為了進行地下探勘，工程師對岩石受到的應力進行深入瞭解
測量自然界中看不見的應力絕非易事，但它可以決定地震發生時一座礦坑或隧道是否坍塌而造成生死之隔。
評估這類事件的風險時，研究人員通常只依據對稱作岩石應力的變量所估計出來的數值。
威斯康辛大學麥迪遜分校的環境土木工程與地質工程的助理教授Hiroki Sone解釋：「岩石應力是岩層在地下受到的壓力，由於我們無法看見造成它的力量，所以只能經由間接方式測量。但由於地下相當深處溫度和壓力增加的幅度極大，使得測量岩石應力的儀器相當難以使用。」
為了解決這項挑戰，Sone和中國及日本的研究人員合作，利用不需使用易受溫度影響的儀器，而將岩石應力測量的極限推展到新的深度，從先前的最大值4.5公里拓展至極深的7公里。
在2017年7月發表於《科學報告》( Scientific Reports)的研究中，研究人員利用從到達此深度的鑽井取出的岩石樣品，證明稱為「非彈性應變回復法」(anelastic strain
recovery method)得到的應力估計值，跟利用影像分析井壁造影(borehole wall images)得到的數值一致。井壁造影法雖然可信，但因為需要專門的掃描儀器造成它通常很難使用。
為了驗證他們的概念，科學家選在中國西北的塔里木盆地進行這項研究。此區域的面積大約是三分之二個阿拉斯加(將近15個台灣)，周遭環繞著僅此於聖母峰的世界第二高山K2以及其他幾座山脈。對歷史學家來說，由於此區跟絲路――中國和地中海之間的古代貿易路線――有重要關係而享負盛名。
今日，除了歷史學家跟登山客之外，由於塔里木盆地含有中亞最大的幾座石油和天然氣產地，使得石油公司也對它深感興趣。由於周圍山區常常發生小型地震，因此石油公司想要瞭解塔里木盆地的地質環境，以評估鑽探動作是否有可能引發地震活動。
對Sone和他的同事來說，這提供了一個絕無僅有的機會來更加精進測量岩石應力的方法。
Sone表示：「我們想要測試在到達7公里深時非彈性應變回復法的可信度有多高，因為它的主要優點是只需要採樣並分析岩石樣品就好。我們測量岩石樣品在取出之後往不同方向的擴張程度，如此就能間接估計岩石受到的應力。」
要把位在此深度夠大的岩石樣品從鑽井中拉上來，岩石可能已經經歷數天的回復過程，因此研究人員證明此方法仍然可以成功作用時感到相當興奮。
即使研究人員在進行完岩芯採樣的65小時之後才把儀器裝設到樣品，他們第一次就成功測量出岩石應力。他們發現結果跟慣用的分析電阻掃描得到的井壁影像結果相符。雖然影像方法在此例中可以成功運作，但掃描儀器的溫度限制使得在這麼深的地方進行測量時並不實用。
除了證實較簡單的方法在大幅增加的深度之下仍然可行，這篇研究還解決了一項懸宕已久關於塔里木盆地地質的謎題：地球最外殼是由分裂成許多塊的冰冷岩石(板塊)組成，漂在非常厚層的高溫岩漿之上，而地殼中的應力在盆地周遭和內部似乎有所差異。
其他科學家之前已經發現此種差異的證據，但目前這項研究確定了它的存在。
雖然塔里木盆地周圍的板塊互相擠壓而破碎彎折，但是盆地內部卻相對穩定，解釋了地震活動的觀察結果。這項結果可以解讀成盆地內部的地震風險較低，並讓石油公司能依此判定鑽井應會處於穩定狀態的深度，而讓結構破壞的風險最小化。
對地球科學家來說，這項新研究是個重要的實測結果，證明更加實用的岩石應力測量方法。「新的結果讓我們有信心非彈性應變回復方法可以運用的深度遠遠超出我們的想像。」Sone表示，「當地殼內部的高溫高壓使得我們能選擇的其他工具備受挑戰時，只要岩石在垂直和水平方向的變形量相同，就可以輕易地運用此方法。」
For under-Earth exploration, engineers
deepen understanding of rock stress
Measuring
unobservable forces of nature is not an easy feat, but it can make the
difference between life and death in the context of an earthquake, or the
collapse of a coal mine or tunnel.
To manage the risk of such events, researchers often rely on estimating a
quantity called rock stress.
“Rock stress—the amount of pressure
experienced by underground layers of rock—can only be measured indirectly
because you can’t see the forces that cause it,” explains Hiroki Sone, an
assistant professor of civil and environmental engineering and geological engineering at the University of
Wisconsin-Madison. “But instruments for estimating rock stress are difficult to
use at great depths, where the temperature and pressure increase tremendously.”
Addressing this challenge, Sone and his colleagues in China and Japan have
now pushed the limits of rock stress measurements that don’t require
temperature-sensitive instruments to new depths, from a previous maximum of 4.5
kilometers (2.8 miles) to a whopping 7 kilometers (4.3 miles).
In a study published in July 2017 in Scientific Reports, the
researchers used rocks sampled from a well bore of that depth to show that
stress estimates obtained by the so-called anelastic strain recovery method
were consistent with a visual analysis of borehole wall images, a reliable but
often infeasible approach that requires a specialized scanner.
The scientists conducted their proof-of-principle study in the Tarim Basin
in northwest China, an area almost two-thirds the size of Alaska that is
surrounded by K2, the world’s second highest mountain after Mount Everest, and
several other mountain ranges. The region is well known to historians because
of its association with the Silk Road, an ancient trade route between China and
the Mediterranean.
Today, in addition to historians and mountain climbers, petroleum
companies have taken an interest in Tarim Basin, as it contains some of the
largest oil and gas resources in Central Asia. These companies want to
understand the region’s geology to assess whether drilling may trigger seismic
activity, given that many smaller earthquakes have occurred in the surrounding
mountains.
For Sone and his colleagues, this presented a unique opportunity to
advance the methodology for measuring rock stress.
“We wanted to test the reliability of the anelastic strain recovery method
at up to 7 kilometers depth because its main advantage is that you only need to
sample and analyze the rock itself,” Sone says. “It estimates stress indirectly
by measuring how much the rock sample expands in different directions after it
has been recovered.”
With that kind of depth, the recovery process—pulling a large enough rock
sample out of a borehole—can take a few days, which is why the researchers were
excited to prove that the method still worked.
For the first time, they measured rock stress even when sensors weren’t
attached to the sample until 65 hours after coring and found that the results
matched a conventional image analysis of the borehole wall, obtained with a
resistivity scanner. While the visual method also worked in this case, it can
be infeasible at such great depths because of the scanner’s temperature
limitations.
In addition to proving the easier method’s validity at greatly increased
depth, the study resolved a longstanding geological puzzle in the Tarim Basin:
The rock stress in Earth’s outer shell—which consists of many large pieces of
cooler rock (tectonic plates) floating on a very thick layer of hot magma—differs
between the Basin’s periphery and its interior.
Other scientists had found evidence for this difference before, but the
current study confirmed it.
In the interior of Tarim Basin, tectonic plates are relatively stable,
even though they crash and fold up against each other in the periphery,
explaining the observed seismic activity. This translates to a lower risk of
earthquakes in the interior and informs a petroleum company’s decisions about
the depth at which boreholes should be stabilized to minimize the risk of
structural collapse.
For earth scientists, the new study is an important validation of a more
practical method for estimating rock stress. “These new results give us
confidence that we can use the anelastic strain recovery method at greater
depths than we thought possible,” Sone says. “As long as the rock deforms the
same amount in vertical and horizontal directions, this method is much easier
to apply when very high temperatures and pressures in the Earth’s crust
challenge the other options in our toolbox.”
原始論文：Dongsheng Sun et al. Stress state measured at ~7 km depth in the Tarim Basin, NW China, Scientific Reports, 2017; 7. DOI:
10.1038/s41598-017-04516-9
引用自：University of
Wisconsin-Madison. “For under-Earth exploration, engineers deepen understanding
of rock stress.”
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