Measuring earthquakes using fiber-optic cables

Nathaniel Lindsey of the University of California, Berkeley, buries fiber-optic cable in a trench near Fairbanks, Alaska, to record earthquake seismicity. Credit: left: Anna Wagner; right: Nathaniel Lindsey.

 

Fiber-optic cables crisscross the world, ferrying digital data and enabling internet access and telecommunication. In a new study, published in Geophysical Research Letters, researchers tested whether fiber-optic cables can also be used to detect and measure earthquakes.

Using fiber-optic cables to monitor ground vibrations is not novel in itself; this method has been developed by the petroleum industry over the past decade. “But this study is one of the first to monitor earthquakes using existing telecommunications infrastructure,” says Robert Mellors, a seismologist at Lawrence Livermore National Laboratory in Livermore, Calif., who was not involved in the study. “It opens up a whole new potential for sensing earthquakes.”

Fiber-optic cables have some advantages over the seismometers most commonly used to monitor earthquakes today, says Nathaniel Lindsey, lead author of the new study and a graduate student at the University of California, Berkeley. “A seismometer sits on the earth at one location and records ground motion only at that point,” Lindsey says. “We are also limited in where we can install seismometers; for example, it is expensive and challenging to place seismometers in urban areas or far offshore.”
There are more than 160,000 kilometers of long-distance fiber-optic cables in the United States, but only part of this network is used for telecommunication; the unused portion, called “dark fiber,” is what researchers like Lindsey aim to use to detect earthquakes. “We can record the motion of a buried fiber-optic cable at essentially every point along it for tens of miles,” Lindsey says, which could allow this approach — called distributed acoustic sensing, or DAS — to yield high-resolution seismic data.
DAS involves connecting a laser “interrogator” unit to one end of a buried fiber-optic cable. The interrogator — about the size of a desktop computer tower — shoots laser light pulses into the cable and then monitors backscattered photons, the portions of that light reflected back toward the source from different points along the cable. Any stretching or bending of the cable, due to ground vibrations from a passing truck or an earthquake, for example, alters the path of the backscattered photons. By measuring these changes, researchers can piece together information about the strain on segments of the cable over time and quantify the speed and direction of the seismic waves responsible.
But fiber-optic cables used for telecommunication are rarely buried in direct contact with the ground. They are usually laid in conduits made of different materials, such as PVC piping coated with cement. So researchers weren’t sure if such cables would be sufficiently “coupled” to the ground to serve as reliable earthquake sensors.

To test decoupled cables, Lindsey and colleagues at Stanford University installed their own fiber-optic cable in an existing telecommunication conduit under Stanford’s campus. While monitoring this embedded fiber-optic cable, they detected several seismic events, including “nearby quarry blasts, small earthquakes within the Bay Area, and larger events from across North America,” the researchers wrote in the study.

How DAS measurements compare to those made with standard seismometers is still an open area of research.

Lindsey and colleagues at Lawrence Berkeley National Laboratory in Berkeley, Calif., compared data collected using fiber-optic cables and conventional seismometers in Fairbanks, Alaska, where they buried about 5 kilometers of cable over an area larger than two football fields directly into the ground.
There were several similarities in the data collected in Fairbanks using the two systems: For example, P and S waves from earthquakes were detected at the same time by both the coupled DAS array and standard seismometers. But there were also some differences. For instance, for seismic vibrations with frequencies lower than 1 Hertz and higher than 10 Hertz, DAS sensitivity falls off more steeply compared to conventional seismometers (broadband seismometers can reliably detect seismic motions with frequencies between 0.01 and 50 Hertz). In addition, data collected by the fiber-optic DAS can be “noisier and not include the quietest seismic signals,” Mellors notes.
“Using this technology, we may be able to convert the same fibers used for home [and] office internet into a huge seismic network that can help better detect and understand earthquakes,” says Zhongwen Zhan, a seismologist at Caltech, who was not involved with the study. However, “DAS only measures extension or compression along the length of a fiber, while seismic waves cause ground motion in three directions,” Zhan says. “So information from DAS alone is incomplete.”

Lindsey acknowledges that current DAS technology has its limitations. For example, “DAS isn’t measuring ground motion in all three dimensions,” he says. However, “having thousands of data points still provides much useful information.” Additionally, some researchers are looking at combining data from cables laid in various directions or configurations to mitigate the one-dimensionality problem.

Lindsey is now testing earthquake detection using existing dark fiber near Sacramento, Calif. “This experiment will be more reflective of how useful pre-existing fiber-optic cables can be in seismic monitoring,” he says.

Adityarup "Rup" Chakravorty

Chakravorty is a science writer at the University of Wisconsin at Madison by day, and a freelancer once the sun goes down.

TRADUÇÃO

Os cabos de fibra óptica cruzam o mundo, transportando dados digitais e permitindo acesso à Internet e telecomunicações. Em um novo estudo, publicado na revista Geophysical Research Letters, pesquisadores testaram se cabos de fibra ótica também podem ser usados ​​para detectar e medir terremotos.
Usar cabos de fibra ótica para monitorar as vibrações do solo não é novidade em si; Esse método foi desenvolvido pela indústria do petróleo na última década. "Mas este estudo é um dos primeiros a monitorar terremotos usando infra-estrutura de telecomunicações existente", diz Robert Mellors, um sismólogo do Laboratório Nacional Lawrence Livermore, em Livermore, na Califórnia, que não esteve envolvido no estudo. "Isso abre um novo potencial para a detecção de terremotos".
Os cabos de fibra óptica têm algumas vantagens sobre os sismógrafos mais usados ​​atualmente para monitorar terremotos hoje, diz Nathaniel Lindsey, principal autor do novo estudo e estudante de pós-graduação da Universidade da Califórnia, em Berkeley. "Um sismógrafo fica na terra em um local e registra o movimento do solo apenas nesse ponto", diz Lindsey. “Também estamos limitados em onde podemos instalar sismômetros; por exemplo, é caro e desafiador colocar sismômetros em áreas urbanas ou longe da costa ”.

Existem mais de 160.000 quilômetros de cabos de fibra óptica de longa distância nos Estados Unidos, mas apenas parte dessa rede é usada para telecomunicações; a porção não utilizada, chamada “fibra escura”, é o que pesquisadores como Lindsey pretendem usar para detectar terremotos. “Podemos gravar o movimento de um cabo de fibra ótica enterrado em praticamente todos os pontos ao longo dele por dezenas de quilômetros”, diz Lindsey, o que poderia permitir essa abordagem - chamada de sensor acústico distribuído, ou DAS - para produzir dados sísmicos de alta resolução.
O DAS envolve conectar uma unidade de “interrogador” de laser a uma extremidade de um cabo de fibra ótica enterrado. O interrogador - mais ou menos do tamanho de uma torre de computador de mesa - dispara pulsos de luz laser no cabo e monitora fótons de dispersão, as porções dessa luz refletidas de volta para a fonte de diferentes pontos ao longo do cabo. Qualquer esticamento ou flexão do cabo, devido às vibrações do solo de um caminhão que passa ou a um terremoto, por exemplo, altera o caminho dos fótons de retroespalhamento. Ao medir essas mudanças, os pesquisadores podem juntar informações sobre a deformação nos segmentos do cabo ao longo do tempo e quantificar a velocidade e a direção das ondas sísmicas responsáveis.