- Scientists have used undersea fiber-optic cables in the Arctic to remotely track the presence and extent of sea ice.
- Sea ice is usually monitored with the help of satellites; however, the lack of high-resolution images and the low frequency of data collection makes it difficult to do in-depth analysis.
- Using a method commonly employed to monitor oil pipelines and highways, the scientists looked for changes in signals sent down a fiber-optic cable in the Beaufort Sea that would indicate the presence of sea ice.
- While promising, the method can’t yet be used to measure the thickness of sea ice or to determine how far the ice extends to either side of the cable.
Andres Peña Castro and his team didn’t set out to track sea ice. Their original plan was to quantify the interactions between ocean, earth and atmosphere to be able to understand phenomena such as the state of the sea or storm surges.
Instead, they ended up piloting a way to use existing undersea fiber-optic cables in the Arctic to track the extent of sea ice and monitor how it’s changing.
“We were not at all expecting to be able to see sea ice changes clearly in the data,” Peña Castro, a postdoctoral researcher at the University of New Mexico, told Mongabay in a video interview.
In a study published in August in the journal The Seismic Record, Peña Castro and his colleagues described how they monitored changes in sea ice in the Beaufort Sea with the help of “ambient noise recorded by fiber-optic sensing technology deployed in an Arctic shallow marine seafloor environment.” While the findings were based on research at one location, they say the method could potentially be scaled up at other sites with existing fiber-optic cables.
Studying how frozen seawater is changing is essential to measuring the impacts of global warming. Sea ice is usually monitored with the help of satellite imagery. However, the lack of high-resolution images often makes it difficult to gauge the reality in detail. Additionally, the data received from satellites often have time gaps.
“They can probably make measurements maybe only once every day because the satellite has to orbit the entire Earth,” Peña Castro said. “The technology we used is more localized, and we can measure it basically every minute, two minutes, 10 minutes — whatever we want to use.”
Fiber-optic cables are most commonly used to transmit large amounts of data over long distances. But they have another, often industrial, application as a real-time sensor, used to monitor linear infrastructure such as highways or oil pipelines. Known as distributed acoustic sensing (DAS), this application relies on the fiber physically deforming under strain — such as pressure build-up in a pipeline, for instance, or an abnormal load at a highway juncture. A light pulse is sent through the fiber and reflected back to a device known as an interrogator unit, which compares the original signal and its reflection. Any strain on a part of the fiber would show up in this comparison, flagging operators to a potential issue on that point of the pipeline or highway.
Or the sea, for that matter. In Peña Castro’s case, what they were monitoring for was the absence of strain. Ocean waves cause vibrations known as microseisms, but a cover of sea ice can dampen, or attenuate, the strength of these vibrations. The working theory, then, is that when a fiber-optic cable passes beneath an area of sea ice, the dampening of the microseisms should be visible in the signal picked up by the interrogator.
“We monitored where the signal was heavily attenuated, and then went ‘Here’s the part where we stopped seeing the oceanic signal. Hence, there must be something that is suppressing that signal,’” Peña Castro said. “In our case, it must be ice because that is the only thing in this region.”
Peña Castro and his colleagues gathered their data over two weeks: once in July 2021 and again in November 2021, both periods representing a time of transitional sea ice cover. For their study, they used a 37.4-kilometer (23.2-mile) section of an existing fiber-optic cable deployed off Oliktok Point in Alaska. They sent light pulses down the fiber, then divided the observations into 30-minute segments and made graphical representations of each segment.
Because of the “overwhelming amount of data” they had to go through, they used a machine-learning algorithm to identify ambient noise patterns that would indicate the presence of sea ice strong enough to suppress ocean waves.
Using this method, the scientists observed abrupt changes in sea ice in the region. The data gathered and the subsequent analysis found changes in sea ice cover of up to 10 km (6 mi) in less than a day.
“It was definitely surprising that the sea ice can change so much in a few hours,” Peña Castro said in a press release announcing the study. “A few colleagues have mentioned that these rapid changes may be common, but the temporal resolution of satellites makes it rare to observe such rapid changes in sea ice.”
The method, however, can’t yet be used to measure the thickness of the sea ice. And while it was able to indicate the presence of ice along the length of the cable, it couldn’t determine how far the ice cover spread to either side.
Additionally, such cables aren’t deployed everywhere across the Arctic, limiting the use of DAS to observe sea ice change in the region. Peña Castro said there are a few more cables in the region, as shown by publicly available data, that could be deployed for the purpose, although that would require the consent of the private cable operators. He also suggested not relying entirely on this method to monitor the presence of sea ice, but to combine it with satellite imagery to get more data and better insights.
“It certainly can complement the observations from satellites,” he said. “We can mesh them together to understand climate change and how sea ice is changing.”
Banner image: Ocean waves cause vibrations known as microseisms, but a cover of sea ice can dampen, or attenuate, the strength of these vibrations. Image by Anders Jildén via Unsplash (Public domain).
Abhishyant Kidangoor is a staff writer at Mongabay. Find him on Twitter @AbhishyantPK.
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Peña Castro, A. F., Schmandt, B., Baker, M. G., & Abbott, R. E. (2023). Tracking local sea ice extent in the Beaufort Sea using distributed acoustic sensing and machine learning. The Seismic Record, 3(3), 200-209. doi: 10.1785/0320230019