Spectral-energy characteristics of sound fields scattered by fine-structured inhomogeneities of sound velocity into the geometrical shadow zone

In analyzing experimental results obtained with explosive sources in the tropical zone of the Indian Ocean, a good agreement was obtained for spectral-energy characteristics of signals observed in the first geometrical shadow zone with computer calculations of the sound field scattered by fine structure inhomogeneities of the fractal type. From the comparison of the results of calculating the frequency characteristics of sound fields in the shadow zone by the wave code and by the method combining ray acoustics with the wave theory of sound scattering, it was found that both methods are appropriate for describing the real processes of scattering and propagation of sound in the ocean with fine-structured stratification and that these methods can be used for solving inverse problems.     

Sound propagation and scattering in media with random inhomogeneities of sound speed,density and medium velocity

Abstract

This review presents both classical and new results of the theory of sound propagation in media with random inhomogeneities of sound speed, density and medium velocity (mainly in the atmosphere and ocean). An equation for a sound wave in a moving inhomogeneous medium is presented, which has a wider range of applicability than those used before. Starting from this equation, the statistical characteristics of the sound field in a moving random medium are calculated using Born-approximation, ray, Rytov and parabolic-equation methods, and the theory of multiple scattering. The results obtained show, in particular, that certain equations previously widely used in the theory of sound propagation in moving random media must now be revised. The theory presented can be used not only to calculate the statistical characteristics of sound waves in the turbulent atmosphere or ocean but also to solve inverse problems and develop new remote-sensing methods. A number of practical problems of sound propagation in moving random media are listed and the further development of this field of acoustics is considered.     

Space-time structure of the low-frequency acoustic field in an underwater sound channel

Experimental data are presented on the fine structure of the sound field in an underwater sound channel for low and infralow sound frequencies. The experiments are performed in the Black Sea, on a 600-km-long path, with explosive sound sources. The intensity, space-time, and frequency characteristics of the sound field are analyzed. The geometric dispersion of the first normal wave is experimentally studied. The role of the channel inhomogeneities in the violation of the sound field coherence is determined for different frequency bands. On the basis of the experimental data, the vertical distribution of the critical frequencies of the waveguide is obtained, and the validity limits are established for the wave and ray calculation methods. The applicability of the phase methods for calculating the sound fields in waveguides with dispersion is discussed. The frequency-angular dependence of the effective sound attenuation coefficient in an underwater waveguide is revealed and explained.     

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.     

Estimation of distortions in the sound field propagating through mesoscale inhomogeneities

Hamiltonian formalism is used to analyze the effect of the mesoscale inhomogeneities of the ocean medium on the ray structure of the sound field. It is shown that the distortions of the structure of the sound field can be successfully estimated by a function that links the values of the canonical variables of the ray action before and after crossing the inhomogeneity. That function is calculated with the use of the standard ray code. Sound propagation through the synoptic eddy and frontal zone is considered.     

On the theory of multiple scattering of sound waves by spherical particles in liquid and elastic media

Two independent systems of equations are derived for describing the scalar and vector potentials of the sound field in a liquid or elastic medium containing discrete inhomogeneities. One of the systems determines the sound field as the sum of the fields scattered by the particles according to the law of scattering by a single particle with the oscillation amplitudes governed by the properties of the inhomogeneous medium. The other system determines the sound field as the sum of the scattered fields formed in the inhomogeneous medium with the oscillation amplitudes of a particle in a homogeneous medium. Expressions relating the fields that occur in a medium consisting of N particles to the fields in a medium consisting of N − 1 particles are proposed. These expressions may simplify and diversify the methods used for computer simulation of sound fields with the aim to verify the calculations. The results of the study are valid for any particle concentrations under the condition that the scattering by a single particle is determined by its monopole, dipole, and rotary oscillations.     

基于声线跟踪法对闭空间声场预测的接收球半径计算

接收球半径是采用声线跟踪法进行声场预测的一个重要参数。以前所用到的接收球半径一般是通过经验估计得到的,缺乏详细的理论分析。本文从声波传播机理角度分析了根据声线密度来确定接收球半径的原因,并推导了矩形闭空间中声线密度和接收球半径的计算方法。声线密度可以通过原始声线数目,声场空间体积与形状,边界吸声系数来确定。在一给定闭空间里,声线密度可看作是均匀分布的,所以接收球半径与空间位置无关,可看作是一个常数。但对不同的声场空间来说,由于空间体积、形状和吸声系数的变化,声线密度是不同的,因此接收球半径也会不同。声线密度越大,接收球半径越小;声线密度越小,接收球半径越大。实验表明,所提出的接收球模型能用来较准确地预测闭空间里的声压级和混响时间等声学参数。     

一种考虑声线弯曲的温度场重建算法

声波在非均匀温度场中传播时会由于声波的折射而产生声线弯曲现象。为提高非均匀温度场声学CT (Computer Tomography)重建精度,提出一种考虑声线弯曲的重建算法。首先用最小二乘法获得一个不考虑声线弯曲的重建温度场,然后用打靶-插值法确定本征声线出射角,用三角形前向展开法追踪声线,获得此温度场中声波发射/接收器间的本征声线,建立本征声线上声波传播时间与温度分布间的关系,进而实现考虑声线弯曲的最小二乘法温度场重建。单峰、双峰温度场仿真重建结果表明:所采用的本征声线追踪法计算简单运行速度快;考虑声线弯曲后,可明显提高温度场重建精度。因此所提重建算法能更好地适应实际温度场重建需求。      

Modeling of bottom backscattering from three-dimensional volume inhomogeneities and comparisons with experimental data

In this paper, backscattering from 3D volume inhomogeneities in the seabed is modeled and the results compared with experimental data at 250-650 Hz. The experiment was part of the Acoustic Reverberation Special Research Program (ARSRP) and the data were obtained in a sediment pond on the western flank of the Mid-Atlantic Ridge. A volume scattering model based on first-order perturbation theory is developed incorporating contributions from both sound speed and density fluctuations. With the propagators, i.e., the Green's functions, handled accurately through numerical wave number integration and random fluctuations generated effectively by a new scheme modified from the spectral method, the model is capable of simulating monostatic, backscattered fields in the frequency domain as well as in the time domain owing to 3D volumetric sediment inhomogeneities. The model compares favorably and consistently with the ARSRP backscattering data over the entire frequency band, with the fluctuations of sound speed and density in two irregular sediment layers, identified from the data analysis, described by a power-law type of power spectrum.     

Caustics and volume prereverberation in a surface oceanic waveguide

A kinematic model of volume prereverberation caused by sound scattering from different types of inhomogeneities in the caustic zones of a surface oceanic waveguide is considered. Numerical estimates are presented for the prereverberation time as a function of range, number of ray cycles, and width of the scattering diagram.     

Phase conjugation in an acoustic waveguide

A method for holographic observation of sound sources and passive inhomogeneities in acoustic waveguides is proposed. The method is based on the principle of coherent phase conjugation. A possibility of visual observation of both sound sources and stationary inhomogeneities of different kinds is experimentally verified.     

Recent Advances in TXRF

Abstract

Total reflection X‐ray fluorescence analysis (TXRF) is a special method of energy‐dispersive X‐ray fluorescence analysis extending EDXRF to the ultra trace element level. The achievable detection limits depend on the excitation source and are in the range of picograms to femtograms. Only small amounts of sample are required and the quantification by adding one element as an internal standard is easy as thin film approximation is valid. In this article, the recent advances in TXRF are reviewed with over 80 references. The principles, advantages, instrumentation, improvements with X‐ray optics, synchrotron radiation as excitation sources as well as various fields of application, wafer surface analysis, depth profiling, absorption spectroscopy, medical samples, biological samples, environmental monitoring, archeological and polymer samples are described. Related techniques are also mentioned and discussed.     

三维复杂温度场高精度声学测量方法

声学温度场检测技术通过多路径声波传播时间数据,反演被测区域的温度分布.提供了一种高精度的三维复杂温度场的声学测量方法.首先从射线声学角度给出了三维非均匀温度场中声波传播路径的数学模型.在此基础上,将三维温度场的重建问题转化为声波传播路径的求解和温度场的反演问题,建立了基于多项式修正径向基函数(RBF-PR)和改进的Ti...     

Sound scattering by random fractal inhomogeneities in the ocean

Sound scattering by random volume inhomogeneities (fluctuations of the refraction index in a medium) with an arbitrary anisotropy is considered using the small perturbation method (Born’s approximation). Surfaces (boundaries) of the inhomogeneities are deemed to be fractal ones: the energy spectra of the refraction index fluctuations follow the power law with a nonintegral exponent. Formulas are obtained for the volume scattering coefficient. Frequency and angular dependences of the scattering coefficient and their relations to the fractal dimension of inhomogeneities with different kinds of anisotropy and different sizes (on the sound wavelength scale) are presented. The fractal dimension of the inhomogeneities is estimated.     

A sound ray tracing algorithm in three-dimensional heterogeneous media based on wavefront traveltimes interpolation

A kind of three-dimensional（3-D） sound ray tracing algorithm in heterogeneous media is studied. This algorithm includes two steps： the first step computes the wavefront traveltimes forward; the second step traces the sound rays backward. In the first step, the computation of wavefront traveltimes at discrete grid points from the sound source, was found on Eikonal equation solutions and carried out by GMM （Group marching method） wavefront marching method based on level set. In the second step, sound ray tracing was proceeded gradually from the receiver to each cell towards the sound source, with wavefront traveltimes computed in the first step. Time values on arbitrary positions in each cuboid cell can be expressed by linear interpolation of wavefront traveltimes at the same cell＇s grid points. Thus, an algorithm of 3-D sound ray tracing in heterogeneous media is put forward. The simulation results indicate that this method can improve both the accuracy and the efficiency of 3-D sound ray tracing greatly.     

Tomography in observation of inhomogenenities in shallow sea during sounding by a focused high-frequency acoustic field: II. Study of the observation efficiency

We study the efficiency of high-frequency acoustic sounding of small-size spatially localized inhomogeneities in shallow sea by a tomographic method. The method is based on that the focusing of the received and radiated fields to the region of the assumed location of the inhomogeneity is matched to the waveguide. During the observation, the statistical check is performed for hypotheses on the object location that are formed using a priori information on the waveguide model and data on the observation-system structure. To decrease the influence of the interference structure of the acoustic fields in the waveguide, we use accumulation of partial images corresponding to separate ray tomographic projections. Using a simulation model of the considered method, we analyze its accuracy and sensitivity under different observation conditions in shallow sea. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 3, pp. 192–207, March 2009.     

In-plane bistatic backward scattering from seabottom with randomly inhomogeneous sediment and rough interface

1 Introduction Backward scattering of sound due to sediment is the main source of shallow waterreverberation. In order to predict the reverberation or detect sediment properties frommeasured reverberation data, a reasonable in-plane bistatic backward scattering (BBS)model is essential. The scattering can be caused by the roughness of water-sediment in-terface or by inhomogeneities within the volume of sediment. A great deal of researchhas been done on sediment backscattering, most of which h…     

One-dimensional numerical simulations of random sound impulses

Abstract

We perform one-dimensional numerical simulations of small-amplitude acoustic pulses in space- and time-dependent random mass density and time-dependent velocity fields. Numerical results reveal that: (a) random fields affect the speeds, amplitudes and, consequently, shapes of sound pulses; (b) for weak random fields and short propagation times the numerical data converge with the analytical results of the mean field theory which says that a space-dependent (time-dependent) random field leads to wave attenuation (amplification) and all random fields speed up sound pulses; (c) for sufficiently strong random fields and long propagation times numerical simulations reveal pulse splitting into smaller components, parts of which propagate much slower than a wave pulse in a non-random medium. These slow waves build an initial stage of a wave localization phenomenon. However, this effect can be very weak in a real three-dimensional medium.     

Application of the WKB method to calculating the group velocities and attenuation coefficients of normal waves in the arctic underwater waveguide

Algorithms based on the WKB approximation are proposed for the fast and accurate calculation of the group time delays and effective attenuation coefficients of normal waves in the deep-water sound channel of the Arctic Ocean. These characteristics of the modes are determined in the adiabatic approximation by integrating the local group velocity and attenuation coefficient over the horizontal distance between the ends of the propagation path. According to the WKB method, the local group velocity is the ratio of two quantities. The first one is the sum of the length of the ray corresponding to the mode and the side displacement of the ray at the reflection by the ice cover. The second one is the sum of the travel time of the sound signal along the ray cycle and the time delay caused by the side displacement. The grazing angle of the ray is determined from the condition of quantization for the phase integral. According to the WKB method, the local attenuation coefficient of the mode is specified as the ratio of the squared modulus of the coherent reflection coefficient at the lower boundary of the ice cover and the sum of the cycle length and the side displacement of the ray. Simple recurrent relations are proposed to estimate, with fair accuracy and short calculating time, the phase integral, the integral that describes the cycle length, and the related local group velocities and attenuation coefficients. The capacity and efficiency of the algorithms are confirmed by the comparison of the aforementioned mode characteristics calculated by using the proposed relations and the precise computer code. The calculations are performed with the sound speed profiles obtained from the temperature and salinity measurements during the SEVER and SCICEX-1995 expeditions.