Known as distributed acoustic sensing (DAS), the method relies on the internal flaws in a long optical fibre, harnessing them as thousands of seismic sensors. An interrogator at one end of the fibre sends laser pulses down the cable that are reflected off the fibre flaws and reflected back to the instrument. When the fibre is disturbed, researchers can examine changes in the timing of the reflected pulses to learn more about the resulting seismic waves.
“With DAS, we’re able to give much more up-to-date and accurate ideas of sea ice conditions over about a 40km stretch of water, versus a few square kilometres resolution that can be accessed up to a couple of days later using satellite imagery,” said researcher Michael Baker of Sandia National Laboratories.
“At a global scale, the annual increase and decrease of fluctuations in sea ice is a good metric for determining the overall health of the Arctic.”
In the Beaufort Sea, Baker and colleagues participating in the Cyrosphere/Ocean Distributed Acoustic Sensing (CODAS) experiment are looking at how DAS technology could be used to monitor Arctic seismoacoustic signals, including sea ice formation and break-up to whale songs.
The CODAS deployment uses an existing telecommunications unused fibre, owned by Quintillion, that lies in the seafloor. During the deployment, Baker and colleagues used DAS signals to observe a change from an open-water to an ice-bound state, watching as a sea ice formation front propagated 20km seaward over a period of eight hours.
Open-water signals tend to be much noisier than ice-bound signals, according to Baker, with wind stirring up the water to generate hydroacoustic noise. “When you put a layer of thick sea ice on top of that, it isolates the water column from the wind, so you don’t really generate wind waves very well,” he said.
The ice generates “exotic floating-plate wave modes that most seismologists don’t have to deal with, but they show up really well on the DAS,” Baker added.
The propagation speed of these plate waves is heavily dependent on the thickness of the ice. With more data, the researchers hope they can use DAS to monitor fluctuations in sea ice thickness as well.
According to the team, the timing and extent of sea ice affects seal breeding and polar bear hunting, which have cultural and commercial importance to the region’s people. The ability to quickly monitor sea ice formation is also vital for commercial shipping lane operations.
Naturally, sea ice is also an important buffer against the impacts of climate change and an indicator of how serious the issue is. “The longevity and thickness of the sea ice gives us an idea of how much any particular area is really going to be impacted by annual changes in the climate,” said Baker. “We’re expecting more coastal erosion, as permafrost thaws out and Arctic Ocean storms increase in strength and frequency.”
Following four more DAS deployments this year, Baker said he and his colleagues would like to follow up with a distributed temperature sensing project to look more closely at the submarine permafrost layer. “We want to see if we can assess the stability of that permafrost and how much if any outgassing of methane is happening, as the permafrost is thawing.”