Scattering of decimetric acoustic waves from the sea’s surface

The influence of the sea’s surface slopes caused by wind waves whose wavelength exceeds that of the Bragg components on the backscattering of acoustic radiation is discussed. The analysis is performed for long waves whose wavelengths are greater than 1.5 m. The study is based on the data of full-scale measurements of the sea’s surface slopes. Expressions relating the backscattering coefficient to the statistical characteristics of the sea’s surface slopes caused by long waves are presented.     

Evaluation of the scattering coefficient for high-frequency sound scattered from the sea surface

The effect of the deviation of the real rough sea surface from the Gaussian isotropic surface on the scattering coefficient of high-frequency sound is analyzed. The analysis relies on the data on the sea surface slopes, obtained from field experiments in the Black Sea with the use of a two-dimensional laser slopemeter. It is shown that the effect of the anisotropy of the surface wave field on the scattering coefficient is small when the angle of incidence is small, but this effect rapidly increases as the angle of incidence grows. The deviation of the real statistical moments of the sea surface slopes from those corresponding to the Gaussian distribution results in a ±20% error in the theoretical values of the scattering coefficient.     

Monostatic and bistatic statistical shadowing functions from a one-dimensional stationary randomly rough surface: II. Multiple scattering

Abstract

The integral equation model (IEM) has been developed over the last decade and it has become one of the most widely used theoretical models for rough-surface scattering in microwave remote sensing. In the IEM model the shadowing function is typically either omitted or a form based on geometric optics with single reflection is used. In this paper, a shadowing function for one-dimensional rough surfaces which incorporates multiple scattering, finite surface length and both monostatic and bistatic configurations is developed. For any uncorrelated process, the resulting equation can be expressed in terms of the monostatic statistical shadowing function with single reflection, derived in the preceding companion paper. The effect of correlation between the surface slopes and heights for a Gaussian surface is studied to illuminate the range over which such correlations can be ignored. It is found that while the correlation between surface slopes and heights in the monostatic statistical shadowing function with single reflection can be ignored, when calculating the average shadowing function with double reflection the correlation between slopes and heights between points must be incorporated.     

Monostatic and bistatic statistical shadowing functions from a one-dimensional stationary randomly rough surface: II. Multiple scattering

The integral equation model (IEM) has been developed over the last decade and it has become one of the most widely used theoretical models for rough-surface scattering in microwave remote sensing. In the IEM model the shadowing function is typically either omitted or a form based on geometric optics with single reflection is used. In this paper, a shadowing function for one-dimensional rough surfaces which incorporates multiple scattering, finite surface length and both monostatic and bistatic configurations is developed. For any uncorrelated process, the resulting equation can be expressed in terms of the monostatic statistical shadowing function with single reflection, derived in the preceding companion paper. The effect of correlation between the surface slopes and heights for a Gaussian surface is studied to illuminate the range over which such correlations can be ignored. It is found that while the correlation between surface slopes and heights in the monostatic statistical shadowing function with single reflection can be ignored, when calculating the average shadowing function with double reflection the correlation between slopes and heights between points must be incorporated.     

Three dimensional analyses of scattering by pressure-release sinusoidal surfaces

Scattering by pressure-release sinusoidal surfaces in three dimensions is analyzed using the Fresnel phase approximation and realistic source and receiver directivity approximations. Geometrical shadowing and second-order scattering are explicitly included to explore the validity of the Kirchhoff approximation. No restrictions on the surface heights and slopes are made. The "goodness" of the resulting expressions is verified by requiring the pressure scattered by a sinusoidal surface to reduce to the image solution as the surface amplitude goes to zero. The first-order scattered pressure achieves a very good approximation to the image solution, and the second-order scattered pressure goes to zero, as expected, under this requirement. The theory is compared with available experimental scattering measurements, and the agreement is good. Because the slopes on the experimental surface are very steep, shadowing corrections are indispensible to achieving accurate first and second order scattering results. Shadowing has a greater impact on the scattering prediction than the second-order scattering contribution. This suggests that the Kirchhoff approximation may be more robust when incorporated into a theory including a detailed shadowing treatment as well as the Fresnel phase approximation and a good directivity approximation.     

Effect of surface roughness on the statistical distribution of image speckle intensity

An experimental investigation is made of the effect of surface roughness on the statistical distribution of image speckle intensity. A speckle pattern is formed by spatially coherent light at the image plane of an object having some surface roughness and its statistical properties are investigated. It is found that the contrast of the speckle is related to the surface roughness. By this relation, a new technique for the measurement of surface roughness is proposed.     

A study of selected slope values in growth fronts of Au thin films

In this work, we compare the experimental slope values measured at the border of the surface protrusions that appear in growth fronts of evaporated Au films with the slope values predicted by the selected slope theories, showing a good agreement between them. For this purpose, a theoretical expression for the histogram of terrace widths at the borders of surface protrusions is developed for surfaces composed of paraboloidal-shaped elements whose shape parameters have a low statistical correlation and a high statistical dispersion between each other. The theoretical histogram was compared with those experimentally measured in Au(111) films grown on single-crystal and polycrystalline substrates. Experimental minimum values of terrace widths at the protrusion borders (that correspond to maximum slopes) obtained from the single-crystal-growth agree with those provided by the selected slope theory. An extension of the selected slope theory has been performed to interpret the additional maximum slope value observed in the polycrystalline films.     

Shadowing function with single reflection from anisotropic Gaussian rough surface. Application to Gaussian, Lorentzian and sea correlations

In this paper, the monostatic (transmitter and receiver are located at the same place) and bistatic (transmitter and receiver are distinct) statistical shadowing functions from an anisotropic two-dimensional randomly rough surface are presented. This parameter is especially important in the case of grazing angles for computing the bistatic scattering coefficient in optical and microwave frequencies. The objective of this paper is to extend the previous work (Bourlier C, Berginc G and Saillard J 2002 Waves Random Media 12 145-74), valid for a one-dimensional surface, to a two-dimensional anistropic surface by considering a joint Gaussian process of surface slopes and heights separating two points of the surface. The monostatic average (statistical shadowing function average over the statistical variables) shadowing function is then performed in polar coordinates with respect to the incidence angle, the azimuthal direction and the surface height two-dimensional autocorrelation function. In addition, for a bistatic configuration, it depends on the incidence angle and azimuthal direction of the receiver. For Gaussian and Lorentzian correlation profiles and practically important power-type spectra such as the Pierson-Moskowitz sea roughness spectrum, the numerical solution, obtained from generating the surface Gaussian elevations (Monte Carlo method), is compared with the uncorrelated and correlated models. The results show that the correlation underestimates the shadow slightly, whereas the uncorrelated results weakly overpredict the shadow and are close to the numerical solution.     

Effective probability density function of rough surface slopes when strong shadowing is present

Abstract

It is shown that at low grazing angles, the slope probability density function (PDF) of the nonshadowed part of a rough surface can differ significantly from the slope PDF of the overall surface, if surface heights and slopes are functionally dependent. If the surface steepness has a tendency to increase with height, the effective slopes of the illuminated part of the surface can be significantly steeper than the average slope of the surface as a whole. This fact can play a crucial role in any theoretical interpretation of experimental results concerning radar scattering by the sea surface at low grazing angles.     

Modulation of radar signal reflected from rough surface by relief slope and soil electrodynamic-parameter inhomogeneities

The authors study variations of the specific effective scattering surface (SESS) of a radar signal reflected from a rough surface with small irregularities due to changes in the slopes of the small-scale relief and inhomogeneities of the complex permittivity of the soil in the case of a surface resolution element whose linear dimensions are much smaller than the characteristic scale of the relief. In the opposite case, formulas are derived for corrections to the SESS that are proportional to the variance of the slopes of the large-scale relief. It is shown that the correction for horizontal polarization is greater than that for vertical; the effect of the three-dimensionality of the relief roughness is also greater. A nonzero SESS for orthogonal polarization is due to the presence of relief slopes across the plane of incidence.Kharkov Radio-Astronomy Institute, Academy of Sciences of Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 4, pp. 444–457, April, 1994.     

Simultaneous two-axis shearographic interferometer using multiple wavelengths and a color camera

Multi-component measurements in shearography and other applications of Electronic Speckle Pattern Interferometry (ESPI) are typically achieved using multiple optical configurations that are activated sequentially to measure each desired quantity separately. A novel optical setup is introduced here where orthogonal shearography measurements are simultaneously made using a single color-camera imaging multiple monochromatic light sources of different wavelengths. The Red–Green–Blue (RGB) sensors of a conventional Bayer type camera are read separately, thereby providing three independent color signals and independent ESPI phase maps. Orthogonal axis shearography is achieved using a modified shearography interferometer where a dichroic filter is added to provide a second wavelength-dependent measurement. The availability of the two surface slopes gives the opportunity for the data to be summed numerically to give the surface displacement shape. This application is of significant practical interest because the surface displacement measurement can be made under field conditions by taking advantage of the well-known optical stability of shearography measurements. The two simultaneously measured surface slopes also offer the possibility to mathematically compensate for non-uniformity and non-orthogonality in the image shear caused by mirror non-flatness and/or mirror misalignments.     

Measurements of sea surface slopes by laser sensing from a space vehicle

The measurement errors of dispersions of sea surface slopes using spacecraft lidar were analyzed. The errors caused by deviations of the actual wave field from the Gaussian isotropic surface were considered. It is revealed that the unaccounted deviations of slope distributions from the Gaussian distribution lead to a systematic underestimation of the calculated slope dispersion by approximately 11–14%. In addition, it is revealed that the anisotropy of slopes during their dispersion calculation using the data of vertical laser sensing from space vehicle can be neglected.     

A monostatic illumination function with surface reflections from one-dimensional rough surfaces

When solving scattering or emissivity problems for rough surfaces, the shadowing effect is often taken into account. Furthermore, for rough surfaces with large root mean square slope, surface reflections of the incidence or emission ray should not be neglected, especially at large observation angles. In this paper, a model of the monostatic statistical illumination function for one-dimensional rough surfaces with single surface reflection is developed, which is based on the Smith illumination function. A Monte Carlo ray-tracing algorithm is used to evaluate the accuracy of the present model. It is shown that, when neglecting the correlation between heights and slopes of the surface, the present model agrees quite well with the Monte Carlo result. Moreover, the result is improved if the correlation between heights and slopes is taken into account. For practical purposes, an empirical factor is introduced to improve the performance of the uncorrelated first-order illumination function to avoid computing the correlated one, which takes a long computation time. Besides, the first-order illumination function is significant at large observation angles, which could be promising to overcome problems in models of surface infrared emissivity where underestimation occurs compared with experimental measurements.     

3D KMC simulations of crater growth during the reduction of oxide nanoislands on metal surfaces

The homoepitaxial growth of Cu nanocraters induced by thermal reduction of Cu₂O nanoislands on Cu(100) surfaces is simulated using a three-dimensional (3D) kinetic Monte Carlo (KMC) model by incorporating surface diffusion, attachment and detachment Cu adatoms dislodged from reducing Cu₂O islands. The craters are observed to grow continuously in rim height and rim slopes while remaining relatively constant in rim width in the course of the oxide decomposition. Such a growth behavior is attributed to the climbing uphill of Cu adatoms released from the perimeter of the reducing Cu₂O island at the crater bottom. The observed decay of the rim height and slopes after completion of the reduction of oxide islands suggests that these surface craters are thermodynamically unstable at high temperatures.     

Effective probability density function of rough surface slopes when strong shadowing is present

It is shown that at low grazing angles, the slope probability density function (PDF) of the nonshadowed part of a rough surface can differ significantly from the slope PDF of the overall surface, if surface heights and slopes are functionally dependent. If the surface steepness has a tendency to increase with height, the effective slopes of the illuminated part of the surface can be significantly steeper than the average slope of the surface as a whole. This fact can play a crucial role in any theoretical interpretation of experimental results concerning radar scattering by the sea surface at low grazing angles.     

基于频域纹理消除的结构性纹理缺陷检测方法

针对金属加工表面等结构纹理表面图像缺陷检测问题,结构纹理的存在会对缺陷（比如划痕）检测带来干扰,该文开展在频率域中消除背景纹理的方法来进行缺陷检测的研究。首先基于傅里叶变换的图像复原技术,空间域图像中的结构性纹理对应傅里叶域中高能频率分量,使用最小二乘法直线拟合操作去除,并将这些能量设置为零,经傅里叶逆变换为空间域图像。在复原的图像中,原始图像中的结构纹理区域将变为近似的均匀灰度级,但其中缺陷部分将被保留下来。再使用统计过程控制来设置阈值的方法就能从复原图像中分离出缺陷。最后在一系列的结构性纹理图像上的实验证实了所提方法可行且有效。     

Polarization dependency of enhanced multipath radar backscattering from an ocean-like surface

Previous analyses of electromagnetic scattering from a two-scale ocean surface assumed the tilts on a large-scale surface to be small. This means that multiple scattering between large-scale roughnesses is insignificant. If the tilts are not small, multipath-enhanced backscattering may occur due to quasi-specular reflections between the opposite slopes of a large-scale surface component. We have considered the simplest situation; this involves one reflection from the large-scale component and one single-scattering from the small-scale component. The coherent addition of this process to the reciprocally reverse one creates multipath-enhanced backscattering. The relative gain in the HH backscattering cross section up to the level of the VV signal was obtained for surface-wave slopes of about 30° and for large incidence angles. This gain occurs because the VV signal experiences an extinction during reflection at incident angles close to the pseudo-Brewster angle. The presented model provides insight into one scattering mechanism that is possibly responsible for the departure of radar sea experimental data from predictions by the conventional two-scale model.     

基于图像区域Lyapunov指数的海面舰船目标检测

为了检测海面背景中的舰船目标,分析了目标存在时背景信号混沌特征的变化,提出了一种基于图像区域Lyapunov指数的目标检测新方法. 新方法定义了图像灰度距离的概念,基于改进的Wolf方法将一维信号Lyapunov指数提取方法扩展到图像信号,利用图像区域最大灰度距离Lyapunov指数的变化检测淹没在混沌背景信号中的目标信号. 实验结果表明海面背景图像信号具有一定的混沌特征,利用新方法能有效检测出海面背景下的舰船目标,检测结果优于基于统计分析的方法. 关键词： Lyapunov指数 灰度距离 混沌特征 目标检测     

A blind image detection method for information hiding with double random-phase encoding

In this paper, a blind image detection method based on a statistical hypothesis test for information hiding with double random-phase encoding (DRPE) is proposed. This method aims to establish a quantitative criterion which is used to judge whether there is secret information embedded in the detected image. The main process can be described as follows: at the beginning, we decompose the detected gray-scale image into 8 bit planes considering it has 256 gray levels, and suppose that a secret image has been hidden in the detected image after it was encrypted by DRPE, thus the lower bit planes of the detected image exhibit strong randomness. Then, we divide the bit plane to be tested into many windows, and establish a statistical variable to measure the relativity between pixels in every window. Finally, judge whether the secret image exists in the detected image by operating the t test on all statistical variables. Numerical simulation shows that the accuracy is quite satisfactory, when we need to distinguish the images carrying secret information from a large amount of images.