An improved algorithm based on equivalent sound velocity profile method at large incident angle

With the development of ultra-wide coverage technology,multibeam echo-sounder(MBES)system has put forward higher requirements for localization accuracy and computational efficiency of ray tracing method.The classical equivalent sound speed profile(ESSP)method replaces the measured sound velocity profile(SVP)with a simple constant gradient SVP,reducing the computational workload of beam positioning.However,in deep-sea environment,the depth measurement error of this method rapidly increases from the central beam to the edge beam.By analyzing the positioning error of the ESSP method at edge beam,it is discovered that the positioning error increases monotonically with the incident angle,and the relationship between them could be expressed by polynomial function.Therefore,an error correction algorithm based on polynomial fitting is obtained.The simulation experiment conducted on an inclined seafloor shows that the proposed algorithm exhibits comparable efficiency to the original ESSP method,while significantly improving bathymetry accuracy by nearly eight times in the edge beam.

Sound speed profiles in high spatiotemporal resolution using multigrid three-dimensional variational method:a coastal experiment off northern Shandong Peninsula

It is essential to ac quire sound speed profiles(SSPs)in high-precision spatiotemporal resolution for undersea acoustic activities.However,conventional observation methods cannot obtain high-resolution SSPs.Besides,S SPs are complex and changeable in time and space,especially in coastal areas.We proposed a new space-time multigrid three-dimensional variational method with weak constraint term(referred to as STC-MG3DVar)to construct high-precision spatiotemporal resolution SSPs in coastal areas,in which sound velocity is defined as the analytical variable,and the Chen-Millero sound velocity empirical formula is introduced as a weak constraint term into the cost function of the STC-MG3DVar.The spatiotemporal correlation of sound velocity observations is taken into account in the STC-MG3DVar method,and the multi-scale information of sound velocity observations from long waves to short waves can be successively extracted.The weak constraint term can optimize sound velocity by the physical relationship between sound velocity and temperature-salinity to obtain more reasonable and accurate SSPs.To verify the accuracy of the STC-MG3DVar,SSPs observations and CTD observations(temperature observations,salinity observations)are obtained from field experiments in the northern coastal area of the Shandong Peninsula.The average root mean square error(RMSE)of the STC-MG3DVar-constructed SSPs is 0.132 m/s,and the STC-MG3DVar method can improve the SSPs construction accuracy over the space-time multigrid 3DVar without weak constraint term(ST-MG3DVar)by 10.14%and over the spatial multigrid 3DVar with weak constraint term(SC-MG3DVar)by 44.19%.With the advantage of the constraint term and the spatiotemporal correlation information,the proposed STC-MG3DVar method works better than the ST-MG3DVar and the SCMG3DVar in constructing high-precision spatiotemporal re solution SSPs.

Matched-field inversion of sound speed profile in shallow water using a parallel genetic algorithm

A sound speed profile plays an important role in shallow water sound propagation.Concurrent with in-situ measurements,many inversion methods,such as matched-field inversion,have been put forward to invert the sound speed profile from acoustic signals.However,the time cost of matched-field inversion may be very high in replica field calculations.We studied the feasibility and robustness of an acoustic tomography scheme with matched-field processing in shallow water,and described the sound speed profile by empirical orthogonal functions.We analyzed the acoustic signals from a vertical line array in ASIAEX2001 in the East China Sea to invert sound speed profiles with estimated empirical orthogonal functions and a parallel genetic algorithm to speed up the inversion.The results show that the inverted sound speed profiles are in good agreement with conductivity-temperature-depth measurements.Moreover,a posteriori probability analysis is carried out to verify the inversion results.

Sound propagation in inhomogeneous waveguides with sound-speed profiles using the multimodal admittance method

The multimodal admittance method and its improvement are presented to deal with various aspects in underwater acoustics,mostly for the sound propagation in inhomogeneous waveguides with sound-speed profiles,arbitrary-shaped liquid-like scatterers,and range-dependent environments.In all cases,the propagation problem governed by the Helmholtz equation is transformed into initial value problems of two coupled first-order evolution equations with respect to the modal components of field quantities(sound pressure and its derivative),by projecting the Helmholtz equation on a constructed orthogonal and complete local basis.The admittance matrix,which is the modal representation of Direchlet-to-Neumann operator,is introduced to compute the first-order evolution equations with no numerical instability caused by evanescent modes.The fourth-order Magnus scheme is used for the numerical integration of differential equations in the numerical implementation.The numerical experiments of sound field in underwater inhomogeneous waveguides generated by point sources are performed.Besides,the numerical results computed by simulation software COMSOL Multiphysics are given to validate the correction of the multimodal admittance method.It is shown that the multimodal admittance method is an efficient and stable numerical method to solve the wave propagation problem in inhomogeneous underwater waveguides with sound-speed profiles,liquid-like scatterers,and range-dependent environments.The extension of the method to more complicated waveguides such as horizontally stratified waveguides is available.

Reconstructing Sound Speed Profiles with Sea Surface Data

Ocean sound speed profile(SSP) is the key factor affecting acoustic propagation. The acquisition of SSPsin real time with high precision is meaningful for underwater activities. By means of the remote sensing method, thesea surface data could be obtained in near-real time. Typically, the subsurface fields are correlated with the sea surfaceparameters. Thus, the SSPs could be obtained by means of satellite remote sensing. In this paper, the history as wellas the current research over the reconstruction of subsurface fields by means of sea surface data is introduced. Thentwo methods to reconstruct the SSPs with sea surface data, including the linear regression method using the empiricalorthogonal function, and the self-organizing method based on the big data theory, are described in detail in the paper.

Comprehensive Study of Inversion Methods for Sound Speed Profiles in the South China Sea

Traditional acquisition method of sound speed profiles using hydro-acoustic instruments is accurate but time-consuming and costly.To overcome this problem,some inversion methods have been developed over the last few decades.In this study,a comprehensive comparison of two inversion methods–the acoustic inversion method(AIM)and the satellite observation reconstruction method(SOR)–is presented.For AIM,the sound speed profile is first parameterized by the empirical orthogonal function(EOF)and the optimal parameters are searched by simulated annealing algorithm with respect to the cross-correlation function of the receiving signal and the simulation signal.For SOR,remotely sensed data are used to construct sound speed profiles.An experiment was conducted in the northeast of the South China Sea to verify the two methods.Both methods can obtain sound speed profiles quickly and cheaply.Compared with the sound speed profiles obtained by a conductivity-temperature-depth(CTD)instrument,the root-meansquare-error(RMSE)of AIM is 0.55 m s^(−1) and that of SOR is 1.71 m s^(−1).It is clear that AIM provides better inversion performance than SOR.Another primary benefit of AIM is that this method has no limitation to the inversion depth.The simulation results of sound propagation in regard to the inversed sound speed profiles show that the transmission losses of AIM and CTD are consistent and that of SOR is adversely affected by the inversion error of the sound speed and the inversion depth.But SOR has particular advantages in the inversion coverage.Together,all of these advantages make the AIM particularly valuable in practice.

Rapid environmental assessment in the South China Sea:Improved inversion of sound speed profile using remote sensing data

Complex perturbations in the profile and the sparsity of samples often limit the validity of rapid environmental assessment(REA)in the South China Sea(SCS).In this paper,the remote sensing data were used to estimate sound speed profile(SSP)with the self-organizing map(SOM)method in the SCS.First,the consistency of the empirical orthogonal functions was examined by using k-means clustering.The clustering results indicated that SSPs in the SCS have a similar perturbation nature,which means the inverted grid could be expanded to the entire SCS to deal with the problem of sparsity of the samples without statistical improbability.Second,a machine learning method was proposed that took advantage of the topological structure of SOM to significantly improve their accuracy.Validation revealed promising results,with a mean reconstruction error of 1.26 m/s,which is 1.16 m/s smaller than the traditional single empirical orthogonal function regression(sEOF-r)method.By violating the constraints of linear inversion,the topological structure of the SOM method showed a smaller error and better robustness in the SSP estimation.The improvements to enhance the accuracy and robustness of REA in the SCS were offered.These results suggested a potential utilization of REA in the SCS based on satellite data and provided a new approach for SSP estimation derived from sea surface data.

Basis functions for shallow-water temperature profiles based on the internal-wave eigenmodes

The shallow-water temperature profile is typically parameterized using a few empirical orthogonal function(EOF)coefficients.However,when the experimental area is poorly known or highly variable,the adaptability of the EOFs will be significantly reduced.In this study,a new set of basis functions,generated by combining the internal-wave eigenmodes with the average temperature gradient,is developed for characterizing the temperature perturbations.Temperature profiles recorded by a thermistor chain in the South China Sea in 2015 are processed and analyzed.Compared to the EOFs,the new set of basis functions has higher reconstruction accuracy and adaptability;it is also more stable in ocean regions that have internal waves.

Acoustic localization with multi-layer isogradient sound speed profile using TDOA and FDOA

In the underwater medium,the speed of sound varies with water depth,temperature,and salinity.The inhomogeneity of water leads to bending of sound rays,making the existing localization algorithms based on straightline propagation less precise.To realize high-precision node positioning in underwater acoustic sensor networks(UASNs),a multi-layer isogradient sound speed profile(SSP)model is developed using the linear segmentation approximation approach.Then,the sound ray tracking problem is converted into a polynomial root-searching problem.Based on the derived gradient of the signal’s Doppler shift at the sensor node,a novel underwater node localization algorithm is proposed using both the time difference of arrival(TDOA)and frequency difference of arrival(FDOA).Simulations are implemented to illustrate the effectiveness of the proposed algorithm.Compared with the traditional straight-line propagation method,the proposed algorithm can effectively handle the sound ray bending phenomenon.Estimation accuracy with different SSP modeling errors is also investigated.Overall,accurate and reliable node localization can be achieved.

基于PIES观测东沙群岛附近声传播变化

本研究旨在将PIES观测数据应用于海洋水声环境的监测。利用一次南海PIES观测数据,反演得到温盐剖面,进而计算声场信息。结合观测期间的暖涡及台风事件对反演及计算结果进行了分析验证。结果表明,PIES能够在长期连续观测中较为敏感地捕获上层水体及表面声道的变化特征,为水声通信、探测及潜航路径规划等提供参考,在水声研究及海洋军事环境保障中都具有良好的应用前景。

利用低声速沉积层反射特性线性反演地声参数

为提高反演效率和降低地声参数耦合,提出一种线性反演低声速沉积层地声参数的方法。从类线性声速剖面的两层海底平面波反射系数出发,分析了全透射角及全透射角下最大反射损失、干涉条纹等反射特性与各相关地声参数的关系,并构建了相应的代价函数。结合敏感性分析和分步反演策略,分步反演了低声速沉积层地声参数。该方法适用于低声速沉积层声速梯度恒定的情况。利用新英格兰泥地实验部分海底反射损失数据进行了实验验证。实验反演结果与相关跨维反演结果基本一致,反演误差分别为声速比0.1%,密度比0.3%,吸收系数14.6%,声速梯度为15.8%。反演效果显示,该方法一定程度上规避了部分地声参数间的耦合,且可给出决定类线性声速剖面形状的幂次项的最佳估计。

实测时变深海声速剖面对声线到达结构影响的分析

在对水下目标进行探测时,往往需要先对目标所在海域的声场进行测量,从而极大地提高了探测目标的成本。该文探讨了同一地理位置处,使用公开声速数据和实测声速数据、不同时间段测量的实测声速数据场景下,声线到达结构的差异,从而揭示时变深海声速剖面变化对声波传播的影响,以期优化声学测量作业流程,降低实验成本。计算结果显示,当存在直达声线时,临近几日测量的声速剖面数据可满足声传播距离测量的要求,否则,需要对声速剖面重新测定。

基于WOA23的日本海声道分布特征分析

本文基于WOA23温盐剖面数据集,利用声速经验公式,计算日本海的声速剖面,根据声速剖面的特点及声速梯度,诊断提取日本海表面声道及深海声道特征信息,研究表面声道及深海声道的区域分布情况和季节变化特征。结果表明:日本海的表面声道受季节变化影响大且具有明显的区域性分布特征。春夏季节,整个海域几乎无表面声道,秋冬季节,表面声道大幅生成,主要分布在东朝鲜湾和日本海东部边界附近海区,冬季与秋季相比,南北两端的表面声道分布面积减少,但东朝鲜湾以及40°N-50°N之间海区表面声道的分布面积进一步扩大。秋季,表面声道厚度基本小于40 m,冬季,表面声道厚度整体增加,特别是在44°N-50°N之间的海区,大部分表面声道厚度超过100m;表面声道梯度季节变化幅度相对较小,除了秋季鞑靼海峡附近的表面声道梯度大于0.2/s,大部分表面声道的梯度小于0.05/s。深海声道与表面声道相比,受季节变化影响较小,声道效应更加稳定,声道轴深度基本在450m以浅,声道轴声速约为1456~1468m/s,且整体表现呈自东南向西北递减之势。

电测深联合剖面法在高铁路基岩溶探测中的应用

采用电测深联合剖面法,对新建莱荣高铁路基岩溶进行综合勘探。该方法结合电测深与联合剖面法的特点,通过测量地下岩层的电性分布,从而准确反映岩溶的位置和形态,且具有横向分辨率高和纵向探测深度可计算等显著优点。将电测深低阻凹陷及联合剖面双线交叉点重叠的位置推测为岩溶发育区,适用于类似地质条件下的路基岩溶探测。探测结果显示,DK11+352~DK11+750段电阻率介于240~560Ω·m,电测深联合剖面法岩溶探测准确率为86.7%;DK11+750~DK12+025段电阻率介于320~1000Ω·m,电测深联合剖面法岩溶探测准确率为50%;路基岩溶综合探测结果准确率为75%。研究结果表明,电测深联合剖面法在路基岩溶探测方面具有较高的精度,为高铁路基岩溶探测提供一种较好的解决方案。

An improved positioning model of deep-seafloor datum point at large incidence angle

The inhomogeneous sound speed in seawater causes refraction of sound waves,and the elimination of the refraction effect is essential to the accuracy of underwater acoustic positioning.The raytracing method is an indispensable tool for effectively handling problems.However,this method has a conflict between localization accuracy and computational quantity.The equivalent sound speed profile(ESSP)method uses a simple sound speed profile(SSP)instead of the actual complex SSP,which can improve positioning precision but with residual error.The residual error is especially non-negligible in deep water and at large beam incidence angles.By analyzing the residual error of the ESSP method through a simulation,an empirical formula of error is presented.The data collected in the sailing circle mode(large incidence angle)of the South China Sea are used for verification.The experiments show that compared to the ESSP method,the improved algorithm has higher positioning precision and is more efficient than the ray-tracing method.

基于BP神经网络的非等效声速剖面反演

声速剖面反演基于历史数据获取声速剖面,是海洋环境声层析中的一种方法。传统声速剖面反演法很难获取大面积海域的非等效声速剖面,得出的结果一般都是等效声速剖面。为解决该问题,提出一种基于反向传播(Back Propagation,BP)神经网络的非等效声速剖面反演方法。对比结果表明,该方法能有效反演非等效声速剖面并且反演结果相比传统声速剖面反演具有更高的精度。

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.

A methodology to achieve the basis function for the expansion of sound speed profile

In order to evaluate the reliability and applicability of the Empirical Orthogonal Functions（EOFs）in the acoustic inversion of sound speed profile（SSP）and reduce EOF＇s dependence on the sample data,a methodology is proposed for the achievement of the basis functions for SSP＇s expansion.By analyzing the oceanographic dynamics which is the main cause of the SSP＇s variation,the basis functions are obtained naming the Hydrodynamic Normal Modes（HNMs）.The HNM basis functions are almost the same as those derived from the EOF method,while HNMs has less dependence on the amount of the sample data.HNMs method has a physically meaningful interpretation,and it could give out the physical parameters which determine the basis functions for the expansion of SSP,and this makes it possible to analyze and evaluate the trustiness and applicability of EOFs.

基于贝叶斯估计的深海GNSS-A定位精度

本文基于贝叶斯估计采用同时估计海底应答器位置和声速水平梯度的策略处理了水深约3 km的GNSS-A观测数据,分别在单应答器模式、多应答器阵列模式下,分析了航迹几何构型、参数初值以及参数估计策略对海底固定应答器和锚系应答器三维定位精度的影响.结果表明:应答器位置后验不确定度与先验不确定度呈正比关系,其比例系数对于单站模式约为0.1,对于多站模式约为0.05;观测时延残差均方差(RMS)在单站模式下约为0.6 ms,多站模式下约为4.5 ms;圆形测线由于测量时间较短,声速水平梯度变化不大,因此同时估计海底应答器位置和声速水平梯度能够获得较高的定位精度;然而,由于井字形航迹的走航时间较长,声速水平梯度变化较大,该估计策略已不再适用,因而精度最低;圆形测线对参数初值先验精度要求较高;当换能器与GNSS天线的臂长先验精度较高时,对于圆形测线,估计该参数并不能显著提升定位精度.

声速剖面对深海远程声脉冲结构的影响

利用东印度洋和南海海域进行的深海远程声传播实验数据,比较分析了声道轴附近深度发射的声信号在两个海域不同声速剖面结构下的远程传播损失和脉冲时间到达结构。通过对比观测发现,两海域的深海声传播损失特性存在一定的差异,声脉冲时间到达结构差异性显著。首先,在东印度洋实验中观测到潜标垂直阵同一接收距离上,靠近声道轴传播的声能量较大,且声道轴附近声速较小但沿其传播的声信号却最先到达,而偏离声道轴传播的声信号延后到达,在整个接收深度上呈现出声道轴附近接收波形早于其他深度到达的分支结构,这与南海典型深海环境下的脉冲时间到达结构存在显著差异。其次,结合深海声道的参数化数学模型,分析了声速剖面对远程脉冲传播时间到达结构的影响机理,并理论解释了两个海域实验中观测到的脉冲声信号时间到达结构现象,其形成原因在于深海声道中决定声速剖面结构的声道轴系列参数的差异。该研究结果对通信声呐在不同海域深海远程环境下的应用具有一定的参考意义。