声速分布对浅海低频声场空间相关的影响研究

针对浅海低频环境下水中声速分布对声场空间相关性的影响问题,利用基于抛物方程方法的RAM程序,仿真分析了声场空间相关特性在浅海不同声速梯度、不同阵元间隔下的变化规律。研究结果表明:在浅海低频环境下,正声速梯度值越大,垂直相关系数衰减越慢,水平纵向相关系数则相反;随着正声速梯度值的增大,垂直相关长度和水平纵向相关长度均呈现剧烈波动。负声速梯度值越大,垂直相关系数衰减越快,水平纵向相关系数则相反;随着负声速梯度值的增大,两类空间相关长度均缓慢波动。     

浅海水平线阵接收海底混响序列仿真

混响序列仿真作为海上实验的替代手段,可为声呐设计及抗混响技术研究提供数据支撑。借助简正波理论研究海底散射声场,将海底散射场视为海洋传输网络形成的传递函数,在此基础上将海底散射微元按方位角均匀划分,依据阵元间的相位关系给出了水平线阵接收海底混响序列的仿真方法。以负梯度浅海环境为例进行了混响仿真,并对仿真序列的频谱特征、统计特征、空间相关特征进行了验证,结果表明该方法获得的混响序列特性与理论预测相符,可以为抗混响技术研究提供参考。     

浅海低频声场水声信号时间相关性研究

起伏的声场接收信号是时变的,浅海声场的时间相关性研究有利于目标深度的判别以及水声信号的数据融合。研究浅海低频声场时间相关性以信号的自相关和互相关原理为基础,完成对信号时间相关半径、相关函数值的影响因素分析,得到不同声场条件下信号传播的作用结果。通过仿真比较在不同频率、不同信噪比条件下低频声场信号的相关性曲线,并结合实际海试数据进行验证。研究表明浅海低频声场随机扰动随着低频波长的增加而减弱,且信号时间相关性研究为声场的实际应用提供了一定参考。     

斜坡海底对声场水平纵向相关性的影响

利用数值仿真分析和总结了不同斜坡海底条件下声场水平相关性随水平间隔和传播距离的变化规律,并利用楔形简正波理论对海底倾斜引起的声场水平纵向相关变化机理进行了解释.研究结果表明,上坡和下坡地形的声场水平纵向相关性具有很大的差别.与水平海底相比,下坡海底会增加声场水平纵向相关性,上坡海底会减小声场水平纵向相关性.有效简正波之...     

用频移补偿方法计算声场的波导不变量

基于提高声场水平纵向相关的频移补偿方法,提出了一种声场波导不变量的计算方法。该方法简单易行,只需处理具有一定纵向间隔的两个水听器接收的信号。该方法通过将波导不变量设定在一定范围内进行搜索,最终选取波导不变量的值为使两个水听器所接收声信号的经频移补偿后的空间相关系数取最大值时对应的数值。浅海声学实验结果表明,利用该方法能实现声场波导不变量的准确估算。实验结果中所得出的波导不变量值随频率的变化特性及相应的理论分析都表明,在频率较低时频率对浅海声场波导不变量的取值有一定影响,因此波导不变量并不是不变的。     

利用声场的水平纵向相关性反演海底参数

1引言 浅海环境中,海底底质的声学特性对浅海声场有决定性影响,进一步影响着海洋声遥感及声纳的作用距离.由于海底参数很难大面积直接测量,而利用声学反演的方法可以大范围的估计海底参数,有海底取样测量无法比拟的优点.近年来,有关海底参数的声学反演方法的探索和研究已成为国内外水声学家研究的热点问题.有多种反演方法相应被提出并应用于某些特定海区,如:匹配场反演、近场脉冲声反演、传播损失反演、简正波过滤技术反演及声场垂直相关性反演等等.但是,基于水平纵向相关来反演海底参数的方法还未见有报道.分析其中的原因,可能是由于在通常海域声场的纵向相关半径远大于实验中用到的水平阵长度.考虑到声场声信息国家重点实验室在2002年8月的一次浅海实验中的海深只有4.5m,而布放于海底的光纤水听器阵列长达100m,水平阵阵长大于20倍的海深.我们在研究其水平相关性时发现:不同频率下的水平纵向相关随水平间距变化呈现出特定的振荡现象,这种振荡现象与海底的声学特性密切相关.基于以上考虑,本文试图通过声场的水平纵向相关性来反演海底参数并给出了初步的研究结果.结果表明该方法有一定的可行性.     

多线阵波束域相关处理方法研究

研究了基阵接收信号的相关性与阵增益和定向精度的关系。在存在干扰条件下,分析了阵增益与信噪比、干噪比以及信号与干扰的到达角间隔的关系。指出只有当信噪比足够大时,阵增益才趋于理想的10log10（M）阵增益（M为阵元数）,并推导了估计定向精度的分析表达式。在特定浅海声传播条件下,分析了多线阵的水平和垂直相关性,并利用信号的相关性提出一种不依赖于多线阵阵形监测的相关波束检测方法。     

浅海混响实验数据垂直指向性简正波建模分析

混响是限制主动声纳性能的主要因素之一。人们利用接收阵的指向性来提高信号／混响比。垂直指向性可用于抑制边界混响。本文利用浅海简正波混响模型进行建模分析,通过仿真计算,研究了海底类型、海深、声速梯度对海底混响的影响。仿真结果表明：海底类型对混响影响最为严重;利用实测的平均声速建模的Pekeris信道的预测海底混响与利用实测SSP的预测海底混响相差很小;海深越深,海底混响越小,海深对海底混响的影响介于上述两种因素之间。在仿真分析的基础上,论文主要进行了浅海混响实验数据垂直指向性的建模分析。对东中国海60元垂直阵采集的混响数据进行了波束形成,并对单个水听器和部分波束的混响曲线与浅海混响模型预测数据进行了比较。经初步分析表明,实验数据与模型吻合得较好。     

孤子内波对声场水平纵向相干特性的影响

运用抛物方程计算模型,研究了浅海孤子内波对声场水平纵向相干特性的影响。仿真结果表明,孤子内波会引起水平纵向相关系数随时间出现起伏。起伏的幅度与收发深度有关,起伏周期与孤子内波的传播速度有关。理论分析给出了在孤子内波的传播方向与水平阵平行时,水平纵向相关系数随时间变化的简明表达式。     

基于垂直线列阵数据提取浅海波导声传播参数的模值追踪法

垂直水听器线列阵接收的声数据或其互谱密度矩阵可以用来提取浅海波导声传播的简正波模深度函数、水平波数和海底参数。在实际应用中,布放在海洋中的垂直水听器线列阵不可避免地产生倾斜。计算机仿真结果表明,垂直阵的倾斜使提取参数误差大幅增大。针对此问题,提出模值追踪法,构建模值追踪代价函数,对垂直水听器线列阵声数据互谱密度矩阵的本征矢量的模值进行逼近。仿真结果表明,模值追踪法可以有效消除基阵倾斜的影响。在仿真条件下,水平波数估计误差小于0.1%,海底相移估计误差在±1.1范围内。     

Hydrophone spatial averaging corrections from 1 to 40 MHz

The purpose of this study was to develop and experimentally verify a practical spatial averaging model for frequencies up to 40 MHz. The model is applicable to focused sources of circular geometry, accounts for the effects of hydrophone probe finite aperture, and allows calibration by substitution to be performed when the active elements of reference and tested hydrophone probes differ significantly. Several broadband sources with focal numbers between 3 and 20 were used to produce ultrasound fields with frequencies up to 40 MHz. The effective diameters of the ultrasonic hydrophone probes calibrated in the focal plane of the sources ranged from 150 to 500 μm. Prior to application of the spatial averaging corrections, the hydrophones with diameters smaller than that of the reference hydrophone exhibited experimentally determined absolute sensitivities higher than the true ones. This discrepancy increased with decreasing focal numbers and increasing frequency. It was determined that the error was governed by the cross-section of the beam in the focal plane and the ratio of the effective diameters of the reference and tested hydrophone probes. In addition, the error was found to be reliant on the frequency-dependent effective hydrophone radius. After applying the spatial averaging correction, the overall uncertainty in the hydrophone calibration was on the order of ±1 dB. The model developed is being extended to be applicable to frequencies beyond 40 MHz, which are becoming increasingly important in diagnostic ultrasound imaging applications     

不同底质海底窄带混响信号空间相关半径研究

海底混响是主动声呐的主要干扰之一,分析海底混响信号的空间相关特性是抗海底混响技术研究的基础。本文依据混响信号的空间相关特性,从理论上解释海底混响信号空间相关半径的定义,利用实验数据研究了在不同海底底质情况下混响信号空间相关半径变化规律以及不同底质混响信号强度的变化。研究表明不均匀海底底质情况下,散射体颗粒增大,混响信号空间相关半径减小且底质较硬时混响信号较强。     

Wideband multipath rejection for shallow water synthetic aperture sonar imaging

Synthetic aperture sonar (SAS) is an emerging technology for seafloor imaging, which has an appealing property of range- and frequency-independent spatial processing resolution. However, for a low-frequency SAS system operated in shallow water environments, there are often strong sea surface and bottom reflected multipath components that interfere with the desired echo signals. Based upon a small vertically displaced hydrophone array, several spatial processing algorithms have been proposed for multipath reduction. Most of these algorithms, however, are only applicable to narrowband signals, whereas wideband signals are usually used in low-to-medium-frequency SAS systems in order to achieve a high range resolution. This paper presents a steered robust Capon beamforming (SRCB) approach, and applies the approach, together with the data-independent delay-and-sum beamforming and the conventional wideband robust Capon beamforming, to wideband multipath rejection for shallow water SAS imaging. Numerical simulations of the proposed SAS processing have verified the performance improvements on output image quality over conventional processing in terms of both ghost target strength reduction and contrast enhancement.     

A nonlinear propagation model-based phase calibration technique for membrane hydrophones

A technique for the phase calibration of membrane hydrophones in the frequency range up to 80 MHz is described. This is achieved by comparing measurements and numerical simulation of a nonlinearly distorted test field. The field prediction is obtained using a finite-difference model that solves the nonlinear Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation in the frequency domain. The measurements are made in the far field of a 3.5 MHz focusing circular transducer in which it is demonstrated that, for the high drive level used, spatial averaging effects due to the hydrophone's finite-receive area are negligible. The method provides a phase calibration of the hydrophone under test without the need for a device serving as a phase response reference, but it requires prior knowledge of the amplitude sensitivity at the fundamental frequency. The technique is demonstrated using a 50-microm thick bilaminar membrane hydrophone, for which the results obtained show functional agreement with predictions of a hydrophone response model. Further validation of the results is obtained by application of the response to the measurement of the high amplitude waveforms generated by a modern biomedical ultrasonic imaging system. It is demonstrated that full deconvolution of the calculated complex frequency response of a nonideal hydrophone results in physically realistic measurements of the transmitted waveforms.     

一种运用倾角传感器的阵形修正算法

垂直水听器阵在实际海洋环境中通常呈现出一定的倾斜状态,各阵元之间存在水平相对位移,对其性能带来很大影响.为了消除阵倾斜带来的影响,提出了一种运用倾角传感器进行阵形修正的算法.具体实现步骤为,首先用多个倾角传感器测得垂直阵上不同位置的倾角,然后通过求解线性方程组以及傅里叶级数分解的方法,利用倾角数据拟合出阵形的函数曲线,...     

250Hz～8kHz频率范围水听器自由场校准补充比对

介绍了以杭州应用声学研究所作为主导实验室与俄罗斯国立物理技术和无线电工程测量研究院开展COOMET.AUV.W-S1:250 Hz~8 kHz频率范围水听器自由场校准国际补充比对的结果。比对校准了两只标准水听器TC 4033和GI 55。互易法和比较法及其校准装置被用于评估中俄两国在水听器自由场低频校准方面的当前水平。结果显示,中俄两国得到的两只标准水听器的自由场灵敏度校准结果具有很好的一致性,绝大多数频率点的校准结果与参考值的差值小于0.40 dB,最大偏差为0.59 dB。结果证明,中俄两国的自由场水听器校准低限可扩展至250 Hz。     

基于深海会聚区声强匹配处理的声源定位方法

匹配场处理技术(Matched Field Processing MFP)近年来在声源被动定位、海洋环境声学参数反演等方面得到广泛的应用。根据深海声道中声场的会聚现象,提出了一种基于深海会聚区声强匹配处理(Matched Convergence Intensity Processing,MCIP)的深海声源定位方法。当声源和接收水听器处在会聚区深度范围内时,MCIP方法利用单个水听器即可实现声源距离和声源深度估计。对单个水听器的实测信号进行相位补偿到会聚区距离上,得到实测场声强,与拷贝场会聚区声强进行相关计算并搜索峰值来确定声源位置。数值仿真实验验证了MCIP方法,并分析了环境失配情况下MCIP的估计性能。     

单向水听器及其在波束形成中的应用

提出常规反向波束形成技术,构建了单向水听器,并探讨其应用于阵列波束形成的可能性。系统地构建单向水听器模型,导出常规反向波束形成技术,证明得到其空间响应单指向性的存在条件,其指向性指数在3~4.8 d B范围内。以单向水听器为基元构建标准线列阵进行常规波束形成,并与全向水听器比较,理论证明了使用单向水听器能够为线列阵提高3~4.8 d B的指向性指数。仿真实验结果验证了单向水听器自身具有心脏形空间响应、3~4.8 d B的指向性指数和空间响应的宽带一致性等三种特性,使用单向水听器能够在主瓣宽度不变的同时降低旁瓣级、提高指向性指数、保持良好的稳健性。因此,提出的常规反向波束形成技术是一种构建单向水听器的有效方法,在阵列波束形成中使用单向水听器是可行的、有利的。     

High-frequency membrane hydrophone

A membrane hydrophone with a 37-μm diameter spot poled electrode has been fabricated on a 4-μm-thick film of the piezoelectric copolymer, polyvinylidene fluoride trifluoroethylene (PVDF-TrFE), and initially characterized. The hydrophone has an effective spot size of less than 100 μm, an on-membrane +7-dB gain buffer amplifier, and a -3-dB bandwidth of 150 MHz. The acoustic properties of the hydrophone were investigated with a transducer equivalent circuit model, the electric fringe fields due to poling were characterized with a finite difference electrostatic field model, and the effective spot diameters 2a₃ and 2a₆ were estimated. Measurements on the bandwidth, effective spot size, and sensitivity are presented. This hydrophone appears suitable for the characterization of both the frequency and spatial parameters of high-frequency transducers such as intravascular ultrasound (IVUS) catheter transducers operating in the 10-40 MHz range