distributed acoustic sensing fiber optics DAS Monitoring Technology for Rail Transit

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Many attempts have been made to provide sensing capabilities in the context of oil exploration, production, and monitoring, with varying degrees of success. Recently, these attempts have included using fiber optic cables to detect sound energy. Because cables typically include multiple optical fibers with uneven backscatter along the length of the fiber, such a system allows for distributed measurement of changes in optical path length along the fiber by measuring the backscattered light input from the laser pulse into the fiber. Because such systems allow for distributed sensing, they can be referred to as “distributed acoustic sensing” or “DAS” systems. One use of DAS systems is in seismic applications, where seismic sources at known locations transmit sound signals to the formation and/or passive seismic sources emit sound energy. These signals are received by seismic sensors after passing through and/or reflecting through the formation. It can process the received signals to provide information about the strata they pass through. This technology can be used to record various seismic information. Another application is in the field of well applications and fluid acoustic monitoring.

Fiber optic distributed acoustic sensing (DAS) is a known type of sensing, where the fiber optic is deployed as a sensing fiber and repeatedly interrogates along its length using electromagnetic radiation to provide sensing of acoustic activity. Typically, one or more input radiation pulses are emitted into the fiber optic. By analyzing the backscattered radiation from within the fiber optic, the fiber optic can be effectively divided into multiple discrete sensing parts that may (but do not necessarily) be adjacent. Within each discrete sensing section, mechanical disturbances in the fiber optic (such as strain caused by incident sound waves) result in changes in the properties of radiation backscattered from that section. This change can be detected and analyzed, and is used to provide a measure of fiber optic disturbance at the sensing section. Therefore, the DAS sensor effectively serves as a linear sensing array for the acoustic sensing part of the fiber optic.

To monitor the railway network, sensing optical fibers can be deployed to extend along the path of one or more tracks of the railway network as a whole. The movement of trains on such tracks adjacent to DAS sensing fibers will generate sound signals/vibrations, which can be used to track the vehicle as it moves, providing real-time location information continuously along the entire length of the monitored section to a resolution of several tens of meters.
DAS has several advantages for this type of railway network monitoring. DAS can be applied to provide many sensing channels on longer fiber lengths, such as up to 40km or more fiber lengths with approximately 10m long adjacent sensing channels. Therefore, a single DAS sensor can be used to monitor the long reach of the railway network. For lengths greater than about 40km, several DAS sensor units can be deployed at various intervals to provide continuous monitoring of any desired length of the transportation network.
The sensing fiber can be a standard telecommunications fiber and therefore relatively inexpensive. Fiber optics can be simply buried side by side with transportation networks (e.g. along edges or under tracks or roads in narrow channels) and relatively easy to install. Fiber optic can be encapsulated in a protective casing (i.e. in a cable) and can survive for a long time without maintenance. Therefore, installation and maintenance costs are low. In many transportation networks, there may already be fiber optic deployed along at least the main route, and this existing communication infrastructure may include redundant fiber optic that can be used for DAS.