基于信道传播理论的多基地远程海底混响研究

针对浅海多基地远程海底混响, 以声传播的线性信道近似理论为基础, 通过确定海底界面的不均匀起伏特性, 对入射声波的远距离异地混响脉冲响应函数进行求解. 对非水平分层介质的浅海波导, 利用绝热简正波求解信道的传输函数, 得出了小角度斜坡海底的远程海底混响强度, 以下坡海底为例, 分析对比了不同海底倾角斜坡海底对远程异地混响强度及其衰减规律的影响. 理论仿真表明, 波导的传输效应对多基地远程海底混响强度的影响不可忽略.     

Low-frequency bottom reverberation in shallow-water ocean regions

A phenomenological model of long-range reverberation in a shallow sea is developed to describe the statistical characteristics and interference of the sound field scattered by bottom inhomogeneities. Experimental data on the scattering of low-frequency sound by the sea bottom are presented for a shallow-water region of the Barents Sea. The results of a numerical simulation of the low-frequency bottom reverberation in a multimode waveguide are described. The simulation is based on experimentally measured values of backscattering strength.     

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.     

Echo-to-reverberation enhancement based on decomposition of time reversal operator with forward and backward reverberation nulling constrains

Combined the decomposition of time reversal operator and the time reversal reverberation nulling, a new time reversal processing approach for echo-to-reverberation ratio enhancement is proposed. In this method, a 2-dimensional signal subspace for the range of the target and two bottom focusing weight vectors for the ranges near the target are obtained by the decomposition of time reversal operator. From the signal subspace and focusing weight vectors, a constrained optimal excitation weight vector of source receiver array can be deduced to null the acoustic energy on the corresponding bottom and maximize the energy at the tar- get. This method remedies the shortages of conventional time reversal processing, time reversal reverberation nulling and time reversal selective focusing method. It focuses sound energy at the target and nulls the energy at the bottom near the target range simultaneously, therefore enhancing the echo-to-reverberation ratio without probe source and prior-knowledge of the relative scattering intensity of target and bottom. Numerical simulations in typical shallow water environments showed the effectiveness of the proposed method and its improved performance for echo-reverberation enhancement than conventional time reversal processing.     

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。      

Long-range reverberation in a randomly inhomogeneous shallow water with the use of focused radiation

The low-frequency bottom reverberation in a randomly inhomogeneous shallow water is investigated within the framework of a numerical experiment using vertical transmitting arrays focusing the acoustic field at various distances from the sea bottom. It is assumed that the main source of sound velocity fluctuations in the medium is represented by background internal waves. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that the reverberation level is mainly determined by the presence of internal waves and may vary by 5–20 dB as the distance from the focusing point to the sea bottom increases up to H/2, where H is the channel depth.     

Waveguide invariance structure for low frequency monostatic bottom reverberation mitigation in shallow water

The stable range-frequency interference structure(waveguide invariance) of the low frequency monostatic bottom reverberation in shallow water is derived theoretically in this paper by using ray-normal mode coherent reverberation model and separable bottom scattering model.And combined the concept of spectral subtraction,a reverberation forecasting and mitigating method based on the waveguide invariance is presented.According to the waveguide invariance,the reverberation intensity for a detecting range can be estimated from the reverberation returns scattered from some other range cells nearby the detecting range with frequency shift.Then the reverberation component for the detecting range in the echo can be canceled mostly by using the estimated reverberation intensity,so as to enhance the echo-to-reverberation ratio and improve the detection capability of the active sonar.Simulations in typical horizontal layered shallow water environment show the stability of waveguide invariance structure in the low frequency monostatic bottom reverberation and the efficiency of the reverberation forecasting and mitigating method based on the waveguide invariance.     

Characteristics of the diffraction of acoustic waves in stratified sound channels

The results of calculations performed in the framework of the approximate approach developed by the authors are presented for the diffraction of sound waves by a stiff spheroid in an acoustic waveguide. The scattered sound field is analyzed as a function of the following parameters of the problem: the spheroid dimensions, its position relative to the sound source and the receiver, the vertical profile of sound velocity in the waveguide, and the acoustic parameters of the waveguide bottom.     

Low frequency seabed scattering at low grazing angles

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].     

Spatial attenuation of different sound field components in a water layer and shallow-water sediments

The paper presents the results of an experimental study of spatial attenuation of low-frequency vector-scalar sound fields in shallow water. The experiments employed a towed pneumatic cannon and spatially separated four-component vector-scalar receiver modules. Narrowband analysis of received signals made it possible to estimate the attenuation coefficients of the first three modes in the frequency of range of 26–182 Hz and calculate the frequency dependences of the sound absorption coefficients in the upper part of bottom sediments. We analyze the experimental and calculated (using acoustic calibration of the waveguide) laws of the drop in sound pressure and orthogonal vector projections of the oscillation velocity. It is shown that the vertical projection of the oscillation velocity vector decreases significantly faster than the sound pressure field.     

Focusing of low-frequency sound fields in shallow water

A numerical experiment is carried out to study the focusing of a low-frequency (100–300 Hz) sound field in a shallow-water acoustic waveguide typical of an oceanic shelf. Focusing with the use of time reversal of broadband acoustic signals, which is called time reversal mirror (TRM) of waves, is considered along with focusing by phase conjugation (PC) of a monochromatic sound field. It is demonstrated that, in the case of focusing by the TRM method in the waveguide of interest, it is sufficient to have a single source-receiving element. The use of a vertical array improves the quality of focusing. The quality achieved in the latter case proves to be approximately the same as that achieved in the case of focusing by phase conjugation of a monochromatic field at a frequency identical to the carrier frequency of the broadband signals. It is also shown that, in a range-independent waveguide, intense surface waves considerably reduce the quality of focusing. This effect is most pronounced in the case of using phase conjugation.     

Finite element modeling of reverberation and transmission loss in shallow water waveguides with rough boundaries

A finite element model for the reverberation and propagation in a shallow water waveguide with a sandy bottom was calculated for five different environments at a center frequency of 250 Hz. The various environments included a rough water/sediment interface, a rough air/water interface, roughness at both interfaces and downward and upward refracting sound speed profiles with roughness at both interfaces. When compared to other models of reverberation such as ray theory, coupled modes, and parabolic equations, finite elements predicted higher levels of reverberation. At early times, this is due to the "fathometer" return, energy that is normally incident on the boundaries at zero range. At later times, the increased reverberation was due to high angle scattering paths between the two interfaces. Differences in reverberation levels among the environments indicated that scattered energy from the air/water interface is transmitted into the bottom at steep angles. This led to a large decrease in reverberation for a rough air/water interface relative to a rough water/sediment interface. Sound speed profile effects on reverberation were minimal at this frequency range. Calculations of the scintillation index of the different environments indicated that most of the reverberation was relatively Rayleigh-like with heavier tailed distributions at longer ranges.     

Experimental studies of low-frequency reverberation on the continental slope in the northwestern Pacific Ocean

Experimental data obtained on the continental slope near the Kamchatka peninsula for the reverberation at the frequencies 230, 600, and 850 Hz in the cases of coincident and spaced source and receiver of sound are presented. The data include the dependences of the reverberation level on time for both directional and omnidirectional receiving systems, as well as the dependences of the reverberation level on the duration of the probing pulses and on the sea depth at the source site. It is shown that, at the frequency 230 Hz, a substantial contribution to the reverberation is made by the reflection and scattering on the shelf near the coastline and in the region of the “depth drop.” At the frequencies 600 and 850 Hz, the predominant mechanism is bottom and surface scattering in the region of the continental slope.     

Interference structure of low-frequency reverberation signals in shallow water

Using numerical simulation, an analysis was conducted of the interference structure of a bottomscattered sound field generated by a wideband point source in shallow water under winter and summer conditions. The scattered signals were received from the place where the source was located and were subjected to Fourier transform with a sliding window. The paper demonstrates the possibility of estimating the waveguide invariant for backscattered signals when processing the sound intensity distributions in wide frequency and distance ranges up to the scattering area. A technique is proposed for reconstructing the twodimensional field of internal waves using variations of the interference pattern of reverberation signals. The influence of wind surface waves on the degree of interference band contrast is illustrated.     

深海本地界面混响平均强度模型与规律分析

为揭示深海多途传播效应及界面参数对本地界面混响强度的影响规律,将界面混响表述为不同多途混响衰落过程的非相干叠加结果,并结合界面散射的若干物理机制,建立了以海面海底物理参数为变量的混响强度模型.通过数值仿真和理论分析研究了不同多途混响强度的衰落特性、到达角的空间分布及部分物理参数对混响强度的影响,并利用南海海盆(典型泥质海底)实验数据对模型有效性进行验证.仿真结果表明,界面混响强度随时间变化呈现多峰结构,峰值时间与多途混响的到达时间分布一致。声源与接收器均位于近海面处时,首峰的峰值强度及其衰落过程由海面风速决定,其它峰的峰值强度由海面海底参数共同决定,但其衰落过程主要受到底质参数的影响.      

浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

Surface reverberation at sound field focusing in shallow water

A numerical experiment is carried out to study the long-range surface reverberation in the presence of intense surface waves for the case of using vertical transmitting arrays providing sound field focusing at different depths. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that surface waves considerably affect the focusing quality at a distance of several tens of kilometers from the transmitting array. This prevents the efficient suppression of long-range reverberation by increasing the focusing depth.     

Characteristics of the reverberation signal at a vertical array with tonal insonification of a shallow-water basin

The angular and spectral characteristics of reverberation signals on a fixed path are studied by using vertical receiving arrays with a tonal insonification of the basin. The choice of the frequency range F ～ 200–300 Hz is caused, first, by the low propagation loss and the availability of high-power acoustic transducers for this range and, second, by the possibility to study the phenomena of sound scattering by the sea surface for comparable wavelengths of surface and acoustic waves. The data of experiments on two paths are considered: the first path is in the Barents Sea with propagation conditions corresponding to a uniform waveguide with sea depths H ～ 110–120 m, whereas the second path is in the White Sea with propagation conditions corresponding to a nonuniform waveguide where the sea depth varies from 30 to 250 m.     

球形传声器阵列下基于主导声源检测MUSIC群时延算法的多声源定位*

针对混响环境中,多径效应、散射、衍射等原因导致声源定位失败或分辨能力不足的现象,提出一种基于主导声源检测MUSIC群时延的邻近多声源定位方法。该方法采用球形传声器阵列,相比平面阵列可以捕获3D声场信息,利用球谐域下信号的频率分量与角度分量解耦的优势,从而可直接利用频率平滑技术处理宽带语声信号而不需要构造聚焦矩阵,并在球谐域下通过设置阈值对一组时频段进行主导声源检测,从而选择出包含直达声的一组时频块来构造MUSIC群时延空间谱。上述举措在提升波达方向估计在高混响环境下定位鲁棒性的同时,也提高了多个邻近声源的分辨能力。仿真实验结果表明,所提出的主导声源检测MUSIC群时延算法,在高混响和低信噪比条件下,仍具有更好的定位精度与更优的邻近多声源分辨效果。     

On the possibility of using acoustic reverberation for remote sensing of ocean dynamics

The two-point correlation function of diffuse noise fields produced by distributed random sound sources carries useful information on the medium of sound propagation. Such information can be used for performing passive acoustic tomography of the ocean. In a number of cases that are important for practice, the noise field in the ocean is predominated by contributions of individual point sources. Here, a theoretical study is presented on the possibility of determining the sound speed and current velocity in the water column by the correlation processing of reverberation signals measured by two vertical receiving arrays. In other words, we study the possibility of replacing the diffuse noise produced by a great number of delta-correlated sources by waves generated by a localized source and scattered at the rough surface and bottom of the ocean for sensing the medium. The correlation function of scattered waves is calculated by using the method of small perturbations. It is shown that the correlation processing of the scattered waves offers an opportunity of measuring the acoustic nonreciprocity and reconstructing the field of sound speed in the fluid, without using any acoustiLc transceivers.