Water-Bearing Body Prospecting Ahead of Tunnel Face Using Moving Electrical-Source Method

Geological hazards such as water inrush often occur during tunnel excavation, which always cause economic loss and construction delay. Predicting the water-bearing structures ahead of tunnel face in advance would reduce these risks substantially. Electrical resistivity is considered as an effective technique to detect water-bearing bodies because of its sensitivity to low-resistivity body. However, abnormity of observed data caused by anomalous body below tunnel bottom or behind tunnel wall, called vicinal interference, is always more serious than that ahead of tunnel face. So the first problem is to design one observe configuration to prompt the sensitivity to the anomalous body ahead of tunnel face. A moving electrical-source method (MESM) is proposed in this study, in which source electrodes move along the survey line, and array of measure electrodes are fixed on the tunnel face. Numerical forward modeling examples show that the abnormality degree of MESM data caused by vicinal interference is much lower than that of a traditional observed mode. Electrical resistivity inversion method with inequality constraints is then presented to reduce non-uniqueness and instability. And prior information involving variation range of material resistivity, expressed as a mathematic constraint, is introduced into traditional resistivity inversion equation. Numerical inversion tests are carried out on several geological models and the imaging features, inversion quality and limitation are analyzed and summarized. The test results from both physical model and field experiments the water-bearing body prospecting utilizing MESM has acceptable error and satisfies the accuracy demand in geological ahead prospecting in tunnels.