利用海浪噪声自相关实现散射体无源探测

提出了一种利用海浪噪声自相关实现散射体无源探测的新方法.将各接收器记录噪声信号的自相关减去所有接收器记录噪声信号自相关的平均值,得到散射信号的到达结构,然后结合基尔霍夫移位算法实现对散射体的探测.与利用背景噪声互相关提取格林函数从而实现散射体探测的方法不同,自相关无需考虑各个接收器之间的大量数据传输及时间同步问题,这为相距较远的多接收器和移动平台目标探测提供了极大的方便.将所提出的方法应用于实验数据中,最终探测结果与实际测量结果相比差别不大,验证了方法的有效性.     

利用海洋环境噪声估计海流流速

为了观测海流在短时间尺度上的变化,基于被动声层析原理,提出了一种利用浅海海洋环境噪声估计海流流速的方法.通过波束形成增加噪声互相关函数的能量积累,从环境噪声互相关函数提取出两个水平阵列间的经验格林函数,利用经验格林函数的时间到达结构反演阵列间的浅海海流流速.海上实验数据处理结果表明,该方法提取了2h时间平均的经验格林函...     

扩散场重建格林函数检测钢轨近表面缺陷*

利用超声相控阵全矩阵捕获功能获取后期时间的扩散场信息,实现钢轨近表面缺陷成像。对扩散场信号进行互相关,重建阵元之间的格林函数,获取阵元之间未延时的响应,恢复被噪声湮没的早期缺陷信息,将波数成像方法用于近表面缺陷的快速成像。采用两种超声相控阵探头在钢轨表面采集实验数据,说明格林函数恢复效果与相控阵阵元的个数和激励频率有关。实验结果清晰地显示了距钢轨表面5～10 mm处的缺陷,证实了互相关方法重建格林函数对近表面缺陷检测的有效性。     

单边噪声源环境下的格林函数提取

对单边噪声源环境下空间中存在散射体时的格林函数到达时间结构提取展开了实验性研究。通过对位于沙滩上的传声器上记录的海浪噪声进行互相关处理,成功提取出两传声器间的格林函数到达时间结构。到达时间结构中观察到伪散射路径,并且伪散射路径在幅值上远高于散射路径。针对上述现象给出了理论及仿真解释:当传声器对受噪声源单边照射时,广义光学定理失效,伪散射路径将会出现;同时,由于伪散射路径由所有方向噪声源贡献,散射路径仅有稳相点附近声源贡献,因此伪散射路径在幅值上可能高于散射路径。结果表明,在单边噪声源环境下,基于互相关技术可以提取伪散射路径。伪散射路径的短时稳定提取可以为散射体无源成像提供更多信息,在海洋声学、地震学等领域具有潜在应用价值。     

水下目标对幅度高斯相关海面环境噪声场扰动特性的研究

理论探讨了水下目标受海面环境噪声场激发产生的散射场对海洋环境噪声场的扰动特性,基于幅度高斯型相关海面噪声源模型,利用任意声源分布声场的积分表达式和单极子源半无限空间格林函数的球面波展开式,获得了刚性球目标在海面单极子和偶极子源海洋环境噪声“照射”下的直达噪声场、散射噪声场、总噪声场,以及二接收点直达、散射和总噪声协方差的理论表达式,表明总噪声场除了和目标阻抗特性、接收点的方向有关外,还受到海面噪声源相关特性,以及直达与散射噪声场之间干涉的较大影响。数值计算结果给出较近距离范围内,刚球目标声学可见度约为4~5dB,并指出在海洋环境噪声场中,刚球目标散射的方向特性明显不同于平面波入射。     

利用环境噪声进行安静目标探测*

该文回顾了近年来该课题组利用环境噪声互相关方法和环境噪声自相关方法进行安静目标探测的主要工作。首先简单回顾了利用环境噪声互相关方法和环境噪声自相关方法进行安静目标探测的理论,然后介绍了两次利用海浪噪声进行安静目标探测的实验。通过海浪噪声实验验证了利用环境噪声互相关和自相关进行安静目标探测的可行性。最后对利用环境噪声互相关进行安静目标探测所需要的累积时间进行了讨论,并给出了累积时间的下界。     

基于二次相关算法的双M-Z光纤传感器的定位方法

双马赫-曾德干涉仪型分布式光纤传感系统因灵敏度高而易受环境噪声及系统噪声的影响,使得利用直接互相关算法计算扰动信号位置存在较大误差.采用二次互相关算法,先对两路接收信号做互相关运算,再与其中一路信号的自相关结果做互相关运算,通过对两路信号的时延估计对扰动信号定位,减小了噪声带来的误差并提高计算准确度.对该算法进行了理论仿真和分析,搭建了实验装置进行了实验验证.实验结果表明,与采用直接互相关定位算法结果比较,采用本文算法能够有效提高系统对扰动信号的定位准确度,且具有较高的实时性.     

利用海洋环境噪声互相关实现阵形校准

阵列信号处理需要准确的阵形,然而海底水平阵的阵形很难精确测量。利用海洋环境噪声可以提取两点间的格林函数,这为无源校准阵形提供了新的可能。通过海洋环境噪声互相关叠加可以获取两个阵元之间的距离信息,一种方法是可以通过该距离与位置之间的关系构建非线性方程组并利用最小二乘法(LS)求解阵形。本文利用多维尺度变换法(MDS)求解,通过对度量矩阵的特征值分解获得阵形。数值仿真和实验数据证明了该方法的可行性,并且MDS相比于LS有更稳健的性能。     

多频带期望值最大声信号时延估计EI北大核心CSCD

通过分析复杂环境中不同频带声信号时延估计的差异,提出多频带期望值最大时延估计方法。为了使各频带之间无重叠,该方法采用独立分带划分声信号不同频带,然后计算各频带广义互相关函数,并对子带广义互相关函数建立最大似然模型,最后利用期望值最大算法将多维优化转为一维优化的迭代式,获得最优子带广义互相关函数,在此基础上估计声信号的时延信息。数据仿真和实际实验结果表明,多频带期望值最大化时延估计相较常规时延估计有效估计值的百分比提升了10%,并将最优频带互相关函数应用到该定位算法中,在网格间距为0.3 m时,得到的峰值区域汇聚更明显,定位效果更好。     

海洋噪声场的时空相关函数

本文借助于方向性点源的平面波展开式导出了海洋中噪声场的时空相关函数的普遍表式。用演绎法将该表式用于分层不均匀浅海与分层不均匀深海,建立了噪声场的时空相关函数与海洋环境参数的有机联系。
文中利用接收器附近点（噪声）源的奇性条件简化了浅海中噪声场时空相关函数的计算。接着用简正波理论计算了均匀浅海中噪声场的时空相关函数。     

Small-scale seismic inversion using surface waves extracted from noise cross correlation

Green's functions can be retrieved between receivers from the correlation of ambient seismic noise or with an appropriate set of randomly distributed sources. This principle is demonstrated in small-scale geophysics using noise sources generated by human steps during a 10-min walk in the alignment of a 14-m-long accelerometer line array. The time-domain correlation of the records yields two surface wave modes extracted from the Green's function between each pair of accelerometers. A frequency-wave-number Fourier analysis yields each mode contribution and their dispersion curve. These dispersion curves are then inverted to provide the one-dimensional shear velocity of the near surface.     

Ambient noise cross correlation in free space: theoretical approach

It has been experimentally demonstrated that the Green's function between two points could be recovered using the cross-correlation function of the ambient noise measured at these two points. This paper investigates the theory behind this result in the simple case of a homogeneous medium with attenuation.     

Extracting the Green's function from measurements of the energy flux

Existing methods for Green's function extraction give the Green's function from the correlation of field fluctuations recorded at those points. In this work it is shown that the Green's function for acoustic waves can be retrieved from measurements of the integrated energy flux through a closed surface taken from three experiments where two time-harmonic sources first operate separately, and then simultaneously. This makes it possible to infer the Green's function in acoustics from measurements of the energy flux through an arbitrary closed surface surrounding both sources. The theory is also applicable to quantum mechanics where the Green's function can be retrieved from measurement of the flux of scattered particles through a closed surface.     

Enhancing the emergence rate of coherent wavefronts from ocean ambient noise correlations using spatio-temporal filters

Extracting coherent wavefronts between passive receivers using cross-correlations of ambient noise (CAN) provides a means for monitoring the seismoacoustic environment without using active sources. However, using cross-correlations between single receivers can require a long recording time in order to extract stable coherent arrivals from CAN. This becomes an issue if the propagation medium fluctuates significantly during the recording period. To address this issue, this article presents a general spatio-temporal filtering procedure to enhance the emergence rate for coherent wavefronts extracted from time-averaged ambient noise correlations between two spatially separated arrays. The robustness of this array-based CAN technique is investigated using ambient shipping noise recorded over 24?h in the frequency band [250-850 Hz] on two vertical line arrays deployed 143?m apart in shallow water (depth 20?m). Experimental results confirm that the array-based CAN technique can significantly reduce the recording duration (e.g., from 22?h to 30?min) required for extracting coherent wavefronts of sufficient amplitude (e.g., 20?dB over residual temporal fluctations) when compared to conventional CAN implementations between single pairs of hydrophones. These improvements of the CAN technique could benefit the development of noise-based ocean monitoring applications such as passive acoustic tomography.     

Ocean acoustic interferometry

Ocean acoustic interferometry refers to an approach whereby signals recorded from a line of sources are used to infer the Green's function between two receivers. An approximation of the time domain Green's function is obtained by summing, over all source positions (stacking), the cross-correlations between the receivers. Within this paper a stationary phase argument is used to describe the relationship between the stacked cross-correlations from a line of vertical sources, located in the same vertical plane as two receivers, and the Green's function between the receivers. Theory and simulations demonstrate the approach and are in agreement with those of a modal based approach presented by others. Results indicate that the stacked cross-correlations can be directly related to the shaded Green's function, so long as the modal continuum of any sediment layers is negligible.     

Using cross correlations of turbulent flow-induced ambient vibrations to estimate the structural impulse response. Application to structural health monitoring

It has been demonstrated theoretically and experimentally that an estimate of the impulse response (or Green's function) between two receivers can be obtained from the cross correlation of diffuse wave fields at these two receivers in various environments and frequency ranges: ultrasonics, civil engineering, underwater acoustics, and seismology. This result provides a means for structural monitoring using ambient structure-borne noise only, without the use of active sources. This paper presents experimental results obtained from flow-induced random vibration data recorded by pairs of accelerometers mounted within a flat plate or hydrofoil in the test section of the U.S. Navy's William B. Morgan Large Cavitation Channel. The experiments were conducted at high Reynolds number (Re > 50 million) with the primary excitation source being turbulent boundary layer pressure fluctuations on the upper and lower surfaces of the plate or foil. Identical deterministic time signatures emerge from the noise cross-correlation function computed via robust and simple processing of noise measured on different days by a pair of passive sensors. These time signatures are used to determine and/or monitor the structural response of the test models from a few hundred to a few thousand Hertz.     

Passive structural health monitoring of a high-speed naval ship from ambient vibrations

Previous studies have used the cross-correlation of ambient vibrations (CAV) technique to estimate the impulse response (or Green's function) between passive sensors for passive imaging purposes in various engineering applications. The technique (CAV) relies on extracting deterministic coherent time signatures from the noise cross-correlation function computed between passive sensors, without the use of controlled active sources. Provided that the ambient structure-borne noise field remains stable, these resulting coherent waveforms obtained from CAV can then be used for structural monitoring even if they differ from the actual impulse response between the passive sensors. This article presents experimental CAV results using low-frequency random vibration data (<50 Hz) collected on an all-aluminum naval vessel (the HSV-2 Swift) operating at high speed (up to 40 knots) during high sea states. The primary excitation sources were strong wave impact loadings and rotating machinery vibrations. The consistency of the CAV results is established by extracting similar coherent arrivals from ambient vibrations between the pairs of strain gages, symmetrically located across the ship's centerline. The influence of the ship's operating conditions on the stability of the peak coherent arrival time, during the 7 days trial, is also discussed.     

Emergence of the acoustic Green's function from thermal noise

The fluctuation-dissipation theorem is used to show how acoustic Green's functions corresponding to sound propagation in opposite directions between any two given points can be extracted from time series of thermal noise recorded at these points. The result applies to arbitrarily inhomogeneous, moving or motionless fluids with time-independent parameters, and demonstrates that the two-point correlation function of thermal noise contains as much information about the environment as can be obtained acoustically by placing transceivers at the two points.