Space-time acoustic holography of moving inhomogeneities

A method for observing weak diffraction responses on the background of a fluctuating signal from a primary cw source is developed and tested. The possibility to visually observe the dynamics of the secondary field caused by the presence of nonstationary perturbations of the water medium is demonstrated.     

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.     

Ocean acoustic tomography with a nonstandard representation of refractive inhomogeneities

The two-dimensional tomography problem of reconstructing a refractive inhomogeneity in the ocean is considered. A distinction of this paper is the expansion of the inhomogeneity under investigation in a nonstandard (nonorthogonal and overfull) basis. This basis makes it possible to overcome some problems inherent in the conventional schemes dividing the ocean into squares, triangles, and other figures with corners. In addition, the perturbation matrix can be easily constructed in such a basis. The proposed approach can be used in its present form for reconstructing flows and solving combined refractive-kinetic problems. The solution of the tomography problem with the use of the proposed basis is carried out in both ray and wave representations.     

A three-dimensional tomography model for reconstruction of oceanic inhomogeneities under unknown antenna positioning

Consideration is given to a scheme for tomographic reconstruction of a scalar-vector inhomogeneity in the ocean, the scheme being based on the measurements of horizontal refraction of modes. Times of mode propagation and differences of angles of modal pulses passing from two transmitting antennas to a single receiving antenna serve as initial data. The suggested approach is distinguished by its weak sensitivity to unknown displacements of antennas in the horizontal plane at a distance of several hundred meters. Results of numerical simulation of reconstruction of a combined scalar-vector inhomogeneity based on the regarded scheme are demonstrated.     

Study of acoustic wave backscattering by discrete inhomogeneities of different sizes

The influence of the distribution statistics on the sizes of discrete random bottom inhomogeneities on the mean intensity of acoustic signal backscattering is studied. The simultaneous presence of two effects leading to an increase in the mean intensity of backscattering are considered: the correlation in the positional relationship of particles on the plane of the bottom and their size distribution.     

Regions of high correlation of acoustic waves in oceanic waveguides

We discuss the role of weakly divergent rays in the formation of regions with high correlation of waves in inhomogeneous stationary random media. The geometrical-acoustic approximation is used. Theoretical study is supported by numerical calculations.     

Imaging with acoustic antenna array through a thick layer of large-scale inhomogeneities

The problem of determining the positions and levels of signals by an array in a thick layer of largescale inhomogeneities is considered. The problem is solved in two steps. At the first step, the method of wave front inversion is used to find the fractions of the spatial spectra at which one of the signals predominates. At the second step, within each spectral fraction, the signal is processed by using both the method [5] of taking the logarithms of complex functions with a subsequent filtering and the method of parameter estimation. The results of numerical modeling are presented.     

Finite difference computation of acoustic scattering by small surface inhomogeneities and discontinuities

The use of finite difference schemes to compute the scattering of acoustic waves by surfaces made up of different materials with sharp surface discontinuities at the joints would, invariably, result in the generations of spurious reflected waves of numerical origin. Spurious scattered waves are produced even if a high-order scheme capable of resolving and supporting the propagation of the incident wave is used. This problem is of practical importance in jet engine duct acoustic computation. In this work, the basic reason for the generation of spurious numerical waves is first examined. It is known that when the governing partial differential equations of acoustics are discretized, one should only use the long waves of the computational scheme to represent or simulate the physical waves. The short waves of the computational scheme have entirely different propagation characteristics. They are the spurious numerical waves. A method by which high wave number components (short waves) in the wave scattering process is intentionally removed so as to minimize the scattering of spurious numerical waves is proposed. This method is implemented in several examples from computational aeroacoustics to illustrate its effectiveness, accuracy and efficiency. This method is also employed to compute the scattering of acoustic waves by scatterers, such as rigid wall acoustic liner splices, with width smaller than the computational mesh size. Good results are obtained when comparing with computed results using much smaller mesh size. The method is further extended for applications to computations of acoustic wave reflection and scattering by very small surface inhomogeneities with simple geometries.     

Optimization of ocean acoustic monitoring systems

We formulate and solve certain problems of the optimal amplitude-phase tuning of antenna complexes that emit and receive acoustic signals in inhomogeneous ocean-type waveguides.     

Illumination of the shadow zone in a two-channel oceanic waveguide with a fine structure of sound speed inhomogeneities

The experimental data on the sound field structure, which were obtained by emitting a continuous pseudonoise signal (a midfrequency of 3.2 kHz) in a two-channel oceanic waveguide, are compared with the calculations performed by the wave program with allowance for the fine structure of the sound speed inhomogeneities. A considerable increase in the intensity of the sound field with a definite angular spectrum is observed in the upper channel in the first shadow zone, and, in the experiment, the increase begins nearer to the sound source than predicted by the wave and ray calculations for a smooth sound speed profile. These features of the field structure are explained by the illumination of the shadow zone by the regular scattering of signals from highly anisotropic fine-structure inhomogeneities of the sound speed profile, which are clearly pronounced in the region of the given oceanic experiment.     

Triple-quantum-filtered imaging of sodium in presence of B(0) inhomogeneities

Triple quantum filtered sodium MRI techniques have been recently demonstrated in vivo. These techniques have been previously advocated as a means to separate the sodium NMR signal from different physiological compartments based on the differences between their relaxation rates. Among the different triple quantum coherence transfer filters, the three-pulse coherence transfer filter has been demonstrated to be better suited for human imaging than the traditional four-pulse implementation. While the three-pulse structure has distinct advantages in terms of RF efficiency, the lack of a refocusing pulse in the filter introduces an increased dependence on the main magnetic field inhomogeneities, which can sometimes lead to significant signal loss. In this paper, we characterize these dependencies and introduce a method for their compensation through the acquisition of a B(0) map and the use of a modified phase cycling scheme.     

Methods for calculation of the fields of parametric acoustic arrays in oceanic waveguides

Methods for calculation of the fields of parametric acoustic arrays operating in an inhomogeneous medium under regimes of low-frequency wave generation similar to the Westervelt and Berktay regimes for free space are discussed. Mode and ray representations of difference-frequency waves are used. Both plane isovelocity waveguides and refractive sound channels are investigated.Nizhny Norgorod State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 8, pp. 752–759, August, 1993.     

Statistics of the ray travel times in oceanic acoustic waveguides under chaos conditions

Data processing for the numerical calculation of the ray paths in both range-dependent and range-independent oceanic acoustic waveguides is performed. The obtained statistical data are used to plot empirical distribution functions of the chaotic-ray random parameters. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 3, pp. 196–211, March 2007.     

Dopant-induced nanoscale electronic inhomogeneities in Ca(2-x)Sr(x)RuO4

Ca(2-x)Sr(x)RuO4 single crystals with 0.1 < or = x < or = 2.0 have been studied systematically using scanning tunneling microscopy (STM) and spectroscopy, low-energy electron diffraction, and angle resolved photoelectron spectroscopy (ARPES). In contrast with the well-ordered lattice structure, the local density of states at the surface clearly shows a strong doping dependent nanoscale electronic inhomogeneity, regardless of the fact of isovalent substitution. Remarkably, the surface electronic roughness measured by STM and the inverse spectral weight of quasiparticle states determined by ARPES are found to vary with x in the same manner as the bulk in-plane residual resistivity, following the Nordheim rule. For the first time, the surface measurements--especially those with STM--are shown to be in good agreement with the bulk transport results, all clearly indicating a doping-induced electronic disorder in the system.     

Features of the energy structure of acoustic fields in the ocean with two-dimensional random inhomogeneities

The paper is devoted to the analytic study and numerical simulation of mid-frequency acoustic signal propagation in a two-dimensional inhomogeneous random shallow-water medium. The study was carried out by the cross section method (local modes). We present original theoretical estimates for the behavior of the average acoustic field intensity and show that at different distances, the features of propagation loss behavior are determined by the intensity of fluctuations and their horizontal scale and depend on the initial regular parameters, such as the emission frequency and size of sound losses in the bottom. We establish analytically that for the considered waveguide and sound frequency parameters, mode coupling effect has a local character and weakly influences the statistics. We establish that the specific form of the spatial spectrum of sound velocity inhomogeneities for the statistical patterns of the field intensity is insignificant during observations in the range of shallow-water distances of practical interest.     

Diffraction focusing of acoustic fields in oceanic waveguides smoothly varying along the track

With the use of adiabatic and WKB approximations for the mode representation of an acoustic field excited by a tone source in an oceanic waveguide smoothly varying along the track, dependences of the characteristic spatial scales of the phenomenon of diffraction focusing are obtained. Conditions that should be satisfied for the formation of zones of diffraction focusing of the acoustic field in such a waveguide are formulated.     

Method for calculating the scattering of solitons from local inhomogeneities

A special mathematical method combining analytical and numerical tools is developed to study how solitons are scattered from local inhomogeneities. As an example the method is used to investigate the scattering of a Toda-lattice soliton from a mass impurity.     

Formation of high-frequency surface reverberation by excitation of directed acoustic pulses in oceanic waveguides

Using the ray approximation, we study the distribution of areas of effective surface reverberation formation as a function of observation conditions, in particular, the parameters of the plane-layered refractive waveguide, the wind waves, the probing pulses, and the transmit-receive antennas. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 5, pp. 369–380, May 2006.     

Statistical characteristics of waves in random media with space-time inhomogeneities (review)

Conclusion Having presented the results one can note that, in recent years, the nonquasistatic theory of waves in media with parameters which fluctuate in space and time has achieved definite results. The effect of various factors on the spectral, energetic and polarization characteristics of waves has been clarified. At the same time, a number of problems remain insufficiently explained. For example, in the works cited above, the attention has been focused mainly on the analysis of correlation functions of a complex field. Other correlation functions (of phase, frequency, etc.) which are also of interest, have not been studied a great deal. The problem of strong fluctuations under a longitudinal propagation if slow waves in flows has not been completely clarified. The role of nonquasistaticity of fluctuations for higher-order moments (in particular, intensity fluctuations) has not been studied. Finally, for applications to realistic situations, it would be desirable to generalize the theory to the case of signals which are initially nonmonochromatic, to the case of statistically nonstationary media and, in addition, to active and nonlinear media.Gorkii State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 30, No. 1, pp. 3–35, January, 1987.