By: Shi En Kim (@goes_by_kim)
Researchers repurpose underground fiber-optic cables for measuring soil properties in urban areas, which helps scientists determine how the soils — and thus the buildings above them — will behave in an earthquake.
Citation: Kim, S. E., 2020, Fiber-optic cables help track soil properties, liquefaction potential, Temblor, http://doi.org/10.32858/temblor.092
What do Mexico City, Los Angeles, the San Francisco Bay Area and Tokyo have in common? They are all earthquake-prone urban areas with populations exceeding several million. To minimize casualties and damage during an earthquake in these areas, engineers need to fully understand the properties of the soil the cities’ buildings stand on so that the buildings can be designed (or retrofitted) to withstand as much ground shaking as possible. Instead of using traditional seismometers, researchers have laid the groundwork for using a newer method of measuring soil properties with a much higher-spatial resolution: the dense network of telecommunication cables already present beneath cities.
In telecommunications, information is transmitted in the form of light zipping through fiber-optic cables, which are bundles of thin glass rods. To measure soil properties, a team of researchers led by seismologist Biondo Biondi at Stanford University in California examined the light bouncing off tiny manufacturing defects in underground fiber-optic cables.
Fiber-optic cables do not lie still in the ground; they are constantly twitching under the relentless strumming of Earth’s ambient vibrations. This ambient noise comes from tidal actions and ocean waves or even human traffic, and the forces propagate through Earth’s crust as a faint background hum. Researchers make use of this background noise in seismological recordings in a technique called distributed acoustic sensing. By shooting a series of light pulses into a fiber-optic cable and tracking how the reflected light wobbles, the researchers discerned how quickly and how much the ground shakes, measures that tell the team a lot about the properties of the soil in which the cable is buried.
Validating the technique
More than 3 miles (5 kilometers) of fiber-optic cables snake beneath the grounds of Stanford University. In 2016, at the behest of the researchers, Stanford IT services installed a new fiber-optic cable in the same PVC sheath as the main telecommunications cables (so they wouldn’t disrupt the regular telecommunications signals). One end of the cable was connected to an interrogator unit, a device that analyzes the reflected light for any changes in the amplitude and phase. It was this simple setup that allowed Zack Spica, a seismologist at University of Michigan, Biondi and their colleagues to collect data uninterruptedly for over a year.
“We laid down the cable and that was it,” says Spica, who conducted this study while he was a post-doc at Stanford. “If we wanted to collect the same density of information with seismometers, it would probably take three to four days to dig holes for about 10 sensors, and then the batteries might need to be replaced every few months. Once a fiber-optic array is installed, it can be there for virtually an unlimited amount of time.”
The researchers measured the ground motions roughly every 26 feet (8 meters) at 620 different points along the cable network. Two important measures — the soil’s measured resonance frequency and ground velocity — were similar to what three nearby seismometers registered, validating the distributed acoustic sensing method, Spica and his colleagues reported in Journal of Geophysical Research: Solid Earth. The resonance frequency indicates which vibration frequencies are amplified and hence the most destructive, so engineers make sure to design buildings with a different natural frequency. The group velocity points to the hardness of the soil. Buildings on top of softer soils are more susceptible to damage during an earthquake as the ground underneath will shake more.
One of the most famous cases of vibrational amplification is the 1985 magnitude-8 Mexico City earthquake. Mexico City sustained considerable damage although it was more than 217 miles (350 kilometers) away from the epicenter. The main reason: Mexico City sits on top of a soft ancient lakebed.
High spatial density
Ethan Williams, a graduate student at Caltech who was not involved in the study, says that the distributed acoustic sensing method is valuable for identifying pockets of high-intensity shaking in urban areas during an earthquake.
“What makes [Spica’s research] so exciting is a future where we could have dense distributed acoustic sensing seismic arrays deployed block-by-block throughout the entirety of [an urban earthquake zone, such as] the Los Angeles Basin,” says Williams. “It’s a huge potential leap forward.”
One conceivable issue Spica and Williams note is that this fiber-optic seismology is only sensitive to lateral vibrations in the soil, whereas a seismometer can measure in all directions. Fortunately, Spica says, soil properties in the vertical direction do not vary as widely with distance. As such, distributed array sensing can be complemented with a seismometer’s single-point measurement in the vertical direction for a full 3D picture at high spatial resolution, as Spica’s research has showed.
Expansion to other areas
Distributed acoustic sensing has already been applied to seafloor seismology and oil and gas extraction. Given its high spatial resolution, the technique has also allowed the Stanford researchers to observe (in a separate study) reduced human traffic in their town due to COVID-19, except in — unsurprisingly — hospital areas.
When universities reopen and research can resume normally, the Stanford research group plans to expand their technique to other earthquake-prone cities, starting with Stanford’s neighbor Redwood City.
“Big companies like AT&T and Google [already own] cables … that could [be used] for seismology,” Spica says. “Soil properties are not characterized in many big cities around the world [with sufficient] spatial density. [Our technique] is applicable and scalable to those cities.”
Shi En Kim is a graduate student in molecular engineering at the University of Chicago. She is also a freelance science writer in her spare time.
Spica, Z. J., Perton, M., Martin, E. R., Beroza, G. C., & Biondi, B. (2020). Urban seismic site characterization by fiber-optic seismology. Journal of Geophysical Research: Solid Earth, 125, e2019JB018656. https://doi.org/10. 1029/2019JB018656
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