Fibre sensor offers tunnel vision

05 Jun 2009

Distributed fibre-optic sensing could help railway companies monitor track beds and tunnels for possible collapse.

US test and measurements specialist Luna Technologies has unveiled a fibre-optic sensor that could form the basis of a 24/7 monitoring system to warn of impending soil collapse inside railway tunnels. The OBR 4400 sensor exploits a technique called Brillouin optical time-domain reflectometry to detect sink holes and soil cavities that lead to instabilities in track beds and tunnel structures.

Current technologies such as ground-penetrating radar, seismic analysis or infrared thermography can be used to locate soil cavities but do not offer round-the-clock monitoring. The Luna sensor comprises a small, easily transportable platform and can measure reflections over lengths of up to 2 km with no dead zone and submillimetre resolution.

Initial laboratory tests of the instrument have been carried out by the French Public Works Research Laboratory and SNCF, the French national railway operator (Measurement Science and Technology 20 034018). "The key advantage of the Luna system is its unique combination of strain sensitivity, accuracy and high spatial resolution," Alex Sang, a project research engineer at Luna, told optics.org. "Basically, we report more strain points along the length of the fibre, allowing for precise and accurate location of the sink hole along with its width and depth."

He added: "The device uses Rayleigh backscattering from an optical fibre to make measurements. This involves taking initial scans from the fibre at 10 µm intervals along its entire length."

A second scan is taken after cavity formation and is compared to the initial scan to determine the induced shift. The shift can be scaled to strain and then graphed as a function of fibre position.