Beneath the soil, there lies a vast network of wires. Installed mostly in the 1990s by telecommunications companies, these hundreds of thousands of miles of fiber optic cables were quickly rendered moot by advances in technology that no longer necessitated such heavily wired communications infrastructure. Today, most of these cables sit dormant. They’re called “dark fiber,” because no electricity sparkles along their lengths.
But scientists at the Department of Energy’s Lawrence Berkeley National Laboratory have found a new use for all of that unused fiber. In two new research papers published Scientific Reports and Geophysical Research Letters, the researchers show the implications of using a dark fiber network to measure seismic activity and other environmental changes. In other words, we’re sitting on a ready-made earthquake sensor that crosses the planet.
An Earth-Sized Sensor
The project began with the research group, led by the geophysicist Jonathan Ajo-Franklin, trying to use dark fiber to measure how fast permafrost is thawing in Alaska for the Department of Defense. The DoD has a lot of infrastructure in Alaska, like runways and roads, and it wanted to know how much maintenance would be required as the ground itself starts to change.
Initially, Ajo-Franklin and his team, including graduate students Nate Lindsey and Shan Dou, installed their own fiber optic cables near Fairbanks, Alaska, to test how much the permafrost was melting. But they also started measuring other things, like seismic activity and the properties of the soil based on vibrations from ground traffic.
Once back in California, Ajo-Franklin decided to try utilizing existing (but dormant) dark fiber to establish a way to measure all kinds of environmental changes, like earthquakes, ground water levels, and soil composition. He started tracking seismic activity on a dark fiber network owned by the Department of Energy, primarily focusing on a 50-mile section running between Sacramento and northern town of Colusa, California.
The process involves an instrument called a laser interrogator, which assesses changes to the strain in the cable that would occur if there are earthquake tremors. Seismographs provide similar measurements with a different technology, but Ajo-Franklin’s idea of using an established network has a lot of advantages. “If you want a very high density network then it’s much less expensive to use the dark fiber,” Ajo-Franklin says. “One thing which large networks do is they give you an ability to do a better job of recording small earthquakes.” That’s because the likelihood you’d have a sensor closer to where an earthquake happens is greater when you have more sensors spread more widely.
There are a few applications for the earthquake sensing technology that Ajo-Franklin is really excited about. For one, there are no seismometers in the ocean, making it difficult to pinpoint exactly where an earthquake occurs, much less give coastal areas any warning about it. But there are offshore fiber optic cables that provide high-speed internet connections around the world, and these cables could potentially do double duty as communications infrastructure and earthquake detectors.
Theoretically, that might not even necessitate a dedicated fiber–one wavelength of laser could send your cat GIFs across the ocean while another wavelength measures earthquakes. Ajo-Franklin says no one has achieved this yet–but it’s a ripe area for research.
More Data About A Volatile World
Repurposing dark fibers on land could be incredibly helpful too, especially in places like California, where earthquakes are common, and in Oklahoma, which has almost no seismic monitoring but has recently become seismically active. In Oklahoma, earthquakes have begun happening over the last five years or so as oil companies have injected wastewater into reservoirs deep in the Earth, changing the amount of pressure beneath the ground.
Dark fiber networks aren’t just good for measuring the thaw rate of permafrost and earthquake activity. They can also be used to measure soil properties, and the depth of the underground water table–something that Ajo-Franklin says could be really helpful for states like California and Arizona that struggle with water management. Other researchers have used dark fiber to detect whether someone is walking or driving on the surface, like ready-made smart city infrastructure.
One of the biggest challenges the researchers face is what to do with the massive amounts of data that this process produces. Ajo-Franklin’s grad students would come back from working in Alaska with suitcases filled with eight-terabyte hard drives; the team has captured 750 terabytes of data over the last two years. “My office is filled with hard drives,” Ajo-Franklin says.
Once you’ve stored all that data, making sense of it is an entirely different challenge. The dark fiber network wasn’t installed with this use in mind, so some of the cables are buried at different depths while others are installed underneath bridges. Vibrations on those cables don’t mean an earthquake is happening–there’s just a semi-truck passing overhead. Ajo-Franklin plans to employ his methodology for seismic sensing to undersea cables next, and has began to look for partners. He also hopes to develop better methods for measuring ground water through the dark fiber networks on land.
As the climate continues to change, any data that helps scientists understand what is happening and why will be incredibly useful. If scientists can figure out how to use dark fiber effectively, this vast system of fiber optic cables that lie sleeping beneath our feet could provide more–and better–data on our planet.