The possibility of using the equivalent plane wave model to increase the efficiency of taking bearings of low-frequency signals in shallow water

The possibility of approximating the sound field in the region of interference maxima using the equivalent plane wave model with the actual amplitude and the average ??effective?? phase velocity calculated or measured by the phase gradient at the array aperture is discussed. The method is substantiated by studying the mode, interference, and phase structures of the low-frequency sound field along with the spatial responses of an extended linear array. For bottom-moored or towed geophysical arrays whose sizes are large compared to the wavelength, both the necessity and the possibility of reducing the error in taking the bearing of a sound source in a waveguide are justified. The use of the proposed model is recommended for approximate matching of the array to the transfer function of the waveguide to reduce the bearing error.     

Possibility of reconstructing hydrological variability of shallow sea by measuring frequency shifts of interference structure of sound field

Variations in the vertical space-frequency interference structure of the sound field in the oceanic waveguide are described. Reconstruction algorithms of sound speed field, based on information on frequency shifts of interference maxima of the sound field, are presented.     

Frequency shifts of sound field maxima in oceanic waveguides

The frequency shifts of sound field maxima that are caused by the variability of the medium in an oceanic waveguide are described. The condition under which the time spectrum of the frequency shifts of interference maxima is proportional to the time spectrum of fluctuations in the dispersion characteristic of the waveguide is determined.     

水平变化波导中的简正波耦合与能量转移

针对海底地形水平变化对声场能量传播和声场干涉结构的影响,对简正波之间的耦合和能量转移进行了研究.建立了一种二维大步长格式的耦合简正波模型和三维楔形波导耦合简正波模型,以便快速有效地分析简正波之间的耦合和能量转移.基于耦合简正波模型,阐述了前向声场能量在水平变化波导中传播时的转移过程.并根据射线简正波理论,解释了海底地形变化对声场能量分布的影响机理.水平变化波导中声场的仿真计算表明,当本征值虚部发生剧烈变化时声场存在着较强的简正波耦合和能量转移,且海底地形变化将导致声场能量的水平传播方向偏转至海水深度增加的方向.在声场能量转移和传播方向变化中,声场的能量趋于保留在波导中而不向海底泄漏.同时,声场能量分布受到类似于压缩或稀疏的作用,从而形成椭圆状的干涉结构.     

The field of an equivalent multipole composite radiator in a waveguide

The possibility of replacing a spatially extended volume low-frequency sound source, which forms a three-dimensional directional field in the far-field zone, by an equivalent multipole source is considered. Developing the ideas of L.M. Brekhovskikh, who obtained the solution for a monopole, exact and simplified models of the source field are constructed in the form of superpositions of multipoles of various orders. The models are considered in the integral, mode, and ray approximations applied to the unbounded space and homogeneous and plane-layered waveguides. The results of computations are presented, and the conclusion is made that the contribution of the vertical dipole and quadrupole components to the interference structure of the field amplitude in a waveguide decreases with an increase in the distance from the source. The phase structure is found to be stable regardless of the type of the complex multipole source.     

利用波导不变量进行运动单线阵的无源测距测速

为克服单线阵难以无源测距、测速的缺点,提出了利用目标辐射低频声场干涉结构和波导不变量的运动单线阵无源测距、测速方法。定义了运动目标的距离距变率比（目标距离与距离变化率的比值）,并利用与目标辐射声场的时间-频率干涉结构和波导不变量相关的变量来表征距离距变率比。根据此定义,通过对目标辐射声场的时间-频率干涉结构图进行Radon变换等处理,实现对距离距变率比的估计。利用所估计的距离距变率比,结合平台速度、目标方位角和目标航向角实现对目标距离和速度的同时估计。仿真实验验证了所提方法在测距、测速时的正确性和有效性。结果表明:所提方法在目标航向角较大时有较高的距离、速度估计精度;同时,目标方位估计误差越小,所提方法的距离、速度估计精度越高。      

Determination of the Source Bearing in a Waveguide by Using the Sound Field Interference Pattern

The feasibility of determining the source bearing in a waveguide by an extended horizontal line array with the use of the sound field interference pattern is studied. The sound field is generated by an omnidirectional monochromatic point source and is represented as a sum of modes. The case of a small number of lowfrequency modes is considered.     

Propagation and resolution of pulse signals in oceanic waveguides

The evolution of mode pulses was described in the second-order approximation of waveguide dispersion theory and their resolution was analyzed. The relationship between the problems of resolution of mode pulses and sound interference maxima was considered. The theoretical concepts were illustrated with an example of perturbation of a medium by background internal waves.     

Vertical amplitude phase structure of a low-frequency acoustic field in shallow water

We obtain in integral and analytic form the relations for calculating the amplitude and phase characteristics of an interference structure of orthogonal projections of the oscillation velocity vector in shallow water. For different frequencies and receiver depths, we numerically study the source depth dependences of the effective phase velocities of an equivalent plane wave, the orthogonal projections of the sound pressure phase gradient, and the projections of the oscillation velocity vector. We establish that at low frequencies in zones of interference maxima, independently of source depth, weakly varying effective phase velocity values are observed, which exceed the sound velocity in water by 5–12%. We show that the angles of arrival of the equivalent plane wave and the oscillation velocity vector in the general case differ; however, they virtually coincide in the zone of the interference maximum of the sound pressure under the condition that the horizontal projections of the oscillation velocity appreciably exceed the value of the vertical projection. We give recommendations on using the sound field characteristics in zones with maximum values for solving rangefinding and signal-detection problems.     

一种基于模态域波束形成的水平阵被动目标深度估计

水面水下目标分辨与识别一直是被动声呐探测领域的难题.利用一种水平阵模态域波束形成算法获得己知方位目标声源的各阶模态强度,将其与不同深度的各阶参考模态强度进行匹配,最终实现了对声源的深度估计.仿真结果表明,该算法可以在信噪比为-10 dB的情况下,用300Hz带宽的信号样本,实现对声源深度的有效估计.系统分析了不同参数和不同波导条件对该方法目标深度估计性能的影响.其中,阵元数越多,模态样本数越多,计算频段越宽,方位估计精度越高,有效阵长越长,深度估计的性能越好.阵元间距和波导深度的变化不会影响该方法的深度估计性能,并且该方法的深度估计性能在声速剖面、海底参数等波导条件存在扰动时具有鲁棒性.     

用光辐照法在SBN:Cr晶体中写入动态阵列平面光波导

利用扩展的准直He-Ne激光束通过菲涅耳双棱镜所形成的干涉光场辐照SBN:Cr晶体， 同时沿晶体光轴方向施加适当的直流电场，可在晶体中形成类似于体相位光栅结构的阵列平 面光波导. 采用马赫-曾德干涉仪光路实时测量了所写入阵列平面光波导的横向折射率分布 ，其峰值接近10-4. 初步的导光测试结果表明，利用周期结构光辐照并辅以适当的外加电场在SBN:Cr晶体中写入阵列平面光波导是可行的. 并且由于SBN:Cr晶体的快速响应特性，所写入的光波导是动态的，可随着写入光的撤除而快速消失，或通过改变双光束夹角 关键词： SBN:Cr晶体 结构光辐照 阵列平面光波导 折射率分布 外电场     

浅海矢量声场干涉结构形成机理及试验研究

浅海低频声场的微观结构特征在于具有可用波导不变量表征的 稳定空间-频率干涉结构.声场兼具标量场和矢量场, 波导条件下二者联合决定声场的全部特性. 本文研究浅海声场空频干涉结构的矢量场特征. 理论分析了声压谱、动能密度谱、声强流谱等矢量场干涉结构的形成机理, 探讨了矢量场干涉结构的波导不变量表征, 数值仿真研究了Pekeris波导中能量和能流密度的干涉特性, 进行了宽带声源辐射矢量声场干涉特性及表征的海上试验.实测结果与理论、仿真分析有较好的一致性. 研究结果表明: 中近程和中远程声场均能模态相干, 有稳定的空频干涉结构, 并且矢量声场空频干涉结构存在多种形式, 除各种能量和能流密度谱图外, 相干系数谱也呈现干涉特征, 这些形式的空频干涉结构均可用波导不变量理论有效表征. 关键词： 矢量声场 干涉结构 波导不变量 浅海低频声场     

Fast estimation of array gain of vertical transmit array in Pekeris waveguide

In order to estimate the sound intensity level of vertical transmit array(VTA) from that of single transducer,a fast estimation method of transmit array gain(TAG) in Pekeris waveguide is studied.At first,the average sound intensity of VTA is calculated on the basis of normal mode theory with the effective depth,and the TAG expressed by the summation of modes is derived.Then by using integral,instead of summation,of modes order,the fast estimation expression of TAG in 3/2 power field is derived along with error correction.Because only the first mode exists in effective shadow zone(ESZ),the fast estimation expression of TAG in ESZ is derived by directly simplifying the former expression.All of the fast estimation errors of computer simulations and experiment in shallow water are less than 1 dB,which can verify the validity of the fast estimation method.This fast estimation method of TAG can provide references for practical application.     

运动小孔径水平基阵估计目标深度

针对浅海动态声场,基于简正波模型提出了一种利用运动小孔径水平基阵估计目标深度的方法。通过合成孔径算法将运动小孔径水平基阵扩展成虚拟的大孔径水平基阵,利用稀疏近似最小方差准则可以在相对较小的合成孔径上估计各阶简正波模态能量,不同深度的模态匹配度由Camberra距离的负指数度量,目标深度估计结果是模态匹配度最大值对应深度。数值仿真与实验结果表明,在简正波声场结构基础上,声源频率越低则实现目标深度估计需要的合成孔径距离越小,当声源与阵列端射方向成一定角度时,对所需合成孔径的影响与其相对速度变化时的影响相同,在典型浅海水平分层波导中,当单阵元输入信噪比为10 dB时,准确估计200 Hz和350 Hz声源的深度,分别要求合成孔径大于12倍和16倍波导深度。利用实验数据验证了该方法对水下低频线谱声源的深度估计能力。      

Interference immunity of an interferometric method of estimating the velocity of a sound source in shallow water

We describe an algorithm for estimating the radial component of the velocity of a sound source based on information about frequency shifts of the interference maxima of the field and consider the problem of its interference immunity. We obtain the limit estimate for the value of the input signal/noise ratio when the algorithm is working effectively. We present results of computational and field experiments using a single receiver and a horizontal array. We compare the experimental data with analytic interference immunity estimates.     

用于浅海有源声呐目标深度估计的匹配相位处理EI北大核心CSCD

针对浅海波导中有源声呐目标深度估计问题,提出了通过单个声源发射低频宽带信号,垂直双接收水听器接收目标回波,利用宽带目标回波比值的相位特征进行匹配相位处理的方法,不仅消除了目标散射特性对有源声呐目标深度估计的影响,而且仅需计算单程的信道传递函数,运算量小,有利于实时处理的实现需求。首先通过理论推导,定义了一个隐含目标深度、且与目标散射特性无关的声场特征——单程传播向量宽带干涉结构,在Pekeris波导条件下仿真分析了简正波阶数与单程传播向量宽带干涉结构的幅度和相位的关系,发现选取较多的简正波阶数贡献的单程传播向量宽带干涉结构进行匹配相位处理可以提高目标深度估计的性能。进一步仿真分析表明,在发射信号选取一定的时间长度和带宽的条件下,利用全部阶简正波且信噪比大于-10 dB时,方法的深度估计误差在5 m之内。最后分析了信号时间长度和处理带宽对有源声呐目标深度估计性能的影响,以及海底声速、海深和声速剖面失配时方法的鲁棒性。     

The possibility of reconstruction of two-dimensional random inhomogeneities in a shallow sea by frequency shifts of the spatial interference structure of the sound field

Fluctuations of frequency shifts of the spatial interference structure of the sound field in the oceanic waveguide, caused by a two-dimensional field of a random perturbation, are described. Various schemes of observation point spacing are considered. The possibility is shown to reconstruct the spatial spectrum of waveguide perturbations by measuring the spatial spectrum of the frequency shift of the interference pattern. The results of the theoretical treatment are illustrated by the examples of background internal waves and bottom roughness. The sensitivity of monitoring based on measurements of frequency shifts of the interference structure of the sound field is estimated. For medium perturbations by background internal waves, the fluctuations of liquid layer vibrations, sound speed, and temperature, which are minimum detectable by frequency shifts of the interference pattern, were estimated.     

The contribution of a lateral wave in simulating low-frequency sound fields in an irregular waveguide with a liquid bottom

A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations.     

Features of the Sound Field Structure in a Two-Channel Oceanic Waveguide

The results of experimental studies of the energy and angular structure of a sound field in the region of the Iberian Basin in the northeastern Atlantic are discussed. The experiments are carried out in a two-channel waveguide whose axes are located at depths of approximately 450 and 2000 m. A continuous pseudonoise signal in the frequency range 2.52–4.0 kHz is emitted. The signals are received by the omnidirectional hydrophones and, simultaneously, by a 10-m-long vertical array, which allows one to realize a narrow beam reception (～2.5°) in the vertical plane. The source and the receiver are located in a 500-m-thick layer within the upper sound channel. The field characteristics are measured in the course of a continuous change of distance from 1 to 65 km. The comparison of the experimental data with calculations shows that the sound field structure formed by the lower channel is much closer to the theoretical results than the structure formed by the upper channel. In the upper sound channel, the shadow zone manifests itself only slightly and the first convergence zone begins approximately 72–11 km nearer to the source than predicted by the calculations. The corresponding angular sound field structure is fairly pronounced in the vertical plane and bears no evidence of the random behavior that is peculiar to the fields scattered by the inhomogeneities.     

Simulation of low-frequency sound propagation in an irregular shallow-water waveguide with a fluid bottom

A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations.