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  3. Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large, slow-moving currents, suc

Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large, slow-moving currents, suc

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问题     Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large, slow-moving currents, such as the Gulf Stream. That view, based on 100 years of observations made around the globe, produced only a rough approximation of the true circulation. But in the 1950’s and the 1960’s, researchers began to employ newly developed techniques and equipment, including subsurface floats that move with ocean currents and emit identification signals, and ocean current meters that record data for months at fixed locations in the ocean. These instruments disclosed an unexpected level of variability in the deep ocean. Rather than being characterized by smooth, large-scale currents that change seasonally(if at all), the seas are dominated by what oceanographers call mesoscale fields: fluctuating, energetic flows whose velocity can reach ten times the mean velocity of the major currents.
    Mesoscale phenomena—the oceanic analogue of weather systems—often extend to distances of 100 kilometers and persist for 100 days(weather systems generally extend about 1,000 kilometers and last 3 to 5 days in any given area). More than 90 percent of the kinetic energy of the entire ocean may be accounted for by mesoscale variability rather than by large scale currents. Mesoscale phenomena may, in fact, play a significant role in oceanic mixing, air-sea interactions, and occasional—but far-reaching—climatic events such as El Nino, the atmospheric-oceanic disturbance in the equatorial Pacific that affects global weather patterns.
    Unfortunately, it is not feasible to use conventional techniques to measure mesoscale fields. To measure them properly, monitoring equipment would have to be laid out on a grid at intervals of at most 50 kilometers, with sensors at each grid point lowered deep in the ocean and kept there for many months. Because using these techniques would be prohibitively expensive and time consuming, it was proposed in 1979 that tomography be adapted to measuring the physical properties of the ocean. In medical tomography X-rays map the human body’s density variations(and hence internal organs); the information from the X-rays, transmitted through the body along many different paths, is recombined to form three-dimensional images of the body’ s interior. It is primarily that this multiplicative increase in data obtained from the multipath transmission of signals that accounts for oceanographers’ attraction to tomography: it allows the measurement of vast areas with relatively few instruments. Researchers reasoned that low-frequency sound waves, because they are so well described mathematically and because even small perturbations in emitted sound waves can be detected, could be transmitted through the ocean over many different paths and that the properties of the ocean’s interior—its temperature, salinity, density, and speed of currents—could be deduced on the basis of how the ocean altered the signals. Their initial trials were highly successful, and ocean acoustic tomography was born.
According to the passage, scientists are able to use ocean acoustic tomography to deduce the properties of the ocean’ s interior in part because______.
选项 A、density variations in the ocean are mathematically predictable
B、mesoscale phenomena are so large as to be easily detectable
C、information from sound waves can be recombined more easily than information from X-rays
D、low-frequency sound waves are well described mathematically
答案D
解析 细节题。根据“the properties ofthe ocean’s interior”定位文章最后一段Researchers reasoned that low—frequency sound waves，because they are so welldescribed mathematically and because even small perturbations in emitted sound wavescan be detected．could be transmitted through the ocean over many different paths andthat the properties of the ocean’s interior--its temperature，salinity，density，and speed ofcurrents--could be deduced on the basis of how the ocean altered the signals．“研究人员推测说，低频声波可以很好地从数学上加以描绘．即使是非常微小的干扰也能够发现，因此可以发射低频声波使它通过多条路径在海洋中行进．从而海洋内部的性质，如温度、盐度、密度和水流速度等，都能够通过海洋对声波信号的影响推断出来。”可知科学家们利用海洋声学断层推断海洋内部的部分性质是因为可以从数学上很好地描绘低频率的声波，和D选项表达一致。所以答案选D。
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