Appraisal of a galvanic-source transient electromagnetic method for hazardous water structures beyond tunnel sidewall

In tunnel construction, hazardous water structures pose significant risks. The transient electromagnetic method (TEM) is widely used for advanced geological prediction due to its sensitivity to conductive targets. Currently, loop sources are generally used to detect these structures, but the space in the tunnel limits the magnetic moment and detection distance. This article proposes using an electric wire source TEM configuration on the tunnel sidewall for lateral exploration. The feasibility of this configuration is preliminarily analyzed using 3D numerical simulations based on tensor grids. The simulations include the six electromagnetic components under the axial electric source and the effects of offset, tunnel spatial conductivity, and tunnel size on the response. The results show that the inline electric field component and the vertical-induced electromotive force component are more sensitive to the target body in this configuration. We also found that when there is a small difference in resistivity between the target body and the surrounding rock, the sensitivity of the induced electromotive force z -component to the abnormal body will be reduced. When the distance between the source and the receiver is large, the broadside-induced electromotive force component can also be used to identify anomalous body electrical properties. Moreover, the conductivity and size of the tunnel have a certain impact on the detection results. In the next research, it is necessary to select more suitable model parameters for numerical simulations based on the actual conditions.