圆形区域多圆孔对SH波散射Green函数解

利用多极坐标变换、Graf加法公式及波函数展开法研究圆形区域内多圆孔缺陷存在时对圆形域边界作用反平面稳态脉冲SH波散射Green函数解。给出各圆孔对SH波散射产生含未知系数的散射波函数,用叠加原理写出圆形域介质内总波场,据边界条件、Graf加法公式及Fourier积分变换获得确定未知系数的无穷线代数方程组;由计算精度进行有限项截断求解,确定问题的Green函数解。用算例结果验证该方法的可行性。     

二维逆Born近似方法对正方形和圆形缺陷的重建

通过分析平面简谐纵波入射二维散射系统散射场远场Ｂｏｒｎ近似解纵波部分的固有性态，给出了二维Ｆｏｕｒｉｅｒ变换法建立散射区域（缺陷）的特征函数与散射场Ｂｏｒｎ近似解形状因子联系的关系式，并对钛合金中正方形铝夹杂和圆形孔穴分别进行了计算机模拟识别。本文的研究对定量无损探伤技术（ＱＮＤＴ）具有重大的理论和应用价值。     

缺陷识别中的逆Born近似方法

利用平面简谐纵波对二维散射远场Ｂｏｒｎ近拟解，给出了二维Ｆｒｏｕｒｉｅｒ变换法建立散射体（缺陷）的特征函数与形状因子的关系式，对钛合金中正方形铝夹杂和圆形孔穴分别进行计算机模拟识别，结果表明这种方法对定量无损检测技术有理论和应用价值。     

基线去相关引起的旁侧声纳系统的测深误差

相位差技术已广泛应用于各种干涉测量系统（ｉｎｔｅｒｆｅｒｏｍｅｔｒｉｃｓｙｓｔｅｍ），如旁侧声纳系统、人造卫星上的地面测高系统等。它利用散射波到达两相近接收器间的相位差，确定散射体的方位，结合波的双程传播时间确定散射界面的高度，但是人们至今对引起相位差估计误差的原因还不是很清楚。我们通过对海底多个散射体的数值模拟发现，在没有噪声干扰的情况下用相位差方法测得的散射体方位的起伏，显著大于对该时刻有贡献的海底照射区域所对应的信号到达角的变化范围。我们着重分析了两个散射体情况下的过量不确定性的原因。这类…     

同步辐射的基本知识第二讲同步辐射中的衍射术及其应用(一)

物质对射线的散射分为弹性散射和非弹性散射。弹性散射是一种无能量损失的散射,换言之,被照射的物体将发出与入射线相同波长的次级射线,并向各个方向传播。如果散射体是理想无序分布的电子、原子或分子,由于向各个方向传播的次级射线没有确定的相位差,则探测不到任何散射峰。如果原子或分子排列具有长程有序的周期性或短程有序,则会发生相互加强的干涉现象,产生相干散射波,这就是X射线衍射现象。如果散射体是短程有序的或散射体中存在某些杂质原子或缺陷,那么散射波很弱,且叠加在背景上,这种相干散射称为漫散射。如果散射体中的原子呈长程有序排列,则在许多特定方向上会产生大大加强的衍射线束,这就是Laue-Bragg衍射现象。非弹性散射是非相干的,且损失能量的散射。     

用于眼球测量的聚焦超声换能器声学参量测量方法的研究

脉冲时域测试系统是一种新颖的宽带测试系统，对高频（例如７ＳＭＨＺ以上）测试尤其适用。通过计算机对脉冲响应的富氏变换，而自动获取换能器的宽频带参数，这种方法速度快、频带宽、功能多及操作简易。本文通过对一种用于眼球测量的８．３ＭＨＺ聚焦超声换能器（进口仪器配套件），用连续波与脉冲波两种测量方法，校正若干主要声学参量。结果表明；二者的接收频响特性相当吻合。     

基于小波变换的裂谱分析法

 为了提高超声无损检测（UNDT）和无损评价（UNDE）中基础数据的信噪比（SNR）,提出了一种基于小波变换多分辨率分析的裂谱分析新方法．该方法在分析传统裂谱分析（SSP）方法原理及其局限性的基础上,通过采用小波变换多分辨率分析能力将原始超声回波信号进行等Q子带分解,然后按照一定的信噪分离规则来消除噪声,达到提高信噪比的目的．实验结果表明,与传统裂谱分析方法相比,该方法提高了消噪性能的稳定性,增强了湮没晶粒(或其他散射体)散射中的缺陷回波信号能力．     

脉冲源反演介质参数的近似波前法

在时域内对弹性波动方程退化的非均匀介质声波方程，引入背景场参数与扰动参数，并化为积分方程形式；针对脉冲源情况，根据射线理论中的传递方程和程函方程，对非均匀介质中的波场形式引入一种波前近似形式，得到波散射点满足散射关系曲线及散射波幅值与介质参数扰动比的代数关系方程式；为求解非均匀介质中散射波场及反演介质参数提供了一种方法，通过对一个完整算例全部过程的模拟，验证了此方法的正确性。     

一般模式点光源下一维PSD的响应函数

本文从Lucovsky方程入手,根据rc传输线模型和理想脉冲响应函数,提出PSD线性时不变系统(LTI)的模型近似,并基于该模型得出一般模式点光源下的PSD的响应函数.光强随时间变化的点光源可以均匀分割成一系列时域脉冲信号,并近似认为是方波脉冲信号,在线性时不变系统情况下,该光源的电流响应近似等于这些方波脉冲信号的线性...     

脊波理论:从脊波变换到Curvelet变换

本文旨较系统地评述继小波理论后,新近发展起来的具有变革意义的脊波理论的发展沿革、研究现状、应用前景和存在的问题。在信号处理、数据压缩、模式识别、统计估值等领域,获得对某些函数类的高的非线性逼近能力是至关重要的。由一维小波张成的二维小波虽然能有效表示含“点奇异”的二维函数,但对于含“线奇异”的二维函数,却不能获得最优的甚至哪怕是“近似最优”的非线性逼近阶。Candes提出的脊波变换巧妙地将二维函数中的“直线奇异”转化为“点奇异”,再用小波进行处理,能获得对含“直线奇异”的二维或高维函数最优的非线性逼近阶。正交脊波,则延续了脊波变换将“直线奇异”转化为“点奇异”进行处理的思想,并且构成一组L^2（R^2）上的标准正交基。单尺度脊波和Curvelet变换由脊波变换发展而来,分别利用了函数局部化和频带剖分的思想,将脊波理论发展到了一个更高的阶段,这两种变换都能“近似最优”的表示直线和曲线奇异,因而具有更好的应用前景。     

Time domain Born approximation

The time domain Born approximation for ultrasonic scattering from volume flaws in an elastic medium is described. Results are given both for the direct and the inverse problem. The time domain picture leads to simple intuitive formulas, which we illustrate by means of several simple examples. Particular emphasis is given to the front surface echo and its use in reconstructing the properties of the flaw.     

Quantitative modeling of the anisotropy of ultrasonic backscatterfrom canine myocardium

Reports extensions and new results of the First Time Domain Born approximation model used by Mottley and Miller (1982) to describe the anisotropy of ultrasonic backscatter measured in canine myocardium. The interaction of an ultrasonic plane wave impulse with a single cylindrical scatterer using time and frequency domain approaches is reviewed. Myocardial tissue is modeled as a suspension of aligned cylindrically shaped scatterers uniformly distributed in a homogeneous medium. The authors propose extensions to this model to deal with nonideal scatterer orientation, by introducing axial distribution functions and scatterer size distributions based on histology, modeled as a uniform distribution. The backscatter coefficient in the range 2.0-8.0 MHz is calculated. An algorithm to compute the average differential scattering cross section is presented. Ultrasonic elastic properties of myocardial tissue are discussed. Results of the anisotropy of the numerically computed backscatter parameters for model media having nominal mechanical and acoustic properties of canine myocardial tissue are presented and compared to available experimental data along with discussion of possible conclusions     

A Modified Born Approximation for Scattering in Isotropic and Anisotropic Elastic Solids

A new modified Born approximation (MBA) is presented that significantly extends the range of validity of the Born approximation to include the pulse-echo responses of strongly scattering inclusions in an elastic solid. The MBA also improves on the doubly distorted Born approximation (DDBA), a similar modification of the Born approximation that has been recently developed. These improvements are demonstrated by comparing the MBA, the Born approximation and the DDBA with the exact separation of variables solution for spherical inclusions in isotropic media. Furthermore, it is shown that the form of the MBA remains valid even for the pulse-echo scattering of an anisotropic inclusion in a general anisotropic elastic medium so that it is potentially applicable to a wide class of flaws and materials.     

Characterization of flaws using the zeroes of the real and imaginary parts of the ultrasonic scattering amplitude

A new method for determining the size of flaws, such as cracks and voids, in structural materials is reported. The method is based on the observation that, for a wide class of flaws, the zeroes in the real and imaginary parts of the scattering amplitude occur at characteristic frequencies which depend on the flaw size. The method of reconstruction using the zeroes is simpler than the inverse Born approximation because the flaw sizes are extracted directly from the characteristic frequencies. The new method employs the same data input as the inverse Born algorithm and has certain common limitations. Following a theoretical treatment, experimental results are presented for a number of different types of flaws.     

Impedance of single-turn coil surrounding a conducting cylinder with a flaw

The impedance of a single-turn coil which surrounds a conducting cylinder with a flaw is calculated employing Green's function technique. The Born approximation is used in order to know how the impedance change due to the presence of a flaw depends on the conductivity and size of a flaw.     

Quantitative characterization of flaws near a surface using inverse born approximation

Quantitative reconstruction of volumetric flaws near a surface of an elastic solid has been carried out experimentally by analyzing the scattered ultrasonic waves. The inverse Born approximation (developed for flaws in bulk materials) was tested for the first time in the determination of the size, shape, and orientation of near-surface flaws. We have studied spherical solid inclusions at various depths below the surface. In addition we examined an approximately 2:1 prolate spheroidal inclusion which was located one major axis below the surface. The determination of the flaw's size, shape, and orientation in terms of an equivalent ellipsoid is realized by performing nonlinear least-squares iteration of the one-dimensional Born inversion results obtained at various scattering directions within a finite aperture. The reconstruction is in good agreement with the actual parameters of the flaw.     

Forward ultrasonic scattering from multidimensional solid or fluid inclusions buried in multilayered elastic structures

The scattering of elastic waves by multidimensional objects buried in a multilayered elastic background medium insonified by ultrasonic forces is studied. The framework developed is based on an integral equation formalism for elastic scattering, where a Born approximation for an inhomogeneous background medium is applied to obtain a closed-form expression for the scattered field. Both compressional and shear waves are allowed to propagate inside the layers, and take into account cross-polarization at the boundaries and multiple reflections within the layers to compute the scattered field. The procedure has been tested in cases where the Born approximation is severely put to test (100% contrast in shear modulus).     

Acoustical tomography based on the second-order Born transformperturbation approximation

Reconstruction algorithms and their numerical examples of acoustical tomography based on the second-order Born transform perturbation approximation are presented. The reconstruction algorithms in the second-order Born approximation are similar in form to those in the first-order Born approximation. Replacing the angular spectrum of the scattered wave in the first-order case by the result of applying a first-order operator to the angular spectrum of the scattered wave or applying a second-order operator to the angular spectrum of the incident wave leads to the second-order reconstruction algorithms. Also, comparisons of reconstruction algorithms of the first- and second-order Born approximations are given, and they show that the second-order Born approximation algorithms have a distinct advantage over the first-order approximations in many cases