Advance in global ocean acoustics

doi:10.1126/science.abe0960

GSTDTAP > 气候变化

DOI	10.1126/science.abe0960
	Advance in global ocean acoustics
	Carl Wunsch
	2020-09-18
发表期刊	Science
出版年	2020
英文摘要	In the past 50 years, the major conceptual revolution in physical oceanography is the transformation from considering the ocean as a large-scale, extremely slowly changing fluid to a fundamentally turbulent one. The ocean changes across a wide range of temporal and spatial scales, from millimeters to 30,000 km and from seconds to multimillennia, with major regional differences. Because ocean exploration relied on a few slow, expensive ships exploring over many decades and depicting only the grossest mappable global properties, observing the variability is a forbidding challenge. The ocean is very noisy, filled with short–spatial scale structures that make obtaining large-scale average properties problematic. The oceanographic community responded by developing altimetric and gravity satellites, the Argo profiling system, and ever more capable models. On page 1510 of this issue, Wu et al. ([ 1 ][1]) demonstrate how an intriguing combination of physical oceanography and classical seismological techniques potentially opens the way for an entirely new and globally capable observation system. The fluid ocean has been known for hundreds of years as a good transmitter of sound. This property contrasts with the opacity of the ocean to electromagnetic radiation at all useful wavelengths. The use of acoustic depth sounding from ships dates back to the beginning of the 20th century. At about the same time, navies recognized that reliable submarine detection required acoustic detection. Acoustic methods have come to be used recently in a variety of ways, including local signaling from ships to release an anchor from an instrument moored to the seafloor, for the tracking of subsurface drifting instruments ([ 2 ][2]), and in measuring devices that use acoustic backscatter techniques over meter scales. Biologists have long listened to the sounds of marine organisms of all sizes and shapes. Nonetheless, observational systems designed to study the physics of the fluid ocean relied initially on purely mechanical devices and, more recently, on instruments that electronically recorded data internally. The use of acoustics was neglected for several reasons. Ocean acoustics were mainly the domain of classified navy work directed at short-range immediate detection of submarines or for hiding submarines from acoustic detection. This situation resulted in acoustics not being part of the normal toolkit for ocean observations. Remarkable and useful as the satellites and Argo systems have proven, these techniques have provided only partially adequate coverage of an electromagnetically opaque ocean with a mean depth of 3800 m. Existing methods produce point values with sometimes uncontrollable spatial distributions, leaving them susceptible to distorted sampling in both time and space ![Figure][3] Temperature profiles from sound The sound speed minimum in the ocean acts as an acoustic waveguide. Many different sources can generate acoustic waves. These include explosions, earthquakes, or perhaps even whale song. Rays emanating from the source at different angles cycle between different depths on their way to the receiver. The varying vertical properties change the travel time for different rays as they move from source to receiver. This different sampling permits tomographic reconstruction of a temperature profile. Intersecting vertical planes permit full three-dimensional results. GRAPHIC: V. ALTOUNIAN/ SCIENCE In 1979, the idea was developed that exploitation of the oceanic transparency to sound coupled with modern acoustic source technology and inverse methods would permit determination of large-scale averages, with useful top-to-bottom resolution of the ocean temperature (heat content) ([ 3 ][4]). The methodology was called “ocean acoustic tomography” for its analog with the mathematics of medical tomography, which also involves using measured integrals to find anomalies inside a patient. Over the next several decades, numerous experiments on spatial scales from hundreds to more than 10,000 km repeatedly demonstrated the fundamental utility of the technique (see the figure) ([ 4 ][5], [ 5 ][6]). These programs used artificial sound sources because these removed the analogous seismological problem of having to determine source location and timing from the same data as was being used to study the medium, and the sources could be placed anywhere. Acoustic tomography was not, as originally envisioned, exploited on a continuing global basis. A federally funded program to establish a Pacific Ocean–wide prototype [Acoustic Tomography of the Ocean Circulation (ATOC)] triggered an outcry that the experiment was going to deafen and thus destroy the marine mammals of the ocean. That led in turn to years of often ill-tempered permitting processes with various U.S. agencies and public hearings. (Tomographic signal levels are a small fraction of those in the ocean from shipping, oil exploration, and naval activities.) A different problem involved the technology of acoustic sources, which had to be (i) low power to work from batteries and (ii) large in size to obtain low frequencies and finite bandwidths. The consequence was that the sources were both cumbersome to handle at sea and relatively expensive in both dollars and shiptime. Despite all of these difficulties, active-source acoustic tomography remains useful, particularly in the Arctic Sea, but in the hands of a comparatively small group of experts. This situation may be about to undergo change. A great variety of natural and artificial sound sources are detectable in the ocean, including earthquakes, shipping noise, breakup of floating ice sheets, oil exploration, rainfall, breaking waves, and large marine mammals, such as blue whales capable of signaling each other over ocean basin distances. From an information extraction point of view, use of natural sources brings back the seismologists' hypocenter problem: finding source timing and three-dimensional space location. Wu et al. have now shown that acoustic integrals from the sound signature of ordinary earthquakes can be used, quantitatively, to determine changes in oceanic temperatures and, consequently, changing heat content. The authors evade the hypocenter problem by relying on “repeating” earthquakes, but more general possible solutions now also exist. Great advantages can accrue from major noise suppression obtained by averaging over months and years. One intriguing possibility is that quantitative deep ocean measurements can be made from purely land-based seismic stations. At the other extreme, Project Mermaid ([ 6 ][7]) has shown the feasibility of detecting tomographic signals on drifting floats. If some fraction of the Argo floats carried relatively cheap hydrophones, developing a true set of intersecting tomographic integrals is possible. These could also serve as ocean rain gauges ([ 7 ][8]). Several quantitative questions will need to be answered to determine how useful the technique may be. The applicability of the technique globally depends on whether repeating earthquake hypocenters are accurate enough globally. If so, historic earthquake records might be used to infer ocean temperatures long before direct measurements became available. Whether noise sources such as ships and ice-sheet breakups, now tracked from space, can be accurately combined with their acoustic signatures is not clear. With enough receivers, the potential exists for blue whales or other marine mammals to be used as tomographic sources. This approach may not only benefit our understanding of the fluid ocean but also help us to understand biological systems in the ocean. 1. [↵][9]1. W. Wu et al ., Science 369, 1510 (2020). [OpenUrl][10][Abstract/FREE Full Text][11] 2. [↵][12]1. T. Rossby, 2. D. Webb , Deep-Sea Res. 17, 359 (1970). [OpenUrl][13] 3. [↵][14]1. W. Munk, 2. C. Wunsch , Deep-Sea Res. 26A, 439 (1979). [OpenUrl][15] 4. [↵][16]1. W. Munk, 2. P. Worcester, 3. C. Wunsch , Ocean Acoustic Tomography (Cambridge Univ. Press, 1995). 5. [↵][17]1. B. M. Howe et al ., Front. Mar. Sci. 6, 426 (2019). [OpenUrl][18] 6. [↵][19]1. F. J. Simons et al ., J. Acoust. Soc. Am. 146, 3067 (2019). [OpenUrl][20] 7. [↵][21]1. J. A. Nystuen , J. Atmos. Ocean. Technol. 13, 74 (1996). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/295416
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Carl Wunsch. Advance in global ocean acoustics[J]. Science,2020.
APA	Carl Wunsch.(2020).Advance in global ocean acoustics.Science.
MLA	Carl Wunsch."Advance in global ocean acoustics".Science (2020).