What if we could monitor what goes on at the inaccessible depths of the oceans – using infrastructure that’s already there?
In a new study published in the journal Optica, researchers show that the millions of kilometres of fibre optic cables crisscrossing the ocean floor can not only be used to send data streaming across the world, but also to sense what’s going on around them, including monitoring geophysical events and observing ocean and seafloor conditions.
This could result in a global network of underwater cables studying otherwise unreachable parts of the planet.
“Once perfected, this new technique will allow geophysical sensing in the ocean depths, which are largely unexplored because of a lack of instrumentation that works in this environment,” explains Zhongwen Zhan, co-author on the study and a geophysicist at Caltech in the US.
“It could one day be used to detect earthquakes with epicenters in the ocean, allowing earlier warnings of earthquakes and tsunamis, for example.”
Zhan, along with researchers from Google and the University of L’Aquila, in Italy, studied the Curie transoceanic fibre optic cable. It’s owned by Google and stretches 10,500 km from Los Angeles, California, to Valparaiso, Chile; it began transmitting only last year.
Cables like Curie encode data into the amplitude and phase of light, which is then transmitted through the link. The team found that changes in the ocean environment around the cable – such as earthquakes or pressure variations – cause tiny but detectable changes in the light’s polarisation, which they observed in the receiver at the end of the cable.
“We developed the theoretical framework required to interpret polarisation data in submarine cables, which will enable further quantitative understanding of submarine geophysical processes,” explains Zhan.
The researchers used the technique to detect deep-sea earthquakes and ocean swells, which aligned with measurements made with land-based seismometers.
Previous research has used submarine links to sense geophysical events in the Mediterranean, but needed to use specialised lasers. The new study required only standard telecommunications equipment.
“In addition,” Zhan notes, “there is no need for a dedicated light channel because the data required for sensing can be collected without disturbing the regular operation of the optical transmission system.”