Sound Propagation in Shallow Water with an Inhomogeneous GAS-Saturated Bottom

We present the methods and results of numerical experiments studying the low-frequency sound propagation in one of the areas of the Arctic shelf with a randomly inhomogeneous gas-saturated bottom. The characteristics of the upper layer of bottom sedimentary rocks (sediments) used in calculations were obtained during a 3D seismic survey and trial drilling of the seafloor. We demonstrate the possibilities of substituting in numerical simulation a real bottom with a fluid homogeneous half-space where the effective value of the sound speed is equal to the average sound speed in the bottom, with averaging along the sound propagation path to a sediment depth of 0.6 wavelength in the bottom. An original technique is proposed for estimating the sound speed propagation in an upper inhomogeneous sediment layer. The technique is based on measurements of acoustic wave attenuation in water during waveguide propagation.     

Simulation of the lower boundary of an acoustic waveguide

The paper discusses simulation of the lower boundary of an acoustic waveguide. The applicability and relationship of rigid bottom and fluid half-space models are studied as a function of the bottom sediment parameters. The influence of bottom losses on the sound field characteristics in a waveguide is studied. Examples of numerical simulation are given.

     

一种低声速沉积层海底参数声学反演方法

软泥底环境下沉积层参数的声学反演是国际水声领域的一个研究热点.浅海中,当高声速基底和海水之间存在一层低声速(小于海水声速)的沉积层时,小掠射角情况下不同频率声传播损失会出现周期性增大现象.基于此现象,提出一种适用于低声速沉积层的海底参数声学反演方法.首先,推导给出小掠射角情况下传播损失周期增大的频率间隔与沉积层声速、厚度及近海底海水声速之间的解析表达式;其次,利用一次黄海实验中软泥底环境下的宽带声传播信号,提取了小掠射角下传播损失增大的频率周期;再次,把该解析表达式作为约束条件,结合Hamilton密度与声速的经验公式,采用匹配场处理反演给出沉积层的声速、密度、厚度及基底的声速、密度;然后,利用声传播损失数据反演得到泥底环境下不同频率的声衰减系数,通过拟合发现泥底声衰减系数随频率近似呈线性关系;最后,给出了双层海底模型和半无限大海底模型等效性的讨论.反演结果为低声速沉积层海底声传播规律研究与应用提供了海底声学参数.     

浅海波导中折射类简正波的warping变换

Warping变换可以实现单水听器浅海低频声波导的简正波分离.本文讨论了海水声速随深度线性减小浅海波导中折射类简正波的warping变换.理论推导了海水折射类简正波水平波数、频域相位及瞬时相位的表达式,由此提出了相应的时域和频域warping变换算子,并由仿真数据进行了验证.本文的理论推导过程同样适用于海水声速线性增大或海水折射系数的平方随深度线性变化的浅海波导.     

Effect of the sediment layer on the excitation of acoustic modes and lateral waves in shallow sea

The simplest model of a shallow sea in the form of an isovelocity water layer and a fluid sediment layer overlying a homogeneous elastic halfspace is used to investigate the effect of the thickness of the sediment layer and the sound velocity in it on the behavior of the frequency dependences of the amplitudes of trapped and leaky modes and shear and longitudinal lateral waves that are excited by an acoustic point source in a shallow-water oceanic waveguide.     

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.     

沉积层声学特性对浅海低频声场分布的影响

已有对浅海低频水声场的讨论多是关注声能量在水体中的分布特性,对水体下沉积层、基底中低频声传播的同步研究相对较少。本文基于波动方程,在柱坐标系下推导了一种浅海水体/海底统一波导下低频声能流的计算模型,在此基础上结合具体仿真算例与波动理论阐述了不同沉积层声学参数对声场能量分布的影响规律及机理。仿真结果表明,在沉积层纵波声速>水中声速>沉积层横波声速的前提下,沉积层中密度与纵波声速数值越大,声能量越趋于保留在水体中而不向海底泄漏,横波声速的影响正好相反;沉积层厚度增加到一定量后,基底对流体层中声传播的影响可近似忽略不计。      

Sound wave scattering by a spherical scatterer located near an ice surface

An echo signal is simulated, which is reflected from a spherical scatterer located near an ice surface. The homogeneous water medium in which the scatterer is located is assumed semi-infinite. For the scattering coefficients of the sphere, asymptotic formulas are obtained by the saddle point method, which can be used for sufficiently large distances between the source emitting a spherical wave and the scatterer. For the occurring branch cut integrals using the steepest descent method, asymptotic expressions are also obtained. Numerical results are obtained for an acoustically rigid sphere and an ice sphere. The density of the ice medium and speed of longitudinal waves in it coincide with the analogous parameters of the ice cover. In a wide frequency range of 8–12 kHz, echo signals are compared that have been calculated for two models of media: a water half-space bordering an ice half-space and an ice-covered homogeneous waveguide with a fluid bottom under the assumption that the source placed in the water layer is directional. It is shown that in a large distance interval between the source and the spherical scatterer, the half-space model sufficiently accurately describes the echo signal while substantially reducing calculation time (by approximately a factor of 10 for the waveguide with a depth of 200 m and a sandy bottom considered in the paper).     

Effects of sediment acoustic characteristics on low-frequency acoustic field distribution in shallow water

Most of the published literatures on low-frequency underwater sound propagation are focused on the sound propagation features in the water column,while studies on sound propagation features in the sediment layer or the semi-infinite basement are rare.In this paper,based on the wave equation,a computational model for sound energy flux in the sediment layer and the basement as well as in the water column is proposed under a cylindrical coordinate system.On this basis,the effects of various sediment acoustic parameters on the sound energy distribution and the corresponding mechanisms are elaborated through numerical examples and acoustic theory.Simulation results reveal that,in a situation where sediment P-wave speed>water sound speed> sediment S-wave speed,the greater the values of density and P-wave speed in sediment,the more likely it is that the sound energy remains in the water column without leaking to the sea floor.Conversely,the influence of the variation of S-wave speed is reversed.Basement influence on the sound propagation in the fluid layer is approximately negligible if the sediment layer is sufficiently thick.     

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.     

Effect of the sound velocity stratification in the bottom sediment layer on the diffraction focusing of an acoustic field in a shallow sea

It is shown that an increase in the sound velocity gradient in the bottom sediment layer results in the formation of additional zones of diffraction focusing of the acoustic field generated by a vertical line array in the water layer. It is also established that, depending on the sound velocity step at the upper boundary of the sediment layer, the diffraction focusing of the acoustic field in the main focusing zones can be enhanced or reduced.     

Sound attenuation on an ocean shelf at short ranges from a source in the presence of surface waves

The effect of surface roughness on the attenuation of low-frequency acoustic waves on a shallow ocean shelf is analyzed using numerical simulation. We focus here on transmission loss during propagation at short (less than 50 water layer depths) ranges from the sound source. The effect is considered both for a soft and hard bottom, when the sound velocity in the bottom is, respectively, lower or higher than the sound velocity in seawater. It is shown that to correctly predict losses at a short range in the presence of a rough upper boundary, it is necessary to take into account the interaction of both propagation and leaky modes. In the case of a hard bottom compared to a low-velocity one, the effect of surface roughness on propagation turned out to be the most pronounced.     

一种基于单水听器的浅海水下声源被动测距方法

针对浅海波导中宽带脉冲声源的被动测距问题,本文在群延迟理论的基础上,与warping变换处理相结合,提出了一种适用于浅海波导中宽带声源的单水听器被动测距方法.利用warping变换可以实现对脉冲声源接收信号各阶简正波的分离提取,对分离后的简正波进行时频分析处理可以得到各阶简正波到达时刻和频率之间的关系,即各阶简正波的频散曲线,从而得到任意两阶简正波到达接收水听器的时延差.海底相移参数P是描述海底地声参数的一个重要参量,包含了海底地声参数信息,在海底环境参数未知而P已知的情况下,利用P和简正波水平波数之间的关系可以求得任意两阶简正波的?S_(g,mn)(群慢差).根据群延迟理论,利用得到的任意两阶简正波的时延和?S_(g,mn)可实现利用单水听器对水下声源进行被动测距.本文提出的测距方法测量简单、计算方便,具有较强的实用意义.数值仿真和海上实验数据处理结果的测距误差都在10%以内,证明了该方法的有效性.     

Features of the energy structure of acoustic fields in the ocean with two-dimensional random inhomogeneities

The paper is devoted to the analytic study and numerical simulation of mid-frequency acoustic signal propagation in a two-dimensional inhomogeneous random shallow-water medium. The study was carried out by the cross section method (local modes). We present original theoretical estimates for the behavior of the average acoustic field intensity and show that at different distances, the features of propagation loss behavior are determined by the intensity of fluctuations and their horizontal scale and depend on the initial regular parameters, such as the emission frequency and size of sound losses in the bottom. We establish analytically that for the considered waveguide and sound frequency parameters, mode coupling effect has a local character and weakly influences the statistics. We establish that the specific form of the spatial spectrum of sound velocity inhomogeneities for the statistical patterns of the field intensity is insignificant during observations in the range of shallow-water distances of practical interest.     

Surface-to-volume wave conversion in shallow water with a gently sloping bottom

Marine sediments support seismoacoustic surface waves, which can propagate along the seafloor, in deep and shallow water, and even onshore. Because of the strong attenuation of compressional and especially shear waves in the sediments, the surface waves can significantly contribute to the acoustic field far from the shore only through their coupling with volume waves in the water. We theoretically study the excitation of acoustic normal modes by seismoacoustic surface waves in a shallow-water waveguide with a sloping bottom consisting of unconsolidated marine sediments. It is found that the coupling primarily occurs in the vicinity of a modal cutoff. The effects of geoacoustic parameters and stratification of soft marine sediments on the efficiency of surface-to-volume conversion are investigated. Published in Russian in Akusticheskiĭ Zhurnal, 2007, Vol. 53, No. 6, pp. 809–816. The article was translated by the author.     

The possibility of reconstruction of two-dimensional random inhomogeneities in a shallow sea by frequency shifts of the spatial interference structure of the sound field

Fluctuations of frequency shifts of the spatial interference structure of the sound field in the oceanic waveguide, caused by a two-dimensional field of a random perturbation, are described. Various schemes of observation point spacing are considered. The possibility is shown to reconstruct the spatial spectrum of waveguide perturbations by measuring the spatial spectrum of the frequency shift of the interference pattern. The results of the theoretical treatment are illustrated by the examples of background internal waves and bottom roughness. The sensitivity of monitoring based on measurements of frequency shifts of the interference structure of the sound field is estimated. For medium perturbations by background internal waves, the fluctuations of liquid layer vibrations, sound speed, and temperature, which are minimum detectable by frequency shifts of the interference pattern, were estimated.     

匹配垂直质点振速的浅海地声参数反演

海底声学参数对海洋波导中的声场特性研究和相关应用具有重要意义。针对一次夏季黄海声传播实验,分析了浅海负跃层环境下垂直质点振速的传播特性和简正波结构,说明当声源和接收器均位于负跃层下时,除海底附近外的大部分深度上垂直质点振速能量较高,且与声压相比,号数高的简正波对垂直质点振速的贡献更大,利用垂直质点振速进行匹配场反演能获得更高的海底参数敏感性。分析了海底吸收系数对垂直质点振速匹配场反演的影响,结果表明只有当进行匹配场反演时设置的海底吸收系数接近真实值时,才能获得准确的海底声速、密度和海深反演结果。利用实验中矢量水听器获取的垂直质点振速信号进行匹配场反演,将海底吸收系数在变化范围内取不同值对海底声速、密度和平均海深进行全局搜索,根据代价函数值最大确定了海底声速、密度及平均海深的反演结果,并利用不同距离上的声压传播损失反演出不同频率下的海底吸收系数。根据反演得到的海底声学参数计算声压传播损失,与实验中声压水听器测量结果符合较好。     

利用简正波频散特征映射方法进行海洋环境参数反演

浅海波导中传播的脉冲信号可以通过频散特征映射方法(warping变换)进行简正波的有效分离,这使得频散曲线的获取更加容易和准确.利用简正波频散特征映射方法分析2010年南海声层析实验中单个阵元接收的声学信号,有效分离出各号简正波,并进一步通过Wigner-Ville分布处理提取出波导中各号简正波的频散曲线.将提取的频散曲线与理论计算到达时差进行匹配,实现了利用单水听器信号的海洋环境参数反演.实验结果表明:将简正波分离与Wigner-Ville分布相结合可有效消除Wigner-Ville分布在估计多分量信号时的交叉干扰,提取的频散特征可很好地用于同时反演海水和海底声速剖面。为评价反演结果的有效性,对反演结果进行了后验概率分析,并将一小时内的海水声速剖面反演结果与温度链实测值作对比,二者符合较好.