Researchers are now studying earthquakes and geology using unused optical fiber cables

Studies are increasingly starting to show that the same optical fibers that provide high-speed internet and our video can be used to study earthquakes and geology

As a student of geophysics, it was exciting to learn how earthquakes are used to study the Earth’s interior. Earthquakes are pretty much the only reason why we know anything about Earth’s internal structure, and they can tell us a lot about the formation of the crust, mantle, and even the core of our planet. It even gets better: you can “fake” an earthquake with special tools (which can range from a large hammer to dynamite) to assess the geological structure of the area beneath. If you have the right tools for listening to the waves, you can see what’s underground. This approach is still used in geological and industrial studies. But researchers are now thinking that they can start using other tools to do this – tools that are already available.

Specifically, researchers are looking at optical fiber cables – cables that are similar to regular electric cables, but contain one or more optical fibers used to carry light. Optical fiber cables are what give you high speed internet (if you are lucky enough to have high speed internet, that is). What researchers learned recently is that you can also convert these cables into seismic sensors.

“Fiber optic cables are the backbone of modern communications, and we have demonstrated that we can convert existing networks into large-scale seismic arrays to assess ground motions during earthquakes,” said Zach Spica, a seismologist at the University of Michigan, who is the first author of a recent study describing an approach.

When an earthquake happens, it makes the cables vibrate. Researchers take advantage of this by attaching an instrument called a laser investigator to one end of the cable. The investigator fires pulses of laser light at the fiber, with the light bouncing off when it encounters any impurities or defects along the length of the fiber, creating a base-level signal called the backscatter signal.

Changes in this backscatter signal can show how fibers are stretched or compressed in response to perturbations such as seismic waves.

Analysis of seismic wave speeds using optical fiber distributed acoustic sensing technology. Image credit: Zack Spica.

The approach draws from the oil and gas industries, which have used this idea for years to monitor pipelines and wells – but applying it to fiber-optic cable is an entirely different deal.

“This is the first time that fiber-optic seismology has been used to derive a standard measure of subsurface properties that seismologists use to predict the intensity of vibration,” said geophysicist Greg Beroza, a co-author of the paper. Wayne Lowell is a professor in the School of Earth, Energy and Environmental Sciences at Stanford University.

The approach called DAS is attractive from multiple perspectives. Optical fiber infrastructure is not very common in many urban areas of the world (with many of it currently unused and thus can be used as a seismic detector), but urban areas are known to be difficult to monitor seismically because it is difficult to add sensors, and you have a lot. From turbulence like traffic.

“The great thing about using fiber for this purpose is that cities already have it as part of their infrastructure, so all we have to do is take advantage of it,” said Peruza.

“In urban areas, it is very difficult to find a place to install seismic stations because asphalt is everywhere,” Spica added. Additionally, many of these lands are private and inaccessible, and you cannot always let a seismic station stand alone due to the risk of being robbed.

So if you have any unused fiber cable (which is often called “dark fiber”), you can convert it into a very long sensor array, and it’s also very inexpensive. It’s not just about monitoring earthquakes – you can also use DAS to map below the surface.

In a separate study, researchers used DAS to study a 27-kilometer stretch of dark fiber near Sacramento, California, and recorded 7 months of readings and obtained a large amount of useful data. They even used ambient seismic noise from traffic to map below the surface. Similar studies have been done in Alaska, California, and Iceland. What makes this even more exciting for geologists is that you only need to place your fancy gear at one end of the cable.

California Institute of Technology seismologist Zhongwen Zhan, one of the study’s authors, believes that DAS can be used in remote locations, or even to study other extraterrestrial bodies.

He added, “You can imagine that on the moon or any other planet, with high radiation or high temperature, the electronics may not withstand this long period in that environment.” “But fiber can.”

The main limitation of the approach is that you need to know exactly how to lay the cable in the ground. Researchers sometimes perform a “tap test” in which they strike a sledgehammer in various places above the cable, and they measure the impact on the cable. Noise is also a problem that needs to be solved: For some reason, some cables seem to be making a great deal of noise, and it’s not clear why.

In any case, while DAS is still far from perfect, it is already proving to be a versatile and useful tool, which can be deployed at low costs and in many situations. We can only look forward to seeing what it will be used for.

“While DAS is still in its infancy, it has already demonstrated itself as the working heart – or perhaps the eardrum – of a valuable new seismic listening tool,” Zan concluded.

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