Dark fiber, subsurface cables | Homeland Security Newswire

InfrastructureDark fiber: Sensors beneath our feet to tell us about earthquakes, water, other geophysical phenomenon

Published 6 December 2017

Scientists have shown for the first time that dark fiber – the vast network of unused fiber-optic cables installed throughout the country and the world – can be used as sensors for detecting earthquakes, the presence of groundwater, changes in permafrost conditions, and a variety of other subsurface activity.

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have shown for the first time that dark fiber – the vast network of unused fiber-optic cables installed throughout the country and the world – can be used as sensors for detecting earthquakes, the presence of groundwater, changes in permafrost conditions, and a variety of other subsurface activity.

In a pair of recently published papers, a team led by Berkeley Lab researcher Jonathan Ajo-Franklin announced they had successfully combined a technology called “distributed acoustic sensing,” which measures seismic waves using fiber-optic cables, with novel processing techniques to allow reliable seismic monitoring, achieving results 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,” said Shan Dou, a Berkeley Lab postdoctoral fellow. “The idea is that by using fiber that can be buried underground for a long time, we can transform traffic noise or other ambient vibrations into usable seismic signals that can help us to monitor near-surface changes such as permafrost thaw and groundwater-level fluctuations.”

Dou is the lead author of “Distributed Acoustic Sensing for Seismic Monitoring of the Near Surface: A Traffic-Noise Interferometry Case Study,” which was published in September in Nature’s Scientific Reports and verified the technique for monitoring the Earth’s near surface. More recently, Ajo-Franklin’s group published a follow-up study led by UC Berkeley graduate student Nate Lindsey, “Fiber-Optic Network Observations of Earthquake Wavefields,” in Geophysical Research Letters (GRL), which demonstrates the viability of using fiber-optic cables for earthquake detection.

What is dark fiber?
LBL says that dark fiber refers to unused fiber-optic cable, of which there is a glut thanks to a huge rush to install the cable in the early 1990s by telecommunications companies. Just as the cables were buried underground, the technology for transmitting data improved significantly so that fewer cables were needed. There are now dense corridors of dark fiber crisscrossing the entire country.