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。      

双基地混响平均强度理论及仿真预报

考察远程双基地界面混响,可以仅考察二维问题,其散射区域为一椭圆环带;对于近程双基地界面混响,必须讨论三维模型,其散射元分布在一椭球面上.对二维和三维双基地混响进行理论建模分析,该模型优点是无需使用无线电通信,借助GPS时钟即可同步工作,实现双基地声呐系统的测距及测向,便于双基地声呐系统的集成.推导出二维和三维模型的双基地声压混响理论预报公式.对三维矢量混响作仿真研究,并讨论矢量传感器组合指向性抗混响. 关键词： 双基地混响 声压混响 矢量混响     

浅海平均混响强度的数值模拟

海中混响是回声探测的严重干扰。利用本文给出的混响模型可较快地计算分层海洋中平均混响强度的衰减规律及深度结构。 对于近程混响,由于海水非均匀性的影响较小,使用均匀层的射线方法计算。对于远程混响,使用简正波方法计算,它保持了平滑平均后的深度结构。在中等距离上两者能很好地衔接起来,由于考虑了复本征值对简正波入射场的影响,提高了计算精度,通过比较不同散射模型对混响强度计算的影响,提出以分离型二元散射模型作为混响数值模拟的基础,使能由界面反向散射系数估算二元后向散射系数,并大大简化混响强度的数值计算。     

一种改进的水下爆炸声混响模型

为了研究水下爆炸声脉冲引起的混响,基于射线理论,利用椭球的几何性质,建立了改进的三维近程双基地混响平均强度模型,推导出爆炸深度任意的双基混响平均强度预报公式。计算分析了爆炸深度和脉冲宽度对混响强度的影响,并与实验结果进行了比较,结果表明:爆炸深度对混响强度的影响相对较小,脉冲宽度越大,混响强度越大。      

基于小斜率近似的深海海面混响

收发设备在海面附近的深海混响实验中,多途时延使得最先到达水听器的为海面混响信号,且不受海底散射声场的干扰.本文利用射线理论描述深海声传播的格林函数,采用粗糙界面一阶小斜率近似方法描述全角度海面散射,给出海面混响声场的表达式.同时考虑了海面表层气泡散射的贡献,获得了海面混响理论.通过数值仿真数据和深海实验数据的比较对海面混响模型进行验证,分析了不同接收深度、频率下的海面混响强度衰减趋势.结果表明:低海况条件下,低频海面混响由粗糙界面散射主导,气泡散射可以忽略,随频率升高,气泡散射对海面混响的贡献逐渐增大,海面附近收发深度的小幅变化对混响衰减曲线的影响不明显.基于该模型提出一种反演海面粗糙界面谱参数的方法,数值计算结果验证了该模型能够在风速已知的前提下,通过海面混响数据提取海面粗糙界面谱参数.     

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。     

浅海平均声场角度谱分析法与回波混响比的距离、深度结构

浅海平均声场角度谱分析法以Ray-mode理论为基础。引进平均声场角度谱的概念以后,可使浅海传播衰减、远程混响和空间相关等平均特性的一些计算变得较为简单。用该法在浅海负梯度和均匀层情况下,计算了远程混响和回波、混响比的距离、深度结构。理论证明,在非均匀层浅海远程探测中,深度可变声呐比固定声呐有较为明显的优越性。本方法和结果将有助于分析小掠角海底(面)散射的角度、频率依赖关系。     

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。     

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.     

浅海远程混响与海底散射强度

利用浅海平均声场角度谱分析法,在海底散射的任意角度关系和浅海均匀层远程混响的距离关系之间建立了变换关系。与国外沿用的深海近程界面混响理论相比,多了多途传播修正因子K(公式18)。利用该变换关系式,由黄海、渤海、东海等5个海区的浅海远程混响资料,推得频率为0.8—4kHz,掠射角为2—10°的低频、小掠角海底散射强度。所得结果与国外深海直接测量的大掠角数据衔接较好。结果同时表明,大陆架高声速海底散射常数(μ)和角度指数(2n)不仅与海区、频率有关,而且与角度范围有关。在掠角很小时,海底散射强度随掠角减小比中等掠角时衰减更快。     

In-plane bistatic backward scattering from seabottom with randomly inhomogeneous sediment and rough interface

1 Introduction Backward scattering of sound due to sediment is the main source of shallow waterreverberation. In order to predict the reverberation or detect sediment properties frommeasured reverberation data, a reasonable in-plane bistatic backward scattering (BBS)model is essential. The scattering can be caused by the roughness of water-sediment in-terface or by inhomogeneities within the volume of sediment. A great deal of researchhas been done on sediment backscattering, most of which h…     

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.     

Determination of the target strength of a scatterer with the use of surface reverberation

Results of measuring the target strength of a scatterer by using the data on the sound propagation and surface reverberation in the basin are presented. The proposed technique implies the use of a receiving antenna array with a narrow directivity pattern and a bistatic scheme of location.     

浅海波导中水平接收阵被动时反混响抑制方法研究

使用基于射线简正波异地混响理论建立的水平接收阵接收海底混响模型,提出了水平阵被动时反混响抑制方法。以水平接收阵接收回波信号的某一时刻作为开始,分别对其前一时刻和后一时刻的回波信号做时反算子分解获得两个混响子空间,将这两个混响子空间的平均作为该时刻的混响子空间的一个估计,然后利用投影处理来抑制该时刻接收信号中的混响成分。该方法可以在一定程度上克服由于海底粗糙引起的局部时间(距离)段内接收回波信号的不平稳性,使得估计得到的混响子空间更加接近真实混响子空间。因此,利用其进行被动时反混响抑制可以增强回波信混比,提高对目标的检测和方位估计能力。针对典型浅海波导环境开展的仿真验证了本文方法的有效性。      

Time structure of bistatic reverberation in the long-range propagation of explosion-generated signals

In experiments on the long-range propagation of explosion-generated signals, a noise background accompanying the classical signal quartets are repeatedly observed. The background smoothly increases before the arrival of the first signal of the quartet, then decreases, and completely vanishes after the arrival of the last signal of the same quartet. The data of an experiment performed in winter in a deep-water region of the northwestern Pacific are considered. These data are used to demonstrate the phenomenon at hand and to show that the noise background of the quartets is the manifestation of the bistatic reverberation caused by sound scattering by the rough ocean surface towards the receiver located a long distance from the source. An easy-to-use technique is proposed for calculating the time relations between the direct signal and the surface-reverberation one. Calculations are performed for the time structure of the bistatic surface reverberation to explain the observed features of the phenomenon.     

Methods for calculating low-frequency surface reverberation at known sea roughness characteristics

The problem of estimating the parameters of a reverberation signal generated due to scattering of acoustic waves on a rough surface is considered for a bistatic localization scheme using the example of tone and tone-pulse signals. Relatively simple calculation schemes that make it possible to recalculate three-dimensional space-time roughness spectra into angular-frequency reverberation characteristics and determine the positions of the surface regions that mainly contribute to the integral reverberation level are found. A reverberation spectrum is calculated for the wind waves typical for shallow-water closed basins, and the positions of the scattering regions forming the components of this spectrum are determined.     

Azimuthal dependence of reverberation from a harmonic signal propagating in shallow water

The problem of scattering from a rough sea surface is considered for a harmonic low-frequency (200–400 Hz) acoustic signal propagating in a shallow water area. An experimental study of the azimuthal reverberation dependence is carried out on the basis of the bistatic location scheme with the use of linear phased arrays. By comparison with calculations, the formation of the frequency-angular characteristics of reverberation is investigated for various wind directions with respect to the path of acoustic signal propagation.