采用小生境遗传算法反演浅海声速剖面研究*

针对浅海声速剖面反演问题,采用小生境遗传算法,结合声线搜索的最快本征声线匹配反演,实现浅海负跃层条件下的声速剖面估计。利用经验正交函数对声速剖面的多参数不确定性降维,依据声场计算模型获取的最快特征声线传播时延与观测声传播时长进行匹配,采用小生境遗传优化算法,获取最优经验正交函数估计,实现声速剖面反演。按上述方法反演处理浅海声传播实验数据,结果表明,该方法针能够有效反演浅海声速剖面,并且显著优于传统遗传算法反演结果。     

南海北部负跃层环境下海底参数声学反演

利用2015年南海北部声学实验的实验数据,对海底声学参数进行反演。在综合分析声速起伏及海底模型对海底反演影响的基础上,通过选择等效的海水声速剖面和海底模型,改进了多参量联合地声反演方法,使得其不仅能解决反演中的多值性问题,还能适用于负跃层起伏环境下的声学反演。即当用匹配场反演海底声速和用传播损失反演海底衰减时,如果温跃层内有内波等随机起伏,可使用传播路径上平均声速剖面和单层等效海底进行声场计算。反演得到的海底声速和密度结果与海底采样测量符合较好,拟合给出了海底衰减系数随频率的非线性经验关系式。反演结果可为南海北部声传播规律研究与应用提供海底参数。      

Geoacoustic inversion for bottom parameters in a thermocline environment in the northern area of the South China Sea

Bottom acoustic parameters have important influence on the application of underwater acoustic propagation and source location.The acoustic parameters of the seabed in the northern of the South China Sea(SCS) were inversed using the experiment data from an acoustic experiment in 2015.Based on the comprehensive analysis of the influence of the sound speed fluctuation and the geoacoustic model on seabed inversion,the multi-parameter hybrid acoustic inversion scheme is improved by selecting the equivalent mean sound speed profile(SSP) and half-infinite liquid bottom model to save the inversion dimensions in the matched field processing(MFP) inversion.The inverted bottom sound speed and density are in good agreement with the core sampling measurements.The nonlinear empirical relationship of the attenuation coefficient with frequency is given out.The inversion results are meaningful to the sound propagation research and application in the northern area of the SCS.     

一种地声参数的联合反演方法

根据地声参数对不同声场物理量影响不同,提出了一种利用简正波频散特征结合声传播损失反演地声参数的联合反演方法。首先,考虑到简正波的频散特性(群速度)对海底的密度和声速较为敏感,而对海底吸收系数不敏感,利用自适应时频分析方法,获得不同频率不同号数简正波的到达时间差,以此作为代价函数,采用全局优化算法,反演出海底密度和海底声速的分层结构,并用概率统计的方法评价反演结果的有效性。反演出海底密度和声速后,利用实验船辐射噪声得到随距离连续的声传播损失来反演出海底吸收系数。最后,把反演的参数很好的用于声源匹配定位验证了反演结果的有效性。      

Tracking of time-evolving sound speed profiles with an auto-regressive state-space model

An approach for time-evolving sound speed profiles tracking in shallow water is discussed.The inversion of time-evolving sound speed profiles is modeled as a state-space estimation problem,which includes a state equation for predicting the time-evolving sound speed profile and a measurement equation for incorporating local acoustic measurements.In the paper,auto-regression(AR) method is introduced to obtain a high-order AR evolution model of the sound speed field time variations,and the ensemble Kalman filter is utilized to track the sound speed field.To validate the approach,the accuracy in sound speed estimation is analyzed via a numerical implementation using the ASIAEX experimental environment and the sound velocity measurement data.Compared with traditional approaches based on the state evolution represented as a random walk,simulation results show the proposed AR method can effectively reduce the tracking errors of sound speed,and still keep good tracking performance at low signal-to-noise ratios.     

Mid-frequency geoacoustic inversion using bottom loss data from the Shallow Water 2006 Experiment

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems many of which operate at mid-frequencies (1-10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data between 2 and 5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters density, compressional wave speed, and attenuation, was used for inversion by fitting the model to data in the least-square sense. Inverted sediment sound speed and attenuation were compared with direct measurements and with inversion results using different techniques carried out in SW06. Inverted results of the present work are consistent with other measurements, considering the known spatial variability in this area. The observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.     

一种基于简正波模态频散的远距离宽带海底参数反演方法

在浅海环境中, 海底环境参数对声传播有着重要的影响. 由于利用单个宽带声源进行海底参数反演时, 随着距离的增大, 误差变大, 本文提出利用warping变换对在浅海波导中传播的, 不同距离上的两个宽带爆炸声源进行简正波的有效分离, 实现了宽带爆炸声源的远距离海底参数反演. 采用全局寻优遗传算法对提取出的模态频散到达时间差与理论计算的模态频散到达时间差进行匹配处理, 并结合随距离连续变化的声传播损失, 实现了利用单水听器进行海底参数的反演. 实验结果表明: 运用反演出的海底参数提取模态频散时间差和实测数据提取出的模态频散时间差吻合得较好; 而通过传播损失反演得到的海底衰减系数与频率呈指数关系. 最后, 对反演结果进行了后验概率分析, 并将本组爆炸声源的反演结果用于另一组不同距离上爆炸声源时仍然有效, 来评价反演结果的有效性.     

利用海底反射信号进行地声参数反演的方法

针对现有反演方法的缺点,提出了一种基于海底反射信号的地声参数高分辨反演方法.它利用短距离声源在不同深度上发射宽带线性调频信号,采用垂直阵进行接收,首先通过匹配滤波方法提取多径到达信息,然后利用海底反射损失曲线,反演海底表层的声速和密度,最后利用浅底层反射信号估计沉积层参数.由于海水中直达波受到内波的强烈影响,选择海底表面反射作为参考,用以可靠地计算浅底层反射的相对到达时间和幅度,从而估计出沉积层的厚度、速度和衰减系数.通过海上实验,验证了利用浅底层反射信号反演参数的有效性. 关键词： 海底参数 反演 浅底层反射信号     

深海近距离声场频率-距离干涉结构反演海底声学参数

地声反演一般存在多解性的问题,不易有效分离沉积层声速与厚度,本文提出了一种当声源与接收水听器分别位于海面与海底附近时匹配声场干涉周期反演深海沉积层声速与厚度的方法。该声场周期结构由直达波与海底沉积层反射回波干涉形成,其干涉周期受沉积层声速和厚度影响,通过匹配实测与理论计算的声场干涉周期可有效反演沉积层声速和厚度。开展了深海实验验证,利用水听器接收到的近距离船舶噪声的声场干涉条纹,反演得到的实验海域海底沉积层声速与该海域海底表层采样平均值差小于20 m/s。数值仿真与实验结果表明该方法可以有效反演沉积层的声速与厚度。      

Sound speed profile inversion using a horizontal line array in shallow water

It is better to use a simple configuration to enhance the applicability of ocean environment inversion in shallow water.A matched-field inversion method based on a horizontal line array(HLA)is used to retrieve the variation of sound speed profile.The performance of the inversion method is verified in the South China Sea in June,2010.An HLA laid at bottom was used to receive signals from a bottom-mounted transducer.Inverted mean sound speed profiles from 9-hour long acoustic signals are in good agreement with measurements from two temperature chains at the sites of the source and receiver.The results show that an HLA can be used to monitor the variability of shallow-water sound speed profile.     

Tomographic inversion of measured cross-correlation functions of ocean noise in shallow water using ray theory

Based on experimental data obtained in 2012 in the Florida Strait, we study the feasibility of employing ray tomography to retrieve sound speed and flow velocity profiles from measured noise cross-correlation functions. We describe the results of numerical experiments that characterize the inversion errors resulting from peculiarities of the ray structure in shallow water, difficulties in unambiguous identification of ray arrivals, and a decrease in accuracy of ray theory at low frequencies. We show that under conditions of low-mode sound propagation, the use of the classical ray tomography scheme can yield only a rough estimate of the sound speed profile, but it allows approximate reconstruction of the current velocity profile. Application of passive ray tomography to the experimental data yields the current velocity profile in the Straits of Florida, which agrees with independent measurements within the inversion error limit.     

Geoacoustic inversion for fine-grained sediments

Fine-grained water saturated sediments like silty clays have the curious property that the speed of sound through its bulk medium is lower than that of the interstitial pore fluid. When a fine-grained sediment is at the water-sediment interface, classical theory predicts that there is an angle at which the reflection coefficient is zero, and there is total transmission of sound into the seabed. This angle is called the angle of intromission and has been directly observed at the seafloor only rarely. Data from a new measurement technique show this phenomenon with remarkable clarity. The presence of the angle of intromission creates an opportunity for a direct (i.e., without search) inversion for the sediment sound speed and density. Though acoustic techniques generally do not estimate sediment density very precisely, this technique is quite sensitive to density. Geoacoustic inversion results from reflection measurements in the Straits of Sicily compare very favorably with independent "ground truth" data indicating that the method is robust.     

利用传播损失反演海底单参数

根据地声反演复杂性随着待反演参数的个数减少不断降低的原理,提出一种利用传播损失反演海底单个参数的方法。通过对海底声阻抗的推导,利用声速、密度和衰减系数拟合出海底反射损失对掠射角的斜率F。基于简正波理论推导了用F描述声场的公式,并据此设计出对传播损失数据进行最小二乘法处理的反演方法。得益于将待反演参数减少至一个,该方法只需单个水听器,避免了复杂测量及多维寻优。对东海实验数据进行了实际反演,并介绍了利用反演结果F进行传播损失预报和海底性质估计的步骤。所获结果与多参数混合反演方法及实测真值进行比较,其一致性验证了方法的有效性。      

Optimized constraints for the linearized geoacoustic inverse problem

A geoacoustic inversion scheme to estimate the depth-dependent sound speed characteristics of the shallow-water waveguide is presented. The approach is based on the linearized perturbative technique developed by Rajan et al. [J. Acoust. Soc. Am. 82, 998-1017 (1987)]. This method is applied by assuming a background starting model for the environment that includes both the water column and the seabed. Typically, the water column properties are assumed to be known and held fixed in the inversion. Successful application of the perturbative inverse technique lies in handling issues of stability and uniqueness associated with solving a discrete ill-posed problem. Conventionally, such problems are regularized, a procedure which results in a smooth solution. Past applications of this inverse technique have been restricted to cases for which the water column sound speed profile was known and sound speed in the seabed could be approximated by a smooth profile. In this work, constraints that are better suited to specific aspects of the geoacoustic inverse problem are applied. These techniques expand on the original application of the perturbative inverse technique by including the water column sound speed profile in the solution and by allowing for discontinuities in the seabed sound speed profile.     

Passive geoacoustic inversion with a single hydrophone using broadband ship noise

An inversion scheme is proposed, relying upon the inversion of the noise of a moving ship measured on a single distant hydrophone. The spectrogram of the measurements exhibits striations which depend on waveguide parameters. The periodic behavior of striations versus range are used to estimate the differences of radial wavenumber between couples of propagative modes at a given frequency. These wavenumber differences are stacked for several frequencies to form the relative dispersion curves. Such relative dispersion curves can be synthesized using a propagation model feeded with a bottom geoacoustic model. Inversion is performed by looking for the bottom properties that optimize the fit between measured and predicted relative dispersion curves. The inversion scheme is tested on simulated data. The conclusions are twofold: (1) a minimum 6 dB signal to noise ratio is required to obtained an unbiased estimate of compressional sound speed in the bottom with a 3 m s(-1) standard deviation; however, even with low signal to noise ratio, the estimation error remains bounded and (2) in the case of a multi-layer bottom, the scheme produces a single depth-average compressional sound speed. The inversion scheme is applied on experimental data. The results are fully consistent with a core sample measured around the receiving hydrophone.     

利用气枪声源数据的地声参数反演*

本文通过对东海采集的气枪声源数据进行处理,基于地声参数对不同声场观测量的敏感性差异,利用分步反演的策略进行分析。首先选择距离接收阵七个不同的距离点数据,通过warping变换准确提取第三阶至第八阶简正波的频散曲线;利用海底声速对频散曲线敏感的特性来反演声速;由Hamilton经验公式求得海底密度;通过传播损失拟合获得海底衰减。不同距离点数据反演的海底声速与密度一致性较高。实验提取的频散曲线和和反演参数仿真结果、实验获取的传播损失与反演参数获得传播损失均拟合较好。     

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.     

浅海复杂环境下等效声速剖面的构建方法*

针对浅海复杂环境声速剖面水平变化情况下的声传播损失预报及目标定位问题,提出了一种基于遗传算法的等效声速剖面重构算法。首先,将声速剖面进行时间和空间上的分解,从而将声速剖面抽象为对声速剖面前三阶正交函数系数的反演;其次,利用遗传算法,以先验声速剖面集为基础,进行参数反演。仿真结果表明,在浅海复杂条件下,传播损失预报受声速剖面及海洋参数的影响,不能直接运用接收或发射位置处的声速剖面进行传播损失预报,否则会对预报结果造成误差。通过构建具有声传播累积效应的等效声速剖面可以提高匹配场定位精度,完成目标定位,且在构建等效声速剖面时,接收位置处即本地声速剖面所占权重较大。     

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.