PAssive COastal TOMography: an Innovative Approach for the Remote Monitoring of Sea Temperature

Abstract. This paper describes a new Ocean Acoustic Tomography (OAT) methodology - a passive tomography - presently in an advanced development phase. This technique has been developed for long-term, extensive, remote monitoring of the seawater temperature spatial distribution, which is estimated from the received noise emitted from ships of opportunity. To test the passive tomographic processor under controlled conditions, the components of the naval noise from different kinds of vessels was analysed and realistic naval noise was simulated. The feasibility of the proposed methodology was confirmed by test-runs on semi-synthetic data; its capability to resolve temperature profiles will be better assessed with the use of real acoustic and environmental data collected during the INTIMATE00 experiment performed in October 2000 in the Atlantic Ocean off the Portuguese coast. An analysis of the space and time variability of the Empirical Orthogonal Function (EOF) decomposition of the sound speed (SSP) in the Mediterranean Sea has been carried out to identify areas where acoustic tomography can be successfully applied. Results from simulations in the South Adriatic Sea, which was identified as a region with a high sound speed variability associated with the seasonal cycle and with the main oceanographic processes, are reported.     

Vertical distribution of the sound speed in the Gulf Stream system

Using archived historical data on the temperature/salinity distribution, this paper describes the structure of the mean seasonal and the actual field of the computed speed of sound in the Gulf Stream region. The formation of acoustic channels of large, medium, and small size is considered, depending on the vertical thermohaline structure of waters. The paper provides statistical characteristics of the sound speed field and acoustic channels of waters interacting in the Gulf Stream system. Translated by Vladimir A. Puchkin.     

基于WOA13数据的海区位置对北大西洋声波导影响分析

应用WOA13季节平均数据和BELLHOP模型,在季节、声源频率等因素确定的情况下,分10 m表面声源和250 m水下声源,分析北大西洋冬季东、西部海区的声波导情况。在给出不同海区位置的声速场和声波导具体信息的基础上,研究其规律:最小声速值和声道轴深度由直布罗陀海峡向外递减扩散,表层声速值和声速梯度由南向北递减,声跃层存在于低纬度海区,混合层在低纬度通常在100 m以内,在高纬度增加至100 m以上。10 m深度表面声源的汇聚区反转深度随纬度增加逐渐减少,西部海区深于东部海区;西部海区的汇聚区跨度大于东部海区,东西部跨度最大值出现在25°N和15°N,传播损失基本一致。250 m水下声源的汇聚区反转深度浅于10 m深度表面声源时,同样是西部海区大于东部海区,汇聚区跨度呈低-高-低规律,东西部跨度最大值出现在35°N和25°N;东部海区25°N以南、西部海区15°N以南,不同接收深度上的传播损失差异较大,以北差异较小。同时简要叙述了声影区对目标探测的影响。     

Large-scale vertical hydroacoustic structure of the North Atlantic and its seasonal variability

A classification based on the number and types of large-scale acoustic waveguides is proposed for the mean seasonal profiles of sound speed propagation. A scheme for North Atlantic zoning, using typical curves of the sound speed vertical distribution, is given. The channel axis's position is shown not to depend on the water mass haline properties, being controlled by the temperature field vertical stratification.Translated by Vladimir A. Puchkin.     

季节因素对大西洋声传播的影响分析

以分析季节对大西洋声传播的影响为研究目的,应用WOA13季节平均数据和Mackenzie声速经验公式,分析了大西洋声道轴和表层声速值的四季分布情况,再利用BELLHOP水声学数值模型,在设定的声源频率1 000 Hz和掠射角15°～-15°情况下,仿真计算选用位置点5 m深度声源的四季声传播情况,研究结果表明:按照实际的季节,大西洋会聚区波导的反转深度,冬季最小,春季增大,夏季最大,秋季再减小.在中低纬度的典型声速剖面下,夏季会聚区跨度最大,秋季和冬季递减,春季最小,第一会聚区的四季跨度差在1 km内.在高纬度的正梯度声速剖面下,夏季声传播距离最远,秋季减小,冬季最近,春季增大,且传播距离的差别较大.各变化规律均以四季循环更替的形式出现.     

Acoustic characteristics of seawater in the area of the coastal discharge front off the Guinean shore

Regularities in the structure and variability of the acoustic characteristics of Guinean shelf waters during the rainy season have been identified as a result of the generalization of the sound speed fields derived from temperature and salinity observations. Three areas with qualitatively different hydroacoustic properties have been revealed, namely, (i) near-shore shallow waters, where a near-surface haline acoustic channel occurs by virtue of intensive seawater freshening; (ii) a discharge front accommodating an intermediate acoustic waveguide in the temperature jump layer; and (iii) off-shore shelf areas displaying a characteristic alteration of the sound speed vertical gradient sign at thec(z) profile.Translated by Vladimir A. Puchkin.     

Seasonal variability of the hydrologic-and-acoustic parameters on the Black Sea shelf off the South Crimea coast

Usingin situ temperature and salinity observations from the Marine Hydrophysical Institute's oceanographic data set, regularities of the formation of structure and seasonal variability of the calculated sound speed field, as well as elements of the latter's vertical stratification in the South Crimea shelf area, have been studied. The ray approximation is applied to consider the influence of seasonal variations of the vertical hydrologic-and-acoustic structure on the propagation of sound in the sea. Translated by Vladimir A. Puchkin.     

菲律宾海的声速剖面结构特征及季节性变化

应用Argo资料研究了菲律宾海的声速剖面结构特征。通过统计分析选取了合理的跃层标准,分析了主跃层、季节性跃层和表面正梯度层的区域性分布及季节性变化。结果表明,菲律宾海主要受赤道流系和北太平洋西边界流系的支配,其环流结构和水团配置对声场结构影响很大;主跃层的经向差异显著,但季节性变化较小,其平均位置由南向北逐渐加深,强度逐渐减弱;季节性跃层的分布及变化主要受混合层的季节性变化以及北部海区冬季温跃层通风过程的影响,夏季较强较厚,冬季较弱较薄;深海声道轴季节性变化较小,南极中层水和北太平洋中层水的温盐差异是其经向分布差异的主要原因。综合考虑海区声速结构区域性和季节性特征,将其归纳为6种典型结构,得出了各类声速剖面的模态特征及垂直结构参数的统计特征值。     

Relationships between the sound speed ratio and physical properties of surface sediments in the South Yellow Sea

Building empirical equations is an effective way to link the acoustic and physical properties of sediments. These equations play an important role in the prediction of sediments sound speeds required in underwater acoustics.Although many empirical equations coupling acoustic and physical properties have been developed over the past few decades, further confirmation of their applicability by obtaining large amounts of data, especially for equations based on in situ acoustic measurement techniques, is required. A sediment acoustic survey in the South Yellow Sea from 2009 to 2010 revealed statistical relationships between the in situ sound speed and sediment physical properties. To improve the comparability of these relationships with existing empirical equations, the present study calculated the ratio of the in situ sediment sound speed to the bottom seawater sound speed, and established the relationships between the sound speed ratio and the mean grain size, density and porosity of the sediment. The sound speed of seawater at in situ measurement stations was calculated using a perennially averaged seawater sound speed map by an interpolation method. Moreover, empirical relations between the index of impedance and the sound speed and the physical properties were established. The results confirmed that the existing empirical equations between the in situ sound speed ratio and the density and porosity have general suitability for application. This study also considered that a multiple-parameter equation coupling the sound speed ratio to both the porosity and the mean grain size may be more useful for predicting the sound speed than an equation coupling the sound speed ratio to the mean grain size.     

Research on the sediment acoustic properties based on a water coupled laboratory measurement system

Abstract

The high-frequency acoustic properties of seafloor sediments are very significant in seafloor study and underwater acoustic study field. In order to measure the sound speed and the attenuation for the small-scale sediment cores more accurately, this study developed a water coupled acoustic laboratory measurement system based on Richardson-Briggs technique. This method used the correlation comparison of waveforms received in sediment core and in identical reference tubes filled with water to measure sound speed and attenuation. The sound speed and attenuation of a clayey silt sediment sample were measured using the water coupled acoustic laboratory measurement system. This frequency dependence of the sound speed and attenuation showed that the clayey silt sediment has a weak positive sound speed dispersion, while the attenuation increases with a strong positive gradient within the measurement frequency range. This study also noted that the measured sound speed ratio match well with the empirical equations from literature. The measured attenuation factor data can fall in the Hamilton’s empirical prediction range.     

A Tactical Approach to Environmental Uncertainty and Sensitivity

The issues of acoustic sensitivity to environmental parameters and the uncertainty in performance prediction due to lack of accurate environmental inputs are addressed. Special emphasis is given to the current state of affairs and to the importance of communicating in a simple, robust means the uncertainty to the sonar operator. A multistaged approach to estimating sensitivity, computing acoustic propagation uncertainty due to environmental variability is presented. A heuristic approach to estimating the most sensitive environmental parameters is developed. A measurements-based statistical approach is applied to environmental data taken in the Mediterranean Sea to estimate performance uncertainty due to sound speed and geoacoustics uncertainty     

Remote sensing of ocean sound speed profiles by a perceptron neuralnetwork

A method is developed to estimate ocean sound speed profiles through synthesis of remotely measured environmental data and historical statistics of sound speed obtained at a remotely sensed location. Sound speed profiles are represented by an expansion of empirical orthogonal functions (EOF) of the historical sound speed variation, while the remotely sensed environmental data provide real-time information to determine the expansion coefficients. Environmental inputs are limited to sea surface temperature available from satellite infrared sensors, acoustic time-of-flight and ocean bottom temperature measurable from bottom mounted acoustic and thermal transducers. A multilayer perceptron neural network is implemented to learn the functional transformation from the measured environmental input to the desired EOF coefficient output on a set of representative sound speed profiles. Sea surface temperature, time-of-year, and time-of-flight from the acoustic multipath that maximally samples the vertical sound speed are found to be the dominant inputs. The trained network is computationally efficient and produces estimates for untrained environmental inputs with a mean error of 1.1-4.4 m/s     

The design of transducers and arrays for underwater datatransmission

Consideration is given to the design of underwater acoustic projector transducers for communication purposes. Particular interest is focused upon broadband designs which provide the potential for high data-rate communication. The author introduces the underlying physics of sound propagation in solid-bar and disc structures typically used for manufacturing underwater acoustic transducers. The variability of sound speed with bar dimensions, in bar resonators, is discussed. Consideration is given to the nature and identification of different modal states in such structures. The possibilities and limitations in modeling and design are reviewed. An examination is made of several specific designs, including thin-disc and broadband high-frequency transducers as well as piston and tonpilz structures modified electrically and mechanically to produce broad, controlled passband responses. A variety of more exotic structures is also included. Attention is given to arrays of transducers and, particularly, to the problems involved in overcoming element interaction and inadequacies in element behavior     

声跃层控制声线传播弯曲模型的构建

基于斯涅耳折射定律,将海水垂向等分成若干层,利用傅里叶步近算法,构建受声速剖面唯一控制的声线传播弯曲模型。将该模型用于模拟研究不同浅海声跃层类型对声线传播弯曲的影响,得出声线波长和轨迹长度按负跃层、无跃层、正跃层的顺序逐渐增加。并利用该模型定量研究跃层深度、跃层强度、跃层厚度三特征参量对声线传播弯曲的影响,得出负跃层强度越大、厚度越大、深度越浅,声线弯曲越大,波长越小。正跃层三特征参量对声线作用相反。     

Estimating climatic temperature change in the ocean with syntheticacoustic apertures

An acoustic tomography simulation is carried out in the eastern North Pacific ocean to assess whether climate trends are better detected and mapped with mobile or fixed receivers. In both cases, acoustic signals from two stationary sources are transmitted to ten receivers. Natural variability of the sound-speed field is simulated with the Naval Research Laboratory (NRL) layered-ocean model. A sequential Kalman-Bucy filter is used to estimate the sound speed field, where the a priori error covariance matrix of the parameters is estimated from the NRL model. A spatially homogeneous climate trend is added to the NRL fluctuations of sound speed, but the trend is not parameterized in the Kalman filter. Acoustic travel times are computed between the sources and receivers by combining sound speeds from the NRL model with those from the unparameterized climate trend. The effects of the unparameterized climate trend are projected onto parameters which eventually drift beyond acceptable limits. At that time, the unparameterized trend is detected. Mobile and fixed receivers detect the trend at about the same time. At detection time, however, maps from fixed receivers are less accurate because some of the unparameterized climate trend is projected onto tile spatially varying harmonics of the sound-speed field. With mobile receivers, the synthetic apertures suppress the projection onto these harmonics. Instead, the unparametrized trend is correctly projected onto the spatially homogeneous portion of the parameterized sound-speed field     

Precise Multibeam Acoustic Bathymetry

The maximum error in ocean depth measurement as specified by the International Hydrographic Organization is 1% for depth greater than 30m. Current acoustic multibeam bathymetric systems used for depth measurement are subject to errors from various sources which may significantly exceed this limit. The lack of sound speed profiles may be one significant source of error. Because of the limited ability of sound speed profile measurement, depth values are usually estimated using an assumed profile. If actual sound speed profiles are known, depth estimate errors can be corrected using ray-tracing methods. For depth measurements, the calculation of the location at which a sound pulse impinges on the sea bottom varies with the variation of the sound speed profile. We demonstrate that this location is almost unchanged for a family of sound speed profiles with the same surface value and the same area under them. Based on this observation, we can construct a simple constant-gradient equivalent sound speed profile to correct errors. Compared with ray-tracing methods, the equivalent sound speed profile method is more efficient. If a vertical depth is known (or independently measured), then depth correction for a multibeam system can be accomplished without knowledge of the actual sound speed profile. This leads to a new type of precise acoustic multibeam bathymetric system.     

Precise Multibeam Acoustic Bathymetry

The maximum error in ocean depth measurement as specified by the International Hydrographic Organization is 1% for depth greater than 30m. Current acoustic multibeam bathymetric systems used for depth measurement are subject to errors from various sources which may significantly exceed this limit. The lack of sound speed profiles may be one significant source of error. Because of the limited ability of sound speed profile measurement, depth values are usually estimated using an assumed profile. If actual sound speed profiles are known, depth estimate errors can be corrected using ray-tracing methods. For depth measurements, the calculation of the location at which a sound pulse impinges on the sea bottom varies with the variation of the sound speed profile. We demonstrate that this location is almost unchanged for a family of sound speed profiles with the same surface value and the same area under them. Based on this observation, we can construct a simple constant-gradient equivalent sound speed profile to correct errors. Compared with ray-tracing methods, the equivalent sound speed profile method is more efficient. If a vertical depth is known (or independently measured), then depth correction for a multibeam system can be accomplished without knowledge of the actual sound speed profile. This leads to a new type of precise acoustic multibeam bathymetric system.     

Acoustic characteristics of the north-eastern Atlantic Ocean

The vertical structure of the climatic and seasonal fields of the estimated speed of sound in the north-east Atlantic Ocean is considered. Zonation according to typicalc(z) profiles allowing for seasonal variations was carried out. It is shown that the water areas with a dual-channel vertical hydroacoustic structure vary essentially throughout the year. The features of the structure of the sound speed field in the lenses of the Mediterranean Sea waters are investigated. The effect produced by a lens on the characteristics of the sound propagation is studied.Translated by Mikhail M. Trufanov.     

Ocean acoustic tomography as a data assimilation problem

The ocean acoustic tomographic (OAT) approach to sound speed field estimation is generalized to include a variety of sources of information of interest such as an oceanographic model of the sound speed field, direct local sound speed measurements, and a full field acoustic propagation model as well as measurements. The inverse problem is presented as a four-dimensional field estimation problem using a variational approach commonly used in oceanographic data assimilation. The current OAT approach is shown to be a special case of the general framework. The matched-field tomography (MFT) approach is also discussed within this context. A simple implementation of this novel approach is then investigated in the absence of a suitable oceanographic model, and acoustic propagation is accounted for using a standard parabolic equation model. The inverse equations derived are validated numerically through a simple inversion example, and some issues on environmental mismatch and computations are discussed. The developments then provide a basic framework for ongoing data-model melding in acoustically focused oceanographic sampling (AFOS) network     

水声定位系统中迭代适应点分层的声线修正算法

水声定位系统中, 声线弯曲是造成定位误差大的主要原因, 本文针对该问题提出了一种迭代适应点分层(IAPL)的声线修正算法, 将声速剖面筛选分层修正声线。首先搭建水声定位模型, 通过拟合目标海域的监测数据, 得到声速高次函数; 其次探究声线弯曲时目标位置与掠射角的关联性, 由此构造出声线插值函数并求解路径参数; 最后提出划分原则, 精简声速剖面分层。仿真结果表明, 所提算法定位误差较低, 分层精简率均维持在48.04%的水平, 使计算量平均下降可达50.27%, 能够最大程度保留声速剖面的原始特征, 减少分层数量, 提高计算效率。