Diffraction imaging by uniform asymptotic theory and double exponential fitting

Seismic diffracted waves carry valuable information for identifying geological discontinuities. Unfortunately, the diffraction energy is generally too weak, and standard seismic processing is biased to imaging reflection. In this paper, we present a dynamic diffraction imaging method with the aim of enhancing diffraction and increasing the signal‐to‐noise ratio. The correlation between diffraction amplitudes and their traveltimes generally exists in two forms, with one form based on the Kirchhoff integral formulation, and the other on the uniform asymptotic theory. However, the former will encounter singularities at geometrical shadow boundaries, and the latter requires the computation of a Fresnel integral. Therefore, neither of these methods is appropriate for practical applications. Noting the special form of the Fresnel integral, we propose a least‐squares fitting method based on double exponential functions to study the amplitude function of diffracted waves. The simple form of the fitting function has no singularities and can accelerate the calculation of diffraction amplitude weakening coefficients. By considering both the fitting weakening function and the polarity reversal property of the diffracted waves, we modify the conventional Kirchhoff imaging conditions and formulate a diffraction imaging formula. The mechanism of the proposed diffraction imaging procedure is based on the edge diffractor, instead of the idealized point diffractor. The polarity reversal property can eliminate the background of strong reflection and enhance the diffraction by same‐phase summation. Moreover,the fitting weakening function of diffraction amplitudes behaves like an inherent window to optimize the diffraction imaging aperture by its decaying trend. Synthetic and field data examples reveal that the proposed diffraction imaging method can meet the requirement of high‐resolution imaging, with the edge diffraction fully reinforced and the strong reflection mostly eliminated.     

Calculation of Ice Crystal Diffraction

The diffraction angular distributions are calculated for different ice crystals (e.g,solid and hollow hexagonal columns,solid plates and single hexagonal bullet) randomly oriented in space,based on the theory of Cai and Liou.Results of solid column and plate are compared with those from formulas of Wendling et al and Liou.The resull comparison shows that all three procedures discussed in this paper have good agreement.After considering the coin putation time and comparison results,it is suggested that the formula of Wendling et al.can be used to calculate the solid column diffraction,and the formula of Liou can be used to calculate the plate diffraction.The comparison also shows that the diffraction results of solid columu,hollow column and single bullet ice particles randomly oriented in space are very close.     

稳定电流场的反射、透射、衍射——波动性

稳定电流场是否有反射、透射现象,它们服从什么规律,以及稳定电流场是否具有波动性等问题,本文将予以探讨。 一、稳定电流场的反射与透射 在用“电像法”解点电流源平界面问题时,一般用反射、透射的概念,并用虚源等效地代替界面,指的是全界面的、源的“反射”与“透射”,要求界面为无限平面,其结果没有普遍性。因此,尚不能当作一般的物理规律来看待。从弹性波与稳定电流场的类比中,有人曾     

The POSEIDON Nearshore Wave Model and its Application to the Prediction of the Wave Conditions in the Nearshore/Coastal Region of the Greek Seas

In the present work, the Poseidon Nearshore Wave Model (PNWM) developed in the framework of the POSEIDON project 1 , and its application to the prediction of the wave conditions in nearshore/coastal regions of Greek seas is presented. The PNWM is based on a one-way energy coupling between a third-generation, phase-averaged, nearshore wave model and a local phase-resolving model, nested in the first model. The local wave model is supported by the consistent coupled-mode theory, based on an enhanced local-mode representation of the wave velocity field, which except for the propagating and the evanescent modes includes an additional mode, permitting the exact satisfaction of the sloping-bottom boundary condition, even in areas with locally steep bottom slope and large curvature. Thus, three-dimensional diffraction effects are fully treated in the local nested area. Numerical results are presented demonstrating the application of the PNWM to nearshore and coastal sites of the Greek seas.     

Diffraction of seismic waves in an elastic, cracked halfplane using a boundary integral formulation

The presence of subsurface cracks in a halfspace excited by elastic waves may give rise to scattered body and surface waves. For many engineering applications, such as non-destructive testing or oil exploration, the scattered field may yield valuable information to detect cracks and other scatterers. We use the Indirect Boundary Element Method (IBEM) to study the diffraction of P, SV waves with various incidence angles and Rayleigh surface waves. This approximate boundary integral technique is based upon the integral representation for scattered elastic waves using single-layer boundary sources. Our approach is usually called indirect BEM as the sources' strengths should be obtained as an intermediate step. This indirect formulation can give to the analyst a deep physical insight on the generated diffracted waves because it is closer to the physical reality and can be regarded as a realization of Huygens' Principle. In any event, mathematically it is fully equivalent to the classical Somigliana's representation theorem. In order to gauge accuracy we test our method by comparing with previous results in the literature. Various crack configurations, including multiple cracks, are investigated. Results in frequency and time domains are displayed. Under certain conditions the amplitude spectra of those waves clearly show conspicuous resonance peaks.     

非均质矿体地震模型的衍射、转换波及次生波研究

采用超声地震模型、数学模型正演试验和矿井试验观测研究了１＜ｋα＜１０范围非均质体散射波场中，衍射波和转换波的运动学和动力学特征．上述方法获得了一致的结果．研究中发现一种次生震源及次生波，其在今后地震勘探中可能有一定的应用价值．     

电解质水溶液的结构

本文叙述了盐湖中主要离子Ｌｉ＋、Ｎａ＋、Ｋ＋、Ｍｇ２＋、Ｃａ２＋、Ｃｌ－、ＳＯ＿４￣２－、ＮＯ＿３和纯水的结构，介绍了主要研究方法Ｘ射线衍射．分析了溶液结构研究历史和现状，对溶液结构研究的未来进行了展望．     

随县—马鞍山地带地壳与上地幔结构及郯庐构造带南段的某些特征

本义利用1979—1981年中国科学院地球物理研究所在随县—马鞍山所做的人工源地震探测的资料,采用二维射线追踪来拟合地震波的走时和振幅,取得了该地带的地壳与上地慢速度结构.结果表明,该地带地壳厚度为32—37km,莫霍面起伏变化显著.地壳速度的横向变化甚为强烈,地壳中部存在低速层.在巢湖以西宽数10km的范围内,地壳结构与其它地方明显不同,上地壳速度明显偏高,莫霍面上隆并有间断;在其两侧,明显可见有两条深断裂.结合其它地球物理场特征及大地构造概况,对结果进行分析,认为郯庐构造带南段被两条深断裂所夹,构造带宽度可达数10km,并在地质历史时期曾有过地幔物质上涌.     

图像的空间频谱识别与分类

本文介绍一种光学－计算机混合处理系统对图像进行空间频谱分析的原理和方法。该系统的光学部分是一个夫琅和费衍射装置，它把光学图像转换成二维的傅立叶频谱，即空间频谱。计算机则负责频谱数据的采集与分析。鉴于遥感地质日视解译中区分岩类的主要识别指标是遥感图像的影纹特征，文中以岩石的遥感图像的空间频谱采样与特征提取，对空间频谱进行识别和分类为例，阐明利用衍射图样对图像进行识别的方法。     

基于拉曼光谱-红外光谱-X射线衍射技术研究斜硅石的相对含量与石英质玉石结晶度的关系

石英质玉石的成分除α-石英外,还或多或少含有一种低温低压的SiO_2矿物"斜硅石",关于斜硅石的相对含量与石英质玉石结晶程度的关系还未见有研究。本文选取中国云南龙陵、安徽霍山、广西贺州和内蒙古阿拉善盟地区所产出的不同类型的石英质玉石为研究对象,采用拉曼光谱、红外光谱、X射线衍射分析技术,确定了斜硅石的相对含量与石英质玉石结晶程度之间的关系。结果表明,石英质玉石中斜硅石的相对含量越高,石英质玉石的结晶程度越低;其中拉曼光谱中能够描述斜硅石相对含量变化的502 cm~(-1)/465 cm~(-1)两峰强度比值(X)与石英质玉石的结晶度指数(Y)基本上呈负相关关系:Y=-0.36X+6.93(r=-0.94)。此结论为利用无损检测手段大致判断石英质玉石的结晶程度提供了新的思路,同时也为定性评价石英质玉石的品级提供了科学依据和理论约束。