How Underground Fiber Optics Spy on Humans Moving Above

DAS could be a powerful tool to track people’s movement: Instead of sifting through cell phone location data, researchers could instead tap into fiber optic cables to track the passage of pedestrians and cars. But the technology can’t exactly identify a car or person. “You can say if it’s a car, or if it’s a truck, or it’s a bike. But you cannot say, ‘Oh, this is a Nissan Sentra, 2019,’” says Stanford University geophysicist Ariel Lellouch, who uses DAS but wasn’t involved in this study but did peer-review it. “Anonymity of DAS is one of the biggest benefits, actually.”

Even if you wanted to track a person as they traveled through a city, they’d have to be continuously walking along the cable you’re monitoring. As soon as they’d veer off-course, you’d lose their seismic signal. “Roughly speaking, if you have a fiber and someone is walking along that fiber—let’s say it’s in the desert—and that’s the only person that’s walking, yes, you can track,” says Lellouch. “But you cannot attribute it to a specific person.” Basically, if you want to track an individual at a distance, you’d be way better off with binoculars or their cell data.

Lately, the use of DAS is booming across the sciences, thanks to “dark fiber.” As the internet grew in the 1990s, telecom companies began laying down a whole lot of fiber optic cable. The cable itself is relatively cheap compared to the labor it takes to dig the holes to lay it, so, in anticipation of the web boom, companies planted more than they needed. Today, much of that fiber is still unused, or “dark,” available for scientists to rent out for experiments.

Its availability depends on the location, though. “So maybe downtown New York, between the stock exchange and New Jersey, there’s a lot of contention for that fiber,” says Rice University geophysicist Jonathan Ajo-Franklin, who wasn’t involved in this new paper but is an associate editor at the journal publishing it. But, he adds, “going across rural Nevada on a long-haul route, maybe there’s extra that you can make use of.”

Unlike traditional seismometers, this cable is inexpensive and doesn’t require a source of power. With DAS, you just need an “interrogator” device that fires the laser and receives the data coming through the fibers. “So it’s really a great opportunity if you want to acquire this closely spaced data to make measurements of earthquakes or surface waves or urban mobility,” Ajo-Franklin says. For instance, Ajo-Franklin once used a 17-mile stretch of dark fiber near Sacramento to record 7 months of earthquakes, large and small.

Civil engineers are already using DAS to study soil deformation, and biologists are even using offshore fiber optic cables to listen in on whales. (Sound propagates as a vibration, after all.) “It’s just really exploding in terms of the applications,” says Ajo-Franklin. “People are embedding fibers in glaciers and dragging them behind boats in the free water column to make temperature measurements. It’s really kind of an amazing set of technologies.”

So the next time you’re out for a stroll, stop to appreciate the science that may be humming along under your feet. Or, if you’re feeling puckish, jump up and down really hard.

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