共查询到20条相似文献，搜索用时 15 毫秒
1.
C.R. Brewitt-Taylor 《Physics of the Earth and Planetary Interiors》1975,10(2):151-158
A model for the coast-effect of geomagnetism is presented, in which the horizontal magnetic field induces currents in a circuit including a thin finite ocean. The currents flow horizontally across the ocean, vertically down into the earth, back through the deep interior of the earth, and vertically up to the ocean to complete the current loop. The upper layers of the earth are given non-zero conductivity, allowing the possibility of such current loops.A two-dimensional model involving such currents has been worked out analytically, and it is found that a significant induced magnetic field at the seafloor can be obtained with a reasonable conductivity in the earth's upper layers. A three-dimensional model has also been worked out numerically. It is found that the induced vertical component of magnetic field is of comparable magnitude to the horizontal component induced normal to the coast, whereas the horizontal component parallel to the coast is small. These relations are required to explain the observation of Parkinson arrows. 相似文献
2.
The digital linear filter method is used to compute the normalized vertical magnetic field for a circular loop in CFS system. Three-layer earth models with resistive and conductive basement are considered. The corresponding field expressions are suitably written, and the multifrequency response is computed and presented in convenient forms. Analysis of theoretical data indicates that for highly resistive basement, the variation in layer conductivity and intermediate layer thickness is well reflected on three-layer amplitude response curves at low frequencies and at high conductivity contrasts between first and second layers. This, however, is not true in the case of conductive basement, where the resolution of the intermediate layer is observed to be comparatively poor. The resolution of an intermediate conductive layer in a three-layer sequence is found to be satisfactory. 相似文献
3.
W. P. Olson 《Pure and Applied Geophysics》1989,131(3):447-462
Since the discovery of the magnetosphere, it has been known that the currents flowing in the magnetosphere contribute toSq, the regular daily variation in the earth's surface magnetic field. The early models, however, were not very accurate in the vicinity of the earth. The magnetospheric contribution toSq has therefore been recalculated by direct integration over the three major magnetospheric current systems; magnetopause, tail and ring. The finite electrical conductivity of the earth, which increases the horizontal and decreases the vertical components of the magnetospheric field at the earth's surface, has been taken into account. The magnetospheric currents are found to contribute 12 nanotesla to the day to night difference in the mid-latitudeSq pattern for steady, quiet magnetospheric conditions. They also contribute to the annual variation in the surface field and must be considered an important source of the observed day to day variation in theSq pattern. 相似文献
4.
5.
Ajit K. Sinha 《Pure and Applied Geophysics》1969,72(1):123-147
Summary The propagation of electromagnetic waves in a two-layer anisotropic earth from an oscillating vertical electric dipole placed over it has been studied. Formal expression for the vector potential in the top layer has been determined for the general case when both the layers are electrically anisotropic. The conductivity and the permittivity tensors of the layers are assumed to have simple diagonal forms. Three special cases of practical interest have been considered in detail, for which the results are given in terms of tabulated functions. It has been shown that at the surface of the earth, only the electric field components are influenced by the anisotropy. 相似文献
6.
Parameters of field-aligned currents reconstructed by ground-based measurements of magnetic field in the Scandinavian countries (IMAGE) and ionospheric conductivity for specific events of the 6 and 8 December 2004 are represented here. Ionospheric conductivity was calculated from precipitating electron flux measured at DMSP-13 satellite and electron density EISCAT incoherent scattering radar direct measurements. There is a high correlation between field-aligned currents, calculated from DMSP-13 satellite data and field-aligned currents calculated from radar measurements for the December 6, 2004 in the presence of developed ionospheric current system. The comparison of field-aligned currents, reconstructed by the proposed method, with the currents calculated by the variation of magnetic field on the DMSP satellites, confirms correctness of the offered algorithm. 相似文献
7.
地磁垂直分量Z日变幅逐日比及其与磁暴和地震的关系 总被引：1，自引：0，他引：1
地磁垂直分量Z日变幅逐日比是基于变化磁场在地球电导率的作用下会产生一个与变化磁场相反的感应磁场, 该感应磁场不仅包含了外空磁场的信息, 还包含了地下电导率的信息。 因此, 相对于地磁场其他成分来说Z分量更多地反应了地下活动的变化, 逐日比是利用前后两日地磁Z分量日变幅相比来提取该异常现象, 但磁暴可能会对逐日比异常判定产生影响。 我们取1984年至今江苏区域范围达到逐日比异常判定指标的29个事件结合磁暴以及该异常事件之后和地震的对应关系进行统计分析。 研究发现, 无磁暴时逐日比异常和预测范围内地震对应关系达到0.77, 有磁暴时逐日比异常和地震对应关系为0.25。 综合分析认为逐日比异常事件发生后, 若该日无磁暴或者Dst指数小于异常第二日, 那么该异常为地震前兆的可能性大概为0.8, 反之则该异常有可能是磁暴的影响而不是来自于地下活动的影响, 需结合其他前兆分析方法进行综合分析, 该研究增强了逐日比异常判定地震地磁前兆信息的可靠性。 相似文献
8.
Harmonic steady-state solutions for magnetic and electric vector components are obtained for a layered transitional earth in a vertical magnetic dipole field. The conduction currents dominate the displacement currents. The time factor eiωt is implied. MKS units are used. 相似文献
9.
V. V. Kalegaev I. I. Alexeev E. V. Makarenkov N. Yu. Ganvushkina 《Geomagnetism and Aeronomy》2006,46(5):563-569
A unified method for calculating the Dst index and its components using models of the magnetospheric magnetic field is proposed. The method is consistent with the procedure for calculating Dst from the ground-based magnetometer data. When calculating Dst, the quiet-day magnetic variation is subtracted from the model variation of the magnetic field of magnetospheric sources. The effect of induced currents flowing in the surface layer of the Earth’s crust is taken into account. The dynamics of the magnetospheric current systems during a storm is studied based on an analysis of the Dst components. The magnetic field components for a “quiet” day in June 1998 are studied. The calculations of the Dst components in the parabolid and T01 models demonstrate that the maximum contributions of the ring current and magnetotail current system to the Dst variation are comparable for the magnetic storm of June 25–26, 1998. 相似文献
10.
Tarkeshwar Lal 《Pure and Applied Geophysics》1970,82(1):259-269
Summary The theory of resistivity sounding on a three layer earth comprising an inhomogeneous interstratum has been presented. The electrical conductivity in the intermediate layer, embedded between two layers of uniform conductivity, is assumed to follow either generalised power law or exponential variation. A point source of current is placed on the surface. The electric potential functions on the ground surface have been obtained. The expressions for apparent resistivity over the above models for the Wenner and Schlumberger electrode configurations have been derived. 相似文献
11.
本文根据苏黎世天文台太阳黑子11年周期资料和太阳黑子磁场磁性变化周期特征，构建了太阳黑子磁场磁性指数MI(Magnetic Index)时间序列.分析表明：太阳活动磁性周期平均长度为222年，但是每个周期长度是不相等的；多数情况周期短时磁性指数较大，对应太阳活动水平强；周期变长时磁性指数较小，对应太阳活动水平较弱；太阳黑子磁场磁性指数序列也具有80～90年的世纪周期. 进一步研究指出，太阳黑子磁场磁性指数曲线由极小值升至极大值时期，太阳磁场南向，行星际磁场磁力线与地磁场磁力线重联，此时磁层为开磁层，太阳风将携带大量等离子体从向阳面进入地球磁层，从而使输入的动量、能量和物质大幅度增加，与北半球对流层增温时期对应；太阳黑子磁场磁性指数曲线由极大值下降至极小值时期，太阳磁场北向，与磁层顶地磁场同向，行星际磁场不会与地磁场发生重联，此时磁层为闭磁层，这种情况下，只有少数带电粒子能够穿越磁力线进入地球磁层，与北半球对流层降温时期对应. 相似文献
12.
Sharp boundary inversion of 2D magnetotelluric data 总被引：6，自引：0，他引：6
Torquil Smith Michael Hoversten Erika Gasperikova & Frank Morrison 《Geophysical Prospecting》1999,47(4):469-486
We consider 2D earth models consisting of laterally variable layers. Boundaries between layers are described by their depths at a set of nodes and interpolated laterally between nodes. Conductivity within each layer is described by values at a set of nodes fixed within each layer, and is interpolated laterally within each layer. Within the set of possible models of this sort, we iteratively invert magnetotelluric data for models minimizing the lateral roughness of the layer boundaries, and the lateral roughness of conductivities within layers, for a given level of data misfit. This stabilizes the inverse problem and avoids superfluous detail. This approach allows the determination of boundary positions between geological units with sharp discontinuities in properties across boundaries, while sharing the stability features of recent smooth conductivity distribution inversions.
We compare sharp boundary inversion results with smooth conductivity distribution inversion results on a numerical example, and on inversion of field data from the Columbia River flood basalts of Washington State. In the synthetic example, where true positions and resistivities are known, sharp boundary inversion results determine both layer boundary locations and layer resistivities accurately. In inversion of Columbia flood basalt data, sharp boundary inversion recovers a model with substantially less internal variation within units, and less ambiguity in both the depth to base of the basalts and depth to resistive basement. 相似文献
We compare sharp boundary inversion results with smooth conductivity distribution inversion results on a numerical example, and on inversion of field data from the Columbia River flood basalts of Washington State. In the synthetic example, where true positions and resistivities are known, sharp boundary inversion results determine both layer boundary locations and layer resistivities accurately. In inversion of Columbia flood basalt data, sharp boundary inversion recovers a model with substantially less internal variation within units, and less ambiguity in both the depth to base of the basalts and depth to resistive basement. 相似文献
13.
The electrical potential due to a point source of current placed on the ground surface is studied for a multi-layered earth consisting of homogeneous overburden of constant conductivity over a stack of transition layers where conductivity varies with depth according to power and exponential laws in even and odd layers, respectively. The general recursion relations are derived and are used to obtain expressions for the apparent resistivities for Schlumberger and Wenner electrode arrays. Their asymptotic behavior has been studied. The solutions for some particular cases are given: (i) odd layers with conductivities exponentially varying with depth while all even layers (and the first) have constant conductivities; (ii) even layer conductivities varying as a power law while odd number layers are of constant conductivity; and (iii) any two successive layers as transition layers and all others having constant conductivities. Further it is shown that Patella's theory is a particular case of the present study. It is concluded, therefore, that the present treatment is more general as all earlier models consisting of trasition layers can be derived from this study. 相似文献
14.
The diffusion of electromagnetic fields is dependent not only on conductivity, but also on magnetic permeability, dielectric permittivity and polarizability, i.e. dispersive conductivity. The long‐offset transient electromagnetic (LOTEM) method is mainly used to determine the spatial distribution of conductivity in the subsurface. However, earlier work on loop‐loop TEM suggests that transient EM methods can also be affected by induced polarization (IP). Numerous 1D forward calculations were carried out to study the IP effect on LOTEM data, using the Cole‐Cole relaxation model to simulate the polarizability of the ground. Besides the polarizability of each layer, the IP effect depends on the LOTEM field set‐up and the spatial distribution of conductivity in the ground. In particular, near‐surface layers with high chargeabilities can significantly distort the late time transients of the electric field components in the vicinity of the transmitter. The influence of polarizable layers on the magnetic field components can be neglected under normal circumstances. In 1997 and 1999, LOTEM measurements were carried out at Mt. Vesuvius in Italy to explore the geological structure of the volcano. Sensitivity studies on the effect of polarizable layers suggest that high chargeabilities in connection with conductive layers at greater depths would result in a detectable distortion of the electric field transients. Although the simultaneous IP measurements revealed high chargeabilities in a near‐surface layer, no evidence of IP effects could be found in the measured LOTEM data. We conclude that the observed chargeabilities are local and that 3D effects are probably present in the data. Another aspect is the measurement of the system response, which is usually measured by placing a receiver very close to the transmitter. Therefore, large distortions can be expected if near‐surface polarizable layers exist. This was verified in practice by field measurements in an area with high chargeabilities in Longerich, Cologne. 相似文献
15.
An earth model with a transition layer (anisotropic inhomogeneous) is considered. The inhomogeneity in σv (vertical conductivity) of the transition layer is represented by a power law variation. Expressions for potential distribution in the upper layer, transition layer and bottom layer are obtained by solving appropriate differential equation for each layer. By utilizing the boundary conditions, expressions of apparent resistivity for Wenner and Schlumberger configurations are derived. Numerical analysis is performed for linear and quadratic variation of σv. The results are presented in the form of theoretical apparent resistivity curves for both configurations. Negative apparent resistivities are the interesting feature of this analysis. 相似文献
16.
The objective of this work is to assess the importance of electrical conductivity and magnetic permeability variations in ground penetrating radar (GPR) reflections commonly interpreted only in terms of permittivity variations. We use the matrix propagator approach to obtain the surface electric field associated with a horizontally layered model whose three electromagnetic properties vary from layer to layer. The solution is based on the plane wave boundary value problem using inverse Fourier transformation to accommodate particular GPR pulses. Our results indicate that while magnetic permeability is unimportant, reflections from electrical conductivity variations can be of the same order as those associated with electrical permittivity boundaries. In particular, we show that a realistic ground model composed of thin conductive layers can produce radargrams similar to those caused by a lossless permittivity contrast. 相似文献
17.
The layered model of the marine environment, including the atmosphere, two seawater layers with different conductivity and density, and the bottom rock layer, has been considered. The geomagnetic field variations, generated by internal and surface waves with different frequency and propagation direction, have been found in the scope of this model. The effect of magnetic permeability and electric conductivity of bottom rocks on induced magnetic field has been taken into account. The transfer functions and spectral densities of these variations have been analytically determined and numerically estimated. 相似文献
18.
基于野外调查和室内试验建立了玉门关段汉长城结构的三维动力分析模型,其中以长城墙体结构中的夯土层、芦苇层和芦苇绳为主要研究对象。采用有限差分法计算了该段长城结构在地震动作用下的抗震性能。计算结果表明:在自重作用下,芦苇层与夯土层变形机制相互补充,芦苇绳的拉结作用明显,整个墙体变形合理;从墙体底部到顶部,地震波的高频成分减少,低频成分增加;在强震作用下,墙体芦苇层的最大应力远小于单根芦苇纤维的抗拉强度,处在比较安全的状态;采用格栅单元和锚索单元分别模拟芦苇层、芦苇绳是合理的。研究成果可为河西地区土遗址保护提供参考。 相似文献
19.
An inspection of the long-term behaviour of the range of the daily geomagnetic field variation from comprehensive modelling 总被引：1，自引：0，他引：1
J. Miquel Torta Luis R. Gaya-Piqu Juan J. Curto David Altadill 《Journal of Atmospheric and Solar》2009,71(13):1497-1510
This paper attempts to reveal whether long-term trends in the ionosphere are reflected in the amplitude range of the geomagnetic daily variation recorded at ground level. The smooth and regular variation observed in the magnetograms on magnetically quiet days is induced by the ionospheric currents flowing in the dynamo region. So it is likely that trends in the conductivity or in the dynamics of this region could produce changes in the current densities, and consequently in the range of the geomagnetic variation. The crucial aspect is how to separate the changes produced by the geomagnetic activity itself, or by secular changes of the Earth's magnetic field, from the part of the variation produced by factors affecting trends in the ionosphere, which could have an anthropogenic origin. To investigate this, we synthesized for several geomagnetic observatories the daily ranges of the geomagnetic field components with a comprehensive model of the quiet-time, near-Earth magnetic field, and finally we removed the synthetic values from the observed ranges at those observatories. This comprehensive model accounts for contributions from Earth's core, lithosphere, ionosphere, magnetosphere and coupling currents, and, additionally, accounts for influences of main field and solar activity variations on the ionosphere. Therefore, any trend remaining in the residuals, assuming that all the contributions mentioned above are properly described and thus removed by the comprehensive model, should reflect the influence of other sources. Results, based on series of magnetic data from observatories worldwide distributed, are presented. Trends in the X and Z components are misleading, since the current system changes in form as well as in intensity, producing changes of the focus latitude in the course of a solar cycle and from one cycle to another. Some differences exist between the long-term trends in the Y component between the real and modelled ranges, suggesting that other non-direct solar causes to the amplitude changes of the solar quiet geomagnetic variation should not be ruled out. Nevertheless, the results also reflect some short-comings in the way that the comprehensive modelling accounts for the influence of the solar activity on the range of the daily geomagnetic variation. 相似文献
20.
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