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.     

Measurements of energy exchange between acoustic fields and non-uniform steady flow fields

This paper describes an experimental investigation of interactions between acoustic waves and a non-uniform steady flow field. Data have been obtained for a resonance tube having a vent at the center in the lateral boundary, an average flow being introduced at the ends. Experiments have been done for both circular and slot vents, over ranges of both frequency and Mach number. According to the one- dimensional linear stability analysis, the interactions between the longitudinal acoustic field and the mean flow in the vicinity of the vent cause a net transfer of acoustic energy from the average flow to the acoustic field. This result has been verified by the experiments reported here. The gain of energy measured is less than that deduced from a one-dimensional analysis.     

Transverse energy flows in vectorial fields of paraxial beams with singularities

A general study of transverse energy flows (TEF) as physically meaningful and informative characteristics of paraxial light beams’ spatial structure is presented. The total TEF can be decomposed into the spin and orbital contributions giving rise to the spin and orbital angular momentums, correspondingly. Definitions and properties of these constituents are discussed in relation with the optical field representation through linear and circular orthogonal polarization bases. With the help of model examples, the results are applied to investigation of TEF singularities in connection with the usual polarization morphology characteristics of paraxial optical fields. An analysis of TEFs near singular points has been carried out; in particular, the behavior of TEF and its partial contributions near polarization singularities (C-points) has demonstrated the special role of a boundary flow in the origin of the spin angular momentum. The analytical and experimental applicability of the introduced concepts are discussed.     

Mapping of the ocean surface wind by ocean acoustic interferometers

Measurements of marine surface winds are crucial to understanding mechanical and thermodynamic forces on the ocean. Satellite measurements of surface winds provide global coverage but are problematic at high wind speeds. Acoustic techniques of wind speed retrieval, and even for tracking hurricanes, have been suggested as an alternative since wind is a strong source of ambient noise in the ocean. Such approaches involve near-local measurements with bottom-mounted hydrophones located close to the area of interest. This paper suggests a complementary approach: measuring directivity of low-frequency ambient noise in the horizontal plane. These measurements would employ long vertical line arrays (VLAs) spanning a significant portion of the ocean waveguide. Two VLAs separated by a distance of some tens of kilometers and coherently measuring acoustic pressure form a single ocean interferometer. By sampling the area of interest from different perspectives with at least two interferometers, marine surface winds might be mapped over horizontal scales of the order of 1000 km with about 10 km resolution (more specifically, the 10 km resolution here means that contribution from the basis functions representing surface wind field with the scale of spatial variations of the order of 10 km can be resolved; independent retrieval of the wind within 10(4) cells of a corresponding grid is hardly possible). An averaging time required to overcome statistical variability in the noise field is estimated to be about 3 h. Numerical simulations of propagation conditions typical for the North Atlantic Ocean are presented.     

基于声能密度模型的中高频复杂声场预报方法

提出并推导了一种基于声能密度分布方程的声场预报方法。在能量和功率流的本构关系基础上建立声能密度平衡方程。应用直达声场和反射混响声场的叠加原理和边界面散射模型,建立了面向中高频复杂声场细节预报的数值计算方法。通过有限元计算结果在一个简单声场模型上对此方法做了验证,对比结果显示了声能密度法预报有可靠的精度和准确度。     

On determining the dimension of chaotic flows

We describe a method for determining the approximate fractal dimension of an attractor. Our technique fits linear subspaces of appropriate dimension to sets of points on the attractor. The deviation between points on the attractor and this local linear subspace is analyzed through standard multilinear regression techniques. We show how the local dimension of attractors underlying physical phenomena can be measured even when only a single time-varying quantity is available for analysis. These methods are applied to several dissipative dynamical systems.     

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.     

Cross-correlation in band-limited ocean ambient noise fields

Observations of ambient noise in the ocean are generally band limited, because of the natural spectral shape of the noise or the restricted bandwidth of the detection system. Either way, the noise may be regarded as white noise to which a band-limiting filter has been applied. An analysis of the two-point cross-correlation function of such filtered noise is presented for two cases, isotropic and surface-generated noise. The most pronounced effects occur with high-pass and bandpass filters when the low-frequency cut-off falls well above the first few zeros in the coherence function. In this situation, the sensor separation is very many times the longest acoustic wavelength (associated with the lowest frequency) in the passband. The filtering then produces sharp pulses at correlation delays equal to the numerical value of the acoustic travel time between the sensors. Although these pulses are narrow, they have a finite width, within which a fine structure appears in the form of multiple rapid oscillations, due to the differentiating action of the filter. The number of such oscillations increases as the low-frequency roll-off of the filter becomes steeper. This fine structure is evident in several recently published experimental determinations of the cross-correlation function of band-limited ocean ambient noise.     

噪声源非均匀分布海洋环境噪声水平声能流理论分析

研究水平非均匀分布噪声源所产生的各向异性海洋环境噪声场声能流。提出一种混合型非均匀分布噪声源模型,理论分析并数值计算了此模型情况下的环境噪声场水平声能流。结果表明:非均匀分布噪声源引起的海洋环境噪声场具有显著非零平均水平声能流;不同接收点的水平声能流明显不同;其幅度和方向取决于各个局部海域不等强度声源产生的合成噪声声能流矢量和。研究了两接收点间噪声的声压和振速水平分量、振速水平分量归一化相关系数随两接收点间距的关系,各量之间表现出较强相关性,为分析水下矢量声场目标探测技术性能提供理论依据。     

Geophysical exploration of the ocean floor using acoustic methods

Sediment patterns on the ocean floor of the continental shelf can be studied using ultrasonic sonar systems, in which the geophysical patterns are mapped on a permanent paper record. Housed within a streamlined fish-like towed body, a transmitter sends an ultrasonic acoustic beam towards the sea bed, illuminating a path of the sea floor of width 1 km parallel to the track of the ship. The strength of the echo reverberation signals increases progressively through the sediment series soft mud, sands, cobbles, rocks and corals, so some measure of sediment identification is possible.     

Fiber-optic tomography of acoustic fields

Institute of Applied Physics, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 31, No. 11, pp. 1388–1393, November, 1988.     

Temporal coherence of acoustic signals in a fluctuating ocean

Temporal coherence of acoustic signals propagating in a fluctuating ocean is important for many practical applications and has been studied intensively experimentally. However, only a few theoretical formulations of temporal coherence exist. In the present paper, a three-dimensional (3D) modal theory of sound propagation in a fluctuating ocean is used to derive closed-form equations for the spatial-temporal coherence function of a broadband signal. The theory is applied to the analysis of the temporal coherence of a monochromatic signal propagating in an ocean perturbed by linear internal waves obeying the Garrett-Munk (G-M) spectral model. In particular, the temporal coherence function is calculated for propagation ranges up to 10(4) km and for five sound frequencies: 12, 25, 50, 75, and 100 Hz. Then, the dependence of the coherence time (i.e., the value of the time lag at which the temporal coherence decreases by a factor of e) on range and frequency is studied. The results obtained are compared with experimental data and predictions of the path-integral theory.     

Characterization of acoustic disturbances in linearly sheared flows

The equation describing the plane wave propagation, the stability, or the rectangular duct mode characteristics in a compressible inviscid linearly sheared parallel, but otherwise homogeneous, flow, is shown to be reducible to Whittaker's equation. The resulting solutions, which are real, viewed as functions of two variables, depend on a parameter and an argument the values of which have precise physical meanings depending on the problem. The exact solutions in terms of Whittaker functions are used to obtain a number of known results of plane wave propagation and stability in linearly sheared flows as limiting cases in which the speed of sound goes to infinity (incompressible limit) or the shear layer thickness, or wave number, goes to zero (vortex sheet limit). The usefulness of the exact solutions is then discussed in connection with the problems of plane wave propagation and stability of a finite thickness shear layer with a linear velocity profile. With respect to the plane wave propagation it is shown that, unlike the compressible vortex sheet, the shear layer possesses no resonances and no Brewster angles, whereas with respect to the stability problem it is shown that, again unlike the compressible vortex sheet, the thin layer is unstable to long wavelength disturbances for all Mach numbers. These results imply that the reflection and stability characteristics of a non-zero thickness but thin shear layer (i.e., the long wavelength characteristics) do not go over smoothly into the results of the compressible vortex sheet as the wave number approaches zero, except for a limited range of generally subsonic relative flow of the two parallel streams bounding the shear layer.     

Features of sound propagation in the ocean with fine-structure inhomogeneities

We analyze the results of an experiment using an explosive sound source in the tropical part of the Indian Ocean. We consider the time structure of sound signals in geometric shadow zones to a distance of 270 km and the scheme of how the sound field in the shadow zone is formed by rays reflected from horizontally extended fine-structured sound velocity layers. From the results of calculation using a wave program that realizes the method of psuedodifferential parabolic equations, we analyze the influence of signal scattering by fine-structure sound velocity inhomogeneities on the sound field distribution in a waveguide. We show that the field formed by spots of light in each of the shadow zones is generated by a regular field and propagates in parallel to it, taking energy from the regular zone in the near field and in each subsequent convergence zone. This mechanism causes an additional decrease in the field in illuminated zones, which can be interpreted as additional attenuation of the regular sound field.     

Acoustic energy in non-uniform flows

The concept of acoustic energy is extended to non-uniform fluid flows. In general, the resulting energy balance equation exhibits acoustic energy production or dissipation within the flow, but in an important special case the production term is zero, implying conservation of acoustic energy. The acoustic energy equations used in classical and geometric acoustics are recovered from the general formulation by making the appropriate assumptions.     

Computer modeling of sound fields in the ocean with fine-structured inhomogeneities

The results of a computer modeling of sound propagation in the ocean with fine-structured inhomogeneities are presented. The modeling was performed using a wave code based on the wide-angle approximation, which allows one to estimate the effects of sound field perturbations. These effects include the insonification of the geometric shadow zones and the abnormal attenuation of low-frequency sound in the course of its propagation in an oceanic waveguide. Calculations clearly demonstrate that the fine-structured inhomogeneities of the sound velocity considerably affect the sound propagation in the ocean.     

Spatial field shifts in ocean acoustic environmental sensitivity analysis

This article examines the effects of spatial field shifts in ocean acoustic environmental sensitivity analysis. Acoustic sensitivity studies are typically based on comparing acoustic fields computed for a reference environmental model and for a perturbed model in which one or more parameters have been changed. The perturbation to the acoustic field due to the perturbed environment generally includes a component representing a spatial shift of the field (i.e., local field structure remains coherent, but shifts in range and/or depth) and a component representing a change to the shifted field. Standard sensitivity measures based on acoustic perturbations at a fixed point can indicate high sensitivity in cases where the field structure changes very little, but is simply shifted by a small spatial offset; this can conflict with an intuitive understanding of sensitivity. This article defines and compares fixed-point and field-shift corrected sensitivity measures. The approaches are illustrated with examples of deterministic sensitivity (i.e., sensitivity to a specific environmental change) and stochastic sensitivity (sensitivity to environmental uncertainty) in range-independent and range-dependent environments.     

Identification of fish vocalizations from ocean acoustic data

A new method for identification of fish vocalizations based on auditory analysis and support vector machine (SVM) classification is presented. In this method, high resolution features have been extracted from fish vocalization data using the amplitude modulation spectrogram (AMS) of the input signals to facilitate the identification of grunts and growls made by a highly vocal wild fish, Porichthys notatus. The comparison results made from ocean audio recordings verify the effectiveness of the proposed method in identifying various types of fish vocalizations. The relationships between signal-to-noise ratio (SNR) and ocean temperature with the accuracy of the proposed method have also been quantified. Moreover, a context-aware prediction algorithm is introduced for estimating the continuous data.