Backscattering from bioturbated sediments at very high frequency

Recent backscattering measurements made in the Gulf of La Spezia, Italy, using a sonar operating at 140 kHz combined with thorough characterization of seabed interface and volume properties illustrate the importance of seabed volume scattering. Three-dimensional fluctuation statistics of density variability and vertical density gradients, both of which are attributed to the level of bioturbation (e.g., sea shell fragments, burrows, pockets of water) have been quantified using X-Ray computed tomography. Two-dimensional interface roughness spectra have also been determined using a digital stereo photogrammetry system. The combined ground truth has allowed a backscattering model to be fully constrained. Measured backscattering strength versus angle is compared to a model that includes the effects of varying density and sound speed. Data-model comparisons show that scattering from the volume of strongly inhomogeneous sediments can often be a primary contributor to seafloor scattering away from normal incidence.     

南黄海砂质海底反向声散射测量（6-24 kHz）

在南黄海某一典型的砂质海底区域,采用全向性声源和全向性接收水听器开展了频率范围为6-24 kHz的海底反向声散射测量。测量结果表明,在避免海面散射干扰并满足远场条件的情况下,本次实验获得了掠射角范围为18~80°的海底反向声散射强度,其数值为-41.1~24.4 dB。在有效掠射角范围内,声散射强度总体上随掠射角的增大呈现出增大趋势,但对于不同的频率,其变化趋势有所不同,反映出不同的散射机理。在20°、40°和60°掠射角处,在6-24 kHz的频率范围内反向声散射强度总体上呈现出正相关的频率依赖性,其线性相关斜率分别为0.2229 dB/kHz、0.5130 dB/kHz、0.1746 dB/kHz。在最大掠射角80°处,反向声散射强度未呈现出明显的频率相关性。     

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     

The analysis of backscattering properties from SAR data of mountain regions

Possibilities and problems for the extraction of backscattering coefficients from airborne and spaceborne Synthetic Aperture Radar (SAR) data are discussed. Corrections for system errors and for imaging geometry are described. Examples are given for airborne SAR data acquired in theX- andC-bands over a test site in the Austrian Alps and for Seasat SAR data of glacierized areas in Iceland. From these data, backscattering coefficients have been derived on a relative scale for wet snow, glacier ice, and several ice-free surfaces. Data of in situ measurements on surface roughness and on dielectric parameters are presented for interpretation purposes and as the basis for backscatter modeling. The SAR-derived results are compared with theoretical backscattering calculations which were based on the Kirchhoff model for surface scattering and on the Born approximation for snow volume scattering.     

A statistical study of acoustic signals backscattered from the seabottom

Backscattering measurements were performed in shallow water on sand, gravel, and clay bottoms. The equipment included a parametric array that emitted pulses of differential frequencies (8 to 40 kHz) with a 3° directivity. The ranges did not exceed 50 m. The grazing angles varied from 4° to 90°. The bottom backscattering strength does not depend on the emitted pulse type (frequency and length). If one fits a Lambert law to the variations of the backscattering strength versus the grazing angle, the value at the origin fluctuates between-15 and-22 dB without any clear effect from the different bottom types. Statistical tests show that under the experimental measurement conditions: (1) the alternative received signal does not generally follow a normal distribution; (2) among five classical distributions in sonar and radar that have been fitted to the detected-integrated signal (exponential, Weibull, chi-2, log-normal, Rice), the best-fitted law is the log-normal; (3) signals backscattered by separated areas of the same bottom can hardly be regarded as stationary and, even less, homogeneous; and (4) with an anisotropic bottom topography the statistical properties depend on the aspect under which this topography is seen     

A comparison of perturbation theory and the small-slope approximation for acoustic scattering from a rough interface for a Biot medium

In this paper, the lowest order small-slope approximation (SSA) scattering cross section for Biot theory is derived. Numerical results are obtained for both backscattering and bistatic scattering using a modified power law spectrum, and these results are compared with those of lowest order perturbation theory (PT). Frequencies ranging from 100 Hz to 3 kHz are used for surfaces with RMS heights h of 0.1 and 1 m and a correlation length l of 10 m. The angle of incidence for the bistatic results is limited to 45/spl deg/. It is found that for the smaller surface height roughness (h = 0.1 m), the SSA and PT give the same results for frequencies up to almost 1 kHz for both backscattering and bistatic scattering. For h = 1 m, the SSA and PT backscatter results are in good agreement at all frequencies for incident grazing angles up to approximately 45/spl deg/. For the bistatic results, the SSA and PT results agree only at low grazing angles of scatter. In the specular region, the results differ significantly.     

非线性随机表面Kirchhoff散射模式

自从Longuet-Higgins(1963)根据非线性作用导出较正态分布为准确的波面高度分布以来,高阶矩在军事、高科技等很多方面得到了应用。Huang等(1980)曾检验了高阶非线性分布,他们发现当波高概率分布中包含直至四阶项时与观测结果相符,但如果考虑更高阶不但效果不好,反而更差。尽管非线性随机过程在海浪理论中获得了广泛的应用,但对海面雷达散射研究仅讨论到三阶矩的影响(Fung et al.,1991;Chen et al.,1992)。电磁随机表面散射理论有适应大尺度随机起伏的粗糙面的Kirchhoff散射模式、轻度粗糙表面的微扰散射模式、大小尺度独立叠加的双尺度散射模式、全波散射模式(Bahar,1987)和积分方程散射模式(Chen et al.,1992)等。对随机粗糙Kirchhoff表面电磁波散射问题,尽管几十年来许多科学家已经进行了大量研究(Fung et al.,1991; Ulaby et al.,1982;Wu et al.,1988),Eom等(1983)曾对Gaussian面和非 Gaussian面的散射特性进行了比较研究,Fung等(1991)将Kirchhoff散射模式推广应用到三阶粗糙随机表面,然而更高阶矩对散射截面的影响还未见报道。本文在Fung等(1991)的基础上将Kirchhoff散射模式推广应用到四阶项,并对模式的应用进行了分析和讨论。     

Non-Rayleigh acoustic scattering characteristics of individual fish and zooplankton

It has long been known that the statistical properties of acoustic echoes from individual fish can have non-Rayleigh characteristics. The statistical properties of echoes from zooplankton are generally less understood. In this study, echoes from individual fish and zooplankton from a series of laboratory measurements from the past decade are investigated. In the experiments, acoustic echoes from various individual organisms were measured over a wide range of frequencies and orientations, typically in 1/spl deg/-3/spl deg/ increments. In the analysis in this paper, the echoes from most of those measurements are grouped according to ranges of orientation, which correspond to typical orientation distributions of these organisms in the natural ocean environment. This grouping provides a distribution of echo values for each range of orientation. This approach, in essence, emulates a field experiment whereby distributions of echoes would be recorded for different distributions of orientations of the organisms. For both the fish and zooplankton data, there are conditions under which the echoes are strongly non-Rayleigh distributed. In some cases, the distribution is quantitatively connected to the physics of the scattering process while, in other cases, the connection is described qualitatively. Exploitation of the animal-specific statistics for classification purposes is suggested.     

Sea-surface acoustic backscattering measurement at 6–25 kHz in the Yellow Sea

Sea-surface acoustic backscattering measurements at moderate to high frequencies were performed in the shallow water of the south Yellow Sea, using omnidirectional spherical sources and omnidirectional hydrophones. Sea-surface backscattering data for frequencies in the 6–25 k Hz range and wind speeds of(3.0±0.5)and(4.5±1.0) m/s were obtained from two adjacent experimental sites, respectively. Computation of sea-surface backscattering strength using bistatic transducer is described. Finally, we calculated sea-surface backscattering strengths at grazing angles in the range of 16°–85°. We find that the measured backscattering strengths agree reasonably well with those predicted by using second order small-roughness perturbation approximation method with "PM" roughness spectrum for all frequencies at grazing angles ranged from 40° to 80°. The backscattering strengths varied slightly at grazing angles of 16°–40°, and were much stronger than roughness scattering. It is speculated that scattering from bubbles dominates the backscattering strengths at high wind speeds and small grazing angles. At the same frequencies and moderate to high grazing angles, the results show that the backscattering strengths at a wind speed of(4.5±1.0) m/s were approximately 5 d B higher than those at a wind speed of(3.0±0.5) m/s. However, the discrepancies of backscattering strength at low grazing angles were more than 10 d B. Furthermore the backscattering strengths exhibited no significant frequency dependence at 3 m/s wind speed. At a wind speed of 4.5 m/s, the scattering strengths increased at low grazing angles but decreased at high grazing angles with increasing grazing angle.     

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%。     

An analysis of the radar backscatter from oil-covered sea surfaces using moment method and Monte-Carlo simulation: preliminary results

An analysis of the radar backscattering from the ocean surface covered by oil spill is presented using a microwave scattering model and Monte-Carlo simulation. In the analysis, a one-dimensional rough sea surface is numerically generated with an ocean waveheight spectrum for a given wind velocity. A two-layered medium is then generated by adding a thin oil layer on the simulated rough sea surface. The electric fields backscattered from the sea surface with two-layered medium are computed with the method of moments (MoM), and the backscattering coefficients are statistically obtained with N independent samples for each oil-spilled surface using the Monte-Carlo technique for various conditions of surface roughness, oil-layer thickness, frequency, polarization and incidence angle. The numerical simulation results are compared with theoretical models for clean sea surfaces and SAR images of an oil-spilled sea surface caused by the Hebei (Hebei province, China) Spirit oil tanker in 2007. Further, conditions for better oil spill extraction are sought by the numerical simulation on the effects of wind speed and oil-layer thickness at different incidence angles on the backscattering coefficients.     

复合雷达后向散射模型与合成孔径雷达、散射计和高度计海面雷达后向散射观测的比较分析

海洋微波散射模型相比于以经验统计建立的地球物理模式函数具有不受特定微波频率限制的优势。组合布拉格散射模型和几何光学模型形成了复合雷达后向散射模型。利用南海北部气象浮标2014年海面风速风向实测值作为散射模型输入,分别比较了复合雷达后向散射模型与RADARSAT-2卫星C波段SAR、HY-2A卫星Ku波段微波散射计的海面后向散射系数,偏差分别为(?0.22±1.88) dB (SAR)、(0.33±2.71) dB (散射计VV极化）和(?1.35±2.88) dB (散射计HH极化);以美国浮标数据中心(NDBC)浮标2011年10月1日至2014年9月30日共3年的海面风速、风向实测值作为散射模型输入,分别比较了复合雷达后向散射模型与Jason-2、HY-2A卫星Ku波段高度计海面后向散射系数,偏差分别为(1.01±1.15) dB和(1.12±1.29) dB。中等入射角和垂直入射下的卫星传感器后向散射系数观测值与复合雷达后向散射模型模拟值比较,具有不同的偏差,但具有相同的海面风速检验精度,均方根误差小于1.71 m/s。结果表明,复合雷达后向散射模型可模拟计算星载SAR、散射计和高度计观测条件下的海面雷达后向散射系数,且与CMOD5、NSCAT-2、高度计业务化海面风速反演的地球物理模式函数的计算结果具有一致性;复合雷达后向散射模型可用于微波遥感器的定标与检验、海面雷达后向散射的模拟。     

Simulation of backscattering of high frequency sound from complexobjects and sand sea-bottom

This paper describes the simulation of backscattering of high frequency sound from complex objects and sand sea-bottom. Backscattering data from complex objects and seabottom were generated using a Universal High Resolution Imaging Sonar Simulation Toolkit (UHRISST) developed by the authors. Our approach here involves the approximation of objects and the sea-bottom through a series of facets that are small compared to the wavelength. The Target Strength of standard objects like the sphere, cylinder, and cone predicted by our model have been compared with various high frequency approximation formulations and produced a good fit. For the prediction of the backscattering strength from the sea-bottom, we have taken into account most of the small-scale features that represent the sand grain properties, intermediate features that represent seabottom roughness and large features that block propagation. Sea-bottom backscattering predictions made using this model were compared with high frequency scattering strength values measured off Panama City, FL, USA, and produced a moderate fit. Nonparametric statistical analysis was carried out on the bottom backscattering signals from a flat isotropic sea-bottom. The probability distribution function and probability of false alarm function of the bottom backscattering data was measured and found to largely follow the normal distribution     

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.     

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.     

渤海海冰微波散射特性研究

Microwave remote sensing has become the primary means for sea-ice research, and has been supported by a great deal of field experiments and theoretical studies regarding sea-ice microwave scattering. However, these studies have been barely carried in the Bohai Sea. The sea-ice microwave scattering mechanism was first developed for the thin sea ice with slight roughness in the Bohai Sea in the winter of 2012, and included the backscattering coefficients which were measured on the different conditions of three bands(L, C and X), two polarizations(HH and VV), and incident angles of 20° to 60°, using a ground-based scatterometer and the synchronous physical parameters of the sea-ice temperature, density, thickness, salinity, and so on. The theoretical model of the sea-ice electromagnetic scattering is obtained based on these physical parameters. The research regarding the sea-ice microwave scattering mechanism is carried out through two means, which includes the comparison between the field microwave scattering data and the simulation results of the theoretical model, as well as the feature analysis of the four components of the sea-ice electromagnetic scattering. It is revealed that the sea-ice microwave scattering data and the theoretical simulation results vary in the same trend with the incident angles. Also, there is a visible variant in the sensitivity of every component to the different bands.For example, the C and X bands are sensitive to the top surface, the X band is sensitive to the scatterers, and the L and C bands are sensitive to the bottom surface, and so on. It is suggested that the features of the sea-ice surfaces and scatterers can be retrieved by the further research in the future. This experiment can provide an experimental and theoretical foundation for research regarding the sea-ice microwave scattering characteristics in the Bohai Sea.     

一种适用于小入射角条件下的白冠海面后向散射模型

本文提出了一种白冠海面的小入射角星载雷达后向散射模型,模型包括海面非波浪破碎部分和波浪破碎部分的后向散射.在风的作用下,海浪破碎形成白冠,对星载雷达的后向散射信号造成影响.文中利用热带降雨测绘任务卫星搭载的降雨雷达(TRMM PR)和欧洲中期天气预报中心(ECMWF)的时空匹配数据集,拟合得出小入射角下星载雷达海面波浪...     

Bragg scattering of centimeter electromagnetic radiation from the sea surface: The effect of waves longer than Bragg components

The effect of sea surface waves longer than Bragg components on the backscattering of centimeter electromagnetic radiation is studied on the basis of data on direct field measurements of sea-surface slopes. A situation in which waves with wavelengths greater than 10 cm are longer than the Bragg components is considered. The increase in the backscattering cross section that is due to the presence of long waves is numerically estimated for sounding at horizontal and vertical polarization. Nonlinear effects in the field of surface waves result in the departure of the distribution of sea-surface slopes from a Gaussian distribution and lead to a change in the backscattering cross section. At a sounding angle of 35°, this change may reach 15% with respect to the cross section calculated for a Gaussian surface.     

Matched field processing of deep-water ambient noise

Matched-field ambiguity surfaces produced by deep-water ambient-noise data are discussed and quantified in terms of power levels and correlation values. Two processors were implemented (Bartlett and minimum variance) using data at 35 and 95 Hz with similar but distinct vertical angular distributions. In general, the ambiguity surfaces have both diffuse and discrete components. The diffuse distribution extends across the sound channel with correlation values increasing with distance. The discrete sources have higher correlation values and are distributed in a convergence-zone sidelobe structure. Local wind conditions appear to affect the received power but not the correlation values or the processor output power