基于简正波理论的浅海低频混响时间序列仿真

混响场时域信号序列的数值实现对主动声纳目标模拟器研制具有重要意义。本文所述适用于水平变化浅海环境混响时间序列的一种实现方法,基于现有的浅海简正波相干混响理论。所述方法的思路是通过相干散射场随机序列与发射信号的离散卷积来构成混响时域序列,文中以Pekeris均匀浅海环境为例,给出了相应的混响时间序列仿真结果,并对仿真结果的频谱特性、统计特性、强度衰减以及垂直相关特性等进行了系统检验。结果表明,由仿真混响时间序列所获得的混响特性与理论结果相符,可用于浅海混响序列的仿真。     

High-resolution angle-Doppler imaging by sparse recovery of underwater acoustic signals

The low resolution of Fourier two-dimensional spatial temporal spectrum estimation and the insufficient sample size of sonar space time sampling data often caused difficulties in high-resolution space time spectrum estimation. Aiming to solve this problem, we proposed a high-resolution angle-Doppler imaging method and designed an anti-reverberation space time filter based on the sparse recovery of underwater acoustic signals. The proposed imaging method established the spatial temporal sparse representation model of array signal under the condition of few observation samples of single measurement vector(SMV), and reconstructed the high-resolution angle-Doppler profile of echo and reverberation through the matching pursuit(MP) algorithm and basis pursuit(BP) algorithm. By utilizing the space time distribution characteristics of echo and reverberation and the prior information of sonar rangecell under test(RUT), a reverberation dictionary composed of special space time guidance vectors was designed, and was used to reconstruct the reverberation and formed an anti-reverberation space time filter to suppress the reverberation interference of angle-Doppler plane. Computer simulations indicated that, under two conditions of forward and side-view array of moving sonar, the single measurement vector of sonar array was successfully used in the reverberation background to reconstruct the high-resolution angle-Doppler profile of low-speed moving target multi-spot echo. Its frequency resolution had a better performance than the Fourier resolution and the angle resolution broke through the Rayleigh limit of the array, the resolution was obviously superior to the Fourier space time spectrum estimation.     

水声信号稀疏重构高分辨角度-多普勒成像

针对傅氏空时二维谱估计分辨率低以及声呐空时采样数据样本数不足给角度-多普勒成像带来困难的问题,提出一种水声信号稀疏重构的高分辨角度-多普勒成像方法和抗混响空时滤波器的稀疏重构方法。该方法在声呐阵列单测量向量的极少观测样本条件下,建立阵列信号的空时稀疏表示模型,应用稀疏表示的匹配追踪算法和基追踪算法重构回波与混响的高分辨角度-多普勒像。并根据运动声呐回波与混响的空时分布规律及声呐待检测距离单元位置的先验信息,沿着混响空时分布脊线设计混响稀疏表示的专用空时导向向量字典,通过重构抗混响空时滤波器来抑制角度-多普勒平面的混响干扰。对运动声呐前视和侧视阵列的计算机仿真结果表明,在混响背景中,该方法采用声呐阵列单测量向量重构了低速运动目标多亮点回波的高分辨角度-多普勒像,频率分辨率突破傅里叶分辨率,角度分辨率突破阵列瑞利限,分辨率明显优于傅氏空时谱估计。      

Sonar inter-ping noise field characterization during cetacean behavioral response studies off Southern California

The potential negative effects of sound, particularly active sonar, on marine mammals has received considerable attention in the past decade. Numerous behavioral response studies are ongoing around the world to examine such direct exposures. However, detailed aspects of the acoustic field (beyond simply exposure level) in the vicinity of sonar operations both during real operations and experimental exposures have not been regularly measured. For instance, while exposures are typically repeated and intermittent, there is likely a gradual decay of the intense sonar ping due to reverberation that has not been well described. However, it is expected that the sound field between successive sonar pings would exceed natural ambient noise within the sonar frequency band if there were no sonar activity. Such elevated sound field between the pings may provide cues to nearby marine mammals on source distances, thus influencing potential behavioral response. Therefore, a good understanding of the noise field in these contexts is important to address marine mammal behavioral response to MFAS exposure. Here we investigate characteristics of the sound field during a behavioral response study off California using drifting acoustic recording buoys. Acoustic data were collected before, during, and after playbacks of simulated mid-frequency active sonar (MFAS). An incremental computational method was developed to quantify the inter-ping sound field during MFAS transmissions. Additionally, comparisons were made between inter-ping sound field and natural background in three distinctive frequency bands: low-frequency (<3 kHz), MFA-frequency (3–4.5 kHz), and high-frequency (>4.5 kHz) bands. Results indicate significantly elevated sound pressure levels (SPLs) in the inter-ping interval of the MFA-frequency band compared to natural background levels before and after playbacks. No difference was observed between inter-ping SPLs and natural background levels in the low- and high-frequency bands. In addition, the duration of elevated inter-ping sound field depends on the MFAS source distance. At a distance of 900–1300 m from the source, inter-ping sound field at the exposure frequency is observed to remain 5 dB above natural background levels for approximately 15 s, or 65%, of the entire inter-ping interval. However, at a distance of 2000 m, the 5 dB elevation of the inter-ping SPLs lasted for just 7 s, or 30% of the inter-ping interval. The prolonged elevation of sound field beyond the brief sonar ping at such large distances is most likely due to volume reverberation of the marine environment, although multipath propagation may also contribute to this.     

改进次最佳检测在侧扫声呐底混响抑制中的应用*

侧扫声呐进行沉底小目标探测时,底混响是主要背景干扰。底混响通常是一种非平稳、非高斯的带限噪声,它使得白噪声条件下的滤波器性能受到限制。在混响背景下常利用自回归模型对接收信号进预行白化处理,但对于实际侧扫声呐应用,白化后直接匹配滤波的处理效果不甚理想。针对此问题,在自回归模型预白化的基础上,提出采用一种次最佳检测与多分辨二分奇异值分解相结合的改进方法。该方法首先对接收信号进行分段处理,利用改进Burg算法估计每段数据自回归模型的系数及阶数;然后构造白化滤波器对分段数据预白化,并对白化后的数据进行多分辨二分奇异值分解;最后应用ostu方法对原始声图和处理后的声图进行目标检测。仿真与实验结果表明,该方法明显提高了信混比,改善了侧扫声呐沉底静态小目标的成图质量,有利于后期实现基于图像的目标自动检测。     

水下目标回波盲分离性能的瞬时频率特征评价方法

在水下主动声呐目标回波与混响盲分离中,针对分离结果顺序不确定性以及缺乏分离有效性衡量手段的问题,提出了以信号瞬时频率特征为指标的盲分离性能评价方法。推导了目标回波与混响的时频分布特性,理论表明目标回波在瞬时频率序列的中心偏离程度以及整体随机程度上低于混响,据此提取信号的瞬时频率方差与瞬时频率熵两种信号特征,并将二者作为从盲分离结果中识别目标回波的依据。海试数据结果表明,在盲分离得到的所有分离信号中,目标回波具有最小的瞬时频率特征值,并且该特征值越小,目标回波与混响的盲分离程度就越高。      

Improved space time prewhitener for linear frequency modulation reverberation using fractional Fourier transform

This letter presents an improved space time prewhitening method for linear frequency modulation (LFM) reverberation. The proposed method transforms the reverberation to fractional Fourier domain to whiten using fractional Fourier transform. The linear varying frequency in LFM reverberation is focused on a stationary frequency, and the adjacent block signal is used as the reference signal of prewhitening. Finally, experiment results with real reverberation data verify that the proposed method improves the detection performance of active sonar in shallow sea significantly.     

浅海低频本地海底混响的波导不变性结构及抑制方法研究

根据浅海混响的射线简正波相干混响理论和可分离的海底散射模型,从理论上推导了浅海低频本地海底混响信号中存在的稳定的距离-频率干涉结构(波导不变性结构),并在此基础上基于谱相减的思想提出了一种基于波导不变性的本地海底混响预测对消方法。针对某个检测距离,其对应的中心频率上的海底混响强度可以根据波导不变性通过频移的方法在另一个与之相邻的距离上估计得到,利用该估计的混响强度可以对消检测距离对应的回波信号中的海底混响强度分量,增强回波信混比,提高目标检测能力。针对典型水平分层浅海波导环境,通过计算机仿真实验分析了低频海底混响存在的稳定波导不变性结构,验证了本文提出的基于波导不变性的混响预测对消方法的有效性。      

采矿混响环境下超声波探测信号设计

针对深海采矿环境中混响干扰对水下微地形探测的影响,设计了一种可以抑制水下混响并提高探测精度的超声波发射信号。通过分析探测目标的接收信号模型,根据Neyman-Pearson准则得到目标回波探测模型的检测性能评价指标。基于高斯点目标最优检测理论,提出采矿混响环境下发射波形的能量谱设计方法,进而得到混响环境下超声探测信号的幅度谱。保持幅度谱分量恒定,利用信号相位谱与信号长度的关系设计了可以满足不同探测系统和探测环境需要的超声波探测信号。利用设计的超声波探测信号及常用声呐信号在模拟深海采矿环境下进行探测实验,结果表明所设计的超声波探测信号在探测精度及抗混响能力方面均优于常用声呐信号。      

The influence of random drift of resonance-scattering harmonics on the frequency spectrum of surface acoustic reverberation in the ocean

We propose a modified model for the frequency spectrum of surface reverberation with allowance for random drift of Bragg resonance harmonics due to long wind-generated waves. The dependence of this effect on the parameters of large-scale surface waves is analyzed. Analytical expressions for the reverberation frequency spectrum are obtained for both narrow-band and wideband sounding signals. It is shown that for matched filtering of wideband signals, the considered effect leads to an increase in the effective pulse reverberation volume. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 20–30, January 2007.     

Beam pattern of sparse array for passive sonar

This work aims at sparse array without grating lobe for passive sonar, in order to increase frequency band of the passive sonar and reduce the cost. According to traditional method, the distance between two contiguous sensors in sonar array should be less than the wavelength of highest frequency of the signal; otherwise the grating lobe appears on beam pattern. The paper realizes that the conclusion of the grating lobe emergence it is not fit to the passive sonar due to the sine wave signal model in traditional method, in fact the input of passive sonar is random signal. Prom three aspect of the computer simulation, real passive sonar signal and theory analysis, it is revealed that the grating lobe does not appear on beam pattern when the distance between two contiguous sensors in passive sonar array is longer than wavelength of highest frequency. Whitening of the radiated noise can widen the frequency band, to ensure the beam pattern without the grating lobe. The conclusion of the paper is: if the band of radiated noise is wide enough (broader than 1 octave), that the wavelength of signal can be shorter than the distance between two contiguous sensors in passive sonar without grating lobe.     

多维波形编码合成孔径声呐技术研究

为了提高测绘效率,提出了两种基于多维波形编码技术的高分辨率宽测绘带合成孔径成像声呐体制,包括使用多发射阵的频分多输入多输出成像方式和方位向波形编码序列的成像方式,并论述了相应的成像处理流程。最后根据高速平台下的成像需求,设计了一种频分双发多收的合成孔径声呐系统,它可以将原有的单发多收系统的测绘效率提高一倍。考虑到水声信号的随机起伏性,利用已有的单发多收系统的实测数据,对多维波形编码技术在水声领域的可行性进行了实际验证。成像结果表明,经水底散射后的相邻频带的回波信号的空间相关性可以满足双发多收体制的合成孔径声呐的成像要求。      

Blind dereverberation of a speech signal

Possibilities to eliminate the reverberation from a speech signal are investigated by applying the method based on the determination of the parameters of the reverberation frequency response from the cepstrum of the reverberation-distorted signal. The delays of reverberating signals and, for the case of a weak reverberation, their amplitudes are determined from the cepstrum of the signal with reverberation. For the cases of medium and strong reverberation, the levels of reverberating signals are refined by adjusting a certain factor. The criterion used for the adjustment of the factor is based on the shape of the speech signal amplitude distribution. By numerical modeling, it is demonstrated that the proposed method can reduce the reverberation level by 30 dB.     

Reverberation of Wideband Signals in Shallow Water when Using Sound Focusing

Within the framework of the normal mode approximation, expressions are obtained for calculating bottom reverberation signals recorded by a horizontal array in an inhomogeneous shallow-water waveguide in a wide frequency band. These expressions can be used to simulate bottom-scattered signals both for a monostatic and bistatic geometry, as well as in the case when sound focusing is applied. The constructed model is used to numerically study the structure of bottom reverberation in a waveguide with different parameters and characteristics of the receiver and source systems. The considered bottom inhomogeneities are the slope of the bottom, change in thermocline depth, and wind waves. The study demonstrates the promise of using sound focusing as time reversal using a single receiver–transmitter element to enhance the reverberation signal arriving from a given bottom area.     

正交多相码连续波主动声呐回波检测算法设计

与常规脉冲式主动声呐相比,连续波主动声呐能够提高目标回波的时间处理增益和目标信息更新速率。该文提出一种由正交多相码合成的连续发射波形,分析了发射信号和目标回波模型,设计了多通道匹配滤波器组完成回波检测。为了进一步提高接收机检测性能,提出了多通道非相干积累的处理方法。通过数值仿真,分析提出的连续波形的多普勒分辨性能和目标信息更新率。通过蒙特卡罗法获取接收机工作特性曲线,比较了脉冲式主动声呐和连续波主动声呐在均匀混响背景下对单目标检测性能的差异。仿真结果表明,该文设计的连续波波形具有较好的多普勒分辨性能,在均匀混响背景下,回波检测算法能够明显提高单个目标的探测能力。     

双基地冰-水界面混响强度的理论预报

针对北极冰层冰水界面具有小尺度粗糙界面的情况,给出由冰层造成的混响平均强度的理论预报公式。首先将北极冰层等效为具有粗糙界面的弹性介质,并且采用小粗糙度微扰理论在精细拟合粗糙度谱的基础上,建立粗糙冰层的三维散射强度模型,然后建立三维双基地声呐几何模型确定有效散射区域,最后给出双基地冰下混响平均强度的估算公式,并在所拟合的粗糙度谱的情况下,利用该预报公式计算双基地声呐配置参数和海冰物理及声学特性对冰下混响强度的影响算例。数值仿真表明该公式能够估算出北极冰下双基地声呐产生的混响平均强度,并且分析出了时延、基线长度等声呐配置参数和冰层声速比、密度比等海冰物理参数会对混响强度造成的影响。声呐的配置参数主要影响散射的有效面积,海冰的物理参数则影响着冰层的散射强度。其中,冰层声速比不仅影响混响强度的大小,还会影响混响强度随时间的衰减速度。      

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.     

Estimation of frequency characteristics of the Tonpilz piezoelectric transducer with a bending piezoelectric disk

A Tonpilz piezoelectric transducer which has a bending piezoelectric disk on the radiation surface for widening the frequency band to improve sonar detection performance in shallow water has been proposed. Variance of transmitting voltage response of this transducer depend on the phase difference between signals which applies to the ring-stack and the bending-disk piezoelectric resonators. Thus, an estimation of an optimal phase difference, which can be applied for obtaining the maximum value of transmitting voltage response, is important factor to assemble sonar array with sonar transducers. In this paper, we applied three methods to estimate transmitting voltage response at an arbitrary phase difference from a few measurements, and the estimation errors of those methods affected by internal impedances of power sources and/or noise are investigated to select the best method. The effectiveness of the best method was verified by water-pool experiments.     

Superluminal Signal Velocity and Causality

A superluminal signal velocity (i.e. faster than light) is said to violate causality. However, superluminal signal velocities have been measured in tunneling experiments recently. The classical dipole interaction approach by Sommerfeld and Brillouin results in a complex refractive index with a finite real part. For the tunneling process with its purely imaginary refractive index this model obtaines a zero-time traversing of tunneling barriers in agreement with wave meechanics. The information of a signal is proportional to the product of its frequency band width and its time duration. The reasons that superluminal signal velocities do not violate causality are: (i) physical signals are frequency band limited and (ii) signals have a finite time duration.     

修正的混响多普勒扩展非对称统计模型

运动声源探测低速目标时,混响谱呈现的非对称性导致正负速度目标的回波具有不同的信混比,需要准确估计双边谱的扩展程度来分别设计检测策略,而传统的对称模型不能描述这种非对称性。基于双边指数分布对称模型,修正的非对称统计模型可用来描述由平台运动、海面运动等引起的混响谱非对称扩展现象。文中推导了非对称模型的数学表达式,并阐述了模型各参数的物理意义。运动声呐平台的湖试实验分析结果验证了该模型对平台在不同速度运动时引起的非对称混响谱都有较高的拟合度,证明混响谱负向多普勒扩展程度随平台运动速度提高而加剧,且非对称模型参数具有量化描述混响谱非对称扩展程度的能力。