## 3 Experimental Tank Facility  
To validate the detection effectiveness of 3D cross-hole ERT for identifying boulders, a physical model experiment was conducted using a hydraulic tank. The experimental setup was meticulously designed to simulate real-world conditions, ensuring the results would be applicable to practical scenarios. The physical model, as illustrated in Figure 3, was constructed with dimensions of 2 m in length, 1.8 m in width, and 0.8 m in height. The water temperature in the tank is maintained at 20 \({}^{\circ}\)C. This sizable model allowed for a comprehensive analysis of the 3D ERT method's capability to detect boulders. Various materials were used to replicate different geological conditions, and multiple electrodes were strategically placed within the model to accurately capture the potential gradient data necessary for 3D resistivity imaging.  
The resistivity and size parameters of each physical body are shown in Table 1. The observation apparatus, depicted in Figure 4, utilizes porous electrode arrays constructed from Ethernet cables and PVC pipes. Each electrode string spans 4 cm with 16 channels. Four electrode arrays are designed and fixed with PVC pipes to maintain the consistent relative positions of the observation system. This setup ensures precise and reliable data collection by keeping the electrodes stable and correctly positioned. The use of Ethernet cables for the electrode arrays allows for robust signal transmission, while the PVC pipes provide a durable and stable structure to support the arrays throughout the experiment. By meticulously controlling the parameters and arrangement of the electrodes, the experiment aims to produce high-quality data that will facilitate a detailed analysis of the 3D ERT method's effectiveness in detecting boulders.  
Figure 3: Physical model test in hydraulic tank.  
In this physical model test, a 12 cm cubic cement block is secured with thin wires and suspended at the center of the observation system. Positioned 45 cm above the lowest electrode, its center aligns with the midpoint of a square formed by four \"boreholes\". Using the first electrode of Hole 1 as the reference point, the coordinates of the cement block's center are (0.3, 0.3, 0.3). The system operates on a 48V power supply for 0.2 s with a sampling interval of 50 ms, employing the AM-BN electrode configuration. This setup ensures that the electric current penetrates the test area effectively, enabling accurate detection of the cement block's resistivity characteristics.  
The experimental results, shown in Figure 5, reveal that the inversion results exhibit a background resistivity value of 2 \(\Omega\cdot\)m in the model. Although the anomaly center is mostly positioned as specified in the model, it displays slight lateral expansion. At the center of the high resistivity anomaly, the resistivity value measures 113 \(\Omega\cdot\)m, demonstrating the effectiveness of the 3D cross-hole resistivity imaging method in detecting high resistivity anomalies, such as isolated boulders. The experiment demonstrates the system's ability to accurately capture and image the cement block's presence within the observation area. These findings validate the effectiveness of the 3D cross-hole ERT method for detecting and characterizing subsurface anomalies, such as boulders, in practical applications.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Model** & **Material** & **Resistivity (\(\Omega\cdot\)m)** & **Model Size (cm)** \\ \hline Background & Tap Water & 1–16 & / \\ \hline Boulder & Concrete Block & 2000–2500 & 12 cm \\ \hline \end{tabular}
\end{table}
Table 1: Model parameters.  
Figure 4: Illustration of 3D cross-hole ERT configuration.