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Terry Ewart

Principal Physicist Emeritus

Professor Emeritus, Oceanography





Research Interests

Medium to small-scale oceanography, wave propagation in random media, stochastic inverse problems between ocean acoustics and oceanography

Department Affiliation

Ocean Physics


B.S. Physics, University of Washington, 1959

Ph.D. Physics, University of Washington, 1965


2000-present and while at APL-UW

Validity of the Markov approximation in ocean acoustics

Henyey, F.S., and T.E. Ewart, "Validity of the Markov approximation in ocean acoustics," J. Acoust. Soc. Am., 119, 220-231, doi:10.1121/1.2130963, 2006.

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1 Jan 2006

Moment equations and path integrals for wave propagation in random media have been applied to many ocean acoustics problems. Both these techniques make use of the Markov approximation. The expansion parameter, which must be less than one for the Markov approximation to be valid, is the subject of this paper. There is a standard parameter (the Kubo number) which various authors have shown to be sufficient. Fourth moment equations have been successfully used to predict the experimentally measured frequency spectrum of intensity in the mid-ocean acoustic transmission experiment (MATE). Yet, in spite of this success, the Kubo number is greater than 1 for the measured index of refraction variability for MATE, arriving at a contradiction. Here, that contradiction is resolved by showing that the Kubo parameter is far too pessimistic for the ocean case. Using the methodology of van Kampen, another parameter is found which appears to be both necessary and sufficient, and is much smaller than the Kubo number when phase fluctuations are dominated by large scales in the medium. This parameter is shown to be small for the experimental regime of MATE, justifying the applications of the moment equations to that experiment.

Internal wave effects on high frequency acoustic propagation to horizontal arrays--Experiment and implications to imaging

Williams, K.L., F.S. Henyey, D. Rouseff, S.A. Reynolds, and T.E. Ewart, "Internal wave effects on high frequency acoustic propagation to horizontal arrays--Experiment and implications to imaging," IEEE J. Ocean. Eng., 26, 102-113, 2001.

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1 Jan 2001

An experiment was carried out over a nine day period from August 18 to 27, 1996 to examine acoustic wave propagation in random media at frequencies applicable to synthetic aperture sonar. The objective was to test experimentally the hypothesized imaging effects of variations in the sound speed along two different acoustic paths as put forth by F.S. Henyey et al. (1997). The focus of this paper is on describing the experiment and carrying out an initial analysis of the data in the context of the effect of ocean internal waves on imaging resolution. The oceanography is summarized to the extent needed to discuss important aspects relative to the acoustics experiment. In the acoustics experiment transmissions at 6, 20, 75, and 129 kHz between sources and receiver arrays were carried out. Source to receiver separation was about 815 m. All sources and receivers were mounted on bottom-deployed towers and were at least 9 m off the seafloor. The analysis concentrates on the 75-kHz data acquired during one day of the experiment. The time span examined Is sufficient to examine a diurnal tidal cycle of the oceanographic conditions. The results indicate the IW phase perturbations would have a significant effect on imaging for even the most benign conditions of the experiment if no autofocusing scheme is used. Also, though autofocusing should be useful in recovering the focus for these conditions, there are conditions (e.g., for the path that has a turning point at the thermocline and during times when solibores are present), where more sophisticated compensation schemes would be needed.

Determining an ocean internal wave model using acoustic log-amplitude and phase: A Rytov inverse

Ewart, T.E., S.A. Reynolds, and D. Rouseff, "Determining an ocean internal wave model using acoustic log-amplitude and phase: A Rytov inverse," J. Acoust. Soc. Am., 104, 146-155, doi:10.1121/1.423264, 1998.

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1 Jan 1998

The feasibility of inverting acoustic field statistics to obtain the parameters of a stochastic internal wave model is demonstrated using numerical simulations. For weak scattering satisfying the Rytov approximation, the parameters of a generalized form of the Garrett–Munk internal wave model can be obtained. A hierarchy of experiment scenarios has been studied. Scenarios range from a densely populated vertical receiving array to single-point measurements. In each case, the intrinsic range-averaging of acoustic measurements provides integral constraints on the environmental model. The success of the inversion improves with increasing experimental complexity. With a vertical array, up to four parameters of the internal wave model can be recovered. For the simplest situation, two parameters can be fit with reasonable accuracy. The implications of these results for understanding oceanographic processes are discussed.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center