Seismic waves produced by thunderstorms, called thunderquakes, can be used as a novel source for seismic imaging, according to a new study led by researchers at Penn State.
Using existing fiber-optic telecommunication cables buried just a few feet below the ground under Penn State's University Park campus, the researchers developed a new way to use distributed acoustic sensing technology, or DAS, to better understand how atmospheric acoustic waves transfer energy into the ground. They also demonstrated how this coupling can be used in seismic imaging. Their findings were published today (Aug. 21) in Science Advances.
"Thunder generates atmospheric acoustic waves that couple into the ground, producing seismic signals," said Tieyuan Zhu, associate professor of geosciences at Penn State and corresponding author on the paper. "We demonstrated the first successful seismic imaging using thunderquakes. Not only does this research serve as a proof of concept for using thunderquakes as seismic sources for tomography, but DAS provided a new way to observe the interaction between the atmosphere and the solid Earth."
To assess seismic events near Earth's surface, researchers use an approach called seismic tomography, which can produce an image of the subsurface from waves recorded by sensors at the surface. In this study, the waves used for imaging were acoustic waves produced by thunder rather than by earthquakes or active seismic equipment.
"Seismic tomography is important because it helps us understand what is happening beneath the surface," said Zhu, who is also a faculty affiliate with the EMS Energy Institute. "Seismic imaging can be used to evaluate additional geohazards like sinkholes or landslides, assess groundwater and mining resources and study volcanoes and magma pockets. Only tomography can give us information about the Earth's subsurface structure that we often cannot observe directly."
Seismic imaging is traditionally done with expensive tools that require human deployment or with passive surveys that rely on seismic sources, like those collected from earthquakes. These passive seismic methods generally require a robust monitoring array, but the thunderquake method allows for passive imaging in places with fewer earthquakes like the central and eastern United States. The method of using existing fiber-optic cables also allows for easier imaging in less accessible locations such as in the Arctic or highly regulated urban areas with minimal disruption to the environment and infrastructure, the researchers said.
In this study, the researchers shot a laser beam down an approximately 2.5-mile preexisting telecommunications fiber-optic cable buried beneath the University Park campus and recorded how the phase of the backscattered light shifts due to tiny strains along the fiber caused by seismic waves moving through the ground.
Lead author Nolan Roth, who conducted this research as part of his doctoral studies at Penn State and is now a postdoctoral researcher at The Ohio State University said that people have tried to use thunder for seismic imaging in the past, but it's been difficult because the process is so complex.
"Without incredibly high-resolution sensing, it's difficult to actually piece together what's going on when the thunder hits the ground," Roth said. "With DAS, we are recording hundreds of samples every second and every few meters along the cable. This allowed us to see what was going on in that transition from atmospheric acoustic source to a seismic signal in very high resolution, which is something that nobody else has been able to do while looking at thunder."
According to the researchers, having a better understanding of the atmosphere-solid earth coupling creates opportunities to further investigate the interaction between Earth's surface and atmosphere.
"Understanding this coupling will allow cross-disciplinary researchers to answer questions in both geosciences and meteorology," Roth said. "Knowing how atmospheres interact with surfaces will also be helpful as we continue to explore outside our own planet. Quakes on other planets and moons aren't well understood, so having a different source for seismic imaging might be necessary."
Whether or not this new technology is eventually employed elsewhere in the solar system, the researchers said they are hoping to expand their work and use their findings beyond Penn State.
"Our research shows that thunderquakes can act a new source for near-surface seismic imaging, especially in regions with limited access to traditional seismic sources," Zhu said. "There are all kinds of weather variability along the East Coast, so the question for the next step is: Can we move forward and think bigger?"
Additional co-authors affiliated with Penn State's Department of Geosciences include postdoctoral scholars Donggeon Kim and Rafał Czarny, who is now at the Institute of Mine Seismology in Australia; graduate student Young Cheol Kim; and Professor of Geosciences Christelle Wauthier.
This work was supported by the Penn State E. Willard and Ruby S. Miller Faculty Fellowship Fund and the U.S. National Science Foundation under award number 2322198. This content is solely the responsibility of the authors and does not necessarily represent the views of the funders.
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