Fibre-optic networks could act as earthquake observatories
Image credit: University of Michigan
Existing networks of buried fibre-optic cables have the potential to become an inexpensive observatory for monitoring and studying earthquakes, a new study has demonstrated.
The study, managed by researchers at the University of Michigan and other institutions, demonstrates that the same optical fibres that deliver high-speed internet to our homes could one day double as seismic sensors.
“Fibre-optic cables are the backbone of modern telecommunications, and we have demonstrated that we can turn existing networks into extensive seismic arrays to assess ground motions during earthquakes,” said Michigan seismologist Zack Spica, first author of a JGR Solid Earth paper describing the work.
The research was conducted using a prototype array at Stanford University.
“This is the first time that fibre-optic seismology has been used to derive a standard measure of subsurface properties that are used by earthquake engineers to anticipate the severity of shaking,” said Stanford geophysicist Greg Beroza.
To transform a fibre-optic cable into a seismic sensor, the researchers connected an instrument to one end of the cable which shoots pulses of laser light down the fibre. The light bounces back when it encounters impurities along the fibre, creating a “backscatter signal” analysed with an interferometer.
Changes in the signal revealed how the fibre stretches or compresses in response to passing disturbances, including seismic waves from earthquakes. The technique is known as distributed acoustic sensing, or DAS, and has been used for years to monitor the health of pipelines and wells in the oil and gas industry.
The new study extends previous work with the Stanford test loop by producing high-resolution maps of the shallow subsurface, which scientists can use to see which areas will undergo the strongest shaking in future earthquakes, Beroza said.
The study also demonstrates that optical fibres can be used to sense seismic waves and obtain velocity models and resonance frequencies of the ground: two parameters essential for ground-motion prediction and seismic-hazard assessment. The results are in good agreement with an independent survey that used traditional techniques, validating the methodology of fibre-optic seismology.
This approach appears to have great potential for use in large, earthquake-threatened cities such as San Francisco, Los Angeles, Tokyo and Mexico City, where thousands of miles of optical cables are buried beneath the surface.
“What’s great about using fibre for this is that cities already have it as part of their infrastructure, so all we have to do is tap into it,” Beroza explained.
According to the researchers, many of these urban centres are built atop soft sediments that amplify and extend earthquake shaking. The near-surface geology can vary considerably from neighbourhood to neighbourhood, highlighting the need for detailed, site-specific information. Collecting that information can be a challenge with traditional techniques, which involve the deployment of large seismometer arrays – thousands of the instruments in the Los Angeles area, for example.
“In urban areas, it is very difficult to find a place to install seismic stations because asphalt is everywhere,” Spica said. “In addition, many of these lands are private and not accessible, and you cannot always leave a seismic station standing alone because of the risk of theft.”
“Fibre optics could someday mark the end of such large scale and expensive experiments. The cables are buried under the asphalt and crisscross the entire city, with none of the disadvantages of surface seismic stations.”
The technique would likely be fairly inexpensive, as well, Spica added. Commercial fibre-optic cables contain unused fibres that can be leased for other purposes, including seismology.
At present, traditional seismometers provide better performance than prototype systems that use fibre-optic sensing, the team said. Seismometers also sense ground movements in three directions, while optical fibres only sense along the direction of the fibre.
The researchers disclosed that the next phase of the project involves a much larger test array. A 43km loop was formed recently by linking optical fibres on Stanford’s campus with fibres at several nearby locations.
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