深海不平海底对声场水平纵向相关性的影响

海底地形变化对与海底作用的声反射区声场空间相关性有重要影响。利用南中国海某深海海域的一次声学实验数据,对深海海底反射区的声场空间相关性进行了分析。实验观测到,在距离声源约29 km至35 km距离范围处,不平海底环境下的水平纵向相关性出现了一些不同于平坦海底环境下的振荡结构,利用射线声学理论分析并解释了这两种不同海底环境产生水平纵向相关差异的原因。结果表明,在深海平坦海底方向一次海底反射到达区,对声场起主要贡献的经过一次海底反射声线间的到达时间差随着水平距离的增加而逐渐减少,从而干涉叠加后的相位差在[0,2π]内发生周期性变化,导致该区域声场的水平纵向相关性随着间距增加呈周期性振荡现象;而不平海底方向的海底小山丘恰对一次海底反射声线的阻挡影响,不仅使得与平坦海底方向相同区域的声传播损失明显增大,而且由于相对更多的本征声线路径的复杂多途干涉,该区域水平纵向相关性不再呈现明显的振荡周期,相关系数也有所下降。研究结果对于分析在深海复杂海底地形环境下的声呐阵列探测性能具有重要意义。      

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。      

利用宽带声场频率—掠射角干涉结构的深海直达声区目标深度估计方法

针对典型深海环境中宽带声源的深度分辨问题,通过研究深海声场随频率起伏的干涉结构与垂直线阵频域波束输出图中的干涉结构,给出一种直达声区内可区分多水下目标的宽带声源深度估计方法.该方法以近水面目标的射线声场模型为基础,推导出近海面宽带声源接收声场的波束输出表达式,阐明了频域波束输出图中干涉结构与声源深度的对应关系.然后利用...     

深海大深度声场垂直相关特性

深海声场垂直相关特性对提高垂直阵阵列增益和水下目标探测性能具有重要意义.基于2014年南中国海实验大跨度垂直阵接收的声信号,分析了深海直达区、影区和会聚区等不同距离下的大深度声场垂直相关特性,并使用射线理论解释了深海垂直相关随空间变化机理.在直达声区内,声场垂直相关半径几乎可以覆盖整个水深,且随着深度增加,直达声和海面反射声到达时间差增加,相关略有下降.在声影区内,声场能量主要来源为经一次海底反射和一到两次海面反射的声线,垂直相关整体偏低.第一会聚区内垂直相关系数随着接收深度的增加而周期性振荡,并且与声能量在深度上的分布具有相似结构,这是高声强区域两组反转声线在垂直方向上周期性干涉的结果.     

浅水近距离测量声场的干涉结构分析

为指导浅水中水平距离在十倍水深范围内进行的近距离声学测量工作,选取对软底浅水声场产生主要贡献的直达声线和一次水面反射声线,经过合理的近似处理,以射线解析方式给出了中低频信号声场干涉结构的表达式,并将表达式由单频信号推广到宽带信号和线性调频等信号形式。所得到的解析关系式定量给出了浅水中声场干涉结构与信号频率、信号带宽、设备布放深度和水平距离等参数的关系,由此进一步得到浅水条件下中近距离干涉声场的一些特性和规律。通过数值模拟和实际试验数据对上述关系式所进行的检验表明:在浅水的常规测量试验条件下,这些关系式可以较为准确地反映近距离干涉声场的起伏特征,由关系式推论得到的声场特性和规律也得到了初步验证。      

典型深海声场频率-距离干涉结构分析及实验研究

基于射线理论分析了在典型深海情况下声源与接收水听器位于海水表层时声场频率-距离干涉结构,给出了直达声作用区与影区情况下声场频率-距离干涉结构的近似理论表达式。数值仿真与实验结果表明:在直达声作用区内,由直达声与海面一次反射声形成声场干涉结构,频率域干涉周期为该两条声线到达时间差的倒数;在影区内,由声源-海底-接收器、声源-海面-海底-接收器、声源-海底-海面-接收器和声源-海面-海底-海面-接收器四条声线形成声场干涉结构,声强随着频率具有两种干涉周期,分别随着声源深度、接收水听器深度的增大而减小,并与收发距离有关。本文给出的理论表达式可以较好的解释实验观测到的声场频率-距离干涉结构。      

南海北部浅海声场起伏及统计特性

水下声场的时间积分声强和峰值声强是声呐检测中的重要物理量,而海洋中的内波等动力学过程会造成声场强度的起伏。对南海北部浅海内波环境下定点声起伏实验数据分析,结合数值仿真,总结了试验海域近海底发射和近海底接收条件下内波引起接收信号强度起伏的统计特性.分析结果表明:接收信号的时间积分声强起伏小于峰值声强起伏;时间积分声强与峰值声强起伏的概率分布与对数正态分布较为接近.时间积分声强和峰值声强起伏大小与信号的频率有关,同一收发距离,中心频率650 Hz信号的声强起伏较中心频率310 Hz和375 Hz信号的声强起伏更为剧烈。对于同一频率,当海底较平坦时,声强起伏的闪烁指数随传播距离的增加而增大;当水深随传播距离逐渐变深时,声强起伏规律受内波和水深变化共同影响而更为复杂,闪烁指数先随传播距离增加而增大,之后又随传播距离的增加而逐渐变小.      

高速垂向运动声源声场干涉特性及其可观测性

以高速垂向运动声源宽带连续谱噪声为研究对象,基于射线理论分析了声场干涉现象的产生机理,推导了干涉条纹的轨迹方程,讨论了影响干涉结构特性的因素,揭示了声场干涉结构的特性和规律。理论和仿真结果表明:干涉周期与多途信号时延差成反比关系;减小接收深度或增大接收距离可以提高干涉条纹的可观测性;较大的海面起伏易使干涉条纹形成断点,可观测性降低。湖试数据分析结果表明,干涉条纹主要由直达声-海面反射声干涉条纹构成,干涉条纹宽度和干涉周期随水平距离的增大而增大。该结果可为高速垂向运动声源运动状态分析、环境参数反演等提供参考。     

浅海远程声场的空间相关性与时间稳定性

本文介绍浅海远程声场空间相关的一些实验结果。实验研究了不同距离、频率与带宽的空间相关,测量数据具有明显的距离与频率依赖关系。实验结果表明,在55至130公里的传播距离上跃变层之下的低频声场有较强的空间相关性与时间稳定性。     

浅海中孤立子内波引起的声能量起伏

当孤立子内波的波阵面与声传播路径所成角度较大时,简正波耦合是导致声信号起伏的主要因素。研究了浅海中孤立子内波引起的声能量起伏规律,给出声场起伏的耦合简正波表达式,并使用抛物方程模型进行仿真。数值分析表明,接收点声强随时间变化呈准周期性。在频谱图中能够得到声强起伏的主导频率,主导频率与孤立子内波沿声传播路径的移动速度成正比,与无扰动波导中简正波在距离上的干涉周期(对应于射线理论中临界声线的跨度)成反比,与孤立子内波的形状无关。此外,对声强频谱的垂直结构进行了分析,该结构与对声场起伏起主要作用简正波的本征函数相关。      

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.     

Calculations of internal-wave-induced fluctuations in ocean-acoustic propagation

Variability in the ocean sound-speed field on time scales of a few hours and horizontal spatial scales of a few kilometers is often dominated by the random, anisotropic fluctuations caused by the internal-wave field. Results have been compiled from analytical approaches and from numerical simulations using the parabolic approximation into an efficient set of algorithms for calculating approximations to internal-wave effects on temporal and spatial coherences, coherent bandwidths, and regimes of acoustic fluctuation behavior. These approximate formulas account for the background, deterministic, sound-speed profile and the anisotropy of the internal-wave field, and they also allow for the incorporation of experimentally determined profiles of sound speed, buoyancy frequency, and sound-speed variance. The algorithms start from the geometrical-acoustics approximation, in which the field transmitted from a source can be described completely in terms of rays whose characteristics are determined by the sound speed as a function of position. Ordinary integrals along these rays provide approximations to acoustic-fluctuation quantities due to the statistical effects of internal waves, including diffraction. The results from the algorithms are compared with numerical simulations and with experimental results for long-range propagation in the deep ocean.     

Temporal coherence of sound transmissions in deep water revisited

This paper examines the signal coherence loss due to internal waves in deep water in terms of the signal coherence time and compare to data reported in the literature over the past 35 years. The coherence time of the early raylike arrivals was previously modeled by Munk and Zachariasen ["Sound propagation through a fluctuating stratified ocean: Theory and observation," J. Acoust. Soc. Am. 59, 818-838 (1976)] using the supereikonal approximation and by Dashen et al. ["Path-integral treatment of acoustic mutual coherence functions for arrays in a sound channel," J. Acoust. Soc. Am. 77, 1716-1722 (1985)] using the path integral approach; a -1 [corrected] power frequency dependence and a -1/2 [corrected] power range dependence were predicted. Recent data in shallow water in downward refractive environments with internal waves suggested that the signal coherence time of the mode arrivals follows a -3/2 power frequency dependence and a -1/2 power range dependence. Since the temporal coherence of the acoustic signal is related to the temporal coherence of the internal waves, based on the observation that the (linear) internal waves in deep and shallow waters have a similar frequency spectrum, it is argued that the modelike arrivals in deep water should exhibit a similar frequency dependence in deep and shallow waters. This argument is supported by a brute-force application of the path integral to mode arrivals based on the WKB relation between the ray and mode. It is found that the data are consistent with the -3/2 power frequency dependence but more data are needed to further test the hypothesis.     

印度洋中北部声速剖面结构的时空变化及其物理机理研究

海洋的声速结构对水下声传播有重要影响,在印度洋中北部复杂多变的海洋物理和水文环境中, 获取声速剖面的时空统计分布规律对水下目标探测和水下声通信有重要意义. 由于垂直梯度法在声速结构分析中的局限性及其在印度洋中北部海域的适用性问题, 采用多元统计分析中的最优分割法对声速跃层进行分析,并应用最近10年的地转海洋学实时观测阵 数据对印度洋中北部海域声速剖面的特征量进行了计算,获得了声速跃层的垂直结构特征和时空变化规律; 还利用经验正交函数(EOF)表示方法,分析了印度洋中北部声速剖面拟合精度随EOF阶次的分布特点. 根据印度洋的海洋物理特征,揭示了声速剖面特征量时空演变的内在物理机理.研究结果表明: 最优分割法是适合印度洋声速结构的跃层判断方法,并提出了相应的判断准则参数; 声速剖面拟合精度随EOF阶次变化的区域性分布特征较明显,其季节性变化较小; 印度洋中北部的深海声道轴只在5°S以南明显存在,在15°S---25°S 附近海域存在三个跃层;印度洋中北部声速剖面结构可分为单跃层、双跃层Ⅰ型、 双跃层Ⅱ型和三跃层四种类型以及春夏秋冬四个季节模态. 声速剖面的分析结果对于水声传播和声纳系统的使用具有一定参考意义.     

深海中利用单水听器的影区声源无源测距测深方法

在典型深海情况下当声源与接收水听器位于海水表层时,在影区内由声源海底接收器、声源海面海底接收器、声源海底海面接收器和声源海面海底海面接收器4条声线形成声场干涉结构,声强随着频率具有两种干涉周期,随着收发距离的增加而增大,分别随着声源深度、接收水听器深度的增加而减小。因此由单水听器记录的声场干涉结构即可实现宽带声源目标的无源测距测深,仿真分析验证了其有效性。在南海深海声学实验中观测到海面宽带噪声源在声场影区所形成的声场干涉结构,数据分析结果验证了深海声场干涉结构用于声源无源定位的有效性。与传统无源定位方法相比,该方法不需要宽带引导声源、精确的海底声学参数和大规模的拷贝场计算。      

An Investigation of the Effects of Internal Waves on Sound Propagation in a Stratified Medium with a Sloping Bed

Internal waves usually cause temporal and spatial changes of density and consequently affect the acoustic wave propagation in the ocean. The purpose of this study is a laboratory investigation of the effects of internal waves generated by oscillation of a cylinder in a large stratified glass tank with a sloping bed on the sound waves propagation. Results showed that sound waves are affected by internal waves that depend on the slope angle to the direction of internal wave propagation angle ratio. When the ratio is subcritical or supercritical, the acoustic signal is much reduced as compared to the case with no sloped bottom. This can be explained in terms of the internal waves energy reaching the sloped bed and their reflections.     

Fresnel tomography and interferometric technique for characterizing Laguerre–Gaussian beams

The Fresnel tomograms of Laguerre–Gaussian beams are found and the relation of the two-point spatial correlation function with the tomographic map is discussed. The spatial distribution and signature of vorticity of Laguerre–Gaussian modes is analyzed by measuring the two-point spatial correlation function of the one-dimensional projection of the vortex field. An interferometric approach, based on a Fourier projection algorithm, for the experimental determination of the two-point correlation function of the full complex vortex field at the cross-section plane of the recorded interference pattern is also described; this approach allows one to characterize the vortex structure and spatial coherence along the direction of propagation.     

Longitudinal purely spatial coherence of a light field

The effects of longitudinal purely spatial coherence of light and the results of observation of these effects in an interference experiment are considered under the condition that the length of temporal coherence l _c is considerably smaller than the length of longitudinal spatial coherence ρ_‖ of the field. It is shown that, for l _c ? ρ_‖, the longitudinal purely spatial coherence of the light field in fact governs the coherence of the wave train in the process of its propagation. The length and the time of coherent (“free”) path of the wave train are considered as new spatial and temporal scales of a partially coherent light field.     

Acoustic propagation through anisotropic internal wave fields: transmission loss, cross-range coherence, and horizontal refraction

Results of a computer simulation study are presented for acoustic propagation in a shallow water, anisotropic ocean environment. The water column is characterized by random volume fluctuations in the sound speed field that are induced by internal gravity waves, and this variability is superimposed on a dominant summer thermocline. Both the internal wave field and resulting sound speed perturbations are represented in three-dimensional (3D) space and evolve in time. The isopycnal displacements consist of two components: a spatially diffuse, horizontally isotropic component and a spatially localized contribution from an undular bore (i.e., a solitary wave packet or solibore) that exhibits horizontal (azimuthal) anisotropy. An acoustic field is propagated through this waveguide using a 3D parabolic equation code based on differential operators representing wide-angle coverage in elevation and narrow-angle coverage in azimuth. Transmission loss is evaluated both for fixed time snapshots of the environment and as a function of time over an ordered set of snapshots which represent the time-evolving sound speed distribution. Horizontal acoustic coherence, also known as transverse or cross-range coherence, is estimated for horizontally separated points in the direction normal to the source-receiver orientation. Both transmission loss and spatial coherence are computed at acoustic frequencies 200 and 400 Hz for ranges extending to 10 km, a cross-range of 1 km, and a water depth of 68 m. Azimuthal filtering of the propagated field occurs for this environment, with the strongest variations appearing when propagation is parallel to the solitary wave depressions of the thermocline. A large anisotropic degradation in horizontal coherence occurs under the same conditions. Horizontal refraction of the acoustic wave front is responsible for the degradation, as demonstrated by an energy gradient analysis of in-plane and out-of-plane energy transfer. The solitary wave packet is interpreted as a nonstationary oceanographic waveguide within the water column, preferentially funneling acoustic energy between the thermocline depressions.