Cross-hole high-density electrical resistivity tomography (ERT) is akin to performing a "CT scan" on the earth. By deploying dense arrays of electrode "probes" within boreholes, it "visualizes" the subsurface from multiple angles and distances. A computer then reconstructs a "2D image" of the internal structure, making it a powerful tool for solving intricate geological problems.
The objective of this project was to verify the position of an underground cut-off wall and identify potential seepage channels beneath it using cross-hole high-density ERT.
（1）The principle of cross-hole high-density ERT is similar to that of conventional surface ERT. Both involve injecting current into the ground through electrodes to detect the electrical characteristics of geological bodies. The key difference is that conventional ERT conducts current injection and data acquisition on the surface, whereas cross-hole ERT performs these operations underground within boreholes. As shown in Figure 3-1, the diagram illustrates the equipotential field generated when current is injected into the ground via electrodes in a borehole. It can be observed that the current flow paths from the cross-hole configuration are omnidirectional (360 degrees). Since electrodes can be positioned at the target depth, dense current lines are maintained even at greater depths. This ensures that the cross-hole method provides superior resolution at depth.
Before conducting cross-hole high-density ERT, corresponding downhole electrode cables must be prepared, arranged at specific intervals, and lowered into the boreholes. Data acquisition follows specific logical patterns. The schematic diagram below illustrates a typical data acquisition setup. Here, A and B are the current electrodes through which current is injected into the ground. M and N are the potential electrodes, which measure the potential difference under the artificial electric field generated by the current injection at A and B.
By switching the positions of the four electrodes (A, B, M, N) within the boreholes, a sufficient amount of data is collected. The data measured from different positions contain coupled information. Geophysical inversion methods are then used to process this data and generate a resistivity distribution map of the investigated area. A geophysical field reflects certain physical properties of geological bodies. Calculating the profile curve of such a field based on known shapes, burial depths, and physical property parameters of the geological bodies is termed the "forward problem." Conversely, determining the shapes, burial depths, and rock properties from a known geophysical field curve, as required in geophysical interpretation, is called the "inverse problem."
Presentation of 2D Profile Results from Cross-hole High-Density ERT
The inversion results of this cross-hole high-density ERT survey indicate a high-resistivity zone (>2000 Ω·m) within the depth range of 22~40 meters. A relatively low-resistivity zone (<500 Ω·m) is identified within the depth range of 40~44 meters. Based on the ERT inversion results and integration with existing materials, it can be inferred that the high-resistivity section at 22~40 meters depth corresponds to the cut-off wall. The low-resistivity section at 40~44 meters depth is inferred to be a seepage/return flow channel beneath the wall.
