A jury-rigged instrument was “listening” when the Penn state university and the neighboring town of state college were quiet, due to the pandemic last spring. A group of university researchers installed a two-and-a-half-mile underground fiber-optic telecommunications cable across campus and converted it into a kind of scientific surveillance equipment.
The scientists were able to detect vibrations from above ground by shining a laser through the fiber optics due to the small deformation of the wire. The ground would send its distinctive seismic signature as a car rolled across the subsurface cable or as a person walked by. The scientists were able to construct an accurate picture of how a once-bustling community came to a halt and then slowly came back to life as the lockdown was lifted without having to look at the ground.
They could see, for instance, that when the lockdown started in April, foot traffic on campus practically stopped and stayed that way until June. Car traffic, on the other hand, increased after initially declining. It’s the signal’s frequency, to be more precise. The vibrations produced by a human footstep range from 1 to 5 hertz, whereas car traffic is closer to 40 or 50 hertz. Vibrations from construction equipment exceed 100 hertz.
How does it work?
Fiber optic cables work by properly capturing light pulses and transferring them as signals over long distances. When a vehicle or individual passes overhead, however, the vibrations cause a disruption, or imperfection, in which a little amount of light reflects back to the source. The Penn State researchers could flash a laser down a single fiber optic strand and quantify vibrations at different lengths of the cable by estimating the time it took the dispersed light to traverse because the speed of light is known. Distributed acoustic sensing, or DAS, is the name given to the approach in geoscience.
A typical seismograph only measures activity at one spot on Earth since it registers to shake with the physical movement of its interior pieces. The scientists were able to sample almost 2,000 places along the 2.5 miles of cable, one every 6 and a half feet, using this method, providing them a superfine resolution of activity above ground. This was done by them between March 2020, when the lockdown was imposed, and June 2020, when State College companies began to reopen.
Because building on the western side of campus, where a new parking garage was under construction, had ceased, DAS could detect there was no industrial activity in April. The researchers were able to distinguish the construction vehicles, which hummed at a lower frequency, from the vibrations from the restarted machinery in June. Even though certain pandemic restrictions had been lifted, they found that pedestrian activity on campus had barely rebounded by this time.
DAS has the potential to be a useful tool for tracking people’s movements: Researchers might use fiber optic cables to follow the movement of pedestrians and autos instead of digging through cell phone location data. However, the technology is unable to precisely identify a vehicle or a person.
Even though you wanted to track someone as they walked across a metropolis, they’d have to stroll along the cable you’re watching all the time. You’d lose their seismic signal as soon as they went off. However, you can’t pin it on a single person. Basically, you’d be far better off using binoculars or their mobile data to monitor an individual at a distance.
Because of “dark fiber,” the use of DAS has recently exploded across the sciences. Telecom firms began putting down a lot of fiber optic cable as the internet expanded in popularity in the 1990s. Companies established more than they required in the preparation for the web boom because the cable itself is very inexpensive compared to the work required to drill the holes to lay it. A great quantity of the fiber is still unused, or “dark,” and can be rented out to scientists for studies.
Its availability, however, is dependent on the location. “So maybe there’s a lot of contention for that fiber in New York, between the stock market and New Jersey,” says Jonathan Ajo-Franklin, Rice University geophysicist who wasn’t involved in the new work but is an associate editor at the journal that published it. “Going into rural Nevada on a long journey, perhaps there’s more that you can use,” he adds.
This cable, unlike standard seismometers, is affordable and does not require a power supply. With DAS, all you need is a device called an “interrogator” that shoots the laser and receives the data sent via the fibers. Ajo-Franklin explains explained that whether you want to conduct measurements of earthquakes, surface waves, or urban mobility, this is a fantastic opportunity. Ajo-Franklin, for example, previously used a 17-mile stretch of black fiber near Sacramento to track 7 months of strong and mild earthquakes.
DAS is already being used by civil engineers to examine soil deformation, while biologists are employing offshore fiber optic connections to listen in on whales. (And besides, sound travels like a vibration.) In terms of applications, it’s just really exploding. To take temperature measurements, they embed fibers in glaciers and tow them behind boats in the free water column. It’s a truly remarkable combination of technologies. So the next time you’re out for a walk, take a moment to appreciate the science that’s going on beneath your feet.