The first semi-airborne transient electromagnetic survey for tunnel investigation in very complex terrain areas

We present an unmanned aerial vehicle (UAV) based semi-airborne transient electromagnetic method (SATEM) for tunnel investigation in tunnels constructed in complex terrains. To the authors' best knowledge, this is the first report on such application. Tunnel investigation must be carried out before its construction, but in some areas, it is extremely challenging due to the complex terrains. To this end, we aim to develop a semi-airborne geophysical method for such cases. Specifically, a grounded-wire source is placed on the surface transmitting an electromagnetic field into the underground. An induction coil is carried by a small UAV in the air to acquire data on top of the tunnel. Low resistivity targets near the tunnel are induced by the transmitter and pure secondary fields are produced with relation to the size and location of the targets. The information is collected by the coil to invert the subsurface resistivity profile. We conducted our first experiment in the Damo Tunnel in Guangxi, China. Here we present the design of this method and the workflow for tunnel investigation. The prospecting results are compared with the actual results exposed by excavations, showing a good agreement. This first attempt of SATEM shows great potential for tunnel investigation in complex terrain areas.