How Underground Fiber Optics Spy on Humans Moving Above
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.
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.”