时间域航空电磁法一维正演研究   总被引：15，自引：6，他引：15       下载免费PDF全文

罗延钟  张胜业  王卫平 《地球物理学报》2003,46(5):719-724

基于电磁勘探理论，导出了层状大地条件下时间域航空电磁法（偶极-偶极装置）的正演计算公式和算法，编制了相应的计算机程序，对若干典型地电断面作了正演计算. 计算结果说明时间域航空电磁法的探测能力和探测条件，进而为时间域直升机航空电磁系统的设计方案提供了依据.  相似文献   

中心回线瞬变电磁法2.5维有限单元算法   总被引：23，自引：6，他引：23       下载免费PDF全文

王华军  罗延钟 《地球物理学报》2003,46(6):855-862

瞬变电磁法的野外资料解释基本上停留在一维水平，本文从生产实践中常用的中心回线法入手，基于电磁场本身的叠加原理，从麦克斯韦方程组出发，导出了中心回线瞬变电磁2.5维二次场（纯异常）的有限单元计算公式. 该算法采用三角形有限元网格，在尽可能拟合地下电性断面的情况下减少有限元网格的节点数和单元数；用选主元的LU分解法求解线性方程组，做到了在移动场源时只需改变右端项；在反傅氏变换中，使用新的波数选取方案，让波数m随时间t滑动. 最后给出的算例表明，该算法直接计算异常场，计算速度快、精度高.  相似文献   

用于多频电磁波测井的负阻抗变换式有源接收天线     

刘海滨  佟文琪 《物探与化探》1997,21(1):58-61

本文对用负阻抗变换电路同小圆锥螺旋天线相结合,构成的有源接收天线进行了理论分析,并给出了该有源天线稳定工作的条件。结果表明,用该方法设计的有源接收天线具有较宽的频带、较高的灵敏度和较小的尺寸。是多频电磁波测井较理想的接收天线。  相似文献   

Analysis of rotational invariants of the magnetotelluric impedance tensor   总被引：1，自引：0，他引：1  

L. Szarka  M. Menvielle 《Geophysical Journal International》1997,129(1):133-142

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The forward problem of electromagnetic induction: accurate finite-difference approximations for two-dimensional discrete boundaries with arbitrary geometry   总被引：3，自引：0，他引：3  

Claudia Aprea  John R. Booker  J. Torquil Smith 《Geophysical Journal International》1997,129(1):29-40

We describe finite-difference approximations to the equations of 2-D electromagnetic induction that permit discrete boundaries to have arbitrary geometrical relationships to the nodes. This allows finite-difference modelling with the flexibility normally ascribed to finite-element modelling. Accuracy is demonstrated by comparison with finite-element computations. We also show that related approximations lead to substantially improved accuracy in regions of steep, but not discontinuous, conductivity gradient.  相似文献   

Transient electromagnetic responses in seafloor with triaxial anisotropy   总被引：2，自引：0，他引：2  

L. Yu  R. L. Evans  R. N. Edwards 《Geophysical Journal International》1997,129(2):292-304

Electrical anisotropy of young oceanic crust at mid-ocean ridges is detectable by observation of the rate and geometry of the diffusion of electromagnetic fields. The anisotropy in electrical properties arises from the presence of conductive seawater in an interconnected network of mostly ridge-parallel cracks. In this paper, we first justify the choice of a triaxial model to represent young oceanic crust, with three distinct electrical conductivities in the vertical, strike and spreading directions. We then present an algorithm to calculate the transient electromagnetic responses generated by an electric dipole source over such a triaxially anisotropic seafloor. We show that if the transient passages are measured with three distinct electric dipole-dipole configurations, it is possible to discern all three unknown conductivities independently of each other.  相似文献   

Recovering magnetic susceptibility from electromagnetic data over a one-dimensional earth   总被引：1，自引：0，他引：1  

Zhiyi Zhang  Douglas W. Oldenburg 《Geophysical Journal International》1997,130(2):422-434

While the inversion of electromagnetic data to recover electrical conductivity has received much attention, the inversion of those data to recover magnetic susceptibility has not been fully studied. In this paper we invert frequency-domain electromagnetic (EM) data from a horizontal coplanar system to recover a 1-D distribution of magnetic susceptibility under the assumption that the electrical conductivity is known. The inversion is carried out by dividing the earth into layers of constant susceptibility and minimizing an objective function of the susceptibility subject to fitting the data. An adjoint Green's function solution is used in the calculation of sensitivities, and it is apparent that the sensitivity problem is driven by three sources. One of the sources is the scaled electric field in the layer of interest, and the other two, related to effective magnetic charges, are located at the upper and lower boundaries of the layer. These charges give rise to a frequency-independent term in the sensitivities. Because different frequencies penetrate to different depths in the earth, the EM data contain inherent information about the depth distribution of susceptibility. This contrasts with static field measurements, which can be reproduced by a surface layer of magnetization. We illustrate the effectiveness of the inversion algorithm on synthetic and field data and show also the importance of knowing the background conductivity. In practical circumstances, where there is no a priori information about conductivity distribution, a simultaneous inversion of EM data to recover both electrical conductivity and susceptibility will be required.  相似文献   

Time-domain electromagnetic migration in the solution of inverse problems   总被引：5，自引：0，他引：5  

Michael S. Zhdanov  Oleg Portniaguine 《Geophysical Journal International》1997,131(2):293-309

Time-domain electromagnetic (TDEM) migration is based on downward extrapolation of the observed field in reverse time. In fact, the migrated EM field is the solution of the boundary-value problem for the adjoint Maxwell's equations. The important question is how this imaging technique can be related to the solution of the geoelectrical inverse problem. In this paper we introduce a new formulation of the inverse problem, based on the minimization of the residual-field energy flow through the surface or profile of observations. We demonstrate that TDEM migration can be interpreted as the first step in the solution of this specially formulated TDEM inverse problem. However, in many practical situations this first step produces a very efficient approximation to the geoelectrical model, which makes electromagnetic migration so attractive for practical applications. We demonstrate the effectiveness of this approach in inverting synthetic and practical TDEM data.  相似文献   

Candidate precursors: pulse-like geoelectric signals possibly related to recent seismic activity in Japan   总被引：1，自引：0，他引：1  

Yuji Enomoto  Akito Tsutsumi  Yukio Fujinawa  Minoru Kasahara  Hiroshi Hashimoto 《Geophysical Journal International》1997,131(3):485-494

Telemetric network observations of pulse-like geoelectric charge signals using a vertical dipole buried under the ground were undertaken at various observation sites over a wide area in Japan from April 1996. From continuous records of the signals during the six months following that, we attempted to select anomalous signals, possibly related to seismic electric activity. Special attention was paid to the elimination of noise resulting from industrial and meteorological electric activity, comparison with other electromagnetic signals in the VLF band and the relation between the precursor period and the distance from the eventual main shock that occurred in Japan. Four candidate precursor electric signals, which were not contaminated by industrial and meteorological electric activity, were then selected, of which the second appeared before the Akita-ken Nairiku-nanbu earthquake swarm, for which the maximum M = 5.9 occurred on 1996 August 11, and the third and fourth before the Chiba-ken Toho-oki earthquake, M = 6.6, on 1996 September 11. A tentative qualitative model explaining why the candidate precursory signal is related to stress building up before an earthquake is presented in terms of the electrification of gases released from fracturing rocks immediately prior to the main shock.  相似文献   

Improved methods for solving for the magnetic field in E-polarization induction problems with fixed and staggered grids     

J. T. Weaver  X. H. Pu  A. K. Agarwal 《Geophysical Journal International》1996,126(2):437-446

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