Fiber optic cables could help scientists understand earthquakes

Wednesday, January 3, 2018
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For over a century, scientists have studied earthquakes using seismographs -- instruments that use magnets to measure vibrations from deep underground.

BERKELEY, Calif. (KGO) -- For over a century, scientists have studied earthquakes using seismographs -- instruments that use magnets to measure vibrations from deep underground.

But now, Lawrence Berkeley National Lab has big plans for something much smaller. In fact, it's about the width of a human hair.

"The (telecom) companies, especially during the first dot-com boom, installed a huge amount of fiber underground, with the expectation that the internet would become a big thing and people would use it all the time," said staff scientist Jonathan Ajo-Franklin, holding up a fiber-optic cable with its outer plastic jacket stripped away to expose the 12 strands inside.

The companies were right -- but they underestimated how quickly their methods of data transmission would improve. Fiber became so efficient that they never wound up needing everything they installed, Ajo-Franklin explained. Now, the unused, so-called "dark" fibers could be the key to learning more about earthquakes.

"The basic idea is to shine a short pulse of laser light into an optical fiber and record the backscattered photons," explained Nate Lindsey, a UC Berkeley Ph.D. student who's been working on the research.

Because light travels at a constant speed, researchers can calculate the position of tiny imperfections along the glass fiber as the laser light bounces off them and comes back to the source. During an earthquake, the fiber stretches -- ever so slightly -- and the timing changes. After a lot of data processing, the measurements come to look just like those from a seismograph: one with sensors placed every 1-2 meters along up to 30km of fiber. Because the fiber is already in the ground, this data can be collected in places where it would be too difficult or expensive for traditional sensors to go.

"Offshore and in urban areas, where we don't have sensors currently," Lindsey said.

The massive amounts of data generated during Berkeley Lab's initial dark fiber studies have quickly filled up a growing mountain of bulky 16-terabyte hard drives, and the team is already running out of room to store them all. But the data could become vitally important for science that goes beyond the study of earthquakes.

"I would say i listen to noise most of the time," said Shan Dou, who was a postdoctoral researcher in Ajo-Franklin's lab during the initial dark fiber studies.

Dou is using vibrations from passing trains and trucks to make a sonogram of what's deep underground -- an area of science known as ambient noise interferometry.

"Just by using this dark fiber, you're getting a picture of where potentially the bedrock interface is, the depth of the water table ... and some of the changes in soil type," Ajo-Franklin explained.

Dou said because moisture affects the way soil responds to vibration, gathering ambient noise data over long periods of time could help monitor water stored in aquifers and even track the thawing of the arctic permafrost. Here in California, it could also help cities prepare for a major earthquake.

"Knowing how stiff or how soft the soil is, that can help us to allocate first aid, emergency response, in a smart way," Dou said.