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.      

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.     

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.     

浅海近程混响的振荡现象*

研究浅海近程混响特性对于评估和提高主动声纳性能具有重要意义。多次浅海混响实验显示,近程混响强度存在稳定的振荡现象,脉宽基本对振荡的幅度和周期没有影响。为解释这一现象,本文基于射线理论和小斜率近似给出了浅海近程混响模型,仿真与实测数据结果基本吻合。数值仿真结果表明：海底反射声场对单站声纳接收到回声信号的贡献远小于海底近垂向大掠射角散射声场的作用;混响强度振荡现象是海底近程散射声场的多途现象造成的,并由此给出了振荡周期与海深及收发深度的关系。     

水下目标回波与混响的时频形态特征域盲分离

水下沉底或掩埋目标识别受海底混响干扰严重,并且目标回波与混响在时-频域上均存在混叠,增大了对目标回波与混响的分离难度。根据目标回波与混响的产生机理差异,将图像形态学与时频分析相结合的盲分离算法用于目标回波与混响的分离。推导了目标几何声散射成分与混响在Wigner-Ville时频面的形态特征表达式,利用图像形态学滤波去除Wigner-Ville时频面中的混响及刚性亮点之间交叉项,在时频形态特征域获取解混矩阵,实现了目标回波和混响的分离。仿真与实验数据处理结果表明,结合图像形态学的时频域盲分离算法提高了目标回波信号的信混比。      

Space anisotropy of bottom reverberation in the coastal zone

A method for calculating the bottom reverberation in a coastal zone with allowance for the three-dimensional bottom topography, which affects the spatial anisotropy of the incoming reverberation signal, is proposed. The method is based on the ray approach in combination with the use of the ray invariant. Examples of calculating the monostatic bottom reverberation level for one of the coastal regions with a complex bottom topography including the coastal line and an underwater elevation are presented.     

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.     

Resolution enhancement and separation of reverberation from target echo with the time reversal operator decomposition

Time reversal operator (TRO) decompositions are performed in a model of an ocean wave guide containing a target and having different kinds of bottom. The objective is to study the effects of bottom reverberation and absorption by means of ultrasonic experiments. It is shown experimentally that the echo from a target can be separated from the bottom reverberation. Reverberation eigenvectors are back propagated in the wave guide leading to focus on the bottom. An amplitude correction is applied to both reverberation and signal eigenvectors to compensate for bottom absorption and thus to improve target resolution.     

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)].     

Effect of noise and occupancy on optimal reverberation times for speech intelligibility in classrooms

The question of what is the optimal reverberation time for speech intelligibility in an occupied classroom has been studied recently in two different ways, with contradictory results. Experiments have been performed under various conditions of speech-signal to background-noise level difference and reverberation time, finding an optimal reverberation time of zero. Theoretical predictions of appropriate speech-intelligibility metrics, based on diffuse-field theory, found nonzero optimal reverberation times. These two contradictory results are explained by the different ways in which the two methods account for background noise, both of which are unrealistic. To obtain more realistic and accurate predictions, noise sources inside the classroom are considered. A more realistic treatment of noise is incorporated into diffuse-field theory by considering both speech and noise sources and the effects of reverberation on their steady-state levels. The model shows that the optimal reverberation time is zero when the speech source is closer to the listener than the noise source, and nonzero when the noise source is closer than the speech source. Diffuse-field theory is used to determine optimal reverberation times in unoccupied classrooms given optimal values for the occupied classroom. Resulting times can be as high as several seconds in large classrooms; in some cases, optimal values are unachievable, because the occupants contribute too much absorption.     

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.     

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。      

浅海海底界面混响平均强度特性仿真

海底粗糙界面是产生混响的主要因素之一。本文通过理论分析和数值仿真的方法,根据浅海全波动混响模型对不同海底粗糙界面所引起浅海混响平均强度特性进行研究,主要考虑Goff-Jordan谱、Gauss谱和指数谱三种不同粗糙界面条件下的海底反向散射强度和混响平均强度特性。计算结果表明:海底粗糙界面会引起海底反向散射强度的频率特性的差异,进而导致海底平均混响强度的频率特性的差异,但随入射角度的变化不大。即使界面起伏的方差和相关长度相同,不同的粗糙度谱也会引起平均混响强度的差异。     

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.     

低频混响信号中的简正波衰减系数提取方法

提出一种从低频混响信号中提取简正波衰减系数的方法。利用简正波过滤技术对垂直阵混响信号进行分析,获得单阶简正波混响声场。假设海底反向散射矩阵可分离,从单阶简正波平均混响强度中提取出有效海底反向散射矩阵元素,最后利用不同距离上的有效海底反向散射矩阵元素计算出简正波的衰减系数。利用该方法从混响信号中提取出的简正波衰减系数预报的声传播损失和相同海域实测声传播损失一致。该简正波衰减系数提取方法有效避免了海底散射衰减和简正波传播衰减耦合的问题,同时对海底参数反演和水声环境的快速评估也具有重要的意义。      

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.     

界面混响的耦合简正波理论研究

利用耦合简正波理论分析了粗糙界面散射引起的混响。引入了粗糙海底以及海面引起的各个模态间的耦合系数表述粗糙界面对声场的散射过程。结合风浪谱Pierson模型,仿真了不同海况下的海面混响,重点在于分析不同海况下海面起伏对海底混响的影响。利用耦合系数的求解从粗糙海面引起模态间能量耦合的角度对这种影响进行了机理性的探究。结果表明,尽管在浅海中海底混响占支配性地位,但随着海况等级的增加,海面散射对海底混响是有影响的,而这种影响可以从粗糙界面对声波模态间的耦合进行机理分析。      

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.