Detection of clandestine tunnels in complex environments using a mobile focused-source electromagnetic data measurement and processing unit

We have developed a new technology for detecting underground tunnels - the Tunnel Detection Focused-Source Electromagnetic (TD-FSEM) method. It uses four horizontal electric dipole transmitters and a five-electrode grounded quadrupole receiver to measure the transient EM field. Such a setup directs the current under the receiver vertically, increasing the sensitivity of the measurement system to a relatively narrow column of subsurface media directly below the receiver. Our previously published feasibility modeling results allowed us to prove the concept by showing that the method provides data sufficient for reliable detection of clandestine tunnels embedded in a homogeneous subsurface. In this paper, we present a 3D EM modeling results showing that our method can be efficiently used in presence of near-surface conductive or resistive obstructions. We present comparisons of GPR, conventional dipole-dipole Controlled-Source EM (CSEM), and our TD-FSEM methods and show that the TD-FSEM, unlike conventional GPR and CSEM, allows for removal of unwanted shallow/near-surface masking effects. The TD-FSEM acquisition and processing unit can be mounted on a vehicle performing large-scale regional and local operations.