In an article in Seismological Research Letters, California Institute of Technology seismologist Zhongwen Zhan describes a method whereby disturbances in long optical fibers are used to detect seismic activity. The method—called Distributed Acoustic Sensing—works by using the tiny internal flaws of a long optical fiber as thousands of seismic sensors along tens of kilometers of fiber optic cable. An instrument at one end sends laser pulses down a cable and collects and measures the "echo" of each pulse as it is reflected off the internal fiber flaws.
When changes in temperature, strain or vibrations disturb the fiber, there are changes in the size, frequency and phase of laser light scattered back to the DAS instrument. Seismologists can use these changes to determine the kinds of seismic waves that might have nudged the fiber, even if just by a few tens of nanometers.
The sensitivity of DAS instruments has improved markedly over the past five years, opening new possibilities for their deployment, according to Zhan. "The sensitivity is getting better and better, to the point that a few years ago that if you compare the waveforms from a fiber section with a geophone, they look very similar to each other."
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Their performance suits them for use across various environments, especially where it would be too expensive to set up a more sensitive or dense seismic network. Researchers can also tap into the large amounts of unused or "dark" fiber that has been laid down previously by telecommunication companies and others. A few strands off a larger cable, said Zhan, would serve a seismologist's purposes.
According to Zhan, the oil and gas industry has been a key driver of the new method, as they used cable down boreholes to monitor fluid changes in deep-water oil fields and during hydraulic fracturing and wastewater injection.
DAS researchers think the method is especially promising for seismic monitoring in harsh environments, like Antarctica—or the moon. With a regular network of seismometers, scientists "need to protect and power each node" of instruments in the network, Zhan explained. "Where for DAS, you lay down one long strand of fiber, which is fairly sturdy, and all your sensitive instruments are only at one end of the fiber."
According to the article, scientists are already using DAS to probe thawing and freezing cycles in permafrost and on glaciers, to better characterize their dynamic motion of ice flows and sliding on bedrock, which could help researchers learn more about how glacial melt driven by climate change contributes to sea level rise.
At the moment, the range of most DAS systems is 10 to 20 kilometers. Researchers hope to extend this in the near future to 100 kilometers, Zhan said, which could be useful for seismic coverage in ocean bottom environments, including offshore subduction zones.
DAS is also suited for rapid response after earthquakes, especially in areas where dark fiber is numerous and seismologists have made arrangements to use the fiber beforehand. After the 2019 Ridgecrest earthquakes in southern California, for instance, Zhan and his colleagues moved quickly to monitor the aftershock sequence in the area using DAS. "We turned about 50 kilometers of cable into more than 6,000 sensors in three days," he said.