Luna Technologies’ fiber optic sensing system, the Optical Backscatter Reflectometer (OBR) 4400 device with distributed Rayleigh sensing, was used in an experiment by the French Public Works Research Laboratory in conjunction with the French National Railway for the possible use as a cavity detection or sink hole warning system on railway track beds and tunnels. Embedded cavities, or sink holes that may induce soil collapse present a major risk to the French railway system. Current technolog...