Submarine cables transformed into seismic network

Researchers use fibre-optic cable that connects a wind farm in the North Sea to detect an earthquake. Credit: Marlinks

An international team of geoscientists led by the California Institute of Technology (Caltech) has used fibre-optic communications cables stationed at the bottom of the North Sea as a giant seismic network, tracking both earthquakes and ocean waves. 

The project was, in part, a proof of concept. Oceans cover two-thirds of the earth’s surface but placing permanent seismometers under the sea is prohibitively expensive. The fact that the network was able to detect and record a magnitude 8.2 earthquake near Fiji in August 2018 proves the ability of the technology to fill in some of the massive blind spots in the global seismic network, says Caltech graduate student Ethan Williams, who is the lead author of a study on the project that was published by Nature Communicationsin December 2019.  

“Fibre-optic communications cables are being used more and moreRather than place a whole new device, we can tap into some of this fibre and start observing seismicity immediately,” he said. 

The technology 

The project relies on a technology called distributing acoustic sensing, or DAS. DAS was developed for energy exploration but has been repurposed for seismology. DAS sensors shoot a beam of light down a fibre-optic cable. Tiny imperfections in the cable reflect miniscule amounts of the light, allowing the imperfections to act as waypoints. As a seismic wave jostles the fibre cable, the waypoints shift minutely in location, changing the travel time of the reflected light waves and thus allowing scientists to track the progression of the wave. The DAS instrument used in this study was built and operated by a team from Spain’s University of Alcalá, led by study co-author Miguel Gonzalez-Herraez. 

“Seafloor DAS is a new frontier of geophysics that may bring orders-of-magnitude more submarine seismic data and a new understanding of the deep Earth’s interior and major faults,” said Zhongwen Zhan, assistant professor of geophysics and co-author of study. 

For the North Sea project, Williams, Zhan, and their colleagues employed a 40,000-m section of fibre-optic cable that connects a North Sea wind farm to the shore. There are millions of tiny imperfections in the cable, so they averaged out the imperfections in each 10-m segment, creating an array of more than 4,000 virtual sensors. 

“With the flip of a switch, we have an array of 4,000 sensors that would’ve cost millions to place,” Williams says. 

Because of the network’s fine degree of sensitivity, the North Sea array was able to track tiny, non-earthquake-related seismic noise or microseisms and found evidence that supports a long-standing theory that the microseisms result from ocean waves. 

In 1950, mathematician and oceanographer Michael Selwyn Longuet-Higgins theorised that the complex interaction of ocean waves could exert enough of a rolling pressure on the sea floor to generate so-called Scholte waves – a type of seismic wave that occurs at the interface of a liquid and a solid. By tracking both ocean waves and corresponding microseisms, the North Sea array revealed that the microseisms could be the result of ocean-wave interactions.