Abstract
An extremely important part of shallow water acoustics is the study of long-range, low-frequency reverberation. In particular, one is studying acoustic wave backscattering by medium inhomogeneities, which are generally separated by a distance of a few to several tens of kilometers from a sound source and receiver. Interest in this area of study is generated by the fact that low-frequency reverberation affects the results of many methods of large-scale acoustic monitoring of the shelf areas of the World Ocean. This reverberation, along with its main role as an undesired noise signal, can also play the role of an additional source of environmental information. It is important to note that long-range reverberation in shallow water is mainly associated with acoustic backscattering by the sea bottom, since the backscattering coefficient of the sea surface at low frequencies (f≤ 500 Hz) is as a rule lower by 10–30 dB than the similar coefficient characterizing the sea bottom. The volume backscattering coefficient is also lower by several tens of decibels. Therefore, this chapter is mostly devoted to bottom reverberation, although the calculation methods described are in large part applicable to surface reverberation. We will only consider long-range reverberation here; that is, the scattered acoustic field is analyzed at a reception point at a distance r from the bottom scattering region, and r satisfies the condition \( r \gg H \), where H is the waveguide depth.
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Notes
- 1.
In this measurement scheme, the value of τ is selected to be sufficiently small so that Δθ << θ.
- 2.
As for the intramode dispersion, it manifests itself at appreciably longer distances than typical distances to the bottom scattering region, affecting long-range reverberation in shallow water (see Chap. 4).
- 3.
To simplify the basic theory, the waveguide is hereafter assumed to be horizontally homogeneous, although the present theory of long-range reverberation is also valid for a smoothly inhomogeneous waveguide.
- 4.
This assumption is not key and was introduced only to simplify the equations. The theory for bistatic scattering, when sound source and receiver are located at distinct points \( (\Delta {\vartheta^{\it{sc}}} \ne 180^\circ ) \), will be described below.
- 5.
Such changes are possible, e.g., in the cases of an underwater relief feature or when a sandy bottom contains regions with rocky outcrops.
- 6.
The intermode dispersion is absent if one has the excitation and reception of individual waveguide modes.
- 7.
The low-frequency, long-range reverberation for a focused sound field was first studied in Kim et al. (2004).
- 8.
We emphasize that P sc depends on time T as a parameter.
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Katsnelson, B., Petnikov, V., Lynch, J. (2012). Low-Frequency Bottom Reverberation in Shallow Water. In: Fundamentals of Shallow Water Acoustics. The Underwater Acoustics Series. Springer, Boston, MA. https://doi.org/10.1007/978-1-4419-9777-7_6
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