Detecting Tunnel Excavation Sites Using Rayleigh Scatter Sensing

Jordan Stringfellow Mechanical Engineer llegal underground cross-border tunnels have been a challenging problem over the last few decades, especially in areas such as the Gaza strip and US-Mexico border. Increased smuggling of drugs, weapons, and people, has made it more important than ever to find an advanced solution to detect and locate these tunnels. Tunnels can range in sophistication, diameter, and depth, depending on their use. Past studies have shown the ability to detect large scale tunnels, but detecting small scale tunnels, on the order of 1m in diameter, is still extremely difficult. A study done by Linker and Klar at the Israel Institute of Technology is the first full scale experiment that has been conducted to show how distributed strain measurements using fiber optic sensors can be used to measure the location and depth of both large and small scale tunnels. A Luna OBR 4600 system was used in this work. The OBR 4600 is able to provide strain measurements down to a 2 mm spatial resolution for a measurement range of up to 70 m. Data here was taken from an array of sensors buried at a shallow depth to reveal tunnels as small as 1 m in diameter, up to 18 m beneath the surface.

Advanced post processing algorithms were developed to filter out disturbances from everyday surface activities (such as from cars, animals, and weather) and enhance disturbances due to actual tunnel excavation. Using additional sensors add repeatability and can reduce the likelihood of false alarms, making it acceptable for practical application. A Brillouin OTDR system was used alongside the Luna system, though the BOTDR system reported measurements at a coarser spatial resolution on the order of 1 m. Results from both the Brillouin OTDR system and the Luna Rayleigh Scatter system correlated well to each other. The figure below was generated by a fiber loop that detected a 1m diameter tunnel using a Luna OBR 4600 system. White gaps represent system downtime.

The full paper can be accessed from the SPIE Digital Library here. For more information about Luna’s fiber sensing solutions visit the Sensing section of our website. Reference: Raphael Linker and Assaf Klar. Detection of tunnel excavation using fiber optic reflectometry: experimental validation. Proc. SPIE 8709, Detection and Sensing of Mines, Explosive Objects, and Obscured Targets XVIII, 87090X (June 7, 2013)