A Method to Extract and Eliminate TEM Interference by Metallic Bodies in Tunnel Geological Anomaly Forecast
The transient electromagnetic (TEM) method is a geophysical exploration technique used in a wide range of applications such as mineral exploration, hydrogeology surveys, and underground geological mapping. In recent years, it has been applied in tunnel construction to forecast geological anomalies in front of the excavation face. However, the TEM response can be affected by metallic objects such as excavation carts and metallic support structures, which makes interpretation more difficult. The purpose of this study is twofold: to introduce a new method for extracting and eliminating TEM interference from metallic bodies in a tunnel excavation environment and to gain insight regarding the influence of metallic objects on the TEM response by physical modeling in the laboratory and studies conducted in the field. The concept of resistive limits is introduced; these limits can be used as eigenvalues in the inversion process, thus achieving more accurate identification of both the location and size of ball-shaped, low-resistivity anomalies (gouge materials) in surrounding rock that has high resistivity. Both the physical laboratory simulations and the field experiments provide insight on the interference effect of metal objects such as a steel arches or excavation carts on the TEM response. Field experiments inside a tunnel indicate that the maximum distance between the excavation cart tested and the tunnel face that can influence the TEM response is 30 m, while the maximum influence distance of a steel arch is 1 m in the vertical plane of the coil and 2 m in the axial direction. The proposed inversion process was shown to be capable of extracting the interference of metallic objects from TEM responses and improve the accuracy of TEM interpretation in identifying geological anomalies.