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.

 

Our journey from the UCMP to South Africa to study fossil monkeys

By Marianne Brasil and Tesla Monson posted February 29, 2016

Mrs. Charles Camp and her son, Charles Camp Jr., in South Africa (1947-48).

At the time we got involved in what has now become for us - the South Africa project - one of us (Tesla) was soon-to-be a second year graduate student, and the other (Marianne) was about to start her senior year as an undergraduate student here at UC Berkeley.

We began working together in the UC Museum of Paleontology (UCMP) during the summer of 2013, making our way through a massive project and cataloguing exceptional fossil material collected during the UC Africa Expedition of 1947 and 1948. This is the story of that project and the journey that followed.

The UC Africa Expedition

A bit of background for those who may not be familiar with this aspect of UC Berkeley history… as World War II ended, a massive research expedition, dubbed The UC Africa Expedition (UCAE) was just beginning to pick up steam on Berkeley campus. From 1947-1948, the extensive research endeavor became an influential force across numerous fields of study.
During this time, the Expedition also attracted plenty of media attention, resulting in dozens of newspaper articles that were published while the expedition was underway. There were two separate branches of the expedition: the northern branch (led by Wendell Phillips) and the southern branch (led by our very own Charles Camp, director of the UCMP from 1930-49). In addition to all of the fossil material that is now housed in the UCMP, the UCAE brought back an enormous amount of material that, to this day, spans a wide range of libraries, museums, and other repositories on the UC Berkeley campus.
The list below gives you an idea of the amount and diversity of non-fossil materials collected by the expedition and stored outside of the UCMP:

• The Museum of Vertebrate Zoology has many mammal specimens that were collected during the UCAE by Thomas Larson, ranging in size from bats and elephant shrews to large antelopes.
• The Phoebe A. Hearst Museum of Anthropology has large amounts of archaeological and ethnographic material, ranging from stone tools to stools, many of which come from the Ovambo people in South Africa. Faunal and archaeological materials collected at the Middle and Late Stone age excavation sites are also stored at Hearst.
• The Music Library has a series of recordings of local traditional music from South Africa, recorded by famed ethnomusicologists Laura Boulton and Hugh Tracey.
• The Bancroft Library holds many photographs documenting the life of Charles Camp and his family during the expedition. The library also has many photos of local people and their traditions, as well as the landscapes on which they lived.
• The UC Botanical Gardens received seeds and living plants that were collected by Robert Rodin, and some of those living plants perpetuate and can still be visited in the African section of the garden.
• The University and Jepson Herbaria also have a considerable number of specimens, as well as Robert Rodin’s field notes and correspondences. A complete list of everything collected can be found in his preserved field notes.

Fossil primates at the Evolutionary Studies Institute in Johannesburg, South Africa. Photo by Tesla Monson

Following our curatorial and historical work with this collection, we narrowed our focus to the Plio-Pleistocene fossil assemblage. For a more extensive historical account of the UCAE, and faunal and locality details for the Plio-Pleistocene fossil assemblage, see our recently published paper in PaleoBios (Monson TA et al. 2015).

As we turned our attention to the Plio-Pleistocene assemblage, two undergraduate students who were involved in the curatorial process took on independent projects. Sandy Gutierrez examined the ostrich eggshells and quantified interspecific variation in shell characteristics. And Bogart Marquez, emphasizing the bovids, studied the faunal composition of the different caves in order to make inferences about deposition, taphonomy, and predatory behavior in and around the caves. Both Sandy and Bogart presented their results at the Society for the Advancement of Chicanos and Native Americans in Science (SACNAS) conference in Spring 2014.

We also dug into the primate material with the goal of assessing the alpha-taxonomy of the UCMP specimens. This part of the assemblage includes specimens that have been very influential throughout the historical course of monkey taxonomy, and many are still quite controversial. We tag-teamed the project, with Marianne working through the mandibular material as part of her honors thesis and Tesla examining the cranial material. Two then-undergraduates in the Hlusko Lab also worked with the primate material: Kevin Roth examined the juvenile craniodental specimens and Sandy Gutierrez looked at the postcranial material.

Tesla poses for a selfie with Sediba, a South African australopithecine.

The whole group (Tesla, Marianne, Sandy, Bogart, and Kevin) presented our results during a UCMP Fossil Coffee seminar back in Spring 2014 and at the American Association of Physical Anthropologist (AAPA) meeting in April 2014. Fortuitously, our Fossil Coffee presentation was attended by Dominic Stratford, a visiting South African geoarchaeologist from University of the Witwatersrand in Johannesburg, South Africa. Dominic has become an invaluable collaborator on the multiple monkey projects that evolved out of our initial work in the UCMP and that are still ongoing. These projects led us (and our advisor – Leslea Hlusko) on the next leg of our journey. In summer of 2015, we journeyed to South Africa to collect more monkey data, a trip graciously funded by a grant from the Palaeontological Scientific Trust and two Desmond C. Clark fellowships from the Human Evolution Research Center at UC Berkeley.

South Africa

Data Collection

The entrance to the hominid vault at the Evolutionary Studies Institute in Johannesburg, South Africa. Photo by Tesla Monson

During our time in South Africa, we studied monkey cranial and dental specimens at University of the Witwatersrand in Johannesburg and at the Ditsong Museum of Natural History in Pretoria. While it was an incredible experience and opportunity, we couldn’t help but feel like some of the days stretched on forever - we were in the museum for nine hours at a time, and some days it felt like all we had to eat was chicken, chicken, and more chicken.... which, according to Dominic, actually qualifies as a vegetable in South Africa. Tesla had to tape her thumbs, followed by her index fingers, followed by almost every other finger, to prevent caliper burn, and Marianne had to squint out of one eye for two weeks straight. (But we made sure to take semi-frequent jellybean breaks to preserve our sanity, thanks Leslea!) It may not have felt like it while we were squinting at calipers and working through the burn, but the amount of data collected made the long hours very worthwhile. Not to mention that we were in good company while at University of the Witwatersrand, since original South African hominid fossil material, including the Taung child, Malapa and Sediba, were displayed (complete with spotlights!) in the vault where we were working.  Yes, that’s correct – a vault. We were stationed in the Hominid Vault at the Evolutionary Studies Institute, a very serious room fully equipped with a 6-foot vault door with rotating handle, locked by a 4-inch key that looked a hundred years old. Serious business indeed.
When we weren’t measuring and photographing monkeys, we got to take tours of some of the famous cave sites, and wow were they incredible! We also got to meet paleoanthropologist Ron Clarke and see the “Little Foot” hominid remains, which are still in the process of being prepared – an opportunity that has only been offered to only a handful of people in the world. Hey, it pays to be a paleontologist!

The surface layers at Sterkfontein Cave in the Cradle of Humankind, South Africa.

Marianne Brasil, Leslea Hlusko and Dominic Stratford underground in Sterkfontein Cave, South Africa. Photo by Tesla Monson

Marianne Brasil and Tesla Monson in Sterkfontein Cave. Photo by Leslea Hlusko.

Famed anthropologist Ron Clarke holding the cranium of “Littlefoot,” a recently discovered South African hominid.

In the evenings while we were in Pretoria, we ate our delivery dinners (mostly chicken) on the floor of Leslea’s room, and sometimes it was in candlelight because of this odd, but normal “it’s just a part of life here,” load-shedding phenomenon that causes small-scale city blackouts. This was only one of the quirks of South Africa that we encountered. Some others included…

1. No picture on a restaurant menu was ever actually replicated in person. Dishes served were a surprise every time!
2. The GPS had a fondness for telling us to “Turn left at unknown road”, as if that’s helpful.
3. On more than one occasion we had to let baby goats get out of the road before we could continue on our way. Ok, that last one wasn’t so bad…

Exploring Africa

Following all of the hard work of data collection, we finally got to explore South Africa. We set off - with Tesla driving on the wrong side of the road, in the wrong side of the car, and with the clutch on the left – to our rental at “Zonk Lake”, which was a lone cottage on a tiny lake. So, we basically rented a lake. It’s not often you get to take a romantic vacation with your labmate…

Giant’s Castle reserve in the Drakensberg. Photo by Tesla Monson

During the couple of days that we were in the Drakensberg region, we went out to enjoy the natural beauty of the landscape as well as the San petroglyphs of Giant’s Castle. We were also able to see our study organisms in their (not so) natural habitat when we ran into chacma baboons in a park area while out for a hike. On a more serious note, it was an honor and a privilege to tour the Apartheid Museum and the Nelson Mandela Memorial while we were in KwaZulu-Natal, and we highly recommend it to any visitors in the area.

San petroglyphs on the rocks at Giant’s Castle, South Africa. Photo by Tesla Monson

Chacma baboons (Papio hamadryas) eating grass at the Giant’s Castle resort in the Drakensberg. Photo by Tesla Monson

A panel from the Apartheid Museum at the Mandela Capture Site near Howick in KwaZulu-Natal. Photo by Tesla Monson

Taking the kayak out on Zonk Lake. Photo by Tesla Monson

Marianne practices the art of braai, South African barbeque. Photo by Tesla Monson

During the evenings, we caught Marianne up on the childhood media she never had, pulling from the random assortment of VHS cassettes that someone left on the shelf of our Zonk cabin: Casper, Mask of Zorro, Daredevil – all the greats. We also went kayaking in the early morning, and had true South African “braai” (AKA barbeque) in the evenings. You know what they say — when in South Africa...
After Zonk Lake, we left early for the nine-hour drive to Kruger National Park. Luckily, awesome street signs and plenty of bad jokes from Tesla dotted our journey. When we finally made it to Kruger, we quickly loaded up on snacks, brewed our coffee at 5:30 in the morning, and set out to drive through the park. The first thing we saw was a rhino (spotted by Tesla). We had heard that some people never see anything, so the mood was gleeful right way.
Then, maybe 20 meters down the road past the rhino, we saw an elephant (spotted by Marianne). The day just got better after that. We saw giraffes, lions, hippo, impala, hyena, kudu, crocodiles, warthogs, TONS of birds, baboons, buffalo, zebra, mongoose, and many other cool critters – including loads and loads of baby animals. Oh the babies!

A white rhinoceros (Ceratotherium simum) in Kruger National Park, South Africa. Photo by Tesla Monson

Southern ground hornbills (Bucorvus leadbeateri) in Kruger National Park, South Africa. Photo by Tesla Monson

Giraffes, impala and warthogs at a watering hole in Kruger National Park, South Africa.

An African elephant (Loxodonta africana) in Kruger National Park, South Africa. Photo by Tesla Monson

A baby spotted hyaena cub in Kruger National Park, South Africa. Photo by Tesla Monson

A zebra in Kruger National Park, South Africa. Photo by Tesla Monson

A warthog  also known as “Radio Africa,” runs with its tail up. Photo by Tesla Monson

A vervet monkey hangs out near a rest area in Kruger Park, South Africa. Photo by Tesla Monson