S. A. Pereselkov

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.

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.

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.

Filtration of groups of single-type modes in shallow seas

A method for filtering groups of single-type sound-field modes in oceanic waveguides is considered. It is based on differences in their dispersion characteristics, which is shown in a spectrogram as a localization of the spectral density in regions, whose positions of maxima cause values of interference invariants corresponding to different groups. Filtration of these groups with subsequent Fourier transform determines the interferograms of single-type modes. Applicability of the method to recording frequency shifts in the interference maxima of different mode groups caused by the motion of an internal-wave soliton is shown. A new approach to passive hydrolocation of sound sources is suggested, based on localization of signals formed by groups of single-type modes.

The Sensitivity of Monitoring by Measuring the Frequency Shifts of the Sound Field Interference Pattern

The sensitivity of the remote method of reconstructing the frequency spectrum of a water medium perturbation from the measured frequency shifts of the sound field interference peaks is investigated. On the basis of the Rayleigh criterion, an expression is derived for estimating the minimal frequency shift that allows the resolution of two neighboring peaks. The detection sensitivity for the sound velocity fluctuations caused by the time variability of the medium along the track is estimated. Results of computations for a perturbation in the form of background internal waves are presented.

Effect of background internal waves on the interference structure of the sound field in a shallow sea

The results of a theoretical analysis of the effect produced by an anisotropic field of background internal waves on the localization of the interference pattern in a shallow sea are presented. The space-time variability of the interference invariant and the smearing of the observation direction of interference fringes are considered in a wide frequency range. The stability of the interference pattern formed by both the superposition of the fields of different mode groups and separate mode groups is analyzed in comparison with the unperturbed waveguide. Numerical calculations are performed for longitudinal and transverse orientations of the acoustic track relative to the propagation direction of internal waves.

Estimation of the velocity of underwater objects in the passive mode using frequency-shift data

A method for estimating the velocity of a broadband noise source at intermediate distances, based on the data for frequency shifts of interference maxima, is described. The effect of interference on the error in determining the object velocity using a horizontal extended receiving antenna is discussed.