Terry E. Ewart

The Mid‐Ocean Acoustic Transmission Experiment, MATE

An experiment to measure phase (travel time) and intensity fluctuations in sound pulses transmitted at 2, 4, 8, and 13 kHz over an 18.1 km wholly refracted Fermat path is discussed. Simultaneously with the acoustic monitoring the index of refraction fluctuations were measured in space and time with sufficient resolution to determine the correlation function of the medium. The site was the Cobb Seamount in the northeast Pacific (46°46′N, 130°47′ W), and the time period was 30 days in June–July, 1977. In terms of both the quality and quantity of acoustic and oceanographic measurements, this experiment represents a significant improvement over an earlier experiment in the same location [J. Acoust. Soc. Am. 60, 46–59 (1976)]. The acoustic measurements cover a wider range of acoustic frequencies and more closely represent measurements from a single Fermat path. Approximately 25% of the acoustic data are discussed here; the representations of the correlation function of the index of refraction are based on all ...

Acoustic fluctuations in the open ocean—A measurement using a fixed refracted path

Amplitude and phase (transit time) fluctuations in pulses sent between a fixed transmitter and a fixed receiver over a wholly refracted 17.2‐km path were recorded for 144.5 h. The sites were the southwest flank of Cobb Seamount and a lesser peak 17.2 km away, both at 1000 m depth. Eight‐cycle pulses at 4166 Hz and 16‐cycle pulses at 8333 Hz were sent alternately every 15.7 sec and received at three receivers located at 0, 5, and 15 m along a horizontal arm located perpendicular to the transmission path. Power spectra have been computed from the time series of phase and amplitude at a single receiver and the phase difference between two receivers. Extreme care was used in the analysis of the data to ensure that the time series obtained represented a single path. The power spectra of the phase data exhibit dominant tidal peaks at 24, 12.4, and 6.2 h; the power spectra of the amplitudes show less evidence of the tides. Between the inertial frequency and the Vaisala frequency, the power spectra of the phase, ...

A model of the intensity probability distribution for wave propagation in random media

Ewart and Percival [J. Acoust. Soc. Am. 80, 1745 (1986)] have shown that the generalized gamma distribution effectively models intensity probability distributions of temporal fluctuations observed in a field experiment and transverse spatial fluctuations simulated in numerical experiments. In both cases the fluctuations are due to wave propagation through a medium with a random index of refraction. Here, the transverse spatial intensity fluctuations of a wave propagating through a medium with a power‐law autocorrelation function of wave speed are modeled over a regime that spans 108 in scattering strength and 106 in scaled range (range divided by the Fresnel length). This scattering parameter regime transforms to ranges between 100 m and 100 km and to frequencies between 100 Hz and 100 kHz when normalizations typical of observed ocean internal wave fluctuations are used. Contour plots of the variance, skewness, and kurtosis of the intensity distribution are presented for the range/frequency plane. It is s...

Forward scattered waves in random media—the probability distribution of intensity

An acoustic wave propagating in a medium with an index of refraction that is random in space and time acquires intensity modulations that can be modeled in terms of a space–time autocorrelation function, a scattering strength parameter γ, and a scaled range X. Over a wide range of γ, X, and medium autocorrelation functions, the probability distributions of intensity vary from lognormal at small X to exponential at large X. The generalized gamma distribution [E. W. Stacy, Ann. Math. Stat. 33, 1187–1192 (1962)] is characterized by three parameters, and reduces to many well‐known distributions. It varies smoothly from lognormal to exponential as the parameters change. It is proposed that this distribution is a general analytic form that represents the probability distribution of intensity as a function of range, depth, and time in forward scattering. This proposition is tested with the measured temporal intensity fluctuations from the Mid‐Ocean Acoustic Transmission Experiment, MATE. It is also tested wtih d...

Effects of internal waves and turbulence on a horizontal aperture sonar

Random variability in the water column will affect the operation of a horizontal aperture sonar. Two sources of variability in shallow water are turbulence and internal waves. In a modeling study, the effects of turbulence and internal waves on a shallow-water imaging system are compared. The operational principles of a large aperture imaging system are first reviewed. A shallow-water internal wave model is developed by modifying the Garrett-Munk model. The internal waves are assumed to dissipate and drive the small-scale turbulence. The two phenomena are predicted to have markedly different effects on a system. Turbulence has short spatial correlation scales whose primary effects will be manifested in the variance of the acoustic phase. By contrast, internal waves will have much larger scattering but also a longer correlation scale. The primary acoustic quantity of interest for internal waves is shown to be the curvature of the phase as observed along the aperture. Properties of shallow-water internal waves are shown to preclude the use of standard acoustic calculations based on the Markov approximation. Using archival environmental data, sample calculations are presented for the site of a planned August 1996 experiment.

Effect of random sea surface and bottom roughness on propagation in shallow water

The potential difficulties associated with modeling acoustic propagation in shallow‐water environments are well documented. Larger scale deterministic features combine with random fluctuations in the water column, sediment, sea surface, and water–sediment interface to produce an extremely complicated propagation regime. The extent to which each of these factors needs to be included in realistic propagation modeling remains to be quantified. Toward this end, results from a series of detailed simulations generated using the parabolic equation method are presented. Beginning with a deterministic downward refracting sound‐speed profile and a known sloping bottom, realizations of the random features are sequentially added to the simulation. Wind‐driven surface gravity waves and power‐law bottom roughness are considered. The individual and cumulative effects of these scattering mechanisms on the acoustic wavefront are quantified. Successive interactions with the random interfaces are studied. A modal decomposit...

Intensity fluctuations. Part I: Theory

This paper deals with the problem of the intensity fluctuations arising in a wave when it propagates through a medium that is randomly inhomogeneous in space and time. It is assumed that multiple scattering can occur and that the intensity fluctuations can become large. The parabolic moment equation for the fourth moment of the wave field is solved for a monochromatic point source immersed in the medium. Approximate expressions are obtained for the space–time spectrum of intensity fluctuations at any distance in the medium. The solution of the fourth moment equation is compared with results of the Rytov method of smooth perturbations, and the limitations of the latter are discussed.

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

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.

Intensity fluctuations. Part II: Comparison with the Cobb experiment

The intensity fluctuations measured at 4 and 8 kHz in the Cobb experiment have been available for nearly 10 years and in that time have not successfully been predicted theoretically. We show that multiple scatter effects must be considered, and that neither the Born nor the Rytov approximation to the scattering formulation is appropriate. A companion paper [J. Acoust. Soc. Am. 74, 1474–14832  (1983)] provides the theoretical background for this work by presenting a general form of the analytical solution to the fourth moment equation in two dimensions for a point source transmission. We use the parameters of the Garrett–Munk model of the internal wave field appropriate to the Cobb experiment oceanographic regime to obtain the correlation functions of the acoustic refractive index field, and then predict the intensity fluctuations. We discuss corrections to the predicted spectrum that are due to fine structure effects and tidal motions. We include a discussion of the scattering parameters Γ and X, which ar...

Reconstruction of oceanic microstructure by tomography: A numerical feasibility study

We consider the feasibility of using small-scale acoustic tomography to reconstruct oceanic microstructure. In contrast to alternate measurement techniques, acoustic tomography can produce quantitative, fully three-dimensional images. Tomography uses acoustic data measured by probing the medium from several different directions. To test the proposed approach, numerical realizations typical of anisotropic microstructure are first simulated and then reconstructed by tomography. Two specific forms are evaluated: conventional computed tomography that uses only travel time delay data, and diffraction tomography that requires coherent field measurements. Conventional tomography is shown to produce high-quality cross-sectional images. The effects of reducing the number of views are studied, and error maps are generated. Issues in experimental implementation are considered. This study can serve as a guide to the design of an experimental device.