17 USC 105 interim-entered record; under review.A look back at a historical article that had a significant impact on the science and pradtice of acoustics. Article: Sound propagation through a fluctuating stratified ocean: Theory and observation Author: Walter H. Munk and Fred Zachariasen Publication Date: April 1976 (JASA 59, 818); https://doi.org/10.1121/1.38093

Colosi, John

Worcester, Peter F.

English

Calhoun, Institutional Archive of the Naval Postgraduate School

Calhoun: The NPS Institutional ArchiveDSpace RepositoryFaculty and Researchers Faculty and Researchers' Publications2020-12A seminal paper linking ocean acoustics andphysical oceanographyColosi, John; Worcester, Peter F.http://hdl.handle.net/10945/69013This publication is a work of the U.S. Government as defined in Title 17, UnitedStates Code, Section 101. Copyright protection is not available for this work in theUnited States.Downloaded from NPS Archive: CalhounARTICLE OVERVIEWIn the early 20th century, the field of ocean acoustics was born outof civil and naval needs. The development of the acoustic echosounder, for example, had a profound impact on safe navigation inharbors and coastal regions, and the use of sound for anti-submarineand mine warfare was critical in countering the Nazi and, later,Soviet Naval threats. In the post-World War Two era, as oceanacousticians made measurements with higher and higher precision, itquickly became apparent that the inherent variability of the oceanwas a major limiting factor. Efforts to predict the ocean’s acousticalimpacts unfortunately utilized methods divorced from ocean dynam-ics and had limited success. At the same time, physical oceanogra-phers with new and better tools to sample the ocean more frequentlyand more accurately were coming to the realization that the oceanhas energetic and dynamic fields of internal waves and eddies onquite short time and space scales. The need for ocean acoustics andphysical oceanography to come together was clear, and the Munkand Zachariasen (MZ) paper,1 as well as a companion paper byDyson et al.2 in 1976, provided the necessary spark and much, muchmore. Not only were they able to show how stochastic and determin-istic physical oceanographic processes, like internal waves and tidesand impact acoustic propagation, but they were also able to showthat ocean acoustic measurements could be expressible in terms ofphysical oceanographic processes and parameters; that is, acousticmeasurements could tell one something about the ocean. In conclud-ing their paper, MZ1 write:“We end up, …., with quite simple and transparent formulae for the acoustical fluctuations. The formulae make explicit thedependence of the various oceanographic and acoustic parameters. The need is to apply these results to a variety ofexperimental situations.”IMPACT OF THE ARTICLEIn treating the problem of ocean sound propagation through the stochastic internal-wave field, MZ1 were revolutionary on manylevels. First, they brought physical oceanographic principles to the problem of sound propagation, describing the ocean sound channeland its fluctuations in terms of oceanographic variables such as temperature and salinity structure, adiabatic gradients, and densitystratification. Second, previous work had attempted to describe ocean sound-channel variability in terms of ad-hoc correlation func-tions or to treat the ocean sound-speed fine structure using ideas of homogeneous isotropic turbulence borrowed from atmosphericoptics. However, MZ correctly identified internal waves as a critical source of the fine structure and, using the newly developedGarrett-Munk (GM) internal-wave spectrum3 and a perturbation method termed the Rytov approximation, they solved a fundamentalproblem in stochastic wave propagation in which the medium was anisotropic, inhomogeneous, had a dispersion relation, and mostComputed (smooth curves) and observed spectra of phase (left) andintensity (right) at Cobb Seamount. Only the 4-kHz results are shownfor phase; at 8 kHz computed and observed values both are higher by6 dB. Reprinted with permission from W. Munk and F. Zachariasen, J.Acoust. Soc. Am. 59, 818–838 (1976). Copyright 1976 Acoustical Societyof America (Ref. 1). Figure to be Reproduced: Fig. 7.The Reflections series takes a look back on historicalarticles from The Journal of the Acoustical Society ofAmerica that have had a significant impact on the scienceand practice of acoustics.Article: Sound propagation through a fluctuatingstratified ocean: Theory and observationAuthor: Walter H. Munk and Fred ZachariasenPublication Date: April 1976 (JASA 59, 818);https://doi.org/10.1121/1.380933A seminal paperlinking oceanacoustics and physicaloceanographyJ. Acoust. Soc. Am. 148 (6), December 2020 R9https://doi.org/10.1121/10.0002761importantly, involved a background waveguide. Their analytic work was a tour de force with over 182 equations and the introductionof several dozens of new parameters. Not content to just present theory, they compared theoretical predictions to numerical simula-tions and observations from two field efforts using controlled electronic sources and precise timing and navigation. Their calculationsof mean-square phase, phase rate, and log-intensity, as well as frequency and wavenumber spectra, were shown to be in agreementwith the available observations mostly within a factor of two, greatly improving upon previous treatments. The successes and tantaliz-ing questions raised by MZ spurred a flurry of observational and theoretical work on the subject over the last four decades lookingat the problem from shallow water and continental shelves to basin and global scale propagation. Interestingly, later work using theFeynman path integral approach, which has much wider applicability than the Rytov approximation used in MZ, showed that, whilesome details changed, the basic physical picture provided by MZ remained.4,5 Last, with the success of their data/theory comparisons,they planted the seed that ocean acoustic measurements could reveal important information about ocean physics. Not long after MZ,Munk and Wunsch6 proposed ocean acoustic tomography, in which acoustic travel times are used to infer large- and mesoscale tem-perature and current structure. With new developments in the field,5 we are looking towards the day when acoustic fluctuation mea-surements will be used to describe the ocean internal-wave field, thus bringing the MZ work full circle.By JOHN A. COLOSI1 and PETER F. WORCESTER21Department of Oceanography, Naval Postgraduate School, 833 Dyer Road, Monterey, California 93943, USA2Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego,9500 Gilman Drive #0225, La Jolla, California 92093, USA(Published online 3 December 2020)REFERENCES1W. Munk and F. Zachariasen, “Sound propagation through a fluctuating stratified ocean: Theory and observation,” J. Acoust. Soc. Am. 59, 818–838 (1976).2F. Dyson, W. Munk, and B. Zetler, “Interpretation of multi path scintillations Eleuthera to Bermuda in terms of internal waves and tides,” J. Acoust. Soc.Am. 59, 1121–1133 (1976).3C. Garrett and W. Munk, “Space-time scales of internal waves,” Geophys. Fluid Dyn. 3(3), 225–264 (1972).4S. M. Flatte, R. Dashen, W. Munk, K. Watson, and F. Zachariasen, Sound Transmission through a Fluctuating Ocean (Cambridge University Press,Cambridge, UK, 1979).5J. A. Colosi, Sound Propagation Through the Stochastic Ocean (Cambridge University Press, Cambridge, UK, 2016).6W. Munk and C. Wunsch, “Ocean acoustic tomography: A scheme for large scale monitoring,” Deep Sea Res. Part A 26(2), 123–161 (1979).R10 J. Acoust. Soc. Am. 148 (6), December 2020

A seminal paper linking ocean acoustics and physical oceanography

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A seminal paper linking ocean acoustics and physical oceanography

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