Rapid advances in fibre optic sensing are revolutionising how we sense the ocean

What is it about?

Distributed Acoustic Sensing (DAS) is a technology that converts regular Fibre-Optic (FO) cables into thousands of tiny microphones or motion sensors. It works by sending short laser pulses down the cable and measuring how reflections from tiny imperfections in the FO cable change when the cable is stretched or disturbed. The basic idea has been around for approximately 30 years. Still, recent improvements have made it useful for many new purposes — including studying earthquakes, measuring sound and motion in the ocean, monitoring ocean conditions, and even developing new types of underwater communication. DAS can collect information along very long distances (up to about 150 km) and can detect changes on short scales (as small as 1 meter). It creates a huge amount of information — roughly 1 gigabit per second. However, each “virtual sensor” created by the system is currently noisier and less precise than traditional instruments like hydrophones or seismometers. Several factors affect how well the system works. The FO cables are usually built into larger telecommunication cables with metal armour, power lines, and plastic coatings, all of which change how the cable responds when something pushes or pulls on it. Each virtual sensor is also directional because of how the laser pulses are created and processed. Finally, the cable’s response changes depending on where it lies on the seafloor — for example, whether it sits on hard rock or is buried in soft sediment. DAS already provides capabilities that no other system can match, but it is not perfect. To fully unlock its potential, researchers need to better understand how the cables respond to stress, how they interact with the environment, how to handle noise, how to process the signals, and how to manage the extremely large data volumes the system produces.

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Photo by Niklas Jonasson on Unsplash

Photo by Niklas Jonasson on Unsplash

Why is it important?

This technology has the potential to completely change the way we sense the environment, if we can harness its power, calibrate its output and find ways to process and absorb the massive amounts of data it creates. The technology finally allows us to sense the ocean on small spatial scales and over the long distances necessary to understand the populations and migrations of marine animals, oceanographic processes at many scales, the impact of shipping and other noise, and to track climate change.

Perspectives