運用海底電信纜線組成超級地震觀測網
By Robert Sanders
組成全球電信網路的海底光纖電纜，或許有朝一日可以幫助科學家研究發生在外海的地震，以及藏在海底深處的地質構造。
就像這張預測到了2021年，會有多少光纖電纜運作中的圖所示，海洋裡縱橫交錯著許多電信纜線，其中許多(黃色)隸屬於Google和微軟這類私人企業。但這些電纜或許還能用做另一種用途：監測地球被水覆蓋的70%面積中，有多少地震與斷層系統的地震站。圖片來源：紐約時報
加州大學柏克萊分校、勞倫斯柏克萊國家實驗室(柏克萊實驗室)、蒙特婁灣水族館附設研究所(MBARI)和萊斯大學的研究人員，在本周發表於期刊《科學》(Science)的論文中敘述了一項實驗：把20公里長的海底光纖電纜轉變成10000座海底地震站。在蒙特婁灣進行試驗的四天期間，他們記錄到一場規模3.5的地震以及其他由海底斷層帶產生的震波活動。
架設在海裡的地震站相當稀少，使得地球表面70%幾乎沒有裝設偵測地震的儀器。而這項研究人員之前用陸上的光纖電纜測試過的技術，或許可以讓科學家得到他們亟需的海底地震資料。
論文主要作者是加州大學柏克萊分校的研究生Nate
Lindsey。他說：「海底地震學對資料有著相當大量的需求。任何可以放到海中的儀器都非常有用，就算只能設置在離岸50公里的地方也是。」
Jonathan Ajo-Franklin是休士頓萊斯大學的地球物理教授暨柏克萊實驗室的研究員，他和Lindsey共同主持了這項實驗。協助進行實驗的還有光纖電纜的所有人，MBARI的助理Craig
Dawe。17年前，MBARI以及加州大學柏克萊分校地球及行星科學研究所的Barbara
Romanowicz教授架設了位在太平洋海床的首座地震站。2009年它和蒙特婁加速研究系統的節點之間連接了一條52公里長的永久電纜。在2018年3月進行年度維修而斷訊的期間，這項研究的人員利用其中20公里來進行實驗。
「這是非常具有開創性的地震學研究，首度有人運用海底光纖電纜來尋找這些海洋學裡的訊號，或是用來描繪出斷層構造。」Ajo-Franklin表示。「在全世界的地震觀測網中，其中一個空白之處便是海洋。」
全世界密集分布的光纖網路在陸上與海裡的長度加總起來說不定有一千多萬公里。Ajo-Franklin說研究團隊盡力達成的最終目標，是讓這道網路變成能夠偵測地球運動的靈敏儀器。某些地區不像常常發生地震的加州與太平洋沿岸地區大部分都部有測站，運用這項方法就能在無法架設昂貴地面測站的地區監測地震活動。
Ajo-Franklin表示：「現有的地震觀測網通常是由精度高、但分布相對零散的儀器組成。而這項方法可以讓我們得到相當密集的陣列。」
光學地震學
研究人員運用的技術稱為分散式聲波感測(Distributed
Acoustic Sensing)，其中的光學裝置可以讓電纜傳輸短脈衝雷射，並且偵測伸張對電纜造成應變後產生的反向散射。利用干涉測量法，他們可以每隔兩公尺便測量一次，相當有效率地把總長20公里的電纜變成10000個各自運作的動作偵測器。
「這些系統靈敏倒可以測到每公尺出現數奈米或數百皮米的變化。」Ajo-Franklin表示。「這相當於十億分之一的變化量。」
他們今年稍早發表了在陸上測試六個月的結果。該次實驗利用了能源局埋在沙加緬度附近的22公里長電纜，其為能源科學網13000英里長的暗纖測試平台的一段。暗纖(dark
fiber)是指埋設在地下但是未使用或者是短期出租的光纖，跟頻繁使用的亮纖(lit
fiber)恰好相反。研究人員運用暗纖成功監測到地震活動與環境中的聲響，此外也取得了高解析度的大範圍地下影像，而且比以往用傳統地震監測網才能得到的影像還好。
「光纖地震學的美好之處在於你不用架設10000台地震儀，只要利用現有的電信電纜就好。」Lindsey說。「只要走到該地，然後把儀器連結到光纖的終端就完成了。」
在水下試驗期間，位於內陸45公里的加州吉爾羅伊附近發生了一場規模3.4的地震。團隊成功測到這場地震傳來許多不同頻率的地震波，並且描繪出已知和未知的海底地震帶，其為聖格雷戈里奧斷層系統的一部分。他們也量到了穩定天氣下的波浪(稱作海洋微震，ocean
microseism)以及風暴造成的大浪，這些都跟浮標以及陸上地震儀的測量記錄相符。
「對於海床上發生的作用以及海洋地殼內部的構造我們還有很多不了解的地方，這是因為要在海底裝設地震儀之類的儀器是相當困難的。」加州大學柏克萊分校的Michael
Manga表示。「在實驗結果中，它們偵測出之前只存在於假設當中而未被偵測出來的震波。研究顯示出現有的光纖電纜具有潛力變成一連串的偵測儀器，使我們能以嶄新的方式得到影像。」
Lindsey表示有越來越多地震學家對地球環境噪訊場的紀錄感到興趣。環境噪訊場是由海洋和陸地的交互作用引起，也就是拍打在海岸的波浪。
他說：「基本上透過沿岸的光纖電纜，我們可以觀察平常從岸上看到的波浪和海床之間有什麼樣的交互作用，此外也能探討波浪是如何牽動地球而產生地震波。」
Lindsey和Ajo-Franklin若要使用世界上的亮纖電纜，需要先證明它們傳輸雷射脈衝時不會干擾電纜中其他帶有各自封包的頻道。他們目前正在進行運用亮纖電纜的實驗，同時也計畫在南加州索爾頓湖南方，布勞利地震帶的地熱區利用光纖來監測地震活動。
Underwater telecom cables make superb
seismic network
Fiber-optic cables that constitute a
global undersea telecommunications network could one day help scientists study
offshore earthquakes and the geologic structures hidden deep beneath the ocean
surface.
In a paper appearing this week in the journal Science, researchers from the University
of California, Berkeley, Lawrence Berkeley National Laboratory (Berkeley Lab),
Monterey Bay Aquarium Research Institute (MBARI) and Rice University describe
an experiment that turned 20 kilometers of undersea fiber-optic cable into the
equivalent of 10,000 seismic stations along the ocean floor. During their
four-day experiment in Monterey Bay, they recorded a 3.5 magnitude quake and
seismic scattering from underwater fault zones.
Their technique, which they had previously tested
with fiber-optic cables on land, could provide much-needed data on quakes that
occur under the sea, where few seismic stations exist, leaving 70% of Earth’s
surface without earthquake detectors.
“There is a huge need for seafloor seismology. Any
instrumentation you get out into the ocean, even if it is only for the first 50
kilometers from shore, will be very useful,” said Nate Lindsey, a UC Berkeley
graduate student and lead author of the paper.
Lindsey and Jonathan Ajo-Franklin, a geophysics
professor at Rice University in Houston and a faculty scientist at Berkeley
Lab, led the experiment with the assistance of Craig Dawe of MBARI, which owns
the fiber-optic cable. The cable stretches 52 kilometers offshore to the first
seismic station ever placed on the floor of the Pacific Ocean, put there 17
years ago by MBARI and Barbara Romanowicz, a UC Berkeley Professor of the
Graduate School in the Department of Earth and Planetary Science. A permanent
cable to the Monterey Accelerated Research System (MARS) node was laid in 2009,
20 kilometers of which were used in this test while off-line for yearly
maintenance in March 2018.
“This is really a study on the frontier of
seismology, the first time anyone has used offshore fiber-optic cables for
looking at these types of oceanographic signals or for imaging fault
structures,” said Ajo-Franklin. “One of the blank spots in the seismographic
network worldwide is in the oceans.”
The ultimate goal of the researchers’ efforts, he
said, is to use the dense fiber-optic networks around the world — probably more
than 10 million kilometers in all, on both land and under the sea — as
sensitive measures of Earth’s movement, allowing earthquake monitoring in
regions that don’t have expensive ground stations like those that dot much of
earthquake-prone California and the Pacific Coast.
“The existing seismic network tends to have
high-precision instruments, but is relatively sparse, whereas this gives you
access to a much denser array,” said Ajo-Franklin.
Photonic
seismology
The technique the researchers use is Distributed
Acoustic Sensing, which employs a photonic device that sends short pulses of
laser light down the cable and detects the backscattering created by strain in
the cable that is caused by stretching. With interferometry, they can measure
the backscatter every 2 meters (6 feet), effectively turning a 20-kilometer
cable into 10,000 individual motion sensors.
“These systems are sensitive to changes of nanometers
to hundreds of picometers for every meter of length,” Ajo-Franklin said. “That
is a one-part-in-a-billion change.”
Earlier this year, they reported the results of a
six-month trial on land using 22 kilometers of cable near Sacramento emplaced
by the Department of Energy as part of its 13,000-mile ESnet Dark Fiber
Testbed. Dark fiber refers to optical cables laid underground, but unused or
leased out for short-term use, in contrast to the actively used “lit” internet.
The researchers were able to monitor seismic activity and environmental noise
and obtain subsurface images at a higher resolution and larger scale than would
have been possible with a traditional sensor network.
“The beauty of fiber-optic seismology is that you can
use existing telecommunications cables without having to put out 10,000
seismometers,” Lindsey said. “You just walk out to the site and connect the
instrument to the end of the fiber.”
During the underwater test, they were able to measure
a broad range of frequencies of seismic waves from a magnitude 3.4 earthquake
that occurred 45 kilometers inland near Gilroy, California, and map multiple
known and previously unmapped submarine fault zones, part of the San Gregorio
Fault system. They also were able to detect steady-state ocean waves —
so-called ocean microseisms — as well as storm waves, all of which matched buoy
and land seismic measurements.
“We have huge knowledge gaps about processes on the
ocean floor and the structure of the oceanic crust because it is challenging to
put instruments like seismometers at the bottom of the sea,” said Michael
Manga, a UC Berkeley professor of earth and planetary science. “This research
shows the promise of using existing fiber-optic cables as arrays of sensors to
image in new ways. Here, they’ve identified previously hypothesized waves that
had not been detected before.”
According to Lindsey, there’s rising interest among
seismologists to record Earth’s ambient noise field caused by interactions
between the ocean and the continental land: essentially, waves sloshing around
near coastlines.
“By using these coastal fiber optic cables, we can
basically watch the waves we are used to seeing from shore mapped onto the
seafloor, and the way these ocean waves couple into the Earth to create seismic
waves,” he said.
To make use of the world’s lit fiber-optic cables,
Lindsey and Ajo-Franklin need to show that they can ping laser pulses through
one channel without interfering with other channels in the fiber that carry
independent data packets. They’re conducting experiments now with lit fibers,
while also planning fiber-optic monitoring of seismic events in a geothermal
area south of Southern California’s Salton Sea, in the Brawley seismic zone.
原始論文：Nathaniel J. Lindsey, T. Craig Dawe, Jonathan
B. Ajo-Franklin. Illuminating
seafloor faults and ocean dynamics with dark fiber distributed acoustic sensing. Science, 2019; 366
(6469): 1103 DOI: 10.1126/science.aay5881
引用自：University of California - Berkeley.
"Underwater telecom cables make superb seismic network”
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