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  • An underground fiber optic fence

Photonics Spectra
Mar 2009
Hank Hogan,

HAIFA, Israel – Tunneling to circumvent borders could be a thing of the past, thanks to a fiber optic-based technique developed here. Researchers Dr. Assaf Klar and Dr. Raphael Linker of Technion, the Israel Institute of Technology, have devised a system that monitors buried fiber for the telltale soil displacements associated with tunneling activity.

By monitoring tiny soil displacements along a fiber optic cable, researchers say they can detect tunneling performed by smugglers. Image courtesy of Technion, the Israel Institute of Technology.

The technology can detect tunnels many feet below ground, with the limit dependent upon tunnel size and the distance between fiber and tunnel. “But when the distance is smaller than about 20 meters, the detection rate is high, regardless of tunnel size,” Klar said.

The method makes use of Brillouin optical time-domain reflectometry, a fiber optic strain-monitoring technology that measures the Brillouin shift of the backscattered light wave. This shift is affected by longitudinal distortion along a cable, which, in turn, is affected by strains in the fiber arising from interaction with the surrounding soil. The system delivers continuous information from the fiber, and a single device can monitor up to 30 km of fiber optic line. That makes for low-cost detection because the cost of standard single-mode communication fiber runs a fraction of a dollar per meter and one-third that per foot.

However, the data from the analyzer, by itself, does not distinguish aboveground activities such as rain or soil loading from underground ones; e.g., tunneling. To make the distinction, the researchers decomposed the complex measured signals into simpler wavelets. They used analytical solutions and numerical simulations to train an automatic detection system based on a neural network, training and validating it with about 50,000 simulations. The resulting system requires no special expertise to operate and does not need a constant presence of personnel along the perimeter, unlike alternatives such as ground-penetrating radar.

The scientists adjusted the neural net to avoid false alarms, a key criterion if the system is to be implemented in a dangerous situation. The trade-off is a greater likelihood of not detecting a tunnel.

In practice, the fiber fence could be arranged in two configurations: buried at a shallow depth of a meter or so, perhaps running parallel to an aboveground fence, or buried in shafts, separated by less than 20 m.

According to the researchers, a prototype tunneling detection system could be deployed within a few months. Moreover, the technique could be used for other applications after refinement and further development. The researchers are investigating some of these alternative uses.

“We have been working on a similar system to detect sinkhole development, which is a major problem in the Dead Sea area,” Klar said.

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