Boris Katsnelson

Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves

During a 12-h period in the 2006 Shallow Water Experiment (SW06), binary phase shift keying (BPSK) signals at the carrier frequencies of 813 and 1627 Hz were propagated over a 19.8-km source-receiver range when a packet of strong internal waves passed through the acoustic track. The communication data are analyzed by time reversal processing followed by a single-channel decision feedback equalizer. Two types of internal wave effects are investigated in the context of acoustic communications. One is the rapid channel fluctuation within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the internal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the receiver to counteract such fluctuation. The other one is the long-term (in the scale of hours) performance degradation in the depressed waveguide when the internal waves passed through the acoustic track. Even with channel tracking, the time reversal receiver experiences average 3-4-dB decrease in the output signal-to-noise ratio (SNR). Such long-term performance degradation is explained by the ray approximation in the depressed waveguide.

Horizontal reflection of a low‐frequency sound signal from a moving nonlinear internal wave front.

Simultaneous measurements of acoustical and internal waves are reported while an internal wave approaches an acoustic track during the SW06 experiment. The incoming internal wave packet acts as a moving layer reflecting and refracting acoustic waves in the horizontal plane. The mechanism of this interaction is shown in received acoustic data on a vertical hydrophone array using a modal approach. It is shown that the wave front of internal waves behaves as selective filter, depending on the mode number and frequency of the broadband signal. In other words the reflection coefficient in horizontal plane depends on mode number and frequency. Experimental data analysis shows good agreement with the theory. [Work supported by ONR 321OA.]

What Is Shallow Water Acoustics

To many ocean acousticians, shallow water is “water a few acoustic wavelengths in depth, where the normal mode description of the sound field is efficient.” To some of our physical oceanographer friends, shallow water is taken as the portion of the sea that extends from the shore to the continental shelf break. A rather jaundiced geologist once described the entire water column (including shallow water) as “a bothersome thin layer of fluid that obscures the really interesting part of the ocean.” All these definitions have some merit (including the last, if you are a geologist), but they are not the definition we will use. Rather, we will be looking at the region from the end of the surf zone out to the continental shelf break (and even onto the slope to ~500 m depth) as our working definition of shallow water. This is done for pragmatic reasons, having to do with the types of sonar systems that work for given purposes in those depths. Acoustically, we will be looking at sound and sonar systems working from ~50 Hz up to about 5 kHz. Incorporating these limits on depth and frequency bounds our technical area of “shallow water acoustics,” but even with these limits, the field is a rather vast one.

Average decay law of sound intensity and surface noise vertical angular distribution in waveguide with random inhomogeneities

Analytical expressions for average decay law are obtained on the basis of a model for sound propagation in shallow water with random inhomogeneities. The directionality of vertical surface noise is also considered.<<ETX>>

Sound Field in Shallow Water with Random Inhomogeneities

Up to this point now we have considered sound propagation in shallow water as being through a deterministic, inhomogeneous medium. It is well known that these inhomogeneities can have a considerable effect on the propagation of sound. Moreover, we know from oceanography that these random inhomogeneities are primarily concentrated in the top kilometer of the water column (e.g., Babii 1983; Shavcho 1982), which leads to a particularly strong effect of random inhomogeneities in shallow water. Recently, great attention has been paid, both experimentally (Zhou et al. 1996; Headrick et al. 2000) and theoretically (Dozier and Tappert 1978; Gorskaya and Raevskii 1984; Ashley et al. 1987; Kuznetsova 1988; Derevyagina and Katsnel’son 1995; Creamer 1996; Beran and Frankenthal 1992, 1996; Frankenthal 1998), to the study of shallow water as a randomly inhomogeneous medium. Also, the propagation of waves of different kinds in a randomly inhomogeneous medium has been studied for a long time (beginning with Rayleigh) [see, for example, the monographs (Chernov 1958; Rytov et al. 1989; Ishimaru 1978; Flatte 1979)], etc. The major theme of this chapter will thus be the transition from the complex, detailed oceanography (see Chap. 2) to appropriate random medium representations, and then the description of propagation and scattering in these representations.

Noise Field in Shallow Water

One of the most important problems in ocean acoustics is the study (measurement, forecasting) of noise, that is, of the sound fields formed by various wideband, distributed sources. These sources include wind-generated waves (surface disturbance noises), various human constructions which generate industrial and mechanical noises (ships, drilling platforms, etc.), marine organisms (biological noises), earthquakes, and volcanic eruptions (seismic noises), meteorological noises (rain, thunder), noises from under the ice, etc. The effects of these sources, and the characteristics of the noise field they generate, depend on many factors including weather, geographical position, time of day, and season, and are manifested in various frequency bands. A good example of course is that mechanical noises have a marked maximum at the “electrical” frequencies (50 and 60 Hz) of the alternating current mains.

Foundations of the Theory of Propagation of Sound

In this chapter, we will consider some simple models of the shallow water waveguide. Such models allow us to obtain and understand the main features of SW sound propagation quickly. Such simple models can also be perturbed to take into account more realistic properties of the environment, thus giving them far more power than one might think at first.

Equipment for Shallow Water Acoustics and Experimental Considerations

In this chapter, we consider the applications of sound signals with frequencies less than 500 Hz. The use of such low frequencies is primarily motivated by the strong attenuation of acoustic waves in shallow water due to the interaction with the bottom. For example, for a negative gradient sound speed profile, the transmission loss of acoustic signals (TL) where \( {{(TL}} = 20\lg (\bar{p}{/}{p_0}) \) can be desibed in the following equation by the three terms inside square brackets, each corresponding to different decay laws for the sound field with distance

Examples Illustrating the Characteristics of Waveguide Propagation

{{TL}} = \left[ {20\lg ({r_0}{/}{r_1})} \right] + \left[ {15\lg ({r_1}{/}{r^{*}}) - {\beta_1}({r^{*}} - {r_1})} \right] + \left[ {10\lg ({r^{*}}{/}r) - \beta (r - {r^{*}})} \right],