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Researchers use fiber optics for tunnel detection

Posted: 14 Jan 2009 ?? ?Print Version ?Bookmark and Share

Keywords:fiber optic? Gaza tunnel detection? optical cable?

Technion-Israel Institute of Technology researchers claim that detecting tunneling activity under the security fence separating Gaza from Egypt could be accomplished through a buried fiber-optic cable.

By combining a neural network with Brillouin optical time-domain reflectometry, small tunnels can be sensed by the buried optical fibers, researchers claimed. "We have proven that our technique works in the lab," said principal researcher Assaf Klar, an engineering faculty member at Technion. "We are ready to cooperate with the [Israeli] army now for field testing of our buried optical cables on the Gaza-Egypt border."

The technique was originally developed to monitor the structural stability of cable tunnels. It measures optical-fiber strain by sending a continuous stream of light pulses into the fiber and measuring the scattered light reflected back down the fiber. By applying frequency spectrum analysis, it is possible to determine the precise location of any distortions.

The reflectometry technique is based on the property that the frequency spectrum of scattered light shifts in proportion to the amount of physical distortion in the fiber.

To detect tunneling activity, the researchers constructed an artificial neural network and trained it to reject all but human digging activities. Their technique can detect vibrations in the soil as small as those caused by falling rain. By training the neural network to only respond to digging activities, the researchers said they were able to tune the system to reject false alarms.

Using fiber-optic cables as long as 15 miles in length, a single device can inject the test signal and then measure its reflected distortions using wavelet decomposition of the continuous signal. After filtering out irrelevant signals, the features detected are submitted to the neural network which determines if the resultant signature indicates tunneling. Thousands of signatures were used to train the neural network to distinguish between distortions caused by tunneling and soil distortions caused by other sources.

The researchers will present the details of their results at an industry conference in April.

- R. Colin Johnson
EE Times

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