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.     

Methods of measuring frequency shifts in the interference structure of the sound field in oceanic waveguides

The efficiency of the correlation method is considered as applied to measuring frequency shifts of maxima in the interference structure of the sound speed under the influence of distortions of the sound-speed profile. The method is based on tracing the position of the maximum of the cross-correlation function corresponding to the spectrum of the transmitted signal in the frequency domain. The distortion is modeled by seasonal variations of the hydrological environment. The noise immunity of the method is analyzed. The correlation method is compared with other known methods of tracing frequency shifts of the interference maxima.     

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.     

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.     

Correlation method of measuring the frequency shifts of the sound field maxima caused by perturbations of the oceanic medium

The correlation method of measuring the frequency shifts of the sound field maxima caused by the nonstationary oceanic medium is considered. The method is based on tracing the frequency shift of the maximum that is observed in the cross-correlation function of signal spectra received at different instants of time. The relation between the correlation method and the method based on measuring the frequency shifts of the maxima observed in the interference pattern of the source is analyzed. The sensitivity of the correlation method is estimated.     

Radiation and scattering of sound waves in oceanic waveguides

The applicability conditions for the concept of a directivity pattern (a scattering amplitude) in the problems of waveguide propagation are formulated. The consideration is based on the solution of the Sturm-Liouville problem. The results of the comparison between these conditions and the analogous conditions obtained earlier in the ray approximation are discussed. The expression for the scattering matrix of waveguide modes is modified on the basis of the suggested conditions in such a way that it involves only the quantities determined from the solution of the Schrödinger equation. This makes it possible to perform numerical calculations by using the results of numerous studies of the propagation in inhomogeneous waveguides and the diffraction by complex-structured bodies in free space.     

On optimal acoustic field excitation in oceanic waveguides

We propose criteria and develop methods for stable optimization when synthesizing acoustic fields in the case of their excitation by a finite set of coherent sources in oceanic waveguides. As an example, we consider methods of amplitude-phase focusing of the field of a vertical emitting array to a given point of a refractive waveguide. We show that the problem of optimal field focusing can be approximately reduced to that of focusing of the most powerful and stable wave components. We use the ray representation of the field.     

Frequency shifts of the spatial interference structure of the sound field in shallow water

A fluctuation model of the frequency shifts of the spatial interference structure formed by the sound field in a randomly inhomogeneous oceanic waveguide is constructed. The relation between the random fields of the perturbation-caused variation of the waveguide’s dispersion characteristic and the frequency deviation of a local interference maximum is analyzed. The applicability of the results to the cases of the waveguide perturbation by the background internal waves and by the roughness of the bottom relief is considered.     

Physical modeling of nonlinear sound wave propagation in oceanic waveguides of variable depth

Results of laboratory experiments aimed at studying the spatial distribution of the difference-frequency acoustic wave field in a shallow-water waveguide with a sloping bottom are presented. It is shown that, in an inhomogeneous waveguide, the direction toward the radiation maximum in the angular spectrum of the low-frequency wave continuously varies as the rib of the wedge is approached, whereas, in a homogeneous waveguide, the angular spectrum is shaped. A spatial filtering of low-frequency modes produced by a parametric radiator and reflected from the coastal wedge is experimentally realized. The results of the experiment are confirmed by numerical modeling. Problems of the physical adequacy of the experimental results obtained under actual and laboratory conditions are discussed.     

Frequency shifts of the sound field interference pattern in shallow water because of second-mode soliton-like internal waves

Numerical simulation is carried out to analyze the effect of an internal soliton of the second gravity mode on low-frequency sound propagation in an oceanic shelf region. The simulation is performed using the data of a full-scale experiment performed on the shelf of the South China Sea near Dongsha atoll, where the aforementioned solitons had been detected by stationary vertical thermistor arrays. The calculations take into account the effect of horizontal refraction of sound waves. It is assumed that a stationary acoustic track is oriented across the predominant propagation direction of internal waves. The results of simulation show that, when the soliton crosses the stationary track, some of the sound field modes are focused, whereas other modes are defocused. It is demonstrated that the soliton parameters can be adequately determined from the frequency shifts of the sound field interference pattern. However, such an estimate of the soliton parameters is only possible for a limited length of the stationary track for which the effect of horizontal refraction is sufficiently weak.     

Correlation reception of a diffraction sound field in an oceanic waveguide

Intensity matched processing is considered for a diffracted signal when the object is detected by the main lobe of the shadow scattered sound field. The consideration is based on the few-parameter model that adequately describes the characteristics of the desired signal. It is shown that, in the absence of noise and fluctuations in the parameters of the medium, the proposed algorithm provides the determination of the exact parameters of the signal and the moving object.     

Spectrograms of single-type modes and their application to problems of sound source localization in oceanic waveguides

The previously proposed method for selecting groups of single-type wave fieldmodes in oceanic waveguides, based on a double Fourier transform of the interference pattern formed by a moving sound source, is theoretically substantiated. This procedure, which is possible due to the invariance of the frequency shift of field maxima with respect to the numbers of interfering single-type modes, leads to spectral density localization. The sizes of acceptable localization domain and spectral density distribution are estimated. The results of a natural experiment on application of this procedure to problems of detection of a sound source and estimation of its radial velocity and distance from receiver are reported.     

Reconstruction of internal waves in oceanic waveguides

Reconstruction of the temporal variability of an intense internal wave field is studied by a numerical experiment. The inverse problem is solved using the data on the frequency shifts of the maxima of the inverted wave field. The influence of the amplitude of the internal wave on the efficiency of reconstruction is analyzed.     

Efficiency of sound field focusing in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning the focal point in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically considered. The localized fields are controlled by varying the transmission frequency without a change in the distribution of the reversed field over the array aperture. The effect of the periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path relative to the propagation direction of internal waves.     

Acoustic-gravity waves in atmospheric and oceanic waveguides

A theory of guided propagation of sound in layered, moving fluids is extended to include acoustic-gravity waves (AGWs) in waveguides with piecewise continuous parameters. The orthogonality of AGW normal modes is established in moving and motionless media. A perturbation theory is developed to quantify the relative significance of the gravity and fluid compressibility as well as sensitivity of the normal modes to variations in sound speed, flow velocity, and density profiles and in boundary conditions. Phase and group speeds of the normal modes are found to have certain universal properties which are valid for waveguides with arbitrary stratification. The Lamb wave is shown to be the only AGW normal mode that can propagate without dispersion in a layered medium.     

Horizontal anisotropy of dynamic noise in oceanic waveguides

A theoretical study of the horizontal anisotropy of dynamic oceanic noise is carried out. It is assumed that an anisotropic noise field distribution arises in the ocean because of noise field scattering from wind waves. The dependence of the magnitude of noise anisotropy on the form of the angular wind wave spectrum is investigated.     

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.     

Caustics and weakly divergent beams in oceanic waveguides

It is found that the weakly divergent beam formed around the reference ray with minimal cycle length does not stand out in intensity against the background of the total field. It is shown that, under the same conditions, another beam stands out in intensity: it is the caustic beam composed of the system of caustic lines, which are the envelopes of the family of rays forming the weakly divergent beam.     

Propagation and resolution of pulse signals in oceanic waveguides

The evolution of mode pulses was described in the second-order approximation of waveguide dispersion theory and their resolution was analyzed. The relationship between the problems of resolution of mode pulses and sound interference maxima was considered. The theoretical concepts were illustrated with an example of perturbation of a medium by background internal waves.