Tunnel face-borehole transient electromagnetic method and its physical experimental studies

In the tunnel surface-borehole configuration,the receiver is put into the borehole based on the advanced drilling on the tunnel face. With the data acquired in different depths in the borehole,the depth to time profile of tran- sient electromagnetic responses can be obtained to determine the position of the water bearing structures. The physical modeling results show that no valid signals are observed in depth with a small magnetic moment excitation,while the acquired electromotive force are direction reversal in deep and late time with a large magnetic moment excitation. This phenomenon can be used to identify the abnormal bodies. The acquired profiles are obvious different on amplitude val- ue and the reversal time with high or low resistivity target exist. This method will have no additional cost with existing borehole and is very fast in acquiring data. It can improve the accuracy in identifying the disaster sources.