一种分层海底反向散射模型

In order to predict the bottom backscattering strength more accurately, the stratified structure of the seafloor is considered. The seafloor is viewed as an elastic half-space basement covered by a fluid sediment layer with finite thickness. On the basis of calculating acoustic field in the water, the sediment layer, and the basement, four kinds of scattering mechanisms are taken into account, including roughness scattering from the water-sediment interface, volume scattering from the sediment layer, roughness scattering from the sediment-basement interface,and volume scattering from the basement. Then a backscattering model for a stratified seafloor applying to low frequency(0.1–10 kHz) is established. The simulation results show that the roughness scattering from the sediment-basement interface and the volume scattering from the basement are more prominent at relative low frequency(below 1.0 kHz). While with the increase of the frequency, the contribution of them to total bottom scattering gradually becomes weak. And the results ultimately approach to the predictions of the high-frequency(10–100 kHz) bottom scattering model. When the sound speed and attenuation of the shear wave in the basement gradually decrease, the prediction of the model tends to that of the full fluid model, which validates the backscattering model for the stratified seafloor in another aspect.     

Analysis of acoustic backscatter in the vicinity of the Dry Tortugas

 Experimental measurements of the bottom backscattering strength from carbonate sediments were made with a 200-kHz multibeam sonar mounted on a remotely operated vehicle. Results were obtained from eight different sites, which may be grouped into three categories, labeled soft, medium and hard, according to measured sediment sound speed. Sediment samples were gathered at or near each site to help interpret the acoustic results. The acoustic results are compared with extant published data and with the BOGGART bottom backscatter model. Backscattering strength values measured in the soft and medium sites fell within the main cluster of previously published values from sediments of similar grain sizes. The values from the hard region fell close to the upper limit. Dependence of the apparent backscattering strength on sonar height above bottom, particularly for the lower values of height above bottom, was observed, which suggests that the scattering process is a multiple-scattering one.     

声波在水-多孔介质海底界面上的反射与透射

声波在海底界面的反射和透射是海底散射、海底混响、海底目标探测的重要问题。利用Biot多孔介质声传播理论对声波在水-多孔海底界面上的反射和透射进行了分析,具体给出反射声波的反射系数,3种透射声波的透射系数以及声能透射系数随入射波入射角和频率(10~40 kHz)的变化关系,分析了各种透射波对透射声能的贡献。多孔海底介质参数分别采用Stoll和Chotiros给出的2组参数进行理论计算。     

On shallow-water bottom reverberation frequency dependence

Determinations of bottom scattering strength in the decade below 1 kHz under downward refracting conditions have been made using acoustic reverberation and transmission data from the 2001 East China Sea Asian Seas International Acoustic Experiment (ASIAEX). The measurements were performed using explosive sources and receiving hydrophones in ship-suspended vertical-line arrays. The focus of this paper has been the dependence of bottom scattering strength on the frequency and characterization of the uncertainties associated with the extraction of scattering strength from reverberation. The derived bottom scattering strength gradually rises with frequency from 100-300 Hz and then more rapidly above 300 Hz. A potential explanation suggests that the frequency variation results from two scattering mechanisms, rough layer scattering at the low end of the band and sediment near-surface volume scattering at the high end. The spatial extrapolation of these results is explored by comparing them with similarly derived scattering strengths using data obtained under the Navy's Harsh Environments Program at a somewhat separated site (56 km) under environmental conditions similar to those during ASIAEX. In the ASIAEX analysis, it has been found that the largest source of uncertainty in the scattering-strength frequency dependence arises from persistence of finite-amplitude effects associated with the source signal.     

Acoustic backscattering experiments in a well characterized sand sediment: data/model comparisons using sediment fluid and Biot models

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10/spl deg/ to 40/spl deg/. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50-150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30/spl deg/) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.     

Overview of SAX99: environmental considerations

A 1-km² area located 2 km off the Florida Panhandle (30°22.6'N; 86°38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18-19 m of water, is characterized by 1-2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics     

海底浅表层沉积物原位声学测量方法探讨

研究透射式和折射式两类海底沉积声学原位测量方法,通过分析10种海底浅表层沉积物声学原位测量仪器的特征,指出不同声学原位测量技术对沉积物声学特性测量结果的影响。比较黄海海底浅表层沉积声学原位测量数据与实验室测量结果的差异,分析原位声学测量数据普遍小于实验室测量数据的原因,指出原位测量的作用和重要性。探讨指出海底浅表层沉积物原位声学测量所需要配合发展的其他物理性质原位测量技术。     

Joint perturbation scattering characterization of a littoral oceanbottom reverberation: theory, scattering strength predictions, and datacomparisons

A joint surface roughness/volumetric perturbation scattering theory is utilized to characterize the reverberation from a littoral ocean bottom. The result is a reflected field spectrum that consists of specular and off-specular components. The predicted scattering strength from the off-specular component is shown to be comprised of interface roughness scattering, sediment inhomogeneity volumetric scattering, and interface roughness/sediment inhomogeneity correlation scattering. The sediment inhomogeneity volumetric scattering is shown to contain two contributions that are due to fractional variations in sediment densities and sound velocities. Both contributions are shown to be affected by the interface effect by a round-trip transmission coefficient factor. These two fractional variations are shown to contribute differently to scattering strength but similarly to backscattering strength. Inversely predicted roughness spectra from various sets of backscattering strength data are shown to be consistent with a generally known roughness spectrum. Both inversely predicted roughness and volumetric scattering physical property spectra are found to be self-consistent. However, the use of only ocean bottom backscattering strength data is found to be insufficient to judge whether the roughness or the volumetric scattering dominates. Reverberation characterizations using bistatic scattering strength data and signal spread data are planned for future studies     

Small-scale fluctuations in acoustic and physical properties in surficial carbonate sediments and their relationship to bioturbation

 Measurements of physical and acoustic properties within the top 40 cm of sediment indicate that there are three classes of parameter variability relevant to acoustic bottom scattering at the Marquesas Keys and Dry Tortugas sites. The three classes of variability, spatially and temporally different in terms of scale, are generated by biological and physical processes acting on sediments. Interplay of bioturbation, trawling and storm events create fine laminations of sand–silt–clay and high gravel-size mollusk shell content at the Marquesas Keys site; the predominance of biological processes creates strong surficial gradients and lateral variability in sediment properties at the Dry Tortugas site.     

海底沉积物的基本地声结构与地声模型

为准确建立海底地声模型,本文探讨地声模型的基本组成和基本结构。通过样品实验室测量,分析南海海底表层沉积物的密度、孔隙度与声速随着埋深变化的关系,得出海底实际存在的低声速表面–声速缓慢变化类型、低声速表面–声速增大类型、高声速表面–声速缓慢变化类型和高声速表面–声速增大类型4种典型地声结构;对比钻探测量,分析黄海海底沉积物的密度、孔隙度与声速随埋深变化关系,得出海底地声模型分层特征与地声结构组合特征。研究表明,地声模型可以归结为4种基本地声结构的组合,通过与底层海水声速、同层内声速剖面以及与上层海底沉积物下表面声速的比较,可以建立各种海底地声模型;基于实验室测量法建立的地声模型可以作为参考地声模型,但需要考虑实际海底温度和压力梯度以及海底沉积物的频散特性等,借助于声速比校正法和频散性理论模型进行计算及修正。     

基于海底表层沉积物声速特征的南海地声模型

在由垂直声速梯度建立的地声模型基础上,通过引入沉积物与海水声速比和沉积物压缩波与切变波声速比两个表征沉积物声学特征参数得到更全面和有实际指导意义的地声模型。在沉积物声波传播FCMCM模型基础上,基于热作用和重力作用下沉积物两相介质的应力应变分析,建立TFCMCM和DFCFCM模型,运用模型校正表层沉积物声速特征来计算和解释地声模型。根据海底表层沉积物存在低声速和高声速两种类型,结合沉积物沿纵深孔隙度不变和变化两种类型,得到南海海底沉积物的两类四种典型地声模型:低声速孔隙度不变型、低声速孔隙度减小型、高声速不变型和高声速孔隙度减小型。运用这四种典型地声模型的组合解释了卢博提出的南海三种典型声速结构。认知声速结构将为南海声学探测海底、划分海底区域提供模型支持。     

The effect of a layer of varying density on high-frequencyreflection, forward loss, and backscatter [seafloor acoustics]

This paper examines the impact that a thin layer of varying density would have on high-frequency reflection, forward loss, and backscattering of acoustic plane waves from the seafloor. A functional form for density stratification was found by examination of several high-resolution density profiles obtained from X-ray computed tomography (CT) scans of seafloor cores. A solution based on these general profiles was used to estimate the reflection coefficient. The influence of the density profile on reflection loss and backscatter was then calculated using the estimated reflection coefficient. Parameter values used in simulations were obtained from the literature or from the CT scans of cores. It was found that inclusion of a density profile adds a strong frequency dependence to estimates of the reflection coefficient and forward loss. The largest effect on total scattering strength is near normal incidence where returns are dominated by interface scattering. The effect of the density profile on the strength of acoustic returns suggests that care should be taken when using high-frequency systems for measuring sediment properties, especially near normal incidence     

An overview of SAX99: acoustic measurements

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30°. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results     

含砂量变化影响海底沉积物压缩波速度的分析研究

本文通过对南海海底沉积物样品的声学物理参数和沉积粒度特征统计分析，发现了高、低含砂量沉积物的声学物理特征存在明显差异，建立了海底沉积物的含砂量与压缩波速度、孔隙度、含水量和密度等经验公式，分析了含砂量变化与沉积物的体积压缩模量和密度变化的关系，从声速理论基础上阐明了含砂量变化引起沉积物压缩波速度变化的内在原因是含砂量变化引起了体积压缩模量和密度发生了变化，说明了含砂量增大引起沉积物压缩波速度增大的内在原因是含砂量增大引起了体积压缩模量变化量大于密度变化量，从而在数据统计和理论分析结合基础上，论证了含砂量是影响海底沉积物压缩波速度的重要因素之一。这一研究对声学方法反演海底沉积物粒度参数和沉积物类型、地声参数转换模型的建立，以及对水声反演海底和海底资源勘探等方面都具有重要理论意义和应用价值。     

Investigation of the relationship between seafloor echo strength and sediment type—a case study in Jiaozgou Bay,China

In recent years, increasing attention is being given to investigating the relationships between seafloor echo strength and sediment type. This aspect was assessed in a course of a marine survey conducted in Jiaozhou Bay near the city of Qingdao, China, where various sediment types are known to exist. Multi-beam echo data and sediment samples were acquired, processed and analyzed. The findings demonstrate that multi-beam echo strengths are strongly related to seabed types such as rock, sand and clay. Thus, echo strength increases progressively with the increase in sand content in gravely, sandy, and clayey sediments. However, echo strength is more sensitive to changes in sand content in fine-grained sediment and less sensitive in coarse-grained sediment. Also, high shell content of sediments greatly enhances the acoustic reflection. These findings help to predict seafloor sediment types by analyzing echo strength.     

Mathematical Modeling and Computer-Aided Manufacturing of Rough Surfaces for Experimental Study of Seafloor Scattering

Diverse aspects of stochastic processes, time-series analysis, fractal geometry, and manufacturing technology are brought together to provide a unified theoretical framework for the mathematical characterization and physical fabrication of scale-model representations of the rough ocean bottom. These scale models can be used to validate the predictions of interface-scattering theories, particularly those relating to the dependence of scattering strength on roughness parameters. In acoustical oceanography, rough surfaces are conventionally described in terms of power-spectral density (PSD) or fractal dimension. Here, recently developed concepts describing modified power-law PSD and approximately self-affine stochastic fractals are used to account for issues of finite sample size and finite manufacturing resolution. The large- and small- bandwidth restrictions that these issues inherently impose are related to their effects on the properties of surfaces as characterized both mathematically and by visual observation. This development provides the groundwork for numerical generation of surfaces, using spectral methods, and their physical manufacture, using computer-aided manufacturing (CAM) techniques. Effects of the spectral method of numerical generation on the statistics of generated topography are detailed. A manufacturing technique using a computer numerically controlled (CNC) milling machine is discussed and topography-specific guidelines for accurate manufacture of rough surfaces are prescribed.     

Seafloor acoustic remote sensing with multibeam echo-sounders and bathymetric sidescan sonar systems

This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.     

小尺度海底沉积物的声衰减特性研究

为研究小尺度海底沉积物样品的声衰减特性,作者提出了用声学探针测量海底沉积物声波幅值的新方法,对沉积物样品扰动小,两个测量点的距离可小于波长,为海底沉积物微观声衰减测量提供了新手段。作者用小于波长的间隔逐点测量了沉积物的压缩波幅值,数据分析表明沿沉积物柱状样全长的声衰减满足指数衰减模型。目前主要用同轴差距衰减测量法获得海底沉积物声衰减数据,但该方法不能辨识声衰减模型,因此不同海区的测量结果难以建立联系。对此作者又提出用声吸收系数反演的幅值比与声衰减系数反演的R值(两种幅值比的比值)作评价依据,分析了垂直轴差距衰减测量法获得的南海海底沉积物声衰减测量数据,发现部分沉积物样品声衰减的R值远大于1,其声衰减不满足指数衰减模型。在声衰减满足指数衰减模型的条件下,用Hamilton的声衰减和频率经验公式预报的南海沉积物声衰减比与作者用声学探针测量海底沉积物所得的声衰减比对比,通过对R值分析得出Hamilton的声衰减和频率经验公式可以预报南海沉积物声衰减比的范围。作者提出的声学探针测量海底沉积物声衰减的方法的优点是既能获得声衰减数据又能辨识声衰减模型,不同海区测量的沉积物声衰减比可用R值建立联系。     

Preference of echo features for classification of seafloor sediments using neural networks

Selection of a set of dominant echo features to classify seafloor sediments using a multilayer perceptron neural network is investigated at two acoustic frequencies (33 and 210 kHz). Several sets of inputs with different combinations of two, three, four, five, and six echo features are exploited with three-layer neural networks. The performances of the networks are analyzed to assess the most discriminating set of echo features for classification of seafloor sediments. The results of the overall average performances reveal that backscatter strength and time spread are the two most important echo features at 33 kHz, whereas backscatter strength has higher discriminating characteristics at 210 kHz for seafloor sediment classification. In addition, a set of four echo features consisting of backscatter strength, time-spread, statistical skewness, and Hausdroff dimension gives the highest success at both the acoustic frequencies.     

Seabed characterization from broad-band acoustic echosounding with scattering models

Acoustic echoes obtained during high-resolution shallow marine seismic surveys contain information about the statistical nature of the sedimentary bottom and its spatial variability. Use of a broad-band seismic source and an appropriately chosen data acquisition window makes the acoustic responses particularly amenable to quantitive analysis. The work reported utilizes experimental frequency-domain spatial coherence functions of along-track acoustic echoes as empirical metrics of bottom character, and by virtue of their correlation with known sediment types, as objective bases for remote sediment classification. Theoretical relationships between parameters describing sediment surface topographies and echo coherence are derived for the case of dominant water-sediment interface acoustic scattering. The diverse experimental data base was acquired from the Grand Banks of Newfoundland using a 1- to 10-kHz Huntec DTS system. Bottom photographs, cores, and grab samples combined with expert geological synthesis provide qualitative and quantitative control.