Detection of underground tunnels with a synchronized electromagnetic wave gradiometer

Cross-border tunnels have been used by drug, people smugglers and terrorist organizations for clandestine entry or exit and transportation of contraband materials under the borders. The ability to detect these tunnels is paramount to successful border control. The Synchronized Electromagnetic Gradiometer uses the enhanced conductivity associated with tunnels, as compared to the surrounding medium, to detect the tunnels. A low-frequency electromagnetic (EM) signal is used to illuminate the area of interest. This signal, in turn, induces current flow in any conductors within the tunnel that generate secondary EM fields observable at a distance from the tunnel. The magnitude of the secondary wave can be orders of magnitude less than the illuminating signal. An efficient detection system has been achieved by using a gradiometer design that suppresses the illuminating signal by more than 70 dB while maximizing the secondary signal with a narrow bandwidth (BW = 1 Hz) synchronized receiver. This paper describes the performance of the Synchronized Electromagnetic Wave Gradiometer during several field studies and demonstrations including the Otay Mesa cross-border tunnel near San Diego, California.