Scientists have discovered for the first time that it is possible to use dark fiber as sensors for the detection of earthquakes, changes in the condition of permafrost, the existence of groundwater and various other activities that take place under the surface of Earth. Department of Energy's Lawrence Berkeley National Laboratory's (Berkeley Lab) scientists have recently conducted the test successfully on dark fiber, which is nothing but the unused massive network of fiber-optic cables that are lying there under the surface of Earth.
A team of scientists led by researcher Jonathan Ajo-Franklin, declared that they have successfully managed to combine a technology, known as "distributed acoustic sensing," with the original processing techniques. These technologies together would allow scientists to detect tremors more accurately than conventional seismometers.
"This has huge potential because you can just imagine long stretches of fibers being turned into a massive seismic network. 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," said Shan Dou, a postdoctoral researcher at Berkeley Lab.
The paper, "Distributed Acoustic Sensing for Seismic Monitoring of the Near Surface: A Traffic-Noise Interferometry Case Study," was published in Nature's Scientific Reports in September 2017. Recently, a follow-up study, "Fiber-Optic Network Observations of Earthquake Wavefields," has been published in Geophysical Research Letters (GRL). In this study, Ajo-Franklin's group of scientists verified the practicality of fiber-optic cables and its use for detecting earthquakes.
Let's come to dark fibers now. As said earlier these are the fiber-optic cables that have been laid out under the planet's surface. The cable companies started installing these cables underground during the early 90s and now there is an overabundance. The more the technology for transmitting data improved over the time, the lesser amount of cables was needed. Now we have impenetrable dark fiber stacks subsurface, interweaving an entire nation.
The researchers conducted a test at Richmond in California, where fiber-optic cables are present underground in a shallow L-shape. They confirmed that seismic waves produced by cars and trains can be used to monitor and picture the thin soil layer's mechanical attributes.
"Imagine a slinky - it can compress or wiggle. Those correspond to different ways you can squeeze the soil, and how much energy it takes to reduce its volume or shear it," stated Ajo-Franklin. "The neat thing about it is that you're making measurements across each little unit of fiber. All the reflections come back to you. By knowing all of them and knowing how long it takes for a laser light to travel back and forth on the fiber you can back out what's happening at each location. So it's a truly distributed measurement," added the scientist.
In a second study, which was published in GRL, the team of researchers measured the data using DAS technique on fiber-optic cables across three places: one in Alaska and two in California. In each of these cases, the DAS technique proved to be relatively responsive to earthquakes, despite the higher level of noise.
Ajo-Franklin also stated that that dark fiber has certain advantages over conventional seismometers. Dark fiber is present almost everywhere, whereas seismometers are difficult to install as they are expensive. Installations under the seas are mainly limited. Also, thick spatial sampling is possible with fiber because of their close proximity. On the other hand, typical seismometers have a distance of several kilometers between each other.