Reconstruction of spatial spectra of the isotropic field of background internal waves

Model reconstruction of the two-dimensional spatial spectra of isotropic background internal waves is considered in the framework of computer modeling. The solution of the inverse problem is based on information about the frequency shifts of interference maxima of an acoustic field. The results of reconstruction of spectra with and without focusing of the inverse wave field are presented. The possibilities of monitoring are illustrated in relation to the interference pattern formed by different modal composition.     

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.     

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.     

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.     

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.     

Horizontal array beamforming in an azimuthally anisotropic internal wave field

A numerical study of beamforming on a horizontal array is performed in a shallow water waveguide where a summer thermocline is perturbed by a time evolving realization of an internal wave field. The components of the internal wave field consist of a horizontally (azimuthally) isotropic, spatially homogeneous contribution, and a horizontally anisotropic, spatially inhomogeneous component. These terms represent a diffuse ("background") internal wave field and a localized solitary wave packet, respectively. Conventional beamforming is performed as a function of time while the internal wave field evolves throughout a computational volume containing the source-receiver paths. Source-receiver orientation with respect to the azimuthally anisotropic component has a significant effect on the beamformed output. When the source-receiver configuration is oriented approximately parallel to the solitary wave crests, beam wander, fading, beam splitting and coherence length degradation occurs in a time-dependent manner as the solitary wave packet passes through the environment. Both horizontal refraction of energy and a time-dependent modal source excitation distribution are responsible for these beamforming effects. In cases where source-receiver orientation is not approximately parallel to the wave crests, these effects are substantially reduced or eliminated, indicating that an azimuthally selective perturbation of the acoustic field can be attributed to the wave packet. Modal decomposition of the acoustic field and single mode starting fields are used to infer that, for the source-receiver orientation along the wave crests and troughs, acoustic propagation is predominantly adiabatic. A modal phase speed analysis explains several features associated with the beamformed power.     

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.     

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.     

Focusing of waves in an anisotropic plane-layered medium

The problem of propagation of electromagnetic waves in an inhomogeneous uniaxial anisotropic medium is discussed. Analytic solutions of Maxwell's equations are found for two particular cases of a plane-layered dielectric. The effect of different parameters of an inhomogeneous distribution of refractive indices on the propagation characteristics of wave beams is analyzed. The corrections of the first approximation of perturbation theory to the propagation constants of natural waves are obtained for a plane-layered focusing medium of general type. This has permitted solving the problem of a distribution of the refractive index which provides for optimal focusing.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1812–1821, December, 1978.In conclusion, the author expresses his gratitude to I. V. Ivanov and N. A. Morozov for their attention to this paper.     

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.     

Information on the inflaton field from the spectrum of relic gravitational waves

After a review of a traditional analysis, it is shown a variation of a more recent treatment on the spectrum of relic gravitational waves (GWs). Then, a connection between the two different treatments will be analysed. Such a connection permits to obtain an interesting equation for the inflaton field. This equation gives a value that agrees with the slow roll condition on inflation.     

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.     

Angular spectrum approach for the computation of group and phase velocity surfaces of acoustic waves in anisotropic materials

The decomposition of an acoustic wave into its angular spectrum representation creates an effective base for the calculation of wave propagation effects in anisotropic media. In this method, the distribution of acoustic fields is calculated in arbitrary planes from the superposition of the planar components with proper phase shifts. These phase shifts depend on the ratio of the distance between the planes to the normal component of the phase slowness vector. In anisotropic media, the phase shifts depend additionally on the changes of the slowness with respect to the direction of the propagation vector and the polarization. Those relations are obtained from the Christoffel equation. The method employing the fast Fourier transformation algorithm is especially suited for volume imaging in anisotropic media, based on holographic detection in transmission of acoustic waves generated by a point source. This technique is compared with measurements on crystals performed by phase-sensitive scanning acoustic microscopy.     

Diffraction of electromagnetic waves by ultrasonic waves in an anisotropic medium. I

Summary The theory of diffraction of electromagnetic waves by an ultrasonic wave in a crystal is discussed above. Expressions are obtained for the diffraction field in the near and the far zones. In the near zone the system is shown to be describable by the scattering matrix S of a 2(N+1)-pole, thus enabling the matrix method of network theory to be used for in vestigating different kinds of cascade connection of such systems. The expressions obtained contain the classical formulas of scalar diffraction in an isotropic layer as particular cases.Izvestiya VUZ. Radiofizika, Vol. 9, No. 1, pp. 16–32, 1966