V. M. Kuz’kin

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.

Method for measuring the frequency shifts of interference maxima in monitoring of dispersion media: Theory, implementation, and prospects

A theory and implementation of the approach to reconstruction of inhomogeneities in dispersion media, based on measuring the frequency shifts of interference maxima (FSIMs) of wave field, are reported. The possibilities of the new approach are compared with the potential of the conventional technique, which is based on measuring the propagation times of pulsed signals. Specific methods for measuring FSIMs in time-dependent media are described, and the noise immunity of these methods is evaluated. The reconstruction (via FSIM monitoring) of typical oceanic inhomogeneities is described and analyzed. Possible applications of this approach in the infra- and ultrasonic ranges are shown.

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.

Frequency shifts of sound field maxima in few-mode propagation, which are initiated by internal wave solitons

The problem of frequency shifts of sound field interference maxima, which are due to the motion of internal wave solitons in the ocean medium and cause intermodal transformation effects, has been solved for few-mode propagation regime. Simulation results are presented and the conditions of applicability of the adiabatic approximation, in which normal waves can be considered as independent, are discussed. The numerical experiment data are used to analyze the possibility of reconstructing the soliton parameters by measuring the frequency shifts of wave field maxima.

Low-frequency bottom reverberation in shallow-water ocean regions

A phenomenological model of long-range reverberation in a shallow sea is developed to describe the statistical characteristics and interference of the sound field scattered by bottom inhomogeneities. Experimental data on the scattering of low-frequency sound by the sea bottom are presented for a shallow-water region of the Barents Sea. The results of a numerical simulation of the low-frequency bottom reverberation in a multimode waveguide are described. The simulation is based on experimentally measured values of backscattering strength.

Estimation of the depth of a stationary sound source in shallow water

The noise immunity of the method for estimating the depth of a stationary sound source in an oceanic waveguide, based on the information about the amplitude ratio of selected wave field modes, is considered. It is shown that the error in recovering the source depth is always limited and tends to a stationary value with an increase in the noise level. The results of a computational experiment with a single detector and a horizontal linear antenna for the low-frequency region are reported. The behavior of the depth estimate in dependence of the mode numbers and detector depth is analyzed for the limiting case of intense noise. The stability of the method with respect to variations in the speed-of-sound profile is demonstrated.

Wave method for estimating the sound source depth in an oceanic waveguide

An experimental estimation of the sound source depth, based on measuring the amplitude ratio for neighboring wave field modes, has been performed on the Pacific shelf under the following conditions: path length about 10 km at a sea depth of about 53m; pneumatic source signal in the band of 200±10 Hz. The predictive parameters of the bottom have been reconstructed from the envelope of the signal received by a single detector. The noise immunity of the algorithm and its sensitivity to variations in the bottom parameters have been analyzed. The experimental results are found to be in good agreement with the theoretical predictions.

Spatial interference of normal waves in oceanic waveguides

Expressions are derived for the frequency shifts of interference maxima that occur at separate observation points because of a two-dimensional anisotropic random perturbation of the oceanic medium. A relation between the frequency shifts of the field maxima at the observation points and the variations in the phase differences between interfering normal waves is revealed. The use of the frequency shifts in solving direct problems of wave propagation is discussed.

Frequency shifts of sound field maxima under the effect of intense internal waves

Numerical simulation is carried out to study frequency shifts of a low-frequency sound field maxima under the effect of solitary internal waves (solitons) propagating along an acoustic track in the presence of mode coupling. The frequency shifts are measured by the correlation method. Simulation data obtained with allowance for mode coupling and data obtained in the adiabatic approximation are compared and analyzed.

Field focusing control in multimode plane-layered waveguides

The possibility of controlling localized fields in multimode regular waveguides on the basis of the interference invariant principle is studied by numerical simulation. It is demonstrated that, by tuning the radiation frequency of an array without changing the initial field distribution at the aperture, it is possible to perform the scanning by a focal spot in a waveguide. Estimates of the efficiency of this method depending on the size of a vertical linear array are presented.