The vast network of unused fiber-optic cables installed throughout the country and the world can be used as sensors to detect earthquakes, the presence of groundwater, changes in permafrost conditions and other subsurface activity.

In a pair of recently published papers, a team led by Berkeley Lab researcher Jonathan Ajo-Franklin says they successfully combined a technology called “distributed acoustic sensing,” which measures seismic waves using fiber-optic cables, with processing techniques to allow reliable seismic monitoring. The say the results are comparable to what conventional seismometers can measure.

“This has huge potential because you can just imagine long stretches of fibers being turned into a massive seismic network,” says Shan Dou, a Berkeley Lab postdoctoral fellow.

The idea is that by using fiber, researchers can transform traffic noise or other vibrations into usable seismic signals that can help monitor near-surface changes such as permafrost thaw and groundwater-level fluctuations.

Dark fiber refers to unused fiber-optic cable, much of it installed in the early 1990s by telecommunications companies. Just as the cables were buried underground, the technology for transmitting data improved so that fewer cables were needed. Dense corridors of dark fiber crisscross the entire country.

Distributed acoustic sensing (DAS) technology measures seismic wavefields by shooting short laser pulses across the length of the fiber. “The basic idea is, the laser light gets scattered by tiny impurities in the fiber,” said Ajo-Franklin. “When fiber is deformed, we will see distortions in the backscattered light, and from these distortions, we can measure how the fiber itself is being squeezed or pulled.”

Using a test array in Richmond, Calif. – with fiber-optic cable placed in a shallow L-shaped trench, one leg of about 100 meters parallel to the road and another perpendicular – the researchers verified that they could use seismic waves generated by urban traffic, such as cars and trains, to image and monitor the mechanical properties of shallow soil layers.

The measurements give information on how “squishy” the soil is at any given point, making it possible to infer information about the soil properties, such as its water content or texture

Having proven the concept under controlled conditions, the team says they expect the technique to work on a variety of existing telecommunications networks. Ongoing research in Alaska is also exploring the same technique for monitoring the stability of Arctic permafrost.

Ajo-Franklin said that dark fiber has the advantage of being nearly ubiquitous. Traditional seismometers can be expensive, are sparsely installed and subsea installations are particularly scarce. Additionally, fiber allows for dense spatial sampling, meaning data points are only meters apart, whereas seismometers typically are separated by many kilometers.