Acoustic wave scattering from two solid boundaries at the oceanbottom: reflection loss

An acoustic wave scattering model is formulated and solved for three homogeneous layers consisting of a thin solid sediment layer sandwiched by semi-infinite water and solid basalt media. The model is applied to two cases to analyze both the physical parameters affecting reflection loss and the effects of interface roughness scattering. It is shown that effects of attenuation in the sediment layer, especially of S-waves, combine with conversion and scattering processes of the basalt interface to constitute the dominant mechanism of reflection loss, especially in the small grazing angle directions. The scattering process is found not only to produce the well-known acoustic energy loss from specular to nonspecular directions, but also to alter the conversion efficiency between P and S waves with a resulting loss or gain     

Class-based target identification with multiaspect scattering data

In underwater sensing applications, it is often difficult to train a classifier in advance for all targets that may be seen during testing, due to the large number of targets that may be encountered. We therefore partition the training data into target classes, with each class characteristic of multiple targets that share similar scattering physics. In some cases, one may have a priori insight into which targets should constitute a given class, while in other cases this segmentation must be done autonomously based on the scattering data. For the latter case, we constitute the classes using an information-theoretic mapping criterion. Having defined the target classes, the second phase of our identification procedure involves determining those features that enhance the similarity between the targets in a given class. This is achieved by using a genetic algorithm (GA)-based feature-selection algorithm with a Kullback-Leibler (KL) cost function. The classifier employed is appropriate for multiaspect scattering data and is based on a hidden Markov model (HMM). The performance of the class-based classification algorithm is examined using both measured and computed acoustic scattering data from submerged elastic targets.     

Fluid-Filled Passive Sonar Calibration Spheres: Design, Modeling, and Measurement

Hollow spheres have long been used as simple underwater targets for testing acoustic projector systems. While spheres offer a mathematically simple shape with a resolvable scattering strength, their usage as a passive target has been less successful due to the complicated manner in which a hollow sphere scatters energy from its exterior and interior as a function of frequency and temperature. Furthermore, a sphere's aspect independent scattering requires a surface area that is physically much greater than a wavelength which in turn requires mechanical support systems that are also large, often with target strengths that rival that of the test target itself. This paper discusses the development of several thin-walled spheres, ranging in diameters from 0.1524 to 0.4953 m, filled with a high-density fluid, to be used collectively as calibrated underwater sonar targets in the 5–50-kHz frequency range and an additional 0.4953-m diameter sphere tested over the range of 5–120 kHz. The combination of the spherical shape and focusing effects of the fluid enhances the acoustic scattering strength of the sphere and produces a significantly greater backscattered response than a rigid sphere. A simple theoretical model is presented to compare several fill fluid possibilities and is then used to compare the chosen fluid, fluorolube, against measured data for each sphere.      

Scattering from objects submerged in unbounded and bounded oceans

We present a study of scattering from objects in a free unbounded ocean and for objects near an interface and in a waveguide. Major emphasis is on results and discussion of theoretical development is limited to the essentials. An examination of backscattered echoes is presented for elastic targets. Angular distributions are examined for all target types, as well as environments, with the exception of objects in a waveguide. In a waveguide, beamforming techniques are most appropriate for detection of objects. We also examine rigid, soft, and elastic objects that are either spheroidal or cylindrical. Aspect ratios studied range from 3:1 to 24:1, andkL/2ranges (kis wavenumber andLis the length of the object) are from 3 to 120.     

Simultaneous acoustic observations of turbulence and zooplankton in the ocean

Models and laboratory experiments show that zooplankton may locate food more easily in turbulent waters, but whether plankton seek or avoid turbulence in the ocean is an open question. It is difficult to measure turbulence and plankton simultaneously and with the necessary spatial resolution using traditional methods (nets and airfoil shear sensors). Acoustics is commonly used to survey zooplankton abundance and recent studies have shown that stratified turbulence can also be a significant source of sound scatter. This may seem like more of a complication than a boon for those aiming to use acoustics to observe plankton in turbulence. We present acoustic data, however, that show that zooplankton and turbulence can be observed simultaneously with a single 307 kHz sounder. The different natures of the two targets (discrete targets versus a volume effect) allow them to be distinguished. The key is sampling the same targets at multiple ranges. The volume scattering strength of a discrete target will increase as the target nears the sounder, because the volume sampled decreases. Turbulence, as a volume scattering effect, has little range dependence to its volume scattering strength.     

全极化SAR图像中溢油极化特征研究

相比于单极化SAR图像,全极化SAR图像不仅能体现海面目标的几何特征、后向散射特征,还能体现目标的极化特征。因此,在溢油检测方面,极化SAR更具优势。特征提取作为溢油检测的关键步骤,直接影响到溢油检测的精度。在本文中,我们分析了全极化SAR图像中海面溢油的极化特征,如极化散射熵、平均散射角等。并提出了新的极化特征P,该特征参数能够反映海面目标电磁散射过程中布拉格散射机制和镜面散射机制的比例。为了研究极化特征溢油检测的能力,本文基于SIR-C/X-SAR和Radarsat-2全极化SAR图像开展了相关实验,并对比分析了溢油的多种极化特征。实验结果显示,在中低风速情况下,C波段溢油探测效果优于L波段;本文提出的极化特征P对海面散射机制敏感;基准高度和特征参数P在C波段比其他极化特征更适于溢油检测。     

Hydrodynamic pressures and forces on quadrant front face pile supported breakwater

Quadrant front face pile supported breakwater is a combination of semicircular and closely spaced pile breakwaters which couples the advantages of these two types. This type of structure consists of two parts. The bottom portion consists of closely spaced piles and the top portion consists of a quadrant solid front face on the seaside. The leeward side of the top portion with a vertical face would facilitate the berthing of vessels. An experimental investigation on this breakwater model in a wave flume is carried out for three water depths. For each water depth, three different spacings between the piles were adopted for the investigation. The dynamic pressures exerted along the quadrant front face due to regular waves were measured. The variation of dimensionless pressures with respect to scattering parameter for different gap ratio (spacing between the piles/diameter of pile) and for relative pile depth (water depth/pile height) are presented and discussed. In addition, the dimensionless total forces exerted on the breakwater model as well as its reflection characteristics as a function of scattering parameter are reported.     

Target strength: Some recent theoretical developments

Some recent developments in the theory of diffraction of transient wavefields with high or moderate, but not excessively low, frequency content are reviewed here. Examples are cited to demonstrate that combinations of wavefronts and resonances, which organize the wave process according to progressing and oscillatory events, respectively, can furnish building blocks for interpretation of transient scattering data, and thereby for classification of target features. Conversely, these same spectral elements hold promise as a basis for physically sound parameterization of scattering data to reconstruct target characteristics. The discussion includes impenetrable and penetrable targets, interior-exterior coupling, real and complex spectra, and suggested directions for future research.     

Real- and synthetic-array signal processing of buried targets

Results from two field experiments aimed at investigating the detection and classification of buried targets are presented. In both experiments a 2-16-kHz parametric source was used. In the first experiment, the source was used in combination with a 12-m horizontal line array and in the second with a 1.4-m vertical line array which was displaced horizontally along an underwater rail to form a 10 m /spl times/ 1.4 m two-dimensional synthetic aperture sonar (SAS). To increase the SAS integration time, the parametric source was electronically scanned in azimuth during the displacement along the rail, as in spotlight mode. It is shown that both arrays allow important signal-to-reverberation gains, enhancing the detection of sub-bottom echoes. A new, environmentally adaptive, matched filter which further improves the signal to reverberation ratio while allowing discrimination between proud and buried targets is presented and validated experimentally. The use of resonant scattering for target classification of buried objects is discussed, in the particular case of spherical shells.     

Progress in Marine Oil Spill Optical Remote Sensing: Detected Targets,Spectral Response Characteristics,and Theories

Different oil spill pollution types could be produced in oil transport and weathering processes. Investigation of these pollution types is beneficial for oil spill recovery and processing. Optical remote sensing techniques play an important role in marine oil spill monitoring and have the ability to identify different oil spill pollution types. Recently, research on oil spill optical remote sensing has made much progress in detecting targets, identifying spectral response characteristics, and formulating theories. Floating black oil, oil slicks, and oil-water mixture in marine oil spill accidents are the main targets to be investigated by optical remote sensors. The visible spectral response differences of these targets are the base of oil spill optical remote sensing research. Bi-directional reflectance distribution function, light interference, absorption, and scattering of targets produce different spectra. Therefore, oil spill optical remote sensing could be used to identify the main oil spill pollution types and estimate oil spill volume.     

极化SAR目标相对最优极化研究

目标相对最优极化就是选取一种极化状态,使得研究目标和背景地物的接收功率对比达到最大。通过讨论了目标相对最优极化问题,提出了改进的模型和解算方法,利用实际数据对其模型和方法进行了实验,验证了其有效性和正确性。     

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     

Polarimetric entropy of the ocean surface with a two-scale scattering model

The relationships among an ocean wave spectrum,a fully polarimetric coherence matrix,and radar parameters are deduced with an electromagnetic wave theory.Furthermore,the relationship between the polarimetric entropy and ocean wave spectrum is established based on the definition of entropy and a twoscale scattering model of the ocean surface.It is the first time that the polarimetric entropy of the ocean surface is presented in theory.Meanwhile,the relationships among the fully polarimetric entropy and the parameters related to radar and ocean are discussed.The study is the basis of further monitoring targets on the ocean surface and deriving oceanic information with the entropy from the ocean surface.The contrast enhancement between human-made targets and the ocean surface with the entropy is presented with quad-pol airborne synthetic aperture radar（AIRSAR） data.     

A model for obliquely incident wave interacting with a multi-layered object

This paper presents an analytical solution for scattering of oblique incident, small amplitude, monochromatic wave trains by a stationary rigid multi-layered objects with rectangular cross-section. The object is infinite long and consists of multilayers, which can be either solid or permeable. This paper extends the previous work by Hu and Liu [1] from normal incident wave condition with a special object configuration to oblique incident waves with multi-layered object. The present model is validated with several existing solutions for normal/oblique waves interacting with a single object; excellent agreement is observed. New numerical results are presented to investigate the effects of incidence angle on reflection, transmission and energy loss coefficients for a combined floating and bottom-mounted permeable breakwater. A new floating board-cage breakwater is developed from the present model and its solutions are discussed in detail. A computer program, AWAS-P, has been updated so that it is applicable for both oblique and normal incident waves, while the object is multi-layered.     

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.     

Radar sensing of the ocean

Radar remote sensing of the ocean has been the subject of research for about 20 years. Spaceborne radar altimetry and scatterometry are approaching maturity, and synthetic-aperture radars (SAR) show great promise. The principles of radar scattering from the sea are outlined here, along with some recently discovered questions. For wind-vector scatterometry, the principle is presented, and remaining uncertainties are outlined. Principles of SAR imaging of moving targets, and particularly the ocean surface, are outlined. The theory of SAR wave imaging is the subject of considerable controversy, and some principles that can be used in evaluating the various theories are presented, along with brief outlines of the major conflicting theories. Other uses of SAR images are discussed briefly, with a theory to explain the bathymetric expression outlined.     

Broadside resonance scattering from elastic spheroids

The authors study the scattering of monochromatic plane acoustic waves incident at 90° angles relative to the axis of symmetry (i.e. broadside or beam aspect) on solid elastic spheroids. In this analysis, the aspect ratios of the spheroids vary in the range 2⩽L/ D⩽5 in steps of one. The nondimensional frequency kL /2 is kept within the band 2⩽kL/2⩽24. A numerical solution based on a modification of the T-matrix method is generated. The authors generate predictions for the backscattered echoes and graphically display their frequency dependence in order to study the resonance features present within them. In this three-dimensional study, the authors identify the (leaky) Rayleigh-type resonances consistent with those present in infinite cylinders     

Wave Action on Structures of Combined Cylinders

1.Introduction Owingtoitssignificanteffectofreductionofwaveloadsandwaverun up,theperforatedwallon anarrayofcylindricalstructureshasreceivedconsiderableattentioninrecentyears.Manyresearches havebeencarriedoutinthisfield.Anexactsolutionforthediffractionoflinearwaterwaveswithan arrayofimpermeablecylinderswasfirstgivenbySpringandMonkmeyer(1974)usinganeigenfunction expansionapproach.Subsequently,LintonandEvans(1990)madeamajorsimplificationtothetheo ry,whichallowedthenear fieldquantitiessuchasload…     

Aircraft measurements of the microwave scattering signature of the ocean

Microwave scattering signatures of the ocean have been measured over a range of surface wind speeds from 3 m/s to 23.6 m/s using the AAFE RADSCAT scatterometer in an aircraft. Normalized scattering coefficients are presented for vertical and horizontal polarizations as a function of incidence angle (nadir to55deg) and radar azimuth angle (0degto360deg) relative to surface wind direction. For a given radar polarization, incidence angle, and azimuth angle relative to the wind direction, these scattering data exhibit a power law dependence on surface wind speed. The relation of the scattering coefficient to azimuth angle obtained during aircraft circles (antenna conical scans) is anisotropic and suggests that microwave scatterometers can be used to infer both wind speed and direction. These results have been used for the design of the Seasat-A Satellite Scatterometer (SASS) to be flown in 1978 on this first NASA oceanographic satellite.     

中肋骨条藻和东海原甲藻的散射特性

浮游藻类是海水中的重要组成成分,对其固有光学特性的研究有助于深入了解水体光学的辐射传输。在过去的30年中,大量的研究都集中在藻类吸收特性上,缺少对其散射特性的认识。本文利用分光光度计设计了在实验室中测量含颗粒水体散射和后向散射特性的方法,并利用标准球形颗粒对该方法的可行性进行验证,结果表明,在400~700 nm范围内,散射测量结果与理论值的一致性较好,最大误差小于3%,而后向散射测量结果在蓝紫光处的一致性较好,在近红外波段处有一定误差。运用这两种测量方法对东中国海常见的赤潮藻种中肋骨条藻Skeletonema costatum和东海原甲藻Prorocentrum donghaiense进行测量,结果显示:中肋骨条藻与东海原甲藻的散射系数幅值相近但谱形差异较大,前者随波长增加散射强度递减,后者则相反;在色素吸收较强的波段,两者散射强度均出现与其它波长位置变化趋势相反的情况,这主要是受细胞物质物理性质的影响。两者的后向散射差异较小,但可以看出其谱形受色素吸收的影响很大,在幅值上,东海原甲藻略高于中肋骨条藻,在550 nm处分别为0.001 74,0.001 43 m²/mg(以藻类叶绿素a浓度归一化),后向散射概率分别为1.104%和0.723%。