Tremors of volcanoes, roar of traffic and songs of whales: fiber optics take the pulse of the planet

Andreas Fichtner removes a cable from its protective sheath, revealing a glass core thinner than a hair—a fragile 4-kilometre-long fiber about to fuse with another.

It's a complicated task, best suited for a lab, but Fichtner and his colleague Sara Klaasen are carrying it out on a frigid, windy sheet of ice.

After a day's work, they have spliced ​​together three segments, creating a 12.5 kilometer long cable.

You will remain buried in the snow and spy on the activity of Grímsvötn, a dangerous glacier-covered Icelandic volcano.

Later, sitting in a hut on the ice, Fichtner's team watches as seismic murmurs from the volcano below flash across a computer screen: tremors too small to be heard, but easily picked up by fiber optics.

"We could see them right under our feet," he says.

"You're sitting there and you feel the heartbeat of the volcano."

Fichtner, a geophysicist at the Federal Polytechnic School in Zurich, is part of a group of researchers using fiber optics to take the pulse of our planet.

Much of this work is done in remote locations, from the tops of volcanoes to the bottom of the seas, where traditional surveillance is too expensive or difficult.

There, in the last five years, fiber optics have begun to shed light on seismic rumbles, ocean currents, and even animal behavior.

Researchers Sara Klaasen and Andreas Fichtner splice optical fibers in the back of a vehicle on an Icelandic glacier.

Like trying to join two hairs together, this is a difficult job for cold hands in a hostile environment. HILDUR JONSDOTTIR

The Grímsvötn ice cap, for example, sits on a lake of water thawed by the heat of the volcano.

Data from the new cable reveals that the floating ice field serves as a natural speaker, amplifying tremors from below.

The work suggests a new way to spy on the activity of volcanoes that are encased in ice — and thus capture the tremors that can herald eruptions.

Like a radar, but with light

The technique used by Fichtner's team is called distributed acoustic sensing, or DAD.

"It's almost like radar on the fiber," says physicist Giuseppe Marra, of the UK's National Physical Laboratory in Teddington, England.

While radar uses reflected radio waves to locate objects, distributed acoustic sensing uses reflected light to detect events, from seismic activity to moving traffic, and determine where they occurred.

It works like this: a laser source located at one end of the fiber fires short pulses of light;

As a pulse travels along the fiber, most of its light continues forward, but a fraction of the light's photons strike intrinsic defects in the fiber—points of abnormal density.

These photons are scattered, some of them traveling back to the source, where a detector analyzes this reflected light for clues about what has happened along the fiber.

An optical fiber for distributed acoustic sensing typically spans several to tens of kilometers, moving or bending in response to disturbances in the environment.

“It wiggles when cars go by, when earthquakes hit, when tectonic plates move,” says earth scientist Nate Lindsey, co-author of a 2021 paper on fiber optics for seismology in the

.

Those wiggles change the reflected light signal and allow researchers to extract information such as how an earthquake has bent a cable at a given point.

An optical cable picks up vibrations, for example, from seismic tremors over its entire length.

In contrast, a typical seismic sensor, or seismometer, transmits information from a single point.

And seismometers can be expensive to deploy and difficult to maintain, says Lindsey, who works for a company called FiberSense that uses fiber-optic networks for applications in urban environments.

Whether under a city or on top of a remote glacier, an optical cable sways when disturbed, for example by moving traffic or seismic waves.

Distributed Acoustic Detection, or DAD, picks up on those tiny movements.

Pulses of laser light are sent from the interrogator to the fiber.

On their way, some photons collide with defects in the fiber, which scatter them, and part of this scattered light returns to the source.

Analyzing this "backscattered pulse" and comparing it to the light that was originally sent allows researchers to detect environmental events. Knowable Magazine.

Distributed acoustic sensing can provide a resolution of about a meter, turning a 10-kilometer fiber into something like 10,000 sensors, Lindsey says.

On occasion, researchers can take advantage of existing or dismantled telecommunication cables.

In 2018, for example, a group that included Lindsey, then at UC Berkeley and Lawrence Berkeley National Laboratory, converted a 20-kilometer cable operated by the Monterey Bay Aquarium Research Institute — which normally used to film corals, worms, and whales—on a distributed acoustic detection sensor while the system was down for maintenance.

“The ability to sink to the seafloor for tens of kilometers—it's remarkable that you can do that,” says Lindsey.

"Historically, deploying a sensor on the seafloor can cost $10 million."

Over the course of their four days of measurements, the team captured a 3.4-magnitude earthquake that shook the ground about 30 kilometers away in Gilroy, California.

For Lindsey's team it was a stroke of luck.

Earth scientists can use seismic signals from earthquakes to get an idea of ​​the structure of the ground that the quake has traversed, and signals from the fiber optic cable allowed the team to identify several hitherto unknown underwater faults.

"We're using that energy to basically clarify the structure of the San Andreas fault," Lindsey says.

Spy on cities and cetaceans

Distributed acoustic sensing was pioneered in the oil and gas industry for monitoring wells and detecting gas in boreholes, but researchers have been finding a variety of other uses for the technique.

In addition to earthquakes, it has been used to monitor traffic and construction noise in cities.

In densely populated metropolises with significant seismic risks, such as Istanbul, distributed acoustic sensing could help map subsurface sediments and rocks to reveal which areas would be most dangerous during a large earthquake, Fichtner says.

A recent study even showed that whale songs could be heard via an optical cable from the seafloor near Norway.

But distributed acoustic sensing has some limitations.

It is difficult to get good data from fibers longer than 100 kilometers.

The same cable defects that cause light to scatter—producing the reflected light that is measured—can impair the signal from the source.

With enough distance traveled, the original pulse would be completely lost.

But a newer and related method may provide an answer — and perhaps allow researchers to spy on a largely unmonitored seabed, using the existing cables that carry the data from billions of emails and

downloads. —.

In 2016, Marra's team searched for a way to compare the timing of ultra-precise atomic clocks at distant points in Europe.

Satellite communications are too slow for this job, so the researchers turned to buried optical cables instead.

At first, it didn't work: environmental disturbances introduced too much noise into the messages the team sent over the wires.

But the scientists sensed an opportunity.

“That noise that we want to get rid of actually contains some very interesting information,” says Marra.

Using state-of-the-art methods to measure the frequency of light waves bouncing along fiber-optic cable, Marra and his colleagues examined the noise and found that—like distributed acoustic sensing—their technique detected events such as earthquakes through changes in light frequencies.

However, instead of pulses, they use a continuous beam of laser light.

And unlike distributed acoustic sensing, laser light travels back and forth in a loop;

then the researchers compare the light coming back with the light they sent out.

When there are no disturbances in the cable, those two signals are the same.

But if heat or vibrations from the environment disturb the cable, the frequency of the light changes.

With its research-grade light source and measurement of a large amount of the initially emitted light—as opposed to what is reflected—this approach works over longer distances than distributed acoustic sensing.

In 2018, Marra's team demonstrated that it could detect earthquakes with underwater and underground fiber-optic cables up to 535 kilometers long, far exceeding the limit of distributed acoustic detection of about 100 kilometers.

This offers a way to keep an eye on the depths of the ocean and Earth systems that are often hard to reach and rarely tracked with traditional sensors.

A cable running near the epicenter of an offshore earthquake could improve seismic measurements on land, perhaps giving people minutes more time to prepare for a tsunami and make decisions, Marra says.

And the ability to sense changes in seafloor pressure could open the door to direct detection of tsunamis as well.

At the end of 2021, Marra's team managed to detect seismicity across the Atlantic in a 5,860-kilometre optical cable that runs along the seabed between Halifax, in Canada, and Southport, in England.

And they did it with a much higher resolution than before, because while the previous measurements were based on the accumulated signals along the entire length of the submarine cable, this work analyzed the light changes in sections of about 90 kilometers between the repeaters that amplify the signal.

Fluctuations in signal strength picked up on the transatlantic cable appear to be tidal currents.

“It's essentially the wire being strummed like a guitar string as the currents rise and fall,” says Marra.

Although it's easy to observe currents at the surface, seafloor observations can improve understanding of ocean circulation and its role in global climate, she adds.

So far, Marra's team is the only one using this method.

They are working on making it easier to deploy and providing more accessible light sources.

Researchers continue to push fiber optic-based sensing techniques to new frontiers.

Earlier this year, Fichtner and a colleague traveled to Greenland, where the East Greenland Ice Core Project is drilling a well deep into the ice sheet to extract an ice core.

Next, Fichtner's team manually lowered a fiber optic cable 1,500 meters—and caught a cascade of ice quakes, rumbles that result from bedrock rubbing against the ice sheet.

Ice earthquakes can deform ice sheets and contribute to their flow towards the sea.

But until now researchers have not been able to investigate how they are produced: they are invisible on the surface.

Perhaps fiber optics will finally bring its hidden processes to light.

Article translated by Debbie Ponchner.

This article originally appeared on

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