## 2 Research Area and Methodology
### Working Principles of High-Density Electrical Resisitivity Tomography  
This investigation is based on electrical resistivity methods, with the primary principle for detecting organic contaminant leakage as follows: In oil-contaminated soil samples,the current conduction pathway can be considered as a composite system comprising soil particles, pore water, and oil. Since the resistivity of oil is significantly higher than that of water, an increase in oil saturation leads to greater occupation of pore spaces by oil, potentially blocking water-dominated conductive pathways, thereby elevating the bulk resistivity of contaminated soil [35]. Petroleum hydrocarbon contamination induces two key alterations: (1) a modification of the pore fluid composition and a restructuring of the double layer at soil-particle interfaces, and (2) resistivity variations caused by differences in the dielectric constant and electrical conductivity between petroleum products and pore water. Although soil resistivity is influenced by multiple factors--including water saturation, oil saturation, porosity, temperature, soil type, salinity, and organic content--the dominant controlling parameters follow this hierarchical order: water content, oil content, and porosity. Electrical exploration methods can be divided into conductive electrical methods and inductive electrical methods. This investigation primarily adopts the high-density resistivity method (HDRM) from the conductive category. As a development of conventional electrical exploration techniques within the DC resistivity domain, HDRM fundamentally relies on electrical property differences in geological media to study the distribution patterns of subsurface conduction currents under applied electric fields [36], as shown in Figure 2. Compared to traditional electrical methods, HDRM is characterized by a high data volume. Through programmable electrode switchers controlled by microcomputers for automatic selection of current injection (A/B) and potential measurement (M/N) electrodes, this method achieves efficient data acquisition capable of rapidly collecting substantial raw data. It demonstrates high measurement accuracy, an extensive data collection capacity, rich geological information content, and superior production efficiency. A single-electrode deployment completes both vertical and horizontal 2D exploration processes, simultaneously reflecting lateral electrical property variations of subsurface media at specific depths and providing vertical electrical characteristic changes of stratigraphic lithology, thereby integrating the detection capabilities of both electrical profiling and electrical sounding methods. Assuming a homogeneous subsurface medium with resistivity (\(\rho\)), when an electric current (\(I\)) is injected and a potential difference (\(\Delta V\)) is measured, the apparent resistivity (\(\rho_{\text{a}}\)) is expressed as follows:  
Figure 1: The testing site in Baoshan district Shanghai. \[\rho_{\rm a}=k\frac{\Delta V}{I} \tag{1}\]  
where \(K\) represents the geometric factor determined by the electrode configuration [37]. Common electrode configurations in high-density resistivity surveys include the Wenner array, dipole-dipole array, and gradient array. The advantages and disadvantages of these configurations have been extensively analyzed in previous studies [38; 39; 40]. This study employs a four-electrode configuration, comprising current injection electrodes (A and B) and potential measurement electrodes (M and N), as illustrated in Figure 2.  
In a homogeneous half-space medium, when current injection electrodes A and B transmit an electrical current into the subsurface, the potential difference (\(\Delta V\)) measured between electrodes \(M\) and \(N\) allows for the calculation of the medium resistivity.