundwater features. Two-dimensional electrical resistivity imaging enhances data resolution at different depths, allowing a comprehensive subsurface analysis. We employed IPINV 3.2 geophysical software for data inversion developed by Central South University [27]. Before inversion, we quality-checked the data to remove noise. Inversion incorporates multiple information sources to refine potential models and match actual environmental conditions. Multiple inversion techniques are applied iteratively to minimize model-data discrepancies.  
Figure 5: Topographic map of the study area.  
Figure 6: Forward and inverse pole-dipole array setup modified after [28].