V. G. Petnikov

Frequency shifts of the sound field interference pattern on oceanic shelf in summer conditions

Considerable shifts Δf/ϕ ≈ 10−1 of the low-frequency sound field interference pattern in the frequency domain, associated with barotropic tide and internal tidal waves, were observed in the Shallow Water’06 experiment on the New Jersey shelf in the summer of 2006. The acoustic frequency shifts appear to be strongly dependent on the modes of the sound field. By examining different modal structure, it is possible to analyze the overall interference pattern and find which part is more sensitive either to the surface tide or the internal waves. This feature can be exploited to acoustically monitor tidal waves of different kinds.

Attenuation of sound in shallow-water areas with gas-saturated bottoms

We investigated the specific features low-frequency (50–300 Hz) sound propagation in shallow-water areas to relatively small distances r ≈ 3H–50H from the sound source, where H is the waveguide depth. The bottoms of water areas were assumed to be fluid homogeneous gas-containing media. Situations were compared in which the sound velocity in the bottom is higher and lower than in the water layer (hard and soft bottom). It was confirmed in experiment that the average effective sound velocity in the bottom may have rather low values (≈100 m/s). The mode description of the acoustic field was used in calculations, and both propagating and outgoing modes, including quasi-modes, were taken into account. The averaged dependences of the field intensity decay on distance were obtained for different frequencies and sound velocities in the bottom. The sound damping factors β in the waveguide were found as functions of frequency and sound velocity in the bottom. It is shown that for a soft bottom, the value of β monotonically increases with an increase in the sound velocity in the bottom, while for a hard bottom, β monotonically decreases. The maximum of β depends on the sound frequency and is reached at the approximate equality of the sound velocities in the bottom and water.

Anisotropic field of background internal waves on a sea shelf and its effect on low-frequency sound propagation

The space-time spectral characteristics of the field of background internal waves (IW) are obtained for two oceanic shelf regions (the Atlantic shelf of the United States and the Kamchatka shelf) and analyzed. Within the framework of a numerical experiment, it is shown that the observed anisotropy of the IW field may considerably affect the low-frequency sound fluctuations in the aforementioned regions and, in particular, may change the interference invariant of the sound field.

Horizontal refraction of low‐frequency acoustic waves in the Barents Sea stationary acoustic track experiment

In this paper, data from the Barents Sea stationary acoustic range experiment carried out in October 1983 are analyzed. The experiment used a tonal acoustic source at 100 Hz and a horizontal array that consisted of 48 hydrophones extending over 450 m, mounted on the bottom. From the measured phase difference between a reference signal and the signal received by the individual hydrophones, phase front fluctuations have been obtained. Continuous measurements of the sound‐speed profile showed fluctuations of the thermocline boundary caused by long period (3.5 h) internal waves with 10‐m amplitude. Simultaneously, acoustic measurements were made of the deviations from linearity of the phase front along the array. Using spatial Fourier analysis, it was seen that the variability of the phase front showed several scales. To explain the spatial phase fluctuations we used results from a horizontal refraction theory for shallow water and a theory of phase dislocation in a waveguide [Kravtsov et al., Sov. Phys. Acou...

Acoustic effects caused by high-intensity internal waves in a shelf zone

The sound field fluctuations caused by high-intensity, solitonlike, quasi-plane internal waves crossing a fixed acoustic path at different angles are numerically modeled for natural conditions of the shelf zone of the Sea of Japan. The horizontal refraction of sound is considered for the case of an acoustic path parallel to the internal wave front.

Focusing of low-frequency sound fields in shallow water

A numerical experiment is carried out to study the focusing of a low-frequency (100–300 Hz) sound field in a shallow-water acoustic waveguide typical of an oceanic shelf. Focusing with the use of time reversal of broadband acoustic signals, which is called time reversal mirror (TRM) of waves, is considered along with focusing by phase conjugation (PC) of a monochromatic sound field. It is demonstrated that, in the case of focusing by the TRM method in the waveguide of interest, it is sufficient to have a single source-receiving element. The use of a vertical array improves the quality of focusing. The quality achieved in the latter case proves to be approximately the same as that achieved in the case of focusing by phase conjugation of a monochromatic field at a frequency identical to the carrier frequency of the broadband signals. It is also shown that, in a range-independent waveguide, intense surface waves considerably reduce the quality of focusing. This effect is most pronounced in the case of using phase conjugation.

Shallow Water Variability and its Manifestation in the Interference Pattern of Sound Fields

Based upon the concept of the interference invariant, the variability in the interference pattern of sound fields in shallow water is described. We have considered the variability in the frequency domain (frequency shift of interference extremum) caused by large‐scale hydrodynamic perturbations. In particular, we have estimated the frequency shift produced by tides, internal waves, surface waves, and diurnal oscillations of the frontal zone. Similar variations of interference pattern can be measured along an invariable acoustic track with stationary sound sources and receivers. The feasibility of such measurements are discussed. Observational data for the frequency shifts of the interference pattern in the Barents Sea are presented and their interpretation is given. It was demonstrated that the measurements of the frequency shifts can be used for monitoring shallow water ocean regions at short distances. To do this, we propose to transmit and receive single‐type modes of underwater sound channels which ar...

The coherence of low-frequency sound in shallow water in the presence of internal waves

The coherence time and transverse coherence length of a low-frequency (100–300 Hz) sound field that is formed by an omnidirectional point source at a distance of 10–30 km in a shallow-water acoustic waveguide, which is characteristic of an open ocean shelf, were estimated analytically and in a numerical experiment. An anisotropic field of background internal waves is considered as a source of spatiotemporal fluctuations. It is shown that the coherence time decreases as the frequency increases, and strongly depends on the perturbation-movement direction. The transverse coherence length is primarily determined by phase incursions that are related to the cylindrical shape of the acoustic-wave front. In the case of transverse propagation, background internal waves may lead to significant variations in this length. The introduction of compensating phase corrections during processing provides a considerable increase in the average transverse coherence length.

On the feasibility of normal wave selection in a shallow-water waveguide

Long-range propagation of low-frequency narrowband sound signals in the near-bottom acoustic channel with random inhomogeneities caused by internal waves and a rough bottom is investigated in the framework of numerical and field experiments. The feasibility of selecting the signal components corresponding to different normal waves is analyzed. The problem of selecting such components is considered for signals of long duration exceeding the characteristic time of the stationary state of the channel.

Determination of the Absorbing and Scattering Properties of the Sea Floor in a Shallow Water Environment by the Spectra of Wide-Band Signals

Sound propagation in a shallow sea is considered within the framework of the two-component model of the sea floor. The porosity and the coefficients of absorption and volume scattering are treated as the parameters characterizing the sea floor. These parameters are determined on the basis of the comparison between the experimental and theoretical frequency spectra of a signal received from a wide-band source. A conclusion is made about the relative contributions of different mechanisms of losses (absorption or scattering) in the sea floor at different sound frequencies.