Range dependence of the vertical structure of the sound field in the ocean

In the ocean without fluctuations, the sound field is calculated by the method of geometrical acoustics with allowance for purely water-path rays in a sound channel of canonical shape with a thickness of 4 km for distances of 500 and 2000 km. The sound field is determined as a sum of individual rays arriving at a given point with their own amplitudes and phases. It is shown that the vertical structure of the sound field consists of a number of caustics separated by regions with a quasi-random distribution of the field whose amplitude is much smaller than that in the caustics. At a fixed distance, the number of caustics is equal to the difference between the numbers of the ray turning points at the boundaries of the departure angle range. As the distance from the source increases, the number of caustics increases proportionally to distance.     

On the vertical structure of the sound field in a canonical waveguide at long ranges

The geometrical-acoustics approach is used to calculate the vertical structure of the sound field in an oceanic waveguide. The profile of the sound speed is specified to be canonical and range-independent along a 1000-km propagation path. A monochromatic sound source lies on the waveguide axis. It is shown that, at long distances from the source, the sound field formed by the water-path rays is mainly concentrated in the caustics, the number of which is determined by the number of the overlapping ray cycles at a given distance. A method for estimating the amplitude of the sound field produced by individual rays is proposed. The amplitudes obtained are used to calculate the total sound field along the vertical. A possible cause of the chaotic distribution of ray coordinates is considered. This cause may consist in the arbitrary choice of the number of rays and their departure angles without taking into account the discrete character of one of the variables. This mechanism of ray chaos formation furnishes an explanation for the fact that the chaos obtained in calculations is mainly associated with the flat rays.     

Calculation of temporal, angular, and intensity characteristics of the sound field in a coastal area with an arbitrary bottom relief and a constant sound speed in the medium

Computational algorithms and some computed data are presented for the total sound field in a waveguide whose depth arbitrarily depends on two horizontal coordinates. The sound speed is supposed to be constant, and the bottom slopes are considered as small. The algorithm involves three steps. First, horizontal rays, i.e., horizontal projections of the real rays multiply bottom-and surface-reflected, are calculated. Second, the horizontal rays are set up to a point. Third, the real rays are set up to a point. The computational accuracy is analyzed for a homogeneous wedge lying on a halfspace. Calculations are carried out for a coastal region of the ocean.     

Low-frequency horizontal acoustic refraction caused by internal wave solitons in a shallow sea

The effect of internal wave solitons on the sound field generated by a point source in a shallow sea is considered. In the framework of the theory of “horizontal rays and vertical modes,” the sound field pattern governed by the aforementioned hydrodynamic effect is investigated. It is shown that solitons can induce time-periodic focusing and defocusing of horizontal rays propagating at shallow angles to the internal wave front. This may result in the formation of “dynamical” horizontal sound channels, which, in its turn, results in considerable temporal fluctuations of the field along the acoustic track oriented along the internal wave front. For the sound field calculations, an approach is developed on the basis of the parabolic approximation in the horizontal plane and the mode representation in the vertical direction. The results obtained can be used for remote monitoring of internal wave packets in a shallow sea.     

Noise field anisotropy of surface sources in a coastal region with arbitrary bottom relief and sound velocity profile

The noise field anisotropy of surface sources in a coastal region with an arbitrary three-dimensional bottom relief and an arbitrary sound velocity profile is investigated. The results of computations performed by a computer code in two stages are presented. The first stage consists in the computation of horizontal rays, i.e., the projections of the real rays multiply reflected from the bottom and sea surface onto the horizontal plane. The second stage summarizes the contributions of the noise sources lying within the surface elements that are cut out on the sea surface by a narrow ray tube launched from the point of observation in a given direction. The computations show that, in the coastal region, the noise field is essentially anisotropic, and this anisotropy occurs not only in the vertical plane, which is characteristic of the deep ocean, but in the horizontal plane as well.     

Horizontal refraction of sound rays due to short-period internal waves in the ocean

The effect of a short-period internal wave measured in field conditions on the horizontal (side) refraction of sound rays is estimated. The angle of horizontal refraction, i.e., the angle between the direction of the signal arrival in the horizontal plane and the true direction to the sound source, is determined. The influence of various factors, such as the position of the receiving system in depth with respect to the layer of high sound velocity gradients, the rotation of the transmitter-receiver track with respect to the internal wave front, etc., on the horizontal refraction is estimated. Numerical calculations are carried out. Conclusions about the possible errors that arise in determining the azimuth direction to the sound source because of the effect of short-period internal waves are derived.     

Investigation of the sound field structure in the ocean with a deep-water reception

The results of studying the angular and energy structures of a sound field at great depths (880 and 1100 m) with the emission of pseudonoise signals (a frequency range of 1–4 kHz) in the upper layers of the ocean below the velocline (at a depth of 200 m) are presented. The results refer to two ocean regions characterized by the presence of a single-axis deep-water sound channel. The sound field structure at great depths is compared with that recorded at the source depth. The experimental data for the first two convergence zones are compared with the calculations by the ray method. A conclusion is made that, on the whole, the basic regularities of the field structure at great depths are adequately described by the ray theory. However, the spatial positions of the convergence zones observed in the experiment differ from those predicted by the ray calculations. Moreover, a closer examination of the field characteristics shows that the parameters of the angular and energy structures noticeably deviate from the calculated values.     

Fresnel zones for modes and analysis of field fluctuations in random multimode waveguides

The notion of Fresnel zones for modes is introduced, which is analogous to the usual Fresnel zones introduced for rays. It is shown that using Fresnel zones for modes one can simplify the analysis of mode scattering at large-scale and random inhomogeneities of a medium in waveguides. Simple formulae to calculate fluctuations of mode amplitudes are obtained. They are similar to well-known formulae of geometrical optics and to those of the Rytov method used to calculate fluctuations of ray complex amplitudes. Relations deduced can be used for calculating field fluctuations both at regular waveguide points and at caustics.     

深海不完整声道下反转点会聚区研究

近期南海远程声传播实验数据的处理分析表明在深海不完整声道中声道轴以下存在一种会聚区,该会聚区相比于海面附近的上反转点会聚区在远距离处具有更高的会聚增益.本文利用射线简正波理论确定了水中反转型焦散线和海面反射型焦散线位置,对比发现实验中观测到的深海大深度会聚区位置与水中反转型焦散线位置一致,证明该会聚区是由大量简正波同相叠加形成的下反转点会聚区,其在深海声道轴以下的一定深度范围内都具有会聚效应,研究了该会聚区的形成条件以及声源深度变化对会聚区焦散结构的影响,对比了远距离处上下反转点会聚区的传播损失以及会聚区宽度,分析表明第七个下反转点会聚区的会聚增益仍不小于10 dB,研究了声速垂直结构变化对下反转点会聚区的影响,理论分析结果与实验数据吻合较好.     

The Ray-Wave correspondence and the suppression of chaos in long-range sound propagation in the ocean

The long-range sound propagation in a horizontally inhomogeneous underwater sound channel is considered. It is shown that the vertical oscillations of inhomogeneity affect the near-axis rays in a resonant way and destroy their stability. As a consequence, the near-axis rays exhibit a chaotic behavior. The ray chaos manifests itself as intensification of interaction between adjacent low-order modes. In this case, a great number of strongly coupled modes appear, and the field structure near the channel axis becomes diffusive. However, as the signal frequency decreases, the inhomogeneity oscillations in depth lead to decoupling of adjacent modes and, hence, to suppression of chaos. As a result, the field structure near the channel axis becomes regular, which is confirmed by numerical simulation.     

基于矢量水听器的深海直达波区域声传播特性及其应用

对于深海近水面声源产生的声场, 处于较大深度处的接收器在一定水平距离范围内能接收到直达波. 2014年在某深海海域进行的水声考察实验中, 应用深度为140 m的拖曳声源发射实验信号, 布放在水下3146 m深处的矢量水听器成功地接收到了直达波信号. 本文应用射线理论, 分析了深海直达波区域声场的传播特性, 得出了水平振速与垂直振速的传播损失与声线到达接收点处的掠射角以及收发水平距离之间的关系. 在以上分析的基础上, 提出了一种利用水平振速与垂直振速的能量差估计声源距离的方法, 并结合2014年实验数据对实验中两条航线上8 km范围内的目标声源进行了测距, 测距结果与目标的GPS数据符合得较好.     

Stable Components of the Wave Field in Underwater Sound Waveguides

A technique is proposed for determining which sound field components are weakly sensitive to variations in the parameters of the speed of sound field in a marine waveguide. Such components are formed by narrow ray beams, with the dispersion of their vertical coordinates on the distance to the point of observation being less than the vertical scale of the perturbation. Since these rays pass through the same inhomogeneities, their phases in the presence of perturbations acquire approximately the same increment. For a monochromatic field, such components in perturbed and unperturbed waveguides differ only by their phase factors. With a pulsed field, perturbations lead only to some additional delays of the stable components. A procedure based on decomposing the field into coherent states is proposed to select stable components from the total field. The solution to the problem of finding the location of a source using the stable components is illustrated by a simple example.     

Rearrangement of the horizontal space-time structure of the sound field in shallow water in the presence of moving internal waves

The rearrangement of the space-time structure of the sound field in a shallow-water waveguide with a moving intense internal wave packet is considered. The analysis is performed in terms of an approach characterized by space-time horizontal rays and vertical modes. It is shown that, within the time the packet travels over the acoustic track (about an hour), considerable spatial and temporal fluctuations occur in the field intensity and interference structure.     

Formation of Brillouin waves in the underwater sound channel

The mechanism of the vertical sound field structure formation in the underwater sound channel is considered. The calculations are performed by the ray method for the rays that have upper turning points at the ocean surface. It is shown that the vertical field structure is formed by the ray pairs producing opposing waves in the vertical. The rays belonging to one pair have the same sign of their departure angles at the source. The pairs are formed because of the presence of a minimum in the ray cycle length as a function of the departure angle. The resulting ray pairs are analogs of Brillouin waves.     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

Space-frequency distribution of sound field intensity in the vicinity of the temperature front in shallow water

The sound field produced by a low-frequency (100–1000 Hz) point source in a shallow water region in the presence of the temperature front is considered. It is shown that the latter anisotropic inhomogeinity leads to a substantial horizontal refraction and to a number of related effects: a redistribution of the sound field in the horizontal plane, changes in the spectral and mode contents of a pulse in the course of its propagation, and changes in the arrival times at the receiver. A theoretical analysis and a numerical modeling are carried out for the Polar Front of the Barents Sea.     

Sound Field Features in the Coastal Zone of a Shallow Sea with an Airborne Source of Excitation

Features of the sound field in the coastal zone are theoretically investigated in the case of a moving airborne sound source. It is shown that two factors govern the drastic increase in the signal level measured at the observation point when the source moves near the shoreline: the directional property of the field transmitted from the air into the water and the structure of the normal wave in the wedge near the caustic. The magnitude of the increase in the sound field level depends on the depth of the reception point and, what is more essential, on the structure of the bottom.     

Effect of the sediment layer on the diffraction focusing of an acoustic field in a shallow sea

Features characterizing the formation of the spatial (in depth and horizontal distance) interference structure of an acoustic field generated by a point tone source or a vertically distributed tone source in a shallow-water oceanic waveguide are considered. The waveguide is modeled by a three-layered fluid medium. The main object of the study is the effect of the acoustic parameters of the sediment layer on the formation and structure of the spatial regions within which the diffraction focusing of the acoustic field takes place.     

南海深海典型不平整地形对声场垂直相关性的影响

海底地形变化对深海环境下的声传播和空间相关性有重要影响。基于2014年南海中南部海域不完全深海声道条件下大跨度垂直阵接收的声信号,选取平坦海底和典型不平整(存在小海底山和海底斜坡)海底两个不同传播方向上传播损失差异较大的距离,对声场垂直相关性进行了对比分析,并应用射线理论对相关性差异予以定性分析和机理解释。在不平整海底方向,在第一影区内小海底山遮挡区附近:在部分一次海底反射声线被小海底山遮挡的距离处,接收声信号主脉冲多途干涉结构相对平坦海底方向更加简单,垂直相关性增强;而在全部一次海底反射声线被小海底山遮挡的距离处,对声场起主要作用的变为二次海底反射声线,接收声信号主脉冲呈现多途干涉结构,垂直相关性降低。在第一会聚区附近:平坦海底环境下,由于两组水体中反转声线在空间垂直方向上的干涉效应,使得声场的垂直相关随接收深度增加呈现出周期性振荡的现象;而不平整海底方向,在50 km处,海底斜坡阻挡了一组水体反转声线,在大深度上只出现单会聚结构,观测不到垂直相关的周期性振荡现象;在57 km附近对声场起主要贡献的成分从水体中反转声线变成了多次海底反射声线,主脉冲多途展宽变宽,垂直相关性显著降低。研究结果对深海复杂海底地形环境下声场垂直相关性的深入分析及声呐阵列增益估计等具有重要意义。