When last spring’s lockdown quieted the Penn State campus and surrounding town of State College, a jury-rigged instrument was “listening.” A team of researchers from the university had tapped into an underground telecom fiber optic cable, which runs two and half miles across campus, and turned it into a kind of scientific surveillance device.
By shining a laser through the fiber optics, the scientists could detect vibrations from above ground thanks to the way the cable ever so slightly deformed. As a car rolled across the subterranean cable or a person walked by, the ground would transmit their unique seismic signature. So without visually surveilling the surface, the scientists could paint a detailed portrait of how a once-bustling community ground to a halt, and slowly came back to life as the lockdown eased.
They could tell, for instance, that foot traffic on campus almost disappeared in April following the onset of lockdown, and stayed gone through June. But after initially declining, vehicle traffic began picking up. “You can see people walking is still very minimal compared to the normal days, but the vehicle traffic actually is back to almost normal,” says Penn State seismologist Tieyuan Zhu, lead author on a new paper describing the work in the journal The Seismic Record. “This fiber optic cable actually can distinguish such a subtle signal.”
More specifically, it’s the frequency in the signal. A human footstep generates vibrations with frequencies between 1 and 5 hertz, while car traffic is more like 40 or 50 hertz. Vibrations from construction machinery jump up past 100 hertz.
Fiber optic cables work by perfectly trapping pulses of light and transporting them vast distances as signals. But when a car or person passes overhead, the vibrations introduce a disturbance, or imperfection: a tiny amount of that light scatters back to the source. Because the speed of light is a known quantity, the Penn State researchers could shine a laser through a single fiber optic strand and measure vibrations at different lengths of the cable by calculating the time it took the scattered light to travel. The technique is known in geoscience as distributed acoustic sensing, or DAS.
A traditional seismograph, which registers shaking with the physical movement of its internal parts, only measures activity at one location on Earth. But using this technique, the scientists could sample over 2,000 spots along the 2.5 miles of cable—one every 6 and a half feet—giving them a superfine resolution of activity above ground. They did this between March 2020, when lockdown set in, and June 2020, when businesses in State College had begun reopening.
Just from those vibrational signals, DAS could show that on the western side of campus, where a new parking garage was under development, there was no industrial activity in April as construction halted. In June, the researchers not only detected the vibrations from the restarted machinery, but could actually pick out the construction vehicles, which hummed along at a lower frequency. Still, they noted, by this time pedestrian activity on campus had barely recovered, even though some pandemic restrictions had eased.
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.