Take a moment to think about the surface ground beneath your feet. Plunge deeper still and think about the world unseen. There are networks of tree roots in the soil, but also networks of another sort: fiber-optic cables that connect us to each other via phone and Internet service.
Researchers from Penn State University have developed an innovative idea for piggy-backing on these existing cables: severe weather monitoring. The team turned to their own campus as a testing ground during a thunderstorm on April 15 of this year. Using distributed acoustic sensing (DAS) array technology, they sent a laser down a hair-thin fiber inside miles of cables 1 meter (3.3 feet) deep under the campus and took measurements every 2 meters (6.5 feet), making for a total of 2,000 sensors.
"If there is any change in the external energy on the ground above, even walking steps, you will have a very small change that's going to stretch or compress the fiber," said Tieyuan Zhu, assistant professor of geophysics at Penn State. "The laser is very sensitive and can detect these small changes."
During a storm such as the one in April, thunder from above smashes the ground and acoustic pressure spreads like a wave in a pond. These thunderquakes are registered and recorded, providing “yet another way to track thunderstorms and help with public safety and emergency response, especially in urban areas," said David Stensrud, co-author of the study published in the Journal of Geophysical Research: Atmospheres.
The team identified 18 thunder-induced seismic events with peak frequencies ranging from 20 Hertz to 130 Hertz. The arrival time of the thunderquakes allowed the scientists to estimate the phase velocity of the near-surface, the back azimuth, and the path of the source as it moved from northwest to south to northeast. The location data was verified with the National Lightning Detection Network (NLDN).
"Severe weather has strong interactions with the ground, but we haven't had the capability to study the coupling between the atmosphere and the solid Earth," added Zhu. "With this new technology, we can utilize existing fiber-optics networks to clearly see how thunderstorm energy passed through campus."
The sensing technology is a relatively recent technique developed to monitor acoustic strain over large distances via scattering sites within the fiber that serve as a distributed interferometer. The use of DAS with fiber-optic cables can extend to other natural hazards such as hurricanes, earthquakes, and flooding. Extreme weather events often transfer energy to the solid Earth and create seismic waves, which can damage buildings and lead to global economic loss, human injury, and more.
"This research is an example of taking an existing technology and using it to serve another purpose," Stensrud said. "Having technologies that are multifunction maximizes the benefits to society.”
A different team earlier this year demonstrated the use of DAS in dark fiber – a term for unused fiber-optic cables – for earthquakes by recording seven months of passive seismic data across a 27-kilometer (17-mile) stretch of land in California.