Distributed Acoustic Sensing (DAS) is a technology that uses the properties of light combined with the material properties of fibre optic cables to measure acoustic vibrations along the cable length
From the Sensonic website:
This technology allows for the continuous monitoring of long lengths of fiber optic cable listening for a variety of events. For railways applications, such events include rockfalls, landslides, track trespass and of course the vibrations a passing train generates.
DAS works by injecting a short pulse of light into the fiber optic cable. As the pulse of light travels down the fiber, it is scattered by tiny imperfections in the fiber. Many of these imperfections are caused by a strain of the fiber, i.e., movement of it. Even extremely small fibre movements such as those caused by acoustic vibrations cause the scattered light signals to change.
The scattered light is collected by a sensor at the same end of the fibre where light was injected. The sensor measures the time it takes for the scattered light to return, which is used to determine the distance from the sensor to the imperfection that caused the scattering.
By measuring the time taken for scattered light to return from the multiple imperfections along the length of the fibre, it is possible to create a fingerprint of the acoustic signature along the fiber, a digital twin/SonicTwin®. This acoustic fingerprint can be used to detect and monitor a variety of events, such as:
- Rockfalls – Rocks falling on or near the fiber/track generate ground vibrations which can be detected and generate alerts for operations teams.
- Landslides: In a similar way large volumes of earth, rock and mud moving near the fiber also generate ground vibrations
- Track trespass: DAS can be very sensitive. It can “hear” the footsteps of people walking on or near the fiber/track.
- Digging: Digging activities from both humans and machines can be detected in the same way to alert staff to potential cable theft, or interference with the track.
- Catenary flashovers: When electrification faults cause a flashover, it makes a brief but distinctive noise. As DAS systems monitor 24/7 and the noise causes fiber vibrations, the fault can be precisely geolocated saving inspection and repair team’s significant time.
- Track condition monitoring: Because a SonicTwin can be created of how the infrastructure responds to trains passing over it, any changes in this performance can be related to changes in track condition.
- Animal intrusion: Just as we can detect humans, we can detect animals too, particularly larger ones which may pose a hazard to train operations such as elephants.