Reconstruction of the spatial spectrum for an anisotropic field of background internal waves

A model reconstruction of the two-dimensional spatial spectrum is considered for an anisotropic field of background internal waves in shallow water. Solution of the inverse problem is based on the data on frequency shifts of the sound field interference maxima. Results of a numerical experiment are presented.     

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.     

Application of time-reversed wave field focusing to reconstructing the frequency spectrum of background internal waves

A numerical experiment is used to analyze the possibility of focusing the time-reversed wave field for reconstructing the frequency spectrum of the vertical displacements of water layers by measuring the frequency shifts of the sound field maximum at the focal spot. The focusing of the field is controlled by varying the transmitted frequency at a fixed distribution of the reversed field, which is formed in the unperturbed waveguide, over the array aperture. The data of computations are compared with those obtained without focusing.     

Acoustic sweep monitoring of background internal waves

The results of a theoretical consideration of fluctuations that occur in the frequency shifts of the interference pattern under the effect of background internal waves are presented. Possibilities of reconstructing the spectrum of vertical displacements of liquid layers from the measured spectrum of frequency deviations of a local interference peak are analyzed within the framework of a numerical experiment. Problems of stability and the efficiency of the proposed monitoring are discussed.     

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.     

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.     

Phase-velocity spectra of background internal friction in polycrystalline solids

The phase-velocity spectrum of background internal friction has been shown to be a material property. The shape of such spectra seems to be different, in particular, for monophasic (e.g. pure metals) and multiphase materials (typically composites and rocks). An explanation for such difference, as yet not known, is proposed in the present work. All spectra are interpreted to be a combination of two or more simple relaxation sources, each emanating from point defect diffusion with a specific diffusion scale length and inducing dissipation into one another. The single dissipation sources are assumed to combine with the laws of linear circuits in parallel. In the case of monophasic metals a model with two sources of dissipation, one in the microstructure and the other in the lattice, fits the experimental spectra qualitatively well. For multiphase materials, including natural rocks and artificial conglomerates, which exhibit flat spectra, a continuous-source model is suggested.     

Efficiency of sound field focusing at a long distance in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning with the focal spot at long distances in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically investigated. 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 periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path with respect to the propagation direction of internal waves. The effect of perturbation on the stability and efficiency of focusing is discussed. A comparative analysis of the data obtained for long and short distances is performed.     

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.     

Radiation of slow electromagnetic waves in an isotropic medium with spatial dispersion

This paper discusses the distinctive features of radiation of electromagnetic waves with anomalously large values of the wave vector and small phase velocity that exist near narrow absorption lines. The distribution of radiated energy with respect to angle and frequency is calculated for Čerenkov radiation and bremsstrahlung of the slow waves. It is shown that the angular distribution of the slow-wave bremsstrahlung exhibits a characteristic maximum in the direction perpendicular to the plane of motion of the particles. Zh. éksp. Teor. Fiz. 112, 1557–1562 (November 1997)     

Effect of intense internal waves on the sound field interference structure

The effect of a train of internal-wave solitons on the formation of space-frequency interference pattern of sound field in an oceanic waveguide has been analyzed. Numerical calculations have been performed for the shallow-water channel parameters corresponding to the conditions of the SWARM’95 natural experiment.     

Energy spectra of the ocean's internal wave field: theory and observations

The high-frequency limit of the Garrett and Munk spectrum of internal waves in the ocean and the observed deviations from it are shown to form a pattern consistent with the predictions of wave turbulence theory. In particular, the high-frequency limit of the Garrett and Munk spectrum constitutes an exact steady-state solution of the corresponding kinetic equation.     

“Hot spots” in the field of internal waves in the ocean

A review of observations of high-amplitude internal waves is presented. These waves play an important role in the global scenario of water mixing in the world ocean and also cause a considerable variability of oceanographic fields, including the sound velocity field. The main sources of intense internal waves and the regions where such waves arise are considered. It is shown that, in addition to the much-investigated trains of soliton-like internal waves, which are formed near large-scale inhomogeneities of the bottom relief because of the nonlinear evolution of internal tides, internal waves encountered in the open ocean also possess considerable amplitudes and nonlinearity.     

Propagation of self-gravitating density waves in the deDonder gauge on a gravitational background field

Arguments are given for using the deDonder instead of the synchronous gauge in describing the propagation of density perturbations in a preexisting gravitational field. Since in the deDonder gauge the corresponding reference frame is fixed on the background, the physical interpretation of results is obvious, while in the synchronous gauge it is at least very difficult to extract the physical consequences from the results. For the propagation of density perturbations, with large spatial extension, a decisive difference is found between the two gauges. While in the synchronous gauge there is a growing mode in the density contrast (at least for adiabatic perturbations on a background matter substratum withp as equation of state), in the deDonder gauge there is not. The calculation in deDonder gauge leads to upper boundaries for the spatial extension of unstable density perturbations, and thus may give a hint for upper boundaries of galaxy masses.     

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.     

New method of spatial superposition of attenuated waves for ultrasound field modelling

The objective of this work is the contrary issues of ultrasonic diagnostics in medicine when modern requirements for resolution are in conflict with strict safety issues. There is only one way to make progress by starting to take into account the attenuation in biological tissues and the wave diffraction phenomena. The aim of this work is to develop the flexible ultrasound field model implemented in routine algorithms of digital signal processing. The method consists of the calculation of plane wave propagation and the calculation of an ultrasound signal field. On the basis of the spatial impulse response of an aperture for calculation of space-spread ultrasound signals and the spectrum decomposition method for modelling plane wave propagation in lossy media, the modified method of spatial superposition of attenuated waves was developed. Using the method of equidistant line calculation the time and frequency features of the ultrasound signal field caused by the geometry and dynamics of the aperture, the attenuation and velocity dispersion in the medium are determined. The method was successfully applied to the investigation of the system for intracranial media monitoring, where a new measurement channel based on the changes of attenuation and dispersion in intracranial medium has been implemented.     

Analysis of spatial resonances in the field of evanescent waves by the discrete source method

Scattering properties of particles placed on a metal film deposited on a glass prism are studied in detail using a computer implementation of the discrete source method. The presence of scattering cross-section resonances in the region of angles of incidence of a plane wave has been established. The possibility of controlling the radiation directionality in the resonance range is demonstrated. Conditions that permit one to increase the resonance intensity by an order of magnitude have been found.     

浅海内波环境下声场时间相关特性

浅海内波是导致声场时间相关半径减小的一个重要原因.利用2015年南中国海声传播起伏实验,对比分析了线性内波以及孤立子内波环境下声场时间相关半径的统计特性.实验数据(175～225 Hz)表明,大振幅孤立子内波的存在极大地降低了声场的时间相关半径,声场时间相关半径从线性内波环境下的1～3 h,降低为孤立子内波环境下的小于...     

Sound field phase front variations in shallow water in the presence of intense internal waves

The method of vertical modes and horizontal rays and the method of parabolic equation in the horizontal plane are used to study space-time fluctuations of the phase front of the sound field in the presence of a train of internal waves passing through the acoustic path. The possibility of measuring the distortions of the phase front with a simultaneous filtering of modes by a horizontal array is analyzed. A numerical simulation is carried out for the conditions corresponding to the Eastern coast of United States and the Barents Sea.