浅海中运动声源径向速度与距离的无源估计

针对浅海波导中声源距离的无源估计问题,提出了一种环境参数和声场模型无关的、不依赖于引导声源的运动声源径向速度以及距离估计方法。该方法首先通过双水听器低频声场强度距离波数谱变换(R-K)的相位差获得干涉简正波水平波数,对水平波数轴定标来估计运动声源的径向速度。进一步地,对接收信号自相关函数进行WARPING变换得到运动声源距离的无源估计。对于反射类简正波为主的声场,在某一假定距离下根据某两时刻接收信号WARPING变换后干涉简正波谱峰频率与假定距离的关系估计距离;对于反射类或折射类简正波为主的声场,根据两个时刻接收信号的β-WARPING变换后干涉简正波的脉冲时延估计距离的值。数值仿真分析了等声速、负梯度以及温跃层3种水文环境下的噪声以及环境宽容性,结果表明径向速度与距离的估计不依赖于环境参数。利用2005年北黄海实验数据验证了方法的可行性,径向速度与距离的估计值与实际值符合良好。      

浅海单矢量传感器宽带声源三维定位仿真

针对水平不变的浅海波导中单矢量传感器对低频宽带声源的三维被动定位问题,首先利用平均声强器估计声源方位；其次,通过分离简正模的声压和水平振速分量联合处理获得运动声源相对速度,进一步基于垂直声能流中简正模相干项特征频率不变性以及长时间窗口中多快拍信号的统一处理,建立WARPING变换频谱作为代价函数的搜索处理器,估计该段信号的初始距离,进而获得各时刻声源距离,所提出的方法避免了对拷贝声场和引导声源的依赖；最后,利用多阶简正模相干项与非相干项能量模基处理方法,当声场中存在三阶以上简正模时,可对声源深度进行匹配估计。仿真分析表明,单个矢量传感器能够给出声源的方位、距离及深度估计结果。     

浅海中利用单水听器的声源被动测距

针对浅海声波导中远距离脉冲声源被动测距问题,提出了一种利用单水听器接收信号自相关函数进行warping变换的声源被动测距方法。理想水下声波导中,接收信号warping变换输出的傅里叶变换频谱中具有不变性频率特征,即与声源距离无关的各简正波截止频率;信号自相关函数中不同简正波相干成分也存在不变性频率特征;推导了未知声源距离时特征频率提取值与不变性频率特征之间的近似关系式。这些规律可推广到实际浅海声波导,并用于声源被动测距。利用声场计算模型来提供具有不变性频率特征的频谱,对2011年12月北黄海海域水声实验中单水听器接收的脉冲声数据进行了处理,验证了方法的有效性,测距结果和实际距离符合良好,平均测距误差在10%以内。      

一种基于warping变换的浅海脉冲声源被动测距方法

针对浅海波导中脉冲声源被动测距问题, 提出了一种利用接收信号的能量密度函数进行warping变换的声源被动测距方法. 对于浅海波导, 接收信号的能量密度函数中不同号简正波相干部分, 经warping变换后输出结果的频谱中包含与声源和接收器位置无关的不变性频率特征. 这些特征频率在数值上等于理想波导中相干的两号简正波的截止频率差, 与海底参数无关, 因此仅需已知海水中的平均声速和海水深度便可计算出特征频率值. 当声源距离未知时, 利用特征频率的提取值与真实特征频率之间的关系可以实现快速测距, 极大地提高了计算速度. 为了验证方法的有效性, 对2011年11月黄海海域水声实验的接收脉冲数据进行了处理, 测距结果与实测距离符合良好, 平均测距误差在8%以内.     

利用自相关函数warping变换的浅海声源深度判别

针对浅海波导声源深度判别问题,提出了一种利用warping变换提取接收信号简正波相关项的深度判别方法。对接收信号自相关函数做warping变换,分离其简正波相关项,利用能量占主导的简正波相关项特征频率之间的比例关系,确定其号数,从而分辨水面声源和水下声源。公式推导和数值仿真结果说明:不同深度声源激发的能量占主导的简正波相关项特征频率不同,可以用于判别声源深度。海试实验结果证明:在海水声速随深度不变或缓变的水文环境,低频条件下,该方法可以分离简正波相关项并确定其号数,在未知声源距离的情况下有效进行声源深度判别,且无须声源相对接收器运动。      

浅海波导中低频声场干涉简正模态的判别

浅海波导中,低频宽带声场中干涉简正模特性可用于声源定位和环境参数反演,然而实际应用中由于存在声源位置不确知、某些简正模激发较弱、模型参数选取失配等因素的制约,导致干涉简正模阶数的判别存在问题.结合水平线列阵应用,根据阵列接收信号中干涉简正模成分的波束输出角度与距离无关但与干涉简正模阶数和频率相关的波导固有频散特性,提出了一种基于阵元域接收信号自相关函数WARPING变换过滤干涉简正模,进而对其波束输出角度进行模基匹配判别简正模阶数的方法.利用2011年北黄海海域声学实验中坐底布放的32元水平线列阵接收的爆炸声脉冲信号,对方法进行了验证.并由仿真数据分析了声速剖面、海底参数和水深等参数失配及信噪比对方法性能的影响.结果表明水深变化14%以上对干涉简正模波束输出角度的提取值影响最大,可引起方法失效;声速剖面和海底参数在一定失配范围内对方法性能的影响可忽略;方法要求单阵元信噪比大于2 dB.     

深海声影区时频谱干涉结构与声源定位

被动声呐探测位于深海声影区的水面舰船辐射噪声时,接收信噪比通常较低,导致声源被动定位方法的性能较差。针对这一问题,提出一种利用接收信号时频谱干涉结构的声源距离和径向速度联合估计方法。首先根据射线声学理论,建立时频谱沿频率轴和时间轴的干涉条纹周期与声源距离和径向速度的关系,然后对接收信号时频谱进行二维傅里叶变换和多频带处理以估计上述干涉条纹周期,最后解算声源距离和径向速度。仿真和海试数据处理结果表明,相比于现有利用接收信号自相关的声源距离估计方法,该文利用时频谱二维傅里叶变换的声源定位方法具有较好的稳健性,比较适用于低信噪比条件下的声源被动定位。     

一种基于β-warping变换算子的被动声源距离估计方法

基于浅海简正波水平波数差与波导不变量之间的关系, 本文提出了一种适用于水平不变浅海声波导中接收信号自相关函数的频域卷绕变换算子. 该算子可以将接收信号自相关函数中的简正波互相关成分变换为时域上可分离的脉冲序列, 且脉冲序列的相对延迟时间包含声源距离信息. 利用已知距离的引导声源, 由单水听器记录的脉冲信号即可实现被动声源距离估计. 对仿真和实验获得的脉冲信号数据处理结果验证了该变换算子用于被动声源距离估计的有效性.     

利用简正模态相位关系的浅海声源深度分辨方法

针对浅海波导声源深度分辨问题,本文提出了一种利用简正波的相位关系进行声源深度分辨的方法。该方法通过对垂直阵接收到的信号进行简正波分解,得到简正波系数,在已知声源距离的情况下通过对简正波系数项的处理得到各阶简正波的相位关系,根据相位关系估计声源的深度区间。该方法在阵元数较少时依然可以分辨声源深度,而且对宽带信号、窄带信号、连续信号和脉冲信号均适用。理论分析和数值仿真的结果说明:只有近水面处声源激发的简正波满足相位一致的特性,利用此特性可分辨水面水下目标。仿真和海试结果证明:在海水声速随深度不变或缓变的水文环境下,该方法可以准确的分辨声源深度区间,并且在深度采样不充分的情况下仍然适用。      

浅海负跃层条件下的warping变换宽带无源测距方法

夏季浅海波导中常常存在强烈的负跃层,研究warping变换无源测距在这种水文条件下的使用方法具有重要的应用价值。对于上发上收、上发下收和跃层强度较小时下发下收的情况,海面海底反射类简正波对声场起主要作用,可以对接收信号的自相关函数进行时域warping变换,利用特征频率匹配的方法进行无源测距。而对于跃层强度较大时下发下收的情况,水体折射-海底反射类简正波对声场起主要贡献,可以对接收信号的自相关函数进行频域β-warping变换进行无源测距。并且能在事先获得粗略的水体声速剖面参数的情况下,通过设置一个已知距离的引导声源,那么即使估计得到的波导不变量存在一定误差,实际测距结果也可以与真实距离符合得较好。利用warping变换对一次夏季浅海实验接收到的目标发出的宽带信号进行无源测距,估计距离与真实距离符合较好,相对误差在20%以内。      

浅海宽带简正模相干/非相干能量比值特征匹配的源深估计

无源声源深度估计一直是水声领域的一个难题。针对浅海波导中低频宽带脉冲声源深度估计问题,提出了利用单水听器接收信号中多阶简正模相干项能量与非相干项能量比值的特征匹配处理方法,不仅消除了声源未知激发谱的影响,而且由于在预先估计声源距离基础上实现对声源深度的独立估计,运算量小。同时,由于利用了实测数据与拷贝声场中具有相同简正模贡献的相干能量特征进行匹配,参数估计具有较高的稳健性。首先通过向量线性相关数学理论,在理想波导条件下讨论了简正模数目与源深估计单值性的关系,证明了仅利用两阶简正模时源深估计具有多值性,而声场含有三阶以上简正模时源深估计存在唯一解。进一步的仿真分析表明,当信噪比高于0 dB时源深估计结果的平均误差整体上在5 m之内。分析了水文参数、沉积层声学参数及声源距离失配时方法的鲁棒性。最后,利用浅海垂直阵采集的低频宽带脉冲实验数据对方法进行验证,声源深度估计结果绝对误差小于5 m的概率在79.1%以上。方法可推广应用于浅海水平或垂直基阵对低频宽带声源的深度估计。      

Acoustic communication in deep water exploiting multiple beams with a horizontal array

A recent experiment showed that coherent long-range acoustic communication (200-300?Hz) is feasible in deep water over a ～550-km range between a source and a horizontal towed array (～100?m aperture). The low input signal-to-noise ratio (SNR) (e.g., -10?dB) at an element level required conventional beamforming to enhance the SNR of the communication signals for subsequent channel equalization and decoding. This paper demonstrates that with a larger aperture (～200?m), multiple adjacent beams can be exploited to further improve the communication performance, achieving an almost error-free data rate of 100 bits/s for QPSK modulation at ～550?km range.     

Method of holographic imaging of inhomogeneities in the density of a substance of a semitransparent medium using plane-parallel laser radiation

A new method for coherent superbroadband sounding of a semitransparent substance with the aim to construct a three-dimensional image of its inner structure is proposed and justified. The method is based on irradiating the studied medium with a plane ray, spectral analysis by the two-dimensional Fresnel–Fourier transform, interferometric comparison of spatial spectra of scattered and reference signals, and holographic construction of a series of two-dimensional images of the medium followed by tomographic reconstruction of its three-dimensional image. The stochastic image model has been justified in a coherent approach for the first time both in the region of high frequencies and after the transfer of the radiation spectrum to the lowfrequency range of spatial frequencies to the surface of the radiation recorder. The spectral correlation characteristics of the speckle structure of the reconstructed image have been studied. Calculations and investigations of the spread function of the structure implementing the proposed method have been carried out.     

Blind deconvolution for robust signal estimation and approximate source localization

Synthetic time reversal (STR) is a technique for blind deconvolution in an unknown multipath environment that relies on generic features (rays or modes) of multipath sound propagation. This paper describes how ray-based STR signal estimates may be improved and how ray-based STR sound-channel impulse-response estimates may be exploited for approximate source localization in underwater environments. Findings are based on simulations and underwater experiments involving source-array ranges from 100 m to 1 km in 60 -m-deep water and chirp signals with a bandwidth of 1.5-4.0 kHz. Signal estimation performance is quantified by the correlation coefficient between the source-broadcast and the STR-estimated signals for a variable number N of array elements, 2 ≤ N ≤ 32, and a range of signal-to-noise ratio (SNR), -5 dB ≤ SNR ≤ 30 dB. At high SNR, STR-estimated signals are found to have cross-correlation coefficients of ～90% with as few as four array elements, and similar performance may be achieved at a SNR of nearly 0 dB with 32 array elements. When the broadband STR-estimated impulse response is used for source localization via a simple ray-based backpropagation scheme, the results are less ambiguous than those obtained from conventional broadband matched field processing.     

Sound source localization in an ocean waveguide

A robust algorithm for sound source localization in shallow sea based on a 2D Fourier transform of source motion interference patterns is developed. A spectrogram in time-frequency coordinates obtained after a Fourier transform contains localized areas of spectral density intensity of constructively interfering modes and a distributed area of noise spectral density. Signal spectral density is localized in an area whose linear sizes are determined by the lowest frequency and spatial scales of field variability. The peak input signal-to-noise ratio is estimated for a single receiver when robust detection is ensured and estimates of speed and initial distance to the receiver are close to the actual values. The high robustness of the algorithm is based on the coherent summation of mode amplitudes occurring at different times. Experimental results demonstrating the effectiveness of the algorithm are presented. For higher than peak signal-to-noise ratios, random estimates of the parameters are close to their statistically average values.     

Time reversal focusing with variable depth and range based on mode extraction

A method for time reversal focusing with variable depth and range based on mode extraction was proposed.First,the normal modes of acoustic propagation in the shallow water are extracted by modal decomposition from the probe signals received by a source receiver array.Furthermore,a diagonal matrix and a vector determined separately by the depth and the range of the probe source are extracted from the received acoustic field data.And time reversal focusing at different depths and ranges can be achieved by modulating the depth-dependent diagonal matrix and the range-dependent vector properly.Then the diagonal matrix and the vector are modulated separately according to the depth and the range of the expected focal location to construct a new acoustic field vector.When this new acoustic field vector is retransmitted by the source receiver array in time reversal order(or phase conjugation in frequency domain),focusing of the resulting acoustic field at the expected location rather than the origin of the probe source can be obtained.Numerical simulations in typical shallow water environment demonstrate the effectiveness of the proposed method.     

Femtosecond dynamics of primary processes in visual pigment rhodopsin

The dynamics of the coherent photoisomerization of the 11-cis-retinal in bovine rhodopsin is studied by femtosecond time-resolved laser absorption spectroscopy with a resolution of 30 fs. Rhodopsin is excited with 500-, 535-, and 560-nm femtosecond pulses to produce different initial Franck-Condon states with different vibrational energies of the molecule in its electronically excited state. The time evolution of the photoinduced differential absorption spectra of rhodopsin is measured upon excitation with a femtosecond pulse in a spectral range from 400 to 720 nm. Oscillations in the time-resolved absorption of the rhodopsin photoproducts, such as photorhodopsin with a vibrationally excited all-trans-retinal and in its initial-state rhodopsin with a vibrationally excited 11-cis-retinal, are examined. It is demonstrated that these oscillations reflect the dynamics of coherent vibrational wavepackets. The Fourier transform of these oscillatory components yields frequencies, amplitudes, and initial phases of various vibrational modes involved in the motion the wavepackets in both photoproducts. The main vibrational modes manifest themselves at frequencies of 62 and 160 cm^?1 for photorhodopsin and 44 and 142 cm^?1 for initial-state rhodopsin. It is shown that these vibrational modes are directly involved in the coherent reaction under the study, with their amplitudes in the power spectrum produced by the Fourier transform of the kinetic curves being dependent on the wavelength of rhodopsin excitation.