共查询到18条相似文献，搜索用时 171 毫秒
1.
本文对地球物理学中的电磁场积分方程正演进行了综述,重点分析和讨论了积分方程正演中的散射场近似求解方法.散射场近似解法可以在保证计算精度的前提下有效的提高计算效率,使积分方程正演突破了传统简单孤立异常体研究的限制,适用于大规模复杂三维电磁场快速正演.本文着重对近年来国内外学者提出的散射场近似求解方法,如扩展Born近似、高阶广义Born近似、准线性(QL)近似和准解析近似(QA)等进行了分析和讨论,指出了各种近似解法的优缺点和适用范围.并在前人工作的基础上总结了地球物理学中的电磁场积分方程正演的基本原理和关键问题及解决方法,包括并矢格林函数、散射场的近似求解方法以及全积分求解方法等.最后,本文提出了积分方程法发展趋势和实际工程应用的前景以及面临的困难和待解决的问题. 相似文献
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从磁场的积分方程出发,应用迭代方法推导了二次迭代条件下粗糙海面的近似电磁散射极化核函数,并对其进行了分析.这种函数的表示是在光学近似即KA近似的基础上的一种扩展形式,因而可称为扩展KA近似(EKA).Elfouhaily采用了与Holliday类似的方法分析了双站配置条件下散射的极化核函数,试图保留所有的高次项,但是由于处理的复杂性他只给出了斜率项在散射项中的表示形式,并称之为KA近似的"补充项".本文在此基础上进一步考虑了Elfouhaily近似的形式,给出了表示双站散射极化核函数的完整表达形式,并以此为基础推算了后向散射条件下极化核函数的理论结果,并与Elfouhaily的结果进行了对比,说明了极化核函数完整表达形式的合理性.近似散射极化核函数的分析对于深入理解粗糙表面(特别是海面)电磁散射特性以及散射近似模型的建立等问题的研究具有重要意义. 相似文献
3.
拟解析近似方法是一种解决电磁场散射问题的快速求解积分方程的近似方法,它绕开了传统数值方法中的求解大型代数方程组或大型矩阵问题,适用于强散射和大扰动问题.本文应用孙建国提出的求解异常电场积分方程的张量拟解析近似理论公式,研究用其求解直流电场积分方程.利用接近实际的地电模型对异常电场进行模拟,研究了均匀场中异常球体的张量拟解析近似解;并对均匀场中的立方体异常体进行了数值计算.效果良好并具有很高的计算速度.研究结果为三维直流电场快速正反演模拟打下基础. 相似文献
4.
在背景模型基础上,求解模型扰动后的地震波散射场,这是目前地震反演中的一个关键步骤.本文将计算数学中求解非线性积分方程的Adomian分解方法,应用到求解标量波散射场的Lippmann-Schwinger积分方程和Ricatti积分方程中,分别得到了散射场的Born序列解和Rytov序列解.通过一维和二维数值算例说明:在满足一定的条件下,散射场的这两种序列解稳定收敛,与传统的Born和Rytov近似解相比,引入散射序列中的高阶项可以更精确地描述地震波散射场. 相似文献
5.
基于长期从事水工建筑物渗漏、管涌等安全隐患检测工作的需要,结合水工建筑物渗漏管涌隐患分布的特点,对水底及水体中的电场分布进行了分析研究.本文从水流场与电流场的相似性、渗流场与电流场的相似性以及动态导体充电法的相关理论三个方面阐述了流场法理论的客观性、合理性以及与动态导体充电法理论的兼容性.首先利用水流场与电流场的相似性解释异常水流场,其次从渗流场与电流场的相似性解释异常水流场,最后利用水工建筑物堤坝渗漏管涌通道地电模型,进一步论述了利用地下动态导体充电法中的不等位导体理论来认识流场法理论更为方便,并可以进行渗漏出口处的电流强度计算及进行正反演计算,同时证明了异常水流场与电流场有着近似的对应关系,以工程实例介绍了流场法在水工建筑物渗漏管涌检测中的应用. 相似文献
6.
在海面波破碎区，假定随机粗糙的海面被两层不同散射性质的球形粒子覆盖，上层是由海面飞沫组成的球形Ｒａｙｌｅｉｇｈ粒子层，下层是由气泡组成的球形Ｍｉｅ粒子层，根据大量观测和实验的统计结果，确定了两层散射粒子的平均半径和单位体积的粒子数；并用迭代法求解随机粗糙海面上的两层粒子的矢量辐射传输方程．根据风波破碎时白冠覆盖率和海面飞沫、气泡的统计模型，给出了一个新的风驱海面的微波散射理论模型．利用这一模型定量地给出了海面后向散射系数和风速、视角、极化和白冠覆盖率等特征参量之间的函数关系，并研究了覆盖在波破碎区的散射粒子对海面后向散射系数影响程度． 相似文献
7.
单程波近似实际上是一种多次前向散射和单次后向散射近似.利用单程波近似来描述波传播可以极大地节省地震数值模拟的计算时间和内存,实现地震波长距离传播模拟和三维地震模拟快速计算.本文基于单程波近似和波动积分方程的分离变量逼近,从广义Lippmann-Schwinger波动积分方程推导出耦合反射/透射系数的单程波传播算子.该算子由两部分构成：分离变量Fourier单程波传播算子和薄板间的反射/透射系数表达.前者将常规的Fourier分裂步单程波传播算子(SSF)推广适应横向强速度变化介质和大角度传播波场.后者是利用垂直波数来表示反射/透射系数,自然耦合到波场传播的计算过程中,其为地质界面倾角的隐式表达,精确描述振幅随入射角的变化,能适应任意复杂的模型.通过两个数值算例和一个实际地质模型的计算,本文将该方法和边界元法进行了比较,结果表明：在算例给出的介质横向速度变化情况下,本文提出的方法在相位和振幅方面与全波数值方法基本吻合. 相似文献
8.
实际海面受风浪的影响是个随机起伏的复杂曲面.一个随机起伏的自由表面对地震波形成相当复杂的散射，进而影响波场在靶区的二次照明与成像结果；此外，起伏海面具有小尺度随机起伏的特性，难以用贴体网格等处理大尺度起伏地表的常规方法和技术对其进行逼近和处理.鉴于此，利用不等距有限差分法实现小尺度起伏海面的波场自由表面边界条件，以此进行正演、波场照明分析；利用逆时偏移（Reverse Time Migration，RTM）进行起伏海面下的成像并对成像结果进行分析.不同模型的试算结果表明：起伏海面引起的复杂散射使靶区地下照明不均匀、成像界面发生弯曲和畸变，从而降低成像结果的分辨率和信噪比. 相似文献
9.
传统的波动方程走时核函数(或走时Fréchet导数)多基于互相关时差测量方式及地震波场的一阶Born近似导出,其成立条件非常苛刻.然而,地震波走时与大尺度的速度结构具有良好的线性关系,对于小角度的前向散射波场,Rytov近似优于Born近似.因此,本文基于Rytov近似和互相关时差测量方式,导出了基于Rytov近似的有限频走时敏感度核函数的两种等价形式:频率积分和时间积分表达式.在此基础之上,本文提出了一种隐式矩阵向量乘方法,可以直接计算Hessian矩阵或者核函数与向量的乘积,而无需显式计算和存储核函数及Hessian矩阵.基于隐式矩阵向量乘方法,本文利用共轭梯度法求解法方程实现了一种高效的Gauss-Newton反演算法求解走时层析反问题.与传统的敏感度核函数反演方法相比,本文方法在每次迭代过程中,无需显式计算和存储核函数,极大降低了存储需求.与基于Born近似的伴随状态方法走时层析相比,本文方法具有准二阶的收敛速度,且适用范围更广.数值试验证明了本文方法的有效性. 相似文献
10.
用二维迭代方法快速地解析计算多导体圆柱目标的散射场,其中各圆柱目标自身的一次散射以各自中心为原点的柱面波函数叠加,匹配各圆柱表面的边界条件获得展开系数;各圆柱间的散射相互作用由坐标变换实现,具体体现在圆柱中心偏移产生的初始相位上.为考虑各个柱目标间的二次散射,将每个柱目标自身的一次散射看成另一柱目标的激励场,利用柱面波函数的加法定理和边界条件匹配来计算各柱目标间的二次散射贡献.类似的处理可获得三次或更高次的散射计算.与矩量法(MoM)的数值结果比较,验证了该方法的有效性,明显提高了后向散射的计算速度.对于随机生成的多个圆柱目标模型,解析计算一定频带f和360°方位角θ范围内均匀离散样点的后向散射场.为抑制数据截断产生的振铃效应,对频率向的散射数据用Hamming窗实现平滑过渡.为充分利用快速Fourier变换(FFT)的高效计算,对f－θ平面上的离散散射数据用二维的三次样条插值处理,得到X－Y平面的均匀离散数据.最后,用FFT方法快速实现逆合成孔径雷达(ISAR)成像模拟,很好地重构了各圆柱体目标的位置和大小尺寸. 相似文献
11.
R.S. Peckover 《Physics of the Earth and Planetary Interiors》1973,7(2):137-142
In addition to the electric potentials induced by the gyral motions in the oceans, horizontal electric fields and currents result from the exchange of water between the depths and the surface in the presence of the main geomagnetic field. In this note a simple model representing such a circulation is considered, and the spatial distribution of the corresponding induced electric fields is calculated. A surface velocity of 2 knots could induce electric currents up to 10?4 Amp/m2. These steady currents in the ocean could be comparable with the normal oceanic daily variation. Since a proportion of the electric current would return through the earth below the sea floor, this calculation provides an upper limit to this component. An indication is also given of the magnetic field distortion and associated electric currents which occur in a highly conducting (Jovian) ocean. 相似文献
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13.
Summary Electromagnetic field response of a homogeneous uniaxially anisotropic conducting medium has been investigated in the presence of a vertical electric dipole placed at a heighth above the conducting surface. Expressions for the horizontal components of electric and magnetic fields and hence that of vertical impedance of the field, as a function of the distance of the place of observation from the source, have been derived. Numerical computations have been done and curves are given to illustrate the influence of anisotropy on the amplitude values of the impedance on the surface of the conducting medium.N.G.R.I. Contribution number 71–279. 相似文献
14.
E. E. S. Sampaio 《Studia Geophysica et Geodaetica》2006,50(1):89-104
The analysis of the primary electromagnetic fields caused by steady state or transient electric current flowing along a current
loop moving with a constant velocity below the sea surface has several applications. It supports the analysis of submarine
physical data and it is useful for protecting ships from the threat of sea mines. The usual approach to the solution for the
primary field starts from a hertz vector potential in the frequency domain due to a magnetic dipole. Subsequently it employs
Fourier, Laplace, and Hankel transforms to describe the time variation of the primary electromagnetic induced field due to
a loop. The result is applicable to both shallow and deep sea water environments. Because of the difference in velocity between
source and receiver, a careful application of the convolution integral is necessary in order to adapt the source pulse solution
to any type of transmitting current waveform. Furthermore, since the scattered field represents a fraction of the primary
field, even minor differences in it caused by the differential velocity renders inadequate interpretation of EMI data. 相似文献
15.
Ocean wave imaging mechanism by imaging radar 总被引：1，自引：1，他引：0
HE Yijun 《中国科学D辑(英文版)》2000,43(6):587-595
Analytical representations of the high frequency spectra of ocean wave and its variation due to the variation of ocean surface
current are derived from the wave-number spectrum balance equation. The ocean surface imaging formulation of real aperture
radar (RAR) is given using electromagnetic wave backscattering theory of ocean surface and the modulations of ocean surface
winds, currents and their variations to RAR are described. A general representation of the phase modulation induced by the
ocean surface motion is derived according to standard synthetic aperture radar (SAR) imaging theory. The detectability of
ocean current and sea bottom topography by imaging radar is discussed. The results constitute the theoretical basis for detecting
ocean wave fields, ocean surface winds, ocean surface current fields, sea bottom topography, internal wave and so on. 相似文献
16.
The detection of a missile target in heavy sea clutter is a significantly challenging problem due to the clutter effects. In this paper, the radar cross sections (RCS) of a pre-assumed generic missile model is computed with multilevel fast multi-pole algorithm (MLFMA), while the RCS of ocean surface is computed by a more reduced form of the fractional Weierstrass scattering model proposed here. At last, the computed RCS of missile model is compared with that of sea surface, and then the comparisons of missile-to-ocean RCS ratios of different incident angles, incident frequencies, and polarization patterns are also presented. The discussion and comparisons of RCS of the missile and ocean surface can help us to plan and design a radar system in the application of detection of a missile target or other analogous weaker targets in the strong sea clutter background. 相似文献
17.
A new transmitting pattern, a buried non-symmetrical electric dipole in a conducting half space, is treated theoretically. A simple formula for the stationary current field of this pattern is derived from the Laplace equation and it is confirmed by the laboratory experiments and a well site experiment. This formula gives the upper limit of the electrical signal strength at the surface, transmitted from the bottom of a drill string. 相似文献
18.
The key processes in marine seismic imaging include (i) removing from seismic data all seismic events (free-surface multiples and ghosts) which contain at least one reflection at the sea surface in their wave-propagation path, and leaving those with no reflection at the free surface (internal multiples and primaries), (ii) removing events with at least two reflections in the subsurface (internal multiples), and leaving events with only one reflection in the subsurface (primaries), and then (iii) locating the scattering points and reflectors inside the subsurface which are the sources of primaries and internal multiple events. All these processes are here explained, derived, and optimized via scattering diagrams (diagrammatica) in a way similar to the way the quantum field theory is often explained via Feynman diagrams. Our discussion of the removal of events with free-surface reflections from the data will be brief, as the diagrammatica of these events are now well understood.The main focus of this paper is the diagrammatica of internal multiples and primaries. Although these events do not contain any reflection at the sea surface, it is important to reconstruct them with scattering points near the sea surface, where seismic data are recorded. So our diagrammatica of primaries and internal multiples include events which are not directly recorded in seismic data but which can be constructed from seismic data. These events have allowed us to construct scattering diagrams of primaries and internal multiples with scattering points near the sea surface. Furthermore, these new diagrammatica of internal multiples and primaries can be used to remove internal multiples from the data. 相似文献