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1.
We present the theory of electromagnetic induction in spherical cap current sheets of arbitrary angular size, with arbitrary axisymmetric integrated electrical conductivity variations and located at any radial position with respect to the surface of observation. The external time-varying magnetic field may be arbitrarily oriented with respect to the current layer cap and the induced fields are derived for vacuum boundary conditions appropriate to terrestrial induction and plasma confinement boundary conditions relevant to lunar induction in the solar wind or magnetosheath plasmas. Numerical evaluations show the induced magnetic field as a function of position over the current sheet cap, depth to the current layer, size of the cap, integrated electrical conductivity of the current sheet, and frequency of the fluctuating external field. The local vertical magnetic field component and the horizontal field component which is normal to the periphery of the cap exhibit peak inductive responses above the edge of the current sheet for external magnetic fields perpendicular to the axis of the cap. Thus, induced magnetic field fluctuations observed over the edge of a conductivity anomaly may exhibit a highly directional, or polarized behavior. This may provide an explanation for the asymmetric character of induced magnetic field fluctuations observed on the lunar surface. 相似文献
2.
The perturbation of alternating geomagnetic fields by conductivity discontinuities is considered. A numerical method is used to solve the two-dimensional induction problem. Models in which the conductivity contrast between conductive regions is great, such as between oceanic and continental regions, are considered. The perturbation field distributions for models with higher conductivity contrasts are compared with lower conductivity contrast models using a method recently developed by Jones (1972). Total field solutions are obtained for several models by using a very large grid as well as a folding-in technique. Results from the two methods for the various models are compared. It is found that the folding-in technique offers an alternative method for handling higher conductivity contrasts. 相似文献
3.
R.J. Banks 《Physics of the Earth and Planetary Interiors》1973,7(3):339-348
This paper is a review of techniques of processing magnetic variation data from the point of view of their effectiveness in determining the parameters that describe a particular electromagnetic induction problem. Among the factors that influence the choice of data-processing technique are: (1) the relative importance of induction by vertical and horizontal magnetic field variations, and (2) the relative importance of local induction in the conductivity anomaly as compared with its influence on the flow of worldwide current systems.The response of an anomaly can be calculated by transfer-function methods, and presented in the form of frequency-dependent induction vectors or ellipses. The usefulness of internal/external field separation procedures is limited by the problems involved in estimating the spatial behaviour of the normal variation fields. 相似文献
4.
在感应测井的有限元模拟中,为了消除源的奇异性,一般将总场分解为背景场和散射场.本文定量研究了不同背景场选择方法对计算精度的影响.首先,分析了在均匀和径向分层介质中不同背景电导率对长短源距线圈系有限元模拟结果的影响;其次,利用三层介质模型对比了选择源点附近和线圈系中点附近地层电导率作为背景电导率的结果.研究结果表明,如果在选择背景场时,只考虑源附近的散射场梯度而不同时考虑源和接收器附近的散射场梯度,计算误差明显增大.在此基础上提出一种利用Gianzero几何因子计算的视电导率作为背景电导率的新的背景场方法.该方法综合考虑了围岩、井眼、线圈距等因素,特别是在介质分界面处,可有效减小计算误差,并取得了满意的精度.本研究为复杂环境下的感应测井模拟的背景场选择提供了指导和依据. 相似文献
5.
Crossing Honghe fracture in Yunnan Province two measuring profiles were set up, each measuring profile consisted of 8 observing
sites. Three component geomagnetic variation observations were carried out continuously and simultaneously along each profile.
Induction arrows were calculated for the periods from 8 min to 60 min. There exists a reversal axis of induction arrow between
Eryuan, Xiaguan, Weishan, Mejiang and Yongping, Yunxian, Lincang, Simao. The real arrows reverse from one side of the axis
to another side and the magnitude of the real induction arrows attenuates with increase in distance from the axis, the attenuation
in northeast side of the axis is slightly less than that in southwest side. The real induction arrows of shorter periods are
greater than those of longer periods at most of the observing sites. According to the features of the real induction arrows
and by use of numerical calculation we tried to look for the restriction which was to be attached to the possible high conductivity
zone in the area under investigation. It shows that there is a high conductivity belt west of Honghe fracture which tilts
northeastward and there is a high conductor cover over the high conductivity belt beneath most of the observing sites
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 201–210, 1992. 相似文献
6.
本文是“地震的感应磁效应（一）--三维电磁感应的数值理论”一文的继续。首先从理论和实际计算两个方面证明了三维电磁感应数值方程解的唯一性、收敛性和稳定性,从而充实了作为研究地壳、上地幔电性结构横向不均匀性理论基础的“三维数值方法”。作为这一理论方法的实际应用,文中对不同源场周期和具有不同埋藏深度的三维电导率异常体进行了模拟计算,以研究地震感应磁效应的大小、空间分布特征和频率特性。模拟计算结果表明,对于周期从数秒到数分钟的地磁短周期变化,若电导率异常体的线度与5至7级地震的震源体积大体相当,其电导率较原背景电导率高近一个量级,其感应磁效应主要特征为:在异常体正上方,地面磁场的水平分量变化最大,相对变化量约30％;在异常体于源场方向一致的两侧,垂直分量变化最大,相对变化量约40-50％。因此,观测短周期地磁场的异常变化,有可能是监测地震孕育过程地下电性变化的一种试验途径。但由于上述异常强度在空间上衰减迅速,所以观测必须接近震源区,这对测点的选择是个不利的因素。 相似文献
7.
本文用E极化的二维有限元法和有限差分法研究导体层边缘附近的虚感应矢量.首先,研究有一定埋深的导体--围岩界面处的情况,其中包括虚感应矢量的频率响应.其次,研究了海岸效应的虚感应矢量的频率响应.在海岸效应中不仅考虑了不同海水层厚度的影响,还考虑了海水与陆地的电导率比值大小对虚感应矢量的影响.最后,研究了被动陆缘（passive continental margin）处的海岸效应,检查磁感应矢量的测量是否能揭示海岸下面深部电导率的横向变化（即与岩石层厚度变化有关联的电导率变化）. 相似文献
8.
PRABHAKAR S. NAIDU 《Geophysical Prospecting》1970,18(Z1):774-785
A point source or a dipole source of electric current, placed on a randomly layered semi-infinite medium, produces an inhomogeneous random potential field on the surface. The variance of the random potential, normalized with reference to the normal field (that is, a field due to a point source or a dipole source on a homogeneous medium), falls off inversely as the distance from the source. The conductivity of the layers is assumed to vary randomly about a mean value (unity) such that the variations can be represented by a zero mean homogeneous random function. Further the variations are assumed to be small compared to the mean so that the first order perturbation is adequate. The analysis shows that the dipole field is more sensitive than the single pole field to the conductivity variations. 相似文献
9.
本文提出了能提高异常体分辨能力,同时得到绝对电导率的地面磁电阻率数据三维反演方法.磁电阻率响应用准直流的低频磁场代替;数值模拟由频率域电场满足的Helmholtz方程出发,采用三维交错网格有限差分法;长直导线源作为发射源,其中源的计算包含在背景场中;结合地面磁电阻率数据各分量的特点,选择y分量进行反演研究;反演采用三维非线性共轭梯度反演技术,为了提高异常体的深度分辨能力,进行迭代重构反演;用印模法对初始模型进行重构,采用的是辅模型在浅部,元模型在深部的组合方式.从合成数据和实际数据的反演结果可以得到以下的认识:(1)由频率域麦克斯韦方程组出发,低频磁场数据反演可以直接得到电导率,而不是相对电导率之比;(2)采用印模法组合初始模型,进行迭代重构反演,可以提高地面磁电阻率数据反演对异常体的分辨能力,确定埋深位置,同时不会丧失对于浅部异常体的分辨能力;(3)在结合印模法的地面磁电阻率数据三维反演中,深部异常体的分辨能力受地表不均匀导电体影响较小;(4)确定印模深度可以采用上一次重构反演结束时的模型变化量,通过相邻两次重构反演结束时的模型变化量之差来确定迭代重构是否终止.因为静磁场与重力场在数学上的相似性,本文的反演方法可以被运用到重力场等位场的地面数据的反演中. 相似文献
10.
F.E.M. Lilley 《Physics of the Earth and Planetary Interiors》1974,8(4):301-316
The geomagnetic induction tensor is a means of summarizing the response of the earth at a given observing site to a geomagnetic variation source field. In this paper the characteristics of the tensor elements are examined, both generally and for the special cases of one-dimensional and two-dimensional geologic structure. The first-order model is taken of uniform source fields originating external to a semi-infinite half-space. Graphical ways of presenting the information contained in an induction tensor are explored, including ellipses of rotation, polar diagrams, and diagrams analogous to the Mohr circles of elasticity theory. Criteria to distinguish “two-dimensional” data from “three-dimensional” data are established. The advantages of simultaneously recording “normal” and “anomalous” variations are demonstrated in terms of the extra tensor elements which may then be estimated. The most practical way of presenting information from many stations on a map may be by drawing, for each site, arrows which summarize the response in the vertical field and quadrics which summarize the response in the horizontal field. 相似文献
11.
M. Menvielle J.C. Rossignol P. Tarits 《Physics of the Earth and Planetary Interiors》1982,28(2):118-128
A numerical thin-sheet model is used to study the coast effect and to determine the magnitude of the near-surface part of the anomalous field. Theoretical induction arrows are computed for Australia, California, Japan and the British Isles and compared with the observed arrows. This comparison shows that the main part of the coast effect, and in some cases the whole coast effect, is due to the deviation of currents flowing in the ocean by the upper part of the resistive continents. It is suggested that the term ‘coast effect’ should refer to the anomalous field whose source is near-surface. The coast effect so defined behaves like a regional anomaly superimposed on other anomalous features of the transient geomagnetic field. A numerical determination of the geometrical features of the coast effect allows consideration of its elimination. This elimination is performed for California and Japan. A correlation between the resulting residual and heat-flow data is apparent in both cases. 相似文献
12.
从电偶源三维地电断面可控源电磁法的二次电场边值问题及其变分问题出发,采用任意六面体单元对研究区域进行剖分,并且在单元分析中同时对电导率及二次电场进行三线性插值,实现电导率分块连续变化情况下,基于二次场的可控源电磁三维有限元数值模拟.这个新的可控源电磁三维正演方法可以模拟实际勘探中地下任意形状及电性参数连续变化的复杂模型.理论模型的计算结果表明,均匀大地计算的视电阻率误差和相位误差分别为0.002%和0.0005°.分层连续变化模型的有限元计算结果表明,其与对应的分层均匀模型解析结果有明显差异.三维异常体组合模型以及倾斜异常体等复杂模型的有限元计算结果也有效地反映了异常形态. 相似文献
13.
Electromagnetic induction in the Vancouver Island region for a uniform inducing source field for 300 s period is investigated with the aid of three-dimensional (3-D) numerical and analogue model results and field site measurements. The thin sheet numerical model, based on the subducting Juan de Fuca plate analogue model ofDosso
et al., consists of a 5km thick non-uniform thin sheet (comprising the lateral conductivity contrasts arising from the land, the varying depth ocean, and the sediment) underlain by a four-layer conductive structure. The four-layer conductive structure beneath the non-uniform thin sheet simulates the effect of the Juan de Fuca plate subducting Vancouver Island. To examine the effects of the ocean channel depth between Vancouver Island and the British Columbia (Canada) mainland, numerical results were obtained for two channel depths (0 and 600 m). The results indicate that the channel plays an important role in the geomagnetic response in the central and inner coastal regions of Vancouver Island. The general agreement of the 3-D numerical model induction arrows with the analogue model and field site induction arrows for 300 s supports the premise of a layered conductive substructure dipping at a small angle, at most, beneath Vancouver Island.Lithoprobe Publication No. 311. 相似文献
14.
This paper tries to formulate the C-response of geomagnetic depth sounding(GDS)on an Earth model with finite electrical conductivity. The computation is performed in a spherical coordinate system. The Earth is divided into a series of thin spherical shells. The source is approximated by a single spherical harmonic P10 due to the spatial structure of electrical currents in the magnetosphere. The whole solution space is separated into inner and external parts by the Earth surface. Omitting displacement current, the magnetic field in the external space obeys Laplacian equation, while in the inner part, due to the finite conductivity, the electromagnetic fields obey Helmholtz equation. To connect the magnetic fields in the inner and external space, the continuity condition of magnetic fields is used on the Earth surface. The external magnetic fields are expressed by the inner and external source coefficients, from which a new parameter called C-response is computed from the inner coefficient divided by the external coefficient, thus normalizing the actual source strength. The inner magnetic fields in each layer can be recursively derived by the continuity boundary condition of both normal and tangential components of the magnetic field from the initial boundary condition at core-mantle-boundary. The consistency of our C-responses with that from a typical 1-D global model validates the accuracy of the proposed algorithm. Numerical results also show that the C-response estimated from the geomagnetic transfer function method will deviate exceeding 5%from the actual response at longer periods than about 106s, which means that ignoring the curvature of the Earth at extreme long periods will make inversion result unreliable. Therefore, an accurate C-response should be computed in order to lay a solid foundation for reliable inversion. 相似文献
15.
B. R. Arora A. Rigoti I. Vitorello A. L. Padilha N. B. Trivedi F. H. Chamalaun 《Pure and Applied Geophysics》1998,152(2):349-375
—Magnetovariational fields recorded by an array of magnetometers in the equatorial region of north-northeast Brazil are analyzed to infer the configuration of internal induced currents in and around the extensive intracratonic Parnaíba basin. Only nighttime magnetovariational fields were used because of the prevailing uniform source field conditions. For periods exceeding 40 min. the vertical fields at all inland sites are dominated by the effects of electric currents originating in the northeast, in the deep Atlantic Ocean. Below this period, although best developed in the 12–15 min. period range, the anomalous signatures are principally controlled by two distinct continental current paths. The first is associated with a N60°E trending graben-like structure in the southeastern part of the basin (named the Parnaíba Basin Conductivity Anomaly—PBCA) and the second appears as a subsurface sedimentary channel, from the NW corner of the array to the central part of the basin. This is named the LINK anomaly, as it connects the northwestern Marajó basin with the Parnaíba basin. While the PBCA is shown to highlight the importance of basement tectonics in the geological evolution of the Parnaíba basin, the LINK anomaly provides strong geophysical evidence of the direction of the sea intrusion into the region of the basin and possibly indicates the connectivity of the Parnaíba basin to the adjoining Amazon basin through the Marajó basin. Frequency and polarization dependence suggest that the induction response of individual structures is not determined by the local conductivity alone but also by their interconnectivity as well as by their linkage to the continental shelf and deep oceanic region. 相似文献
16.
Integral equation of relationship on the apparent conductivity and true conductivity in induction logging 总被引：2，自引：0，他引：2
JianHao Li 《中国科学:地球科学(英文版)》2014,57(8):1972-1978
This paper investigates the relation between the induced electromotive force measured by induction logging tool and the apparent conductivity, and the relation between the apparent conductivity and the formation true conductivity. Assuming the conductivity in Green’s function to be the function of the field point coordinate, the apparent conductivity expression of electric-field intensity is derived using Green’s formula, and the integral equation has been established representing the relationship of the apparent conductivity with the true conductivity under this condition. The integral equation is analyzed and then leads to the conclusion that the equivalent conductivity is the apparent conductivity and the values of the apparent conductivity function contain the true conductivity, and the method derived the true conductivity from the apparent conductivity around the well axis is put forward. The validity of the approach given in this paper is verified through numerical calculation. On the basis of above means, the transmitter coil produces an electric-field distribution in formation at every point where the induction logging tool moves along a wellbore, and a number of the receiving coils obtain an apparent conductivity distribution; this is what induction electric-field logging is. 相似文献
17.
The data of long-term electromagnetic monitoring are used for studying the dynamics of electric conductivity of the medium
and the electric field of the terrestrial sources. The electric conductivity of the medium is estimated from the magnetotelluric
transfer functions (impedance tensor and telluric tensor). The electric field of terrestrial sources is identified by filtering
the variations of the observed electric field of the Earth. The magnetotelluric parameters and the electric field of terrestrial
sources feature anomalous changes of supposedly earthquake-related origin. The anomalies associated with the same earthquake
are not simultaneous. It is shown that these anomalies are generated by processes occurring at different depths. The strong
earthquake is preceded by the appearance of surface anomalies several months before the event and accompanied by a deep coseismic
anomaly. The probable nature of the recognized anomalies is discussed. 相似文献
18.
Jichun Wu Bill X. Hu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(6):665-682
The unconditional stochastic studies on groundwater flow and solute transport in a nonstationary conductivity field show that
the standard deviations of the hydraulic head and solute flux are very large in comparison with their mean values (Zhang et al.
in Water Resour Res 36:2107–2120, 2000; Wu et al. in J Hydrol 275:208–228, 2003; Hu et al. in Adv Water Resour 26:513–531, 2003). In this study, we develop a numerical method of moments conditioning on measurements of hydraulic conductivity and head
to reduce the variances of the head and the solute flux. A Lagrangian perturbation method is applied to develop the framework
for solute transport in a nonstationary flow field. Since analytically derived moments equations are too complicated to solve
analytically, a numerical finite difference method is implemented to obtain the solutions. Instead of using an unconditional
conductivity field as an input to calculate groundwater velocity, we combine a geostatistical method and a method of moment
for flow to conditionally simulate the distributions of head and velocity based on the measurements of hydraulic conductivity
and head at some points. The developed theory is applied in several case studies to investigate the influences of the measurements
of hydraulic conductivity and/or the hydraulic head on the variances of the predictive head and the solute flux in nonstationary
flow fields. The study results show that the conditional calculation will significantly reduce the head variance. Since the
hydraulic head measurement points are treated as the interior boundary (Dirichlet boundary) conditions, conditioning on both
the hydraulic conductivity and the head measurements is much better than conditioning only on conductivity measurements for
reduction of head variance. However, for solute flux, variance reduction by the conditional study is not so significant. 相似文献
19.
Malcolm R. Ingham 《Physics of the Earth and Planetary Interiors》1985,39(3):182-193
Magnetovariational measurements have been made at 10 sites on the northern side of the Cook Strait, New Zealand. Single-station transfer functions have been calculated for the sites and indicate that the effect of induction in the shallow water of the Cook Strait is most important at around 1000 s period. At longer periods the effect of induced currents in the Pacific Ocean predominates.A two-dimensional electrical conductivity model including local conductivity structure has been shown to satisfy the measured responses at sites about 60–80 km distance from the strait. Closer to the strait the inductive process is strongly three-dimensional. A simple d.c. line current model of current flow has been shown to reproduce some of the features of the observed responses.Induction arrows indicate the existence of conductivity anomalies associated with a known lateral seismic boundary and with one of the two principal faults in the region. 相似文献
20.
井间电磁测量的2．5维层析成像方法 总被引：1，自引：0，他引：1
利用正则化最小二乘反演方法实现了井间电磁测量数据的层析成像,对井间地层电阻率进行了重建。在成像算法中,我们假设了井间电磁的激发与接收采用电磁偶极子源,井间介质仅在二维（xoz）平面内变化。在数值模拟中,通过对构造走向（y方向）的Fourier变换,将三维电磁场问题转化为一系列二维问题,用等参有限元方法在波数域求解,使实际地层模型的处理得以实现。对于波数域中每个波数对应的电磁场方程采用等参有限元求解,并用高斯积分将波数域解变换为空间域电磁场。利用源与接收器电磁场的互易原理,实现了电磁场响应对电导率分布灵敏度的快速计算。针对正演模拟中源点的奇异性,我们采用具有一定面积的伪艿函数表达源电流分布,使数值解精度得到提高。用层状介质的解析解与数值计算结果的对比,验证了模拟算法的精度。用介质扰动产生的电磁场变化检验互易性定理计算灵敏度的有效性。对简单块状模型、斜向裂缝带模型及“大”字模型的模拟数据成像结果表明,本文介绍的层析成像方法是正确有效的。 相似文献