While this prolonged lockdown has quieted all the places and surrounding towns of the world, a jury-rigged instrument continues “listening.” A team of researchers from the Penn State campus university had tapped into an underground telecom fiber optic cable, which runs two and half miles across the campus, and turned it into a kind of scientific surveillance device.
By shining a laser through the fiber optics, the scientists were able to detect vibrations above ground thanks to its ever so slight deformation. As a car rolled over the subterranean cable or as a person walked by, the ground would transmit their unique seismic signature. Scientists were able to produce a detailed portrait of how a once-bustling community ground to a halt, and slowly began to return to life as the lockdown eased.
For instance, foot traffic on campus almost disappeared in April following the lockdown, and remained gone through June. Despite initially declining traffic, vehicle traffic then began to increase. In spite of the limited traffic on the street, the vehicle traffic is almost back to normal as well. This fiber optic cable is able to distinguish such a “subtle signal” , says Penn State seismologist Tieyuan Zhu in a new paper describing the research.
Vibrations from cars and pedestrians create unique signals in cables. Now scientists have used the trick to show how Covid-19 brought life to a halt.
A signal’s frequency is what determines its quality. Footsteps produce vibrations between 1 and 5 hertz, while car traffic is closer to 40 or 50 hertz and machine vibrations exceed 100 hertz when working on construction sites. Fiber optic cables operate by perfectly trapping pulses of light and transmitting them across vast distances as signals.
Nevertheless, when a car or person passes overhead, the vibration introduces a disturbance: a tiny amount of that light scatters back to the source. Because the speed of light is known, the Penn State researchers were able to shine a laser through a single fiber optic strand and calculate the time it took the scattered light to travel at various lengths of the cable. This is Geosciences, it refers to the technique as distributed acoustic sensing, or DAS.
Traditionally, seismographs measure earthquakes where they register physical movements in their internal parts. Using this technique, the scientists were able to sample over 2,000 spots every 6 and a half feet along the 2.5 miles of cable, providing a very fine resolution of activity above ground. This was done between March 2020, when the lockdown began, and June 2020, when businesses in State College began to reopen.
As a result of these vibrational signals, DAS could determine that there was no industrial activity on campus’ western edge, where a parking garage was being built, in April as construction halted.In June, researchers not only detected vibrations from the restarted machinery, but were able to identify the construction vehicles, which hummed along at a lower frequency. Despite some easing of pandemic restrictions, they noted that pedestrian activity on campus had barely recovered by this point.
DAS could be a powerful tool to track people’s movements. Rather than sifting through cell phone location data, researchers could instead tap into fiber optic cables to track pedestrians and cars. However, the technology can’t exactly identify a car or person. There’s no way to say “Oh, this is a Nissan Sentra, 2019” says Stanford geophysicist Ariel Lellouch, who uses DAS but was not involved in the study, but did peer review it.
In fact, what makes DAS so valuable is its anonymity. It wouldn’t be possible to track someone moving through a city unless they were constantly walking along the cable.As soon as they veered off-course, their seismic signal would be lost.Lellouch explains, “Roughly speaking, if you have a fiber and somebody is walking along it – let’s say it’s in the desert – and that’s the only person who is walking, yes, you can track.” Basically, if you want to track someone at a distance, you’d be better off using binoculars or cell data.
Recent studies show that the use of DAS is booming across the sciences, thanks to dark fiber. In the 1990s, as the internet grew, telecom companies laid down a lot of fiber-optic cable.In anticipation of the web boom, companies planted more than they needed because the cable itself is relatively cheap.
Currently, much of that fiber is still unused, or “dark,” and available for scientists to rent. Availability depends on the location, however. According to Rice University geophysicist Jonathan Ajo-Franklin, who wasn’t involved in this paper but worked on its journal publishing. “Going across rural Nevada on a long-haul route, maybe there’s extra there you can use.
Seismometers typically require a source of power, but this cable doesn’t need one. DAS simply requires an “interrogator” device that fires the laser and receives the data coming through the fiber. Ajo-Franklin says this is a good opportunity to gather closely spaced data about earthquakes, surface waves, and urban mobility.
For example, Ajo-Franklin once recorded 7 months of earthquakes using a 17-mile stretch of dark fiber near Sacramento. Scientists are even experimenting with fiber optic cables buried offshore to listen to whales, and civil engineers are already using DAS for studying soil deformation (since sound propagates as a vibration, after all). The applications for Fiber Optics have just exploded; people are embedding fibers into glaciers and dragging them behind boats in the water column to measure temperature. It’s quite a remarkable set of technologies!
Next time you’re out for a stroll, appreciate the science that may be humming along beneath your feet. Next time you’re out for a stroll, appreciate the science that may be humming along beneath your feet.
