Distributed acoustic sensing with optical fibres: Fundamentals and applications

Abstract

Because of their intrinsic geometrical features and extraordinary transparency, optical fibers allow for a continuous and dynamic monitoring of strain along all their length. In state of the art systems, 100 000 points can be resolved over a range of the order of 50 km, providing an extremely cost effective solution per sensing unit. In its basic implementation, a light pulse is injected into the sensing optical fibre and the Rayleigh backscattered light is continuously monitored, in a process similar to a radar system. While the phase relation between different point along the fiber is used to reconstruct the strain (thus the acoustic information), the time of flight (e.g., delay between injected pulse and received backscattered light) provides information about the physical location along the fiber. In the last decade, research on novel detection schemes and fibers has allowed to extend the detection range to over 170km, to decrease the spatial resolution below 10 cm and significantly increase the signal-to-noise-ratio. Because of this, distributed acoustic optical fiber sensing (DAS) has found applications in the oil and gas industry for oil well monitoring and geoseismics and more widely for the monitoring of border and perimeter security, resulting in market of the order of $1 billion. More recent applications include monitoring of railways and road traffic, structural health, earthquakes and other geophysical events.