Locating Tunnel Excavations

An illustration of the proposed design (Credit:Technion Institute of Technology)

“Smuggling tunnels enable uncontrolled movement of people, drugs, and weapons and are a security threat to Israel,” the scientists said earlier this month. “Recent advances in distributed strain [an indicator of distortion] measurements using fiber optics enable the development of smart, underground security fences capable of identifying and locating tunnel excavations. Tunnel excavation is accompanied by the release of stresses that cause permanent displacements and strains – although very tiny – in the ground. By measuring the developed strains in the soil with sensitive equipment, one can find the tunnel’s location.”

The scientists hope that their research will provide a solid groundwork for a future development of an “underground fence” which will be based on BOTDR (Brillouin optical time-domain reflectometry) technology. Originally developed by the Japanese NTT Group, this technology measures the longitudinal distortion taking place in an optical fiber and features measurement of distortion occurring in any part of the entire length of more than 10 km. Effective not only for measuring incipient distortion in soil, the technology can also be used to monitor concrete and large steel structures including river banks, cable tunnels, road or rail tunnels, bridges, and industrial plants.

BOTDR – Fiber-optic strain-monitoring technolog (Credit: Nippon)

The technology is based on an ordinary optical fiber used as a sensor that is fixed to the monitored object; any distortions in which are instantly transmitted to the sensor. The measuring device launches light pulses into the optical fiber, receives the scattered light reflected in the fiber, and applies frequency spectrum analysis to determine the existence and location of distortion – making use of the fact that frequency spectrum of the scattered light shifts in proportion to the amount of distortion. According to NTT, the main challenge in applying their technology to practical use is the necessity to develop a variety of optical fiber sensors, each of which should be optimized for the specific object it monitors. The company says it has already developed durable optical fiber sensors optimized for concrete structures and river banks, as well as having applied the system in the hull of a yacht in order to detect hidden damage.

Researchers are currently looking at two possible alternatives to embed sensors in the ground – one advocating placement of optical fibers at a shallow depth, while the other suggests placing fiber optics in small-diameter vertical piles. The detection system itself is based on wavelet decomposition of the incoming BOTDR signals, which are characterized by a neural network that was trained to locate tunnels using computer simulation of tens of thousands of cases. According to the scientists, tunnel excavation has a unique spatial characterization pattern which differs significantly from other disturbances such as rain or surface loads. This, they say, is what makes the system especially accurate in its assessments.

When testing the performance of their design, Technion scientists found that it is capable of locating very narrow tunnels at depths greater than 20 meters, with a relatively small number of false alarms. Another advantage of BOTDR technology, they said, is the possibility to simultaneously monitor as much as 30 km of the border using a single device. Moreover, since the required optical fiber is just the kind that is used in conventional telecommunication applications, its cost is relatively low – only a few dollars per meter.

TFOT has previously covered a number of innovative technologies with defense-related applications including a DARPA-funded project that aims to develop guided bullets and rifles that will allow snipers to actively control the flight path of the bullets fired from them and the “Flying Beer Keg” – an XM156 Class I UAV, which is capable of hovering over areas traditional surveillance methods cannot adequately cover.

More information on the BOTDR technology can be found here.