Influence of Electrical Anisotropy on Apparent Resistivity Responses in Tunnel Advance Detection: A Three-Dimensional Forward Modeling Study

Water-bearing faults pose critical hazards in tunnel excavation due to sudden water inrush, making reliable advance detection essential. This study presents a systematic forward modeling framework that integrates COMSOL Multiphysics and MATLAB to simulate three-dimensional direct current (DC) responses ahead of the tunnel face. A three-dimensional finite-element model was developed to investigate the influence of electrical anisotropy on apparent resistivity under controlled geological conditions. Electrical anisotropy of both surrounding rock and water-bearing faults is incorporated to evaluate its influence on apparent resistivity. Numerical experiments investigate the effects of surrounding-rock and fault anisotropy and reveal the mechanism behind hourglass-shaped low-resistivity anomalies. The results also reveal systematic biases when anisotropy is neglected, including forward-shifted anomaly positions, overestimated lateral extents, and more diffuse anomaly boundaries. When anisotropy is considered in both the surrounding rock and the fault, the simulated anomaly closely matches the preset fault location, demonstrating improved localization accuracy. The modeling results clarify the effects of key parameters on apparent resistivity responses and improve the interpretation of low-resistivity anomalies. They also provide a theoretical basis for enhancing the reliability of DC resistivity-based tunnel advance detection.