Cheap and rugged optical fibers are revealing Earth’s hidden motions | Science
A version of this story appeared in Science, Vol 374, Issue 6573.
In August, as Iceland’s long summer days began to wane, Sölvi Thrastarson made his 10th visit to a volcano so young it lacks a name. Since it began to erupt in March on a peninsula a short drive south of Reykjavík, the volcano has drawn flocks of tourists to its fiery but relatively tame outbursts. Thrastarson, a geophysics graduate student at ETH Zürich, joined a throng of sightseers on a ridge of weathered basalt 2 kilometers from the summit, and watched as globs of lava leaped out of the caldera. Camera shutters clicked as the volcano surged. “This really is the perfect tourist eruption,” Thrastarson says.
The swelling crowds have been a boon for Thrastarson and his science. In April, Neyðarlínan, the Icelandic emergency telecom company, extended a fiber optic line to the volcano, providing internet access to a region lacking cell service. For the tourists, the fiber is a digital lifeline. For the researchers, it is a way to take the volcano’s very pulse.
For several weeks in late spring, from a small utility shack in an impromptu parking lot plowed out of an old lava field, Thrastarson and colleagues tapped the fiber with an “interrogator”—a box that fired laser pulses along the cable and recorded the response. Every few days, a team member drove in from Reykjavík to retrieve the data set. From the comfort of their cozy homes, the researchers sifted through the data for clues to the interior fluctuations driving the restive volcano. It was about as easy as volcanology can get. The hardest part, Thrastarson says with a laugh, was fending off tourists who thought their shack was a bathroom.
The demands of the internet era have draped the world in a web of fiber. Fiber now connects neighborhoods and utilities, traffic cameras and mass transit. For geoscientists, fiber had always been a means to an end—a way to hook up a weather station or an undersea pressure sensor. But that has changed in the past few years. Now, the fiber itself is the sensor, says Jonathan Ajo-Franklin, a geophysicist at Rice University. “We’re seeing data where we’ve never seen data before.”
The principles of fiber optic sensing are relatively simple. The cables are bundles of glass fibers, each no thicker than a human hair, that carry information encoded in light. Small, randomly oriented defects within the fibers act like tiny mirrors, scattering light. The interrogators—or boxes, as most researchers call them—work much like radars. They fire a laser pulse into an unused fiber and record the pattern of reflections coming back from defects along the length of the cable. When an external pressure wave crosses a section of the fiber—be it from an earthquake or a footfall—it stretches and squeezes the defects. The reflections in that section are displaced by nanometers, leading to a phase shift in the rebounding light. By firing thousands of pulses per second, researchers can build up a picture of a passing seismic wave, at a distance of up to 100 kilometers or more along the fiber.
Unlike traditional seismometers, which are spaced many kilometers apart, fiber offers the equivalent of a seismometer every meter or two along the cable. This density, combined with the low cost and ruggedness of fiber, has prompted researchers to lay cables on glaciers, volcanoes, permafrost, and earthquake fault zones—any place the earth might crack or crunch, grind or grate. They have also tapped into unused “dark” fibers in existing telecom cables to pick up vibrations from sources as faint as pedestrians and cars. “This fiber is really everywhere, and that is a game changer,” says Biondo Biondi, a geophysicist at Stanford University. “We can have seismic sensors everywhere on the cheap.”
For the past few years, many fiber measurements were mere proofs of principle. But the field is now maturing, says Andreas Fichtner, a seismologist at ETH Zürich and Thrastarson’s adviser. Fiber is revealing previously unknown earthquake faults, the hidden mechanics of glaciers and avalanches, and volcanic gurglings that could aid in predicting eruptions. “It’s getting beyond the hype, to where people start doing science with it,” Fichtner says.
Like many breakthrough scientific techniques, fiber sensing has its origins in U.S. military research. Beginning in the 1980s, the Navy towed fiber optic cables behind ships to sense the sounds of enemy submarines—one reason the technique is still sometimes called distributed acoustic sensing (DAS). By the late 2000s, the oil and gas industry was lining its pipelines and boreholes with fiber. Technicians used backscattered laser light to look for sharp temperature changes—a sign of a ruptured well or pipe—or to detect artificial seismic waves from air guns on the surface, in order to probe the structure of the surrounding rock. Today, the fracking industry uses borehole fibers to monitor rock fracturing and the microearthquakes caused by the high-pressure injection of water.
By the mid-2010s, academic scientists were adopting the technique. A pioneering effort came in 2015, when a team of scientists at the German Research Centre for Geosciences (GFZ Potsdam) took advantage of unused fibers on a 15-kilometer cable connecting two geothermal power plants in Iceland. Philippe Jousset and his colleagues were able to not only detect distant earthquakes, but also locate the rupture sources by measuring differences in the arrival times of earthquake waves on either side of small bends in the cable.
In California, meanwhile, Eileen Martin, then a student with Biondi, was running a 2.5-kilometer loop of fiber in utility tunnels under Stanford. In that hushed milieu, the array picked up not only earthquakes, but also the vibrations of traffic, footfalls—even waves on the ocean. It only went offline once, Martin adds, when someone jostled the fiber in the computer room.
The nascent field’s biggest splash came when Ajo-Franklin and colleagues tapped a fiber the Monterey Bay Aquarium Research Institute runs off the California coast to undersea instruments. They deployed their interrogator box in 2018, during a 4-day maintenance shutdown of the undersea instruments, and detected a small earthquake that struck California at the time. More interesting was what the arrival times of the earthquake waves revealed in the surrounding rock: a previously unknown fault zone under the cable, just 10 kilometers off the coast. The finding showed that fiber could detect unknown earthquake threats. For fiber, says Martin, now at the Colorado School of Mines, “that was the single most convincing case anybody has made.”
Fichtner had specialized in using data collected by others to image the planet’s deep interior. But in DAS, he saw a potential paradigm shift in the making. The promise of the technique turned him into a field seismologist. He decided to target remote, frozen environments where traditional seismometers are especially expensive and difficult to deploy.
Those efforts started close to home with the Rhône Glacier in the Swiss Alps, where 15 square kilometers of ice is retreating in the face of global warming. In 2019, near the front of the glacier, Fichtner’s group laid a kilometer-long cable in the shape of triangle, covered it with snow, and left it in place for 1 week. Despite the short deployment, the fiber captured a set of fast motions that were traced to occasional icequakes—previously seen only on ice sheets, not smaller glaciers. The fiber triangle traced the source of the quakes to the same slip patch—suggesting some parts of the glacier bed behave almost like earthquake fault zones. “This changes our understanding of how glaciers move significantly,” Fichtner says.
In January, the researchers studied a far faster flow of snow and ice: avalanches. They attached their box to a cable running up a snow-draped mountain in southwestern Switzerland at the Sionne Valley avalanche test site, where researchers use helicopter-dropped explosives to trigger artificial avalanches. As the avalanches thundered down the slope, the fiber detected internal structures in the flows, including “roll waves” produced by instabilities in their dense flowing core. “You see waves that had only been predicted theoretically,” Fichtner says. “You see them immediately.”
Buoyed by these results, the team shifted its sights to one of Iceland’s largest and most dangerous volcanoes, Grímsvötn, which is buried beneath an ice cap but monitored only by a lone seismic station. It last erupted for 4 days in 2011, blowing a hole in the ice cap and lofting ash into the stratosphere that grounded hundreds of transatlantic flights. As global warming accelerates melting of the ice cap, it could uncork Grímsvötn; the declining weight could allow pressurized magma to more easily fracture its rock jailhouse and escape. The researchers wanted to know whether Grímsvötn was growing more restless.
That led Fichtner, Thrastarson, and others to make the 2-day trek to the caldera in April. Using a sled weighed down by an oil barrel and towed by a snowcat, they plowed a half-meter trench in the ice and snow, burying a 12-kilometer-long cable that circumscribed the caldera before bending into the center. They left the instrument box running for several months, beaming its data back. Fichtner was astonished at the upheaval they detected: some 1800 small earthquakes within 10 kilometers of the caldera, more than 10 times the number seen by the seismometer.
They also picked up a distinct hum, smoother than any known kind of volcanic tremor. Their best guess is that it is a resonance, generated as tremors ring the 300-meter-thick ice cap like a bell. If so, changes in the hum could signal changes in ice thickness, or provide warning of increased volcanic activity, Fichtner says.
Fichtner isn’t alone in his focus on frozen worlds. In 2016, Ajo-Franklin led a team to Fairbanks, Alaska, to see whether fibers could monitor permafrost, the subsurface layer of frozen soil, chock full of preserved organic matter, that is threatened by climate change across much of the Arctic. They laid a 4-kilometer-long fiber cable in a crisscrossing array at a military research site. More than 100 small borehole heaters warmed the surrounding soil, and an instrument that wobbles like a badly loaded washing machine set off vibrations at the surface. The resulting data—hundreds of terabytes, flown back in hard drives in students’ luggage—showed the fibers could indeed detect thawing: Meltwater significantly slowed the speed of the seismic waves. The study, to be published this year, raises the possibility of using dark fiber and ambient earthquake waves to track permafrost thaw—which could inform projections of how much carbon these thawing soils will release with continued Arctic warming.
Two years ago, a 7.1 magnitude earthquake struck Ridgecrest, California, 180 kilometers north of Los Angeles—the most powerful quake to hit California in 20 years. Zhongwen Zhan, a seismologist at the California Institute of Technology, was elated. He knew Ridgecrest would ring for days with aftershocks; by deploying a fiber array to detect and analyze them, he could learn about other earthquake hazards. The local telecom helped his team hook up four boxes on unused fibers, including one stretching 8 kilometers under the town. In just a few days, the team had installed the equivalent of thousands of seismic sensors. And then they watched while the ground continued to shake.
On a traditional seismic hazard map, Ridgecrest was a single pixel, the whole area lumped together with the same risk. The aftershocks caught by the fiber array revealed the drastic variations within that pixel, with one side of the town shaking three times more strongly than the other. Using a different set of vibrations— mini–seismic waves from traffic—to image the shallowest parts of subsurface, Zhan and colleagues found that the shaky side of town sat on far looser sediments, a risk unknown to residents. The array also identified faults that were misplaced or missing in geological maps. “We were able to see it all,” Zhan says.
In the future, Zhan hopes such a rapid response won’t be necessary, because DAS arrays exploiting unused telecom fibers will be integrated into the permanent seismic networks that continuously monitor earthquake-prone regions and generate early warnings. Those telecom fibers will be especially useful in remote regions and offshore, where traditional seismic sensors are expensive to establish and maintain. “DAS really shows strong potential for detecting earthquakes earlier than conventional sensors,” he says.
Offshore fibers could detect not just earthquakes, but also shifting pressures from tides and currents. Add those capabilities to the standard instruments that are usually attached to ocean cables, and you have a new standard for ocean observation, says Charlotte Krawczyk, geophysicist at GFZ Potsdam. “This will be a new kind of global monitoring,” she says.
Its reach may extend far from shore. Last year, researchers set a new record for DAS length with a next-generation interrogator that detected vibrations at a distance of 120 kilometers on a fiber cable running off Svalbard, the world’s northernmost inhabited island. The team, led by Martin Landrø at the Norwegian University of Science and Technology, sensed earthquakes from the Mid-Atlantic Ridge, storms, ocean swells, ship traffic, and the lonesome calls of blue and fin whales, raising hopes for an unobtrusive way of monitoring the mammals, Landrø says. “These are numbers we’d like to get a better grip on.”
And in 2020, a seafloor cable deployed from the Canary Islands picked up waterborne sound waves from an earthquake that shook the ocean floor. Zhan was part of a team that used such ocean sound waves, which travel faster in warmer water, to measure the ocean’s temperature change over time. If fibers start to pick up these waves regularly, it would be “amazing,” he says—a way to identify where the deep ocean is heating up, and whether its capacity to absorb 90% of the heat from global warming is diminishing.
Fiber has made inroads on ice, land, and water. Tieyuan Zhu, a geophysicist at Pennsylvania State University (Penn State), University Park, thinks ground-based fibers can even be used to study the air. With a small array at Penn State, he has shown how they can capture and locate thunder while singling out lightning strikes, which shake the ground in a way that’s distinct from thunder. They can also distinguish pelting rain from gusty winds. In June at the nearby Shale Hills Critical Zone Observatory, he and his colleagues deployed a fiber array to probe how storms, growing more severe with global warming, deepen the ground’s “weathering layer”—the tens of meters or so of soil and rock that sit above bedrock. Zhu says the fiber easily heard the sounds of rain soaking into the weathering layer from the remnants of Hurricane Ida, which passed through the eastern United States in September. “We saw a beautiful signal variation.”
Fiber is poised to break out in science, but its potential in the applied world may be even greater, says Nathaniel Lindsey, a geophysicist at FiberSense, a startup in California targeting city-sensing applications. Already, small university arrays, sensitive to car traffic and human footsteps, have witnessed the stark falloff in activity because of the coronavirus pandemic, and its resumption as lockdowns are lifted. They can detect fires, landslides, and all sorts of natural threats. Fibers laid into bridges and buildings can detect when infrastructure is close to failure, and they could replace the motion sensors and cameras used to monitor borders and fences. Unlike cameras, DAS “doesn’t care if it’s pouring or there’s fog,” Biondi says. The next step will be getting a city to allow a large, sustained network of DAS arrays. In San Jose, California, Biondi is targeting just that, pitching the city on opening up unused fibers in its telecom network for monitoring.
The downside to fiber is the staggering amount of data it produces. In the past, seismologists could gather data from their isolated, widely spaced instruments and crunch it later, on a laptop. The thousands of distinct “sensors” along a single fiber, in contrast, can gather hundreds of terabytes of data in a matter of days. Long-term arrays will produce petabytes, requiring processing in real time, Fichtner says. “Then we can’t store our data anymore,” he says. “We will have to learn how to do science on the fly.” Many labs are developing artificial intelligence algorithms to efficiently sift through the data and look for patterns.
Fichtner’s lab already has a data problem—it has done so many deployments that it doesn’t have enough researchers to work through the collected data. They’ve just started to tease apart data captured from the unnamed Icelandic volcano. One mystery they have yet to unravel: low-frequency pulses of activity every few seconds, which could correspond to small eruptive bursts or the collapse of bubbles in the magma.
Despite the data deluge, Fichtner’s team isn’t stopping. The researchers just wrapped up a monthlong deployment in Greece, where they tapped 30 kilometers of cable running through the north of Athens, overnight adding 10,000 seismometers to the 20 in the region, hoping to capture hidden faults. In October, they interrogated a submarine cable that connects the islands of Santorini and Ios. It runs right past Kolumbo, a large, mysterious underwater volcano that last erupted in 1650 but has shown worrying signs of activity.
With the fibers watching, Fichtner says, Kolumbo is putting on a show. The vibrations just keep coming.