“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.
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.