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　　Pour some water into a partly full bathtub and the level in the tub will rise. Calculating how much it will rise(how引导的名词性从句充当名词) is straightforward, as long as the surface area of what was already there and the amount being added are known. The same should apply to working out how much the sea level will rise as the world’s ice sheets melt in response to rising global temperatures. But in practice it is not that easy. (预判接下来作者会告诉我刚才讨论的原则并不适用) Though geographers know the surface area of the oceans, measuring how the masses of the world’s ice sheets are changing has proved hard. Aurélien Mordret of the Massachusetts Institute of Technology, however, thinks he has found a way to make it simpler. He proposes to do it using the vibrations created by ocean waves (预判接下来作者将会重点阐述该方法) .

　　At the moment, geographers monitor the mass of ice sheets in two ways. One employs aircraft to fly over such sheets and reflect laser beams from their surfaces, to record their topography. The other uses satellites to track gravitational fluctuations caused by variations in the amount of ice present. Both techniques work, but both have limitations (介绍了两个传统方法以及它们的局限性) . Laser overflights create high-resolution images but are expensive, so can be done only a few times a year. Satellites pass overhead more often, but generate fuzzier pictures.

　　Dr Mordret, however, knew from work conducted (此处 "conducted" 用作过去分词，表示被动) by other research groups that(that此处作为noun modifier修饰work, 但由于that从句过长所以后置) vibrations created by waves crashing onto the shore are transmitted inland through the Earth’s crust, sometimestravelling (comma + ving为邻近的句子提供额外的信息) thousands of kilometres from the coast. These vibrations can be picked up by seismometers of the sort used to monitor earthquakes. He also knew that the speed at which the vibrations propagate varies with the amount of pressure being exerted on the crust by mountains or glaciers sitting above. That gave him his idea (预判接下来作者要阐述新方法的来源/原理).

　　Propagation speeds vary because many types of rock have small voids within their structure. These lower the velocity of passing vibrations. The greater the proportion of a rock’s volume that is void, the more slowly vibrations will travel through it. If a piece of rock is dry, compressing it will shrink the voids and speed the vibrations up. Water, though, is famously incompressible—more so(so作为代词指代incompressible) than most rock-forming minerals. If the voids are filled with water, they will thus resist compression. So, putting pressure on wet rock increases the relative volume of the voids, which slows down any passing vibrations.

　　Since an icy overburden will both fill the voids with water and compress the rock, Dr Mordret reasoned that changes in the speed of transmission in rocks under ice sheets will reflect the thickness of the overlying sheet (提出假设的具体内容) . To test this idea, he and a team of colleagues set up an experiment which made use of existing earthquake sensors at seven monitoring stations in western Greenland, an island almost completely covered by ice sheets....

　　以下是结构分析

　　介绍一个测量的基本原则，但并不适用于测量海平面的上升 → 引出新方法(P1)

　　↓

　　介绍两个旧方法以及它们的弊端(P2)

　　↓

　　得出新方法之前的相关背景信息(P3)

　　↓

　　阐述新方法的来源/原理(P4)

　　↓

　　基本新方法的来源/原理提出假设(P5)

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