Subsea cables turned into sensor arrays

According to the team, the technique could allow scientists to acquire continuous, real-time environmental data from the bottom of seas and oceans for the first time. The results are published in Science.

In a statement, Giuseppe Marra, principal research scientist, NPL said: “This new technique opens a new era for Earth monitoring by providing for the first time a feasible solution to the lack of environmental data from the bottom of seas and oceans. We can now harness existing underwater cables as a valuable tool for Earth sciences and beyond. This breakthrough is a perfect example of how ultra-stable optical frequency metrology can transition from the laboratory to improve our understanding of the world and also deliver tangible benefits to society.”

Installing and maintaining permanent ocean-floor sensors is challenging and expensive, so only a handful exist globally. This has left a gap in geophysical data, limiting scientists’ understanding of the Earth’s structure and its dynamic behaviour. 

Previous work by NPL and its partners in 2018 showed that submarine cables could be repurposed as sensors to detect underwater earthquakes by using ultra-stable interferometric techniques. However, one cable could act only as a single sensor, and measurements were limited only to the integrated changes over the entire length of the cable.

Davide Calonico, researcher, INRiM, said: “Our seminal work in 2018 turned coherent laser interferometry from a laboratory technique to a powerful tool for geophysical sensing, and today a new step forward confirms it can be extended to thousands of kilometres, reaching even the most remote areas of our planet.”

The NPL-led team, which included researchers from Edinburgh University, the British Geological Survey, the Istituto Nazionale di Ricerca Metrologica (INRiM), and Google, tested the technique on a 5,860km-long intercontinental submarine optical fibre link between the UK and Canada.

The team showed the detection of earthquakes and ocean signals, such as waves and currents, on individual spans between repeaters spread across the entire transatlantic connection. The optical fibre in each span acted as a sensor. 

In this research up to 12 sensors were implemented along the cable. Future upgrades will increase this number to 129 and the data from these sensors can be recorded continuously and in real time.

By applying this new method to the existing network of submarine cables, huge areas of the ocean floor can potentially be instrumented with thousands of permanent real-time environmental sensors, transforming underwater telecoms infrastructure into an array of geophysical sensors. 

Integrating this cable-based approach with current seismometer-based networks means the method has the potential to substantially expand the global earthquake monitoring infrastructure from land to the seafloor where only a handful of permanent seismometers are currently installed. The method does not require any change to the underwater infrastructure, providing for the first time an affordable and scalable solution for sea floor monitoring on a global scale.

Due to optical fibre cable’s sensitivity to environmental perturbations, this research also opens up the possibility of monitoring for other natural phenomena, such as improving the understanding of deep-water flows.

Research results provide evidence that the method could potentially be used for detecting tsunamis. Enabling the real-time detection of tsunami-inducing earthquakes closer to their off-shore epicentre could save lives by giving national governments crucial extra time to warn of an impending incident.

The research team now plans to test the method on multiple submarine cables, including those in more seismically active areas such as the Pacific Ocean.