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。      

单水听器波导不变量被动测距

本文主要讨论利用声场干涉现象由单水听器被动测距的可行性及测距性能,为此提出了单水听器波导不变量被动测距方法,通过提取LOFAR图上干涉条纹、使用简正波模型计算波导不变量、频域相关法估计相对速度,最后依据干涉条纹方程得到声源距离估计量。数值仿真和海上实验结果验证了单水听器被动测距的可行性,并具备一定测距性能,在3 dB信噪比环境中,对于7 km处的运动声源,平均测距误差小于5%。本文方法具有设备简单、易于推广至阵列信号处理等特点,为声纳信号处理环境宽容性的提高及环境适配声纳的设计开拓了思路。     

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。      

深海脉冲信号簇到达结构特征及其在水下声源定位中的应用

为了实现对深海水下声源的定位,对典型深海环境中的脉冲信号到达结构特征进行了理论分析,给出了声源和接收位置位于近海面时到达信号的簇信号形式近似表达式。当声源和接收位置处于近海面深度时,接收到的信号呈簇状结构形式。在提取到达信号中各簇信号到达时间、幅值等特征参数的基础上,提出了一种通过对到达信号中簇信号特征参数匹配搜索进行水下脉冲信号定位的方法,仿真分析了不同距离和不同深度处的簇信号特征,并利用一次南海海域爆炸声源的声传播实验数据进行了实验验证。结果表明:各簇信号对应的到达时间、幅值等特征参数可用于对声源位置的匹配定位;海上实验中利用单水听器得到的水下声源的距离估计结果与实测距离结果较为一致,对实验中2.0 km至90.4 km内声源的距离估计误差不大于8%。      

点噪声源在近程声场中传播损失的仿真研究

在分层介质条件下，用本征声线法仿真计算了点声源这距离噪声场的传播损失曲线，仿真结果表明，其传播损失不仅依赖于声速分布，海区深度，海底反射特性等环境条件，也依赖于声源及接收水听器的深度，声传播损失显著不同于球面波衰减规律，其传播损失率差别可达８ｄＢ，这表明在测量舰船目标的辐射噪声声源级时必要的声场校正测量是必须的。     

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。      

薄壁球形导流罩对矢量水听器接收性能的影响

在实际使用时,为了减小流噪声对矢量水听器的影响,将其放置在流线型较好的导流罩内,因此对导流罩内矢量声场的计算是非常必要的。以水下弹性球壳为研究对象,从理论上研究了弹性薄球壳声透射对矢量水听器声场测量的影响;推导了平面声波对弹性球壳声透射的矢量声场数学表达式,计算了不同壳体参数下内充水弹性球壳的声透射矢量声场对矢量水听器各通道声波接收响应幅值和相位以及各通道接收指向性的影响。理论分析表明,弹性薄球壳透射声场对矢量水听器接收声场的影响与壳体机械阻抗和水听器安装位置密切相关,水听器安装偏差对其声场测量影响比较大。     

方位远探测声波测井仪接收声系实验研究

方位远探测声波测井仪接收声系中各通道接收换能器及相应采集电路的综合一致性极大地影响处理结果的精确度,必须测试并进行必要的校正。在实验室构建了接收声系测试系统,使用主频10 k Hz的单极子声源辐射声场,测量了阵列化声系中每个换能器单独正对单极声源时所有通道的采集波形,分析了波形的时域和频域特征,对声系站内、站间的各通道综合一致性进行了评价并给出接收通道一致性不好时的校正公式;对比了相控接收子阵与单个阵元的接收特性。实验结果表明,该声系各通道综合一致性较好,不需要校正;软件相控处理后,接收阵的能量比单个阵元强、指向性更好,有利于提高有用信号的信噪比和空间的探测分辨率。     

基于矢量水听器的深海直达波区域声传播特性及其应用

对于深海近水面声源产生的声场, 处于较大深度处的接收器在一定水平距离范围内能接收到直达波. 2014年在某深海海域进行的水声考察实验中, 应用深度为140 m的拖曳声源发射实验信号, 布放在水下3146 m深处的矢量水听器成功地接收到了直达波信号. 本文应用射线理论, 分析了深海直达波区域声场的传播特性, 得出了水平振速与垂直振速的传播损失与声线到达接收点处的掠射角以及收发水平距离之间的关系. 在以上分析的基础上, 提出了一种利用水平振速与垂直振速的能量差估计声源距离的方法, 并结合2014年实验数据对实验中两条航线上8 km范围内的目标声源进行了测距, 测距结果与目标的GPS数据符合得较好.     

空气中声源激发浅海水下声场传播特性的实验研究

为了获得空气中远距离声源激发水下声场的精细结构,2013年3月,声场声信息国家重点实验室在南海海域进行了一次空气中声源激发水下声场的实验。采用汽笛作为空气声源,海底放置水听器作为接收,在实验过程中,发射船由距离水听器2.4 km处行驶至9.8 km。本文对该次实验数据进行分析,获得了收发距离远达9.8 km、频率分别为128 Hz和256 Hz的声传播损失曲线,该曲线随传播距离变化存在清晰的震荡结构.利用波数积分方法计算实验环境下的水下声场理论值,并对获得的声场传播特性进行了较好的物理解释。      

The sonar equation and the definitions of propagation loss

A rigorous application of the traditional definition of sonar equation terms leads to the appearance of an unexpected factor, not routinely included, equal to the ratio of the characteristic impedance at the receiver to that at the source. An omission of this factor can lead to non-negligible errors for realistic conditions. It is further argued that a gradual change in the de facto definition of propagation loss occurred between 1965 and 1980. Two alternatives to the traditional sonar equation are suggested, each using one of the two propagation loss definitions and both eliminating the unwanted impedance ratio.     

Single- and multi-channel underwater acoustic communication channel capacity: a computational study

Acoustic communication channel capacity determines the maximum data rate that can be supported by an acoustic channel for a given source power and source/receiver configuration. In this paper, broadband acoustic propagation modeling is applied to estimate the channel capacity for a time-invariant shallow-water waveguide for a single source-receiver pair and for vertical source and receiver arrays. Without bandwidth constraints, estimated single-input, single-output (SISO) capacities approach 10 megabitss at 1 km range, but beyond 2 km range they decay at a rate consistent with previous estimates by Peloquin and Leinhos (unpublished, 1997), which were based on a sonar equation calculation. Channel capacities subject to source bandwidth constraints are approximately 30-90% lower than for the unconstrained case, and exhibit a significant wind speed dependence. Channel capacity is investigated for single-input, multi-output (SIMO) and multi-input, multi-output (MIMO) systems, both for finite arrays and in the limit of a dense array spanning the entire water column. The limiting values of the SIMO and MIMO channel capacities for the modeled environment are found to be about four times higher and up to 200-400 times higher, respectively, than for the SISO case. Implications for underwater acoustic communication systems are discussed.     

Matched-field depth estimation for active sonar

This work concerns the problem of estimating the depth of a submerged scatterer in a shallow-water ocean by using an active sonar and a horizontal receiver array. As in passive matched-field processing (MFP) techniques, numerical modeling of multipath propagation is used to facilitate localization. However, unlike passive MFP methods where estimation of source range is critically dependent on relative modal phase modeling, in active sonar source range is approximately known from travel-time measurements. Thus the proposed matched-field depth estimation (MFDE) method does not require knowledge of the complex relative multipath amplitudes which also depend on the unknown scatterer characteristics. Depth localization is achieved by modeling depth-dependent relative delays and elevation angle spreads between multipaths. A maximum likelihood depth estimate is derived under the assumption that returns from a sequence of pings are uncorrelated and the scatterer is at constant depth. The Cramér-Rao lower bound on depth estimation mean-square-error is computed and compared with Monte Carlo simulation results for a typical range-dependent, shallow-water Mediterranean environment. Depth estimation performance to within 10% of the water column depth is predicted at signal-to-noise ratios of greater than 10 dB. Real data results are reported for depth estimation of an echo repeater to within 10-m accuracy in this same shallow water environment.     

Trading detection for resolution in active sonar receivers

This paper proposes an active sonar receivers that offers a smooth trade-off between detection and resolution. A matched filter is the optimal detector of known signals in white Gaussian noise but may fail to resolve the targets if the time separation of targets is less than the mainlobe width of the autocorrelation function of the transmitted signal. An inverse filter achieves optimal resolution performance for multiple targets in the absence of noise, but amplifies the noise outside the signal bandwidth in a manner that makes it impractical in many realistic scenarios. The proposed active sonar receiver, the variable resolution and detection receiver (VRDR) combines the matched and inverse filter properties to achieve a smooth trade-off between detection and resolution. Simulated receiver operating characteristics demonstrate that for a range of dipole sonar targets, the performance of the VRDR is superior to the matched and inverse filter, as well as another previously proposed bandlimited inverse filter.     

Probing waveforms and adaptive receivers for active sonar

Active sonar systems involve the transmission and reception of one or more probing sequences, which provide a basis for extraction of target information in a region of interest. The probing sequences at the transmitter and signal processing at the receiver play crucial roles in the overall system performance. In this paper, CAN (cyclic algorithm-new) is employed to synthesize probing sequences with good aperiodic autocorrelation properties. The performance of the CAN sequences will be compared with those of pseudo random noise and random phase sequences. Two adaptive receiver designs, namely the iterative adaptive approach (IAA) and the sparse learning via iterative minimization (SLIM) method, will also be considered. IAA and SLIM will be compared with the conventional matched filter method. The performances of the algorithms will be illustrated via numerical examples, which show that CAN, IAA, and SLIM can contribute to the overall performance improvement of the active sonar systems.     

预测特征误差映射及其在多基地水下目标识别中的应用

针对水下多基地目标识别问题,提出了基于特征预测和误差映射的多基地融合识别算法。推导并简化了基于贝叶斯公式的多基地目标识别条件概率公式,利用BP神经网络对最后一个节点的特征向量进行预测,并计算得到预测值与实际值误差的概率密度,将其与前面每个节点的条件概率累乘,以得到目标识别的条件概率。将利用特征预测计算条件概率的方法从单个节点推广到多个节点上,同时针对误差概率分布模型不准确的问题,提出了利用混合高斯分布模型代替单高斯概率分布模型的改进方法。对每个目标重复此过程,取结果最大值对应的目标类别为最后的识别结果.在消声水池开展多基地模拟实验,对四类目标进行识别,在一定声呐节点数目及信噪比条件下,与单基地声呐相比,多基地目标融合识别得到的识别正确率最大可提高40%,采用改进方法以后,识别正确率得到进一步提高。      

Investigation of long-range sound propagation in surface ducts

Understanding the effect of source-receiver geometry on sound propagation in surface ducts can improve the performance of near-surface sonar in deep water. The Lloyd-mirror and normal mode theories are used to analyze the features of surface-duct propagation in this paper. Firstly, according to the Lloyd-mirror theory, a shallow point source generates directional lobes, whose grazing angles are determined by the source depth and frequency. By assuming a part of the first lobe to be just trapped in the surface duct, a method to calculate the minimum cutoff frequency （MCF） is obtained. The presented method is source depth dependent and thus is helpful for determining the working depth for sonar. Secondly, it is found that under certain environments there exists a layer of low transmission loss （TL） in the surface duct, whose thickness is related to the source geometry and can be calculated by the Lloyd-mirror method. The receiver should be placed in this layer to minimize the TL. Finally, the arrival angle on a vertical linear array （VLA） in the surface duct is analyzed based on normal mode theory, which provides a priori knowledge of the beam direction of passive sonar.     

The detection by sonar of difficult targets (including centimetre-scale plastic objects and optical fibres) buried in saturated sediment

This paper reports on a laboratory study into the use of sonar to detect objects, two of which exhibit a poor acoustic impedance mismatch with the water-saturated sediment in which they are buried to depth of about 30 cm. The targets are solid cylinders having diameters of 20-25 mm and 50 cm length, made of polyethylene, of telecommunications optical fibre, and of steel. Steel spheres are included for comparison. A poor acoustic impedance mismatch between the target and the host sediment is one factor that can make buried targets difficult to detect with sonar, but such detection is increasingly becoming an issue in a range of applications from archaeology to defence to telecommunications. Attention is paid to those signal processing techniques which could be of potential benefit. For this range of test objects, comparisons are made between use of optimal filtering and synthetic aperture sonar. In addition, the potential of a range of acousto-optical effects (optical time domain reflectometry, Raman and Brillouin scattering, and fibre optic hydrophones) is assessed in the Appendix for the particular application of detecting non-metallised fibre optic telecommunications cables. A web page dedicated to this paper hosts movies and reports at http://www.isvr.soton.ac.uk/fdag/uaua/target_in_sand.htm.     

A unified model for reverberation and submerged object scattering in a stratified ocean waveguide

A unified model for reverberation and submerged target scattering in a stratified medium is developed from wave theory. The advantage of the unified approach is that it enables quantitative predictions to be made of the target-echo-to-reverberation ratio in an ocean waveguide. Analytic expressions are derived for both deterministic and stochastic scattering from the seafloor and subseafloor. Asymptotic techniques are used to derive expressions for the scattering of broadband waveforms from distant objects or surfaces. Expressions are then obtained for the scattered field after beamforming with a horizontal line array. The model is applied to problems of active detection in shallow water. Sample calculations for narrow-band signals indicate that the detection of submerged target echoes above diffuse seafloor reverberation is highly dependent upon water column and sediment stratification as well as array aperture, source, receiver, and target locations, in addition to the scattering properties of the target and seafloor. The model is also applied to determine the conditions necessary for echo returns from discrete geomorphologic features of the seafloor and subseafloor to stand prominently above diffuse seafloor reverberation. This has great relevance to the geologic clutter problem encountered by active sonar systems operating in shallow water, as well as to the remote sensing of underwater geomorphology.     

Clutter from non-discrete seabed structures

Clutter, or discrete target-like returns, is the most significant problem in the employment of active sonar. It is well understood that discrete objects, which are of the same spatial scale as the target and which possess a significant impedance contrast to the surrounding ocean, can lead to clutter. Here a somewhat counter-intuitive result is shown: that discrete target-like returns can occur from slowly varying seabed structures. The range dependence of the seabed can be weak and smooth-due to changes in layer thicknesses, sound speed, or both. Thus, this clutter mechanism may be a viable hypothesis for areas in which seabed clutter has been observed, but no discrete features, buried or proud, could be found. By using a broadband source, the time-frequency evolution of this clutter could be a useful way to discriminate against other kinds of clutter; e.g., that due to discrete objects.