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.     

海底沉积物参数对浅海中低频声传播的影响

推导出了考虑海底沉积物中声衰减和横波后浅海中点声源激发声场的积分表达式,并用实轴积分法和快速傅里叶交换（ＦＦＴ）数值计算模拟了爆炸声源在浅海中激发的全波列波形。考察了海底沉积物参数、声源激发主频等对低频声传播的影响。研究表明：在远场接收到的波群为声源主频处的模式波。最后讨论了通过几个不同声源激发主频处的某一模式波的群速度的测量值用最小平方和估计法来反演海底沉积物的纵波速度和横波速度的可能性。     

浅海中沉积层参数对多途到达时差反演声速剖面的影响

本文以简正波模型和声线模型为基础，通过计算机仿真，研究了浅海多途到达时延差结构与沉积层密度、声速、传播损失系数等参数的关系，针对100m左右浅海和600-1300Hz的信号频率，对两种模型仿真结果进行了比较，结果表明，声线模型仿真得到的多途到达时延差结构与简正波模型仿真的结果基本一致，沉积层参数失配对基于声线模型的多途到达时差反演SSP的影响为：反演声速剖面的均方根误差的变化小于1.45m/s，平均误差的变化小于2.60m/s，在所使用的频段内，浅海沉积层参数对多途到达时差反演浅海声速剖面的影响不大，工程应用中可以忽略不计。     

基于大掠射角海底反射特性的深海地声参数反演

海底声学参数的获取对于海洋声学研究有着重要意义.通过推导分层吸收介质下的海底反射系数,理论分析了大掠射角条件下吸收系数对海底反射系数的影响.海底反射系数随频率振荡过程中,将其等于海水-沉积层界面反射系数模时所对应的频点定义为1/4振荡周期频率.在该频率下,沉积层吸收系数与基底地声参数的耦合程度小于其他频点.本文基于大掠射角下的海底反射特性,提出一种深海地声参数分步反演方法.首先,利用相关法提取得到海底反射系数的干涉周期,利用干涉周期反演了沉积层声速和厚度.声速的反演结果结合Hamilton经验公式反演密度.第二步,通过结合基底声速的穷举边界,给出沉积层吸收系数的假设值,利用1/4振荡周期频率下的海底反射系数对基底声速进行一维反演.最后利用半波层频率下的海底反射系数对沉积层吸收系数进行一维反演.大掠射角海底反射特性结合分步反演,实现了基底声速和沉积层吸收系数一定程度的解耦合.实验结果表明,在大掠射角测量条件下,该方法反演的地声参数可有效应用在一定范围内的传播损失预报.     

水平分层低马赫数运动介质中的声传播

针对海流对水声传播的影响,基于高斯波束追踪方法,利用高频近似的低马赫数亥姆霍兹方程,建立了三维低马赫数运动介质下的声传播模型,并将该模型应用于浅海与深海水平分层介质下的声传播问题。仿真结果表明,海流能定性定量地改变声传播模式。浅海环境下,声速梯度大于流速梯度,导致顺逆流声线反转点的差别随着距离增加而增大;此外声线到达结构也会发生较大改变。深海环境下,流速梯度大于声速梯度,顺流方向表面声道消失,使得表层顺逆流传播损失相差10 dB以上,同时水平分层环境中也会产生三维声传播效应。因此在流速梯度与声速梯度相当甚至更大时,海流对声传播的影响不能被忽略。     

水平不变浅海环境随机扰动对声传播的影响

本文主要讨论浅海水平不变波导中的低频声传播问题,为环境适配声纳设计奠定基础。通过仿真分析了海水及海底环境参数对传播损失的影响,根据仿真及实验结果,建立了声速剖面随机扰动及海面、海底起伏条件下的声强分布概率模型,并利用模型中的形状参数α和尺度参数β,提出了局部蒙特卡洛模拟加曲线拟合的声场敏感性分析方法。仿真及实验结果表明声场传播损失服从伽马分布,良好水文条件下10 km距离的声场能量起伏也达到10 d B,随着距离增加,声场敏感性增加。本文所提出方法通过对近场声场的测量和统计实现对远场声场敏感性的预报,与全声场蒙特卡洛模拟相比计算量减小一个数量级。     

南中国海海域存在孤立子内波条件下的声场统计特性

利用南中国海浅海海域低频声传播起伏实验中获取的水文数据,结合二维平流模型重构出声传播路径上的动态声速场,使用蒙特卡洛方法研究了有无孤立子内波经过时声传播路径上的声传播损失统计特性,并与实验结果进行了对比分析。仿真和实验结果表明:当孤立子内波经过声传播路径时,声传播损失起伏剧烈;与"下发上收"相比,"下发下收"情况下传播损失的概率分布更加分散。     

三维浅海环境下孤立子内波对低频声能流的传播影响

针对三维浅海环境下孤立子内波对低频声信号传播特性的影响问题,基于Oxyz坐标系下的三维浅海低频声场有限元计算方法,以声能流为研究对象,仿真分析了内波存在对低频声信号传播特性的影响规律.研究结果表明:受内波影响,在xOz平面,声能流垂直分量的传播偏转角度呈现周期性的起伏规律;随着声源深度的增加,内波对声能流偏转角度的影响...     

舰船辐射噪声贝叶斯方法反演浅海底层结构及地声参数EI北大核心CSCD

针对以舰船辐射噪声为参考声源的浅海海底分层结构及地声参数反演问题,研究了一种基于贝叶斯理论的浅海多层海底地声参数反演方法。反演中以舰船辐射噪声的线谱成分为研究对象,进而采用非线性贝叶斯反演方法反演浅海底层结构、层中声速、声速衰减和密度,并对反演结果的不确定性进行分析。反演结果的最大后验概率估计值和边缘概率分布分别通过拨正模拟退火算法和Metropolis-Hastings采样法在各参数先验区间内计算获得,并根据贝叶斯信息准则确定最佳海底分层结构。海上实验表明:根据该方法反演获得海底分层结构及地声参数,计算得到的声压场与实测舰船辐射噪声传播损失误差不超过10%,反演结果能够准确表征实验海区海底特征。反演结果不确定性分析表明:海底纵波声速、横波声速以及密度的不确定性更小,对声压场变化更加敏感,反演结果更有效、准确。     

浅海环境下数据同化声层析方法研究

讨论了适用于浅海的基于声速局部测量模型、声传播模型及声压场局部测量模型的声学数据同化方法,并给出了具体的执行算法。该算法根据最优化准则把局部声速和声压场的先验测量信息,以及声传播模型进行了有效地融合,提高了海洋环境参数估计的精度,为浅海声层析提供了新思路。同时,利用实验浅海声速测量数据,通过经验正交函数对实际海洋声速剖面进行了估计,并分析了各类噪声对水体声速场及海底声学参数估计精度的影响,验证了该执行算法的有效性。      

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.     

三点弯曲试验 条件下花岗岩声发射横纵波特征及损伤演化*

开展花岗岩三点弯曲试验,利用横纵波两种传感器接收花岗岩破坏过程的声发射信号,对比分析声发射横纵波信号的时频特征,在此基础上,基于声发射横纵波建立损伤变量,探讨花岗岩三点弯曲条件下损伤演化规律。研究结果表明：花岗岩破坏过程声发射横纵波事件率变化趋势较为相似,临近峰值载荷时均加速上升,破坏时达到峰值,但纵波事件率加速点早于横波事件率;声发射横纵波能率变化趋势一致,呈现峰前低水平稳定变化、峰后陡升到峰值的特征。声发射横纵波主频在临近峰值载荷逐渐形成主频条带,但其频率存在差异,纵波主频分布在0~10 kHz、30~50 kHz和100~110 kHz,而横波主频集中分布在0~10 kHz。相比纵波损伤变量,横波损伤变量能够刻画峰后损伤急速发展的过程,能较好地表征花岗岩三点弯曲作用下损伤变化规律。     

Hybrid geoacoustic inversion of broadband Mediterranean Sea data

This paper describes an acoustic experiment (PROSIM'97) carried out to investigate inversion for seabed properties at a site off the west coast of Italy where previous acoustic and geophysical studies have been performed. Acoustic fields were measured at a vertical hydrophone array due to a swept-frequency source towed over weakly range-dependent bathymetry. Based on the known geology, the seabed is modeled as a sediment layer overlying a semi-infinite basement with unknown model parameters consisting of the sediment thickness, sediment and basement sound speeds, source range and depth, water depth at the source and array, and array tilt. A hybrid inversion algorithm is applied to determine the model values that minimize the mismatch with the measured acoustic fields. Multiple data sets are analyzed to examine the consistency of the inversion results. It is found that the low sound speed of the sediment layer, together with a large uncertainty in bathymetry, leads to strong correlations between the water depths and sediment thickness. This precludes reliable estimation of these parameters individually; however, the total depth to the basement can be estimated reliably. In addition, the basement speed and geometric parameters are estimated consistently, and all parameters compare favorably with the geophysical ground-truth information and with previous inversion results.     

The influence of optical properties of shallow shelf seas on temperature structure and acoustic propagation

The penetration of solar radiation below the sea surface is not always taken into account in numerical ocean models, even though its influence on the temperature (and therefore sound speed), structure of the water column can be significant, especially in shallow shelf seas where water turbidity tends to be high. Variations in sound speed structure have a consequent effect on the propagation of sound underwater which, in turn, influences the performance of sonar systems. A double exponential parameterization for the penetration of solar radiation with depth was implemented in an N × 1D turbulence closure model of the UK shelf seas. The model was run along a section through the Celtic Sea Front, with sets of optical extinction coefficients representing different water clarities, for one month to simulate the generation of the front, and the results were compared with a control model run in which all solar radiation was absorbed in the surface layer. Temperature structure and sound propagation were only affected by changes to the optical parameterization on the stratified side of the front, where consequent variations in acoustic propagation loss of up to 10 dB relative to the control were simulated using an acoustic model. Changes in propagation loss were greatest for acoustic sources placed in the stratified water above the thermocline. Similar changes to the optical parameterization made on the well-mixed side of the front had no discernible effect.     

浅海不确定声场的随机多项式展开法研究

为获得求解浅海不确定声场的普适模型,建立了随机多项式展开法与Helmholtz方程的非嵌入式耦合模型,其间运用概率配点法求解多项式系数。针对仅当海水深度不确定时的Pekeris波导、声速剖面和海深均不确定时的Pekeris波导以及下限深度不确定温跃层等几种情形,计算了传播损失概率密度分布。结果表明所建模型对声场计算模型普适性强,计算精度和计算效率高,可用于研究含多个不确定环境参数、声速剖面复杂的浅海环境中声传播的不确定性。      

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.     

Propagation in a waveguide with range-dependent seabed properties

The ocean environment contains features affecting acoustic propagation that vary on a wide range of time and space scales. A significant body of work over recent decades has aimed at understanding the effects of water column spatial and temporal variability on acoustic propagation. Much less is understood about the impact of spatial variability of seabed properties on propagation, which is the focus of this study. Here, a simple, intuitive expression for propagation with range-dependent boundary properties and uniform water depth is derived. It is shown that incoherent range-dependent propagation depends upon the geometric mean of the seabed plane-wave reflection coefficient and the arithmetic mean of the cycle distance. Thus, only the spatial probability distributions (pdfs) of the sediment properties are required. Also, it is shown that the propagation over a range-dependent seabed tends to be controlled by the lossiest, not the hardest, sediments. Thus, range-dependence generally leads to higher propagation loss than would be expected, due for example to lossy sediment patches and/or nulls in the reflection coefficient. In a few instances, propagation over a range-dependent seabed can be calculated using range-independent sediment properties. The theory may be useful for other (non-oceanic) waveguides.     

Acoustic propagation through anisotropic internal wave fields: transmission loss, cross-range coherence, and horizontal refraction

Results of a computer simulation study are presented for acoustic propagation in a shallow water, anisotropic ocean environment. The water column is characterized by random volume fluctuations in the sound speed field that are induced by internal gravity waves, and this variability is superimposed on a dominant summer thermocline. Both the internal wave field and resulting sound speed perturbations are represented in three-dimensional (3D) space and evolve in time. The isopycnal displacements consist of two components: a spatially diffuse, horizontally isotropic component and a spatially localized contribution from an undular bore (i.e., a solitary wave packet or solibore) that exhibits horizontal (azimuthal) anisotropy. An acoustic field is propagated through this waveguide using a 3D parabolic equation code based on differential operators representing wide-angle coverage in elevation and narrow-angle coverage in azimuth. Transmission loss is evaluated both for fixed time snapshots of the environment and as a function of time over an ordered set of snapshots which represent the time-evolving sound speed distribution. Horizontal acoustic coherence, also known as transverse or cross-range coherence, is estimated for horizontally separated points in the direction normal to the source-receiver orientation. Both transmission loss and spatial coherence are computed at acoustic frequencies 200 and 400 Hz for ranges extending to 10 km, a cross-range of 1 km, and a water depth of 68 m. Azimuthal filtering of the propagated field occurs for this environment, with the strongest variations appearing when propagation is parallel to the solitary wave depressions of the thermocline. A large anisotropic degradation in horizontal coherence occurs under the same conditions. Horizontal refraction of the acoustic wave front is responsible for the degradation, as demonstrated by an energy gradient analysis of in-plane and out-of-plane energy transfer. The solitary wave packet is interpreted as a nonstationary oceanographic waveguide within the water column, preferentially funneling acoustic energy between the thermocline depressions.