Simple fiber-optic cables that are essential for powering the internet can be a useful tool in spotting geohazards such as sinkholes, according to a new study led by researchers at Penn State.
Using existing communications cables buried just a few feet below the ground under Penn State's University Park campus, the team developed a new way to use existing acoustic sensing technology. The approach, recently detailed in the Journal of Geophysical Research: Solid Earth, can plot fractured zones - such as sinkholes - as deep as hundreds of feet below the Earth's surface.
The researchers' method involves a tool they developed called the distributed acoustic sensing (DAS) interrogator, which they attached to preexisting telecommunications fiber optic cable that stretched roughly four miles across University Park. The DAS interrogator shot a beam of light across the cable, capturing acoustic signals throughout. Because campus is bustling with activity, researchers developed a series of computational methods for isolating sound waves that correlated to rock density and omitting sounds generated by cars, students or construction.
"We geoscientists often think of inputs such as everyday traffic as noise in the data. However, our research shows that the so-called 'garbage noise' is very useful," said Tieyuan Zhu, associate professor of geophysics and corresponding co-author of the paper. "We can rely on these signals to spot geohazards in a way that's much more affordable and effective than traditional methods."
Traditionally, instruments called geophones are used for measuring ground density. These tools are expensive, require human deployment and create a lone data point. The new approach, turning everyday traffic noises into seismic surface waves using cross correlation, paints a detailed picture of the surface wave speed below the cable and offers a mesh of data points, researchers said.
Imagine shouting into the Grand Canyon. The distance your voice travels before it echoes back can reveal a lot about the depth and distance of the canyon. Sound travels through the ground in a similar way. The denser the rock, the slower the sound wave moves through it.
The proof-of-concept work really hit home after the researchers flagged a low-density area that has the potential to produce a sinkhole deep below the surface. In modeling of the acoustic signals, the area on campus displays as a low-velocity structure that's much less dense than the ground surrounding it at the same depth, the researchers suggested in the paper.
The region's Karst geology - the caves, low-density rocks, springs and other common landscape features across Pennsylvania - is known for its soluble rocks, primarily limestone and dolomite, which are weakened by acidic water, so it wasn't a big surprise to spot what appears to be a low-density area, Zhu said.
Professionals at Penn State's Office of Physical Plant and their contractors reviewed the researchers' data and determined there is no imminent danger to structures on campus due to the potential underground void.
Zhu added that the technique could prove useful in future planning. DAS technology is already being deployed on a larger scale to help prevent disasters in Pittsburgh through a Civic Innovation Challenge grant from the U.S. National Science Foundation.
"Sinkholes are widespread in Pennsylvania and beyond," Zhu said. "What makes this research especially powerful is that it turns everyday traffic noise - something completely free - into a tool for locating geohazards. By using the existing fiber optic cables already in place as sensors, we can provide an affordable and scalable way to assess risks and help prevent future threats for Pennsylvanians."
Zhinong Wang, a postdoctoral scholar in geosciences, co-authored the research that was funded by U.S. National Science Foundation. The research is part of the Fiber-Optic foR Environmental SEnsEing (FORESEE) project, led by Zhu, that collects high-resolution acoustic vibration data using underground telecommunication fiber-optic cables. Prior research has demonstrated the ability to aid in energy extraction, severe weather forecasting and even tracking the score of a Penn State football game at Beaver Stadium.
