原文網址：https://eos.org/research-spotlights/fiber-optic-cables-can-produce-high-resolution-underground-maps
將電信纜線改造成分散式聲波感測陣列之後可以用來繪製近地表構造，或許還能改良都市地區的地震災害潛勢圖。
By Jack Lee
地震發生時的地面搖晃程度很大一部份取決於地表下方不遠處的土壤和岩石的性質。模擬研究指出沉積盆地會放大地面搖晃程度，而人口密集的市區通常就坐落在這種環境。然而，都市地區的近地表構造繪製起來卻是相當困難。
Yang等人運用分散式聲波感測(distributed
acoustic sensing, DAS)發展出一套新方法，可以用來繪製近地表構造的高解析度圖像。DAS這種新興技術可以把現存的光纖纜線轉變成地震儀陣列，透過監測光脈衝通過纜線時的散射方式，科學家可以計算纜線附近的物質發生的微小應變變化。除了記錄地震之外，DAS也已經證明有許多應用，像是判定2020年玫瑰花車遊行中音量最大的樂儀隊，並且發現COVID-19居家令期間的車流量出現了驚人的變化。
研究人員之前在2019年7月加州的里奇克萊斯特發生規模7.1的地震過後，重新利用一條10公里長的纜線來偵測餘震。在三個月的期間之內，他們的DAS陣列相較於一般的儀器，偵測到的小型餘震多出了五倍左右。
在這篇新研究當中，研究人員分析了車輛持續製造出來的震波數據。團隊經由DAS的數據建立起近地表的剪力速度模型，其解析度可以達到公里以下，比一般的模型高出二個數量級。他們的模型顯示出沿著纜線，餘震發生時地表晃動程度較強的地方，通常可以對應至剪力速度較低處。
作者認為這種尺度更加精細的地震災害潛勢圖可以加強都市地區對地震的風險管理，尤其是那些已經建立起光纖網路的城市。
Fiber-optic cables can produce high-resolution
underground maps
Telecom fiber repurposed as
distributed acoustic sensing arrays can image near-surface structure and
potentially improve seismic hazard mapping in urban areas.
How much the ground moves during an earthquake
strongly depends on properties of rock and soil just beneath Earth’s surface.
Modeling studies suggest that ground shaking is amplified in sedimentary
basins, on which populated urban areas are often located. However, imaging
near-surface structure around urban areas at high resolution has been
challenging.
Yang et al. have developed a new approach of using
distributed acoustic sensing (DAS) to construct a high-resolution image of
near-surface structure. DAS is an emerging technique that can transform
existing fiber-optic cables into seismic arrays. By monitoring changes in how
light pulses scatter as they travel through the cable, scientists can calculate
small strain changes in the material surrounding the fiber. In addition to
recording earthquakes, DAS has proven useful in a variety of applications, such
as naming the loudest marching band at the 2020 Rose Parade and uncovering
dramatic changes in vehicular traffic during COVID-19 stay-at-home orders.
Prior researchers repurposed a 10-kilometer stretch
of fiber to detect aftershocks following the magnitude 7.1 Ridgecrest
earthquake in California in July 2019. Their DAS array detected about 6 times
as many small aftershocks as conventional sensors did during a 3-month period.
In the new study, the researchers analyzed continuous
seismic data produced by traffic. The DAS data allowed the team to develop a
near-surface shear velocity model with a subkilometer resolution 2 orders of
magnitude higher than typical models. This model revealed that along the length
of the fiber, sites where aftershocks produced more ground motion generally
corresponded with where shear velocity was lower.
Such fine-scale seismic hazard mapping could improve
urban seismic risk management, especially in cities where fiber-optic networks
may already be present, the authors suggest.
原始論文：Yan
Yang, James W. Atterholt, Zhichao Shen, Jack B. Muir, Ethan F. Williams, Zhongwen
Zhan. Sub-Kilometer Correlation Between
Near-Surface Structure and Ground Motion Measured With Distributed Acoustic
Sensing. Geophysical Research
Letters, 2022; 49 (1), e2021GL096503 DOI: 10.1029/2021GL096503
引用自：EOS.
“Fiber-Optic Cables Can Produce High-Resolution Underground Maps.” By Jack Lee.
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