共查询到20条相似文献，搜索用时 31 毫秒
1.
The scale factors to permit a laboratory analogue model study of the problem of magnetic fields induced by ocean waves in the earth's field are derived. An analogue model employing surface fluid waves in mercury to simulate ocean waves is described. In the analogue model, magnetic field measurements were made 1 cm above a 2 cm deep model mercury ocean for a wave period of 0.21 s. This model simulates measurements 38 m above the surface of a shallow ocean 78 m deep for a wave period of 13 s. The validity of the analogue modelling technique is established by the good agreement obtained in comparing the analogue model measurements of the induced magnetic fields with fields using Podney's expression for an ocean of finite depth. 相似文献
2.
John F. Hermance 《Physics of the Earth and Planetary Interiors》1973,7(3):349-364
The processing of magnetotelluric data involves concepts from electromagnetic theory, time series analysis and linear systems theory for reducing natural electric and magnetic field variations recorded at the earth's surface to forms suitable for studying the electrical properties of the earth's interior.The electromagnetic field relations lead to either a scalar transfer impedance which couples an electric component to an orthogonal magnetic component at the surface of a plane-layered earth, or a tensor transfer impedance which couples each electric component to both magnetic components in the vicinity of a lateral inhomogeneity.A number of time series spectral analysis methods can be used for estimating the complex spectral coefficients of the various field quantities. These in turn are used for estimating the nature of the transfer function or tensor impedance. For two dimensional situations, the tensor impedance can be rotated to determine the principal directions of the electrical structure.In general for real data, estimates of the apparent resistivity are more stable when calculated from the tensor elements rather than from simple orthogonal field ratios (Cagniard estimates), even when the fields are measured in the principal coordinates. 相似文献
3.
R.C. Hewson-Browne 《Physics of the Earth and Planetary Interiors》1973,7(2):167-186
The tidal motion of sea water across the earth's magnetic field is known to induce small electric currents to flow in the oceans and the surrounding land. There has been recently a great deal of interest in this phenomenon. Here we consider a model ocean in the form of an infinitely long channel with a tidal wave passing along it. It is shown that for the lunar tidal frequency of 12.45 hours the solution of this problem for typical ocean depths exhibits characteristics which are essentially due to the high frequency. The solution therefore differs appreciably from low-frequency solutions which are therefore only applicable as models of the shallow seas. In our model we observe concentrations of electric current near the coast and phase changes relative to the oceanic tide. A new “equivalent field exclusion principle” is presented and used to explain some of the results we have obtained. The method used is simple and applies readily to waves of any frequency, and results are given for waves of higher frequencies. The amplitude and phase lag of the magnetic field of a channel simulating the Atlantic Ocean are discussed. 相似文献
4.
A. V. Kuvshinov 《Surveys in Geophysics》2008,29(2):139-186
Electromagnetic induction in the Earth’s interior is an important contributor to the near-Earth magnetic and electric fields.
The oceans play a special role in this induction due to their relatively high conductivity which leads to large lateral variability
in surface conductance. Electric currents that generate secondary fields are induced in the oceans by two different processes:
(a) by time varying external magnetic fields, and (b) by the motion of the conducting ocean water through the Earth’s main
magnetic field. Significant progress in accurate and detailed predictions of the electric and magnetic fields induced by these
sources has been achieved during the last few years, via realistic three-dimensional (3-D) conductivity models of the oceans,
crust and mantle along with realistic source models. In this review a summary is given of the results of recent 3-D modeling
studies in which estimates are obtained for the magnetic and electric signals at both the ground and satellite altitudes induced
by a variety of natural current sources. 3-D induction effects due to magnetospheric currents (magnetic storms), ionospheric
currents (Sq, polar and equatorial electrojets), ocean tides, global ocean circulation and tsunami are considered. These modeling
studies demonstrate that the 3-D induction (ocean) effect and motionally-induced signals from the oceans contribute significantly
(in the range from a few to tens nanotesla) to the near-Earth magnetic field. A 3-D numerical solution based on an integral
equation approach is shown to predict these induction effects with the accuracy and spatial detail required to explain observations
both on the ground and at satellite altitudes.
On leave from Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Sciences, 142190
Troitsk, Moscow region, Russia. 相似文献
5.
6.
R.G. Roberts 《Physics of the Earth and Planetary Interiors》1983,33(3):198-212
The composition of the upper mantle is of great significance to our understanding of plate tectonics and global evolution. Information about the physical properties of the Earth at upper mantle depths, including lateral variations in electrical conductivity, can be deduced from measurements of the electric and magnetic fields at the Earth's surface. Electromagnetic methods appear to give poorer resolution than do some other methods, for example seismics, but as they are sensitive to quite different properties of a medium they provide a different and complementary class of information.The basic theory of electromagnetic sounding methods is briefly reviewed below, and evidence regarding lateral conductivity inhomogeneities in the Earth's upper mantle is examined. While lateral electrical conductivity inhomogeneities appear to be the rule rather than the exception, the interpretation of electromagnetic data still presents difficulties and the results from many regions are not as yet unambiguous. Where the data are of sufficient resolution, a rapid increase in electrical conductivity can usually be identified within the upper mantle. The depth to this highly conductive zone is different in different tectonic environments, but is broadly consistent between analogous but widely separated tectonic environments. A comparatively shallow conducting region is found beneath the ocean lithosphere. The depth of this region is dependent on lithospheric age. Many of the more shallow conducting regions in both continental and oceanic environments are associated with high heat flow values and seismic low velocity zones. These highly conducting regions may be zones of partial melt. 相似文献
7.
Maurice Aubert 《Physics of the Earth and Planetary Interiors》1974,9(4):364-366
The pluri-annual variations of the earth's magnetic field in France increase their range of influence from East to West. This leads to presume a discontinuity of electric conductivities at the transition from the continental to the oceanic area. 相似文献
8.
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
9.
M.C. Frazer 《Physics of the Earth and Planetary Interiors》1974,8(1):75-82
The equation of motion (Navier-Stokes equation) for a uniformly rotating, compressible, magnetic, viscous fluid is analyzed in terms of infinite series of spherical surface harmonics. Differential equations are obtained for the radial functions of the poloidal and toroidal harmonics of the velocity, corresponding to those obtained by Bullard and Gellman for the magnetic field from the electromagnetic induction equation. This new analysis opens the way for the dynamical problem of electromagnetic induction in the earth's core to be considered by the spherical harmonic method. 相似文献
10.
We are reporting some results and future planning of the Magnetic and Electric fields Monitoring Project. The principal purpose of the project is to create a network of observatories for monitoring the electromagnetic signals in the 0.001 Hz–100 kHz frequency bands in Central Italy. These bands include both natural and artificial signals. The technological objectives of the project are the development of new instrumentation and the know-how transfer to the industry. These instruments were tested in the Geomagnetic Observatory of L'Aquila, the first station of the network. Here we show some examples of the developed electric and magnetic sensors. These instruments are designed for the automatic long-term recording of the electromagnetic fields in a wide band of frequency. The preliminary results of the first operating station of the network are shown taking into account, in the signal elaboration, the single station approach. Here we are reporting some examples concerning the calculation of the ground electric resistivity profile, the evaluation of the magnetic induction vectors and the study of the magnetic sources by the eigenvectors calculation. When all the stations of the network will be in operation, the use of an innovative technique, the wideband interferometry will be proposed. Combining the simultaneous observations of the electromagnetic field measured in each station, we will be able to obtain detailed information about the investigated electromagnetic sources. 相似文献
11.
An introduction to electromagnetic induction in the ocean 总被引：1，自引：0，他引：1
J.C. Larsen 《Physics of the Earth and Planetary Interiors》1973,7(3):389-398
The governing equations for the induction of electromagnetic fields in the ocean by ionospheric and oceanic sources are presented. A uniformly conducting layered model and a nonuniformly conducting thin-sheet model are discussed with reference to the interpretation of fields observed in the ocean. A procedure for the separation of the electric field continuum into parts of ionospheric and oceanic origin is presented. 相似文献
12.
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
13.
为了探索我国海域油气和水合物等高阻目标体CSEM勘探的可行性和方法技术,本文研究了在海水中水平电性源激励下有限水深海洋地电模型的频率域电磁响应,为进一步的1D和3D仿真计算奠定了理论基础.在推导电磁响应公式时,首先给出了各层介质的Lorentz势,然后根据Coulomb势与Lorentz势的关系,得到了各层介质的Coulomb势.各层介质中的电磁场均可以由Lorentz势或者Coulomb势计算得到,但在有限元计算时Coulomb势具有优势.长导线源的电磁场和势函数可以由电偶源的电磁场和势函数沿导线长度积分得到.文中具体给出了海水中水平电偶源和长导线源在海水层的电磁场公式,并根据该公式计算了不同水深环境下海底表面的电磁场分布,分析了海水深度对海底油气储层电磁异常的影响.结果表明,随着水深减小,异常幅度和形态特征发生明显变化.当水深很浅时(如50 m),只有同线方向的Ex和Ez两个电场分量存在明显异常.最后,以两个已知海底油田为例,计算了不同水深环境下可观测到的电场异常,展示了电性源频率域CSEM在海底勘探中(包括浅海环境)的良好应用前景.对于该方法实用化过程中还需进一步解决的问题,文中结尾部分也进行了初步探讨. 相似文献
14.
Goesta Wollin William B.F. Ryan David B. Ericson 《Earth and Planetary Science Letters》1978,41(4):395-397
Our investigation of deep-sea climatic and magnetic records showing that high eccentricity of the earth's orbit, low magnetic field intensity and warm climate occur together indicates the relative importance of eccentricity as perhaps the phenomenon which has most consistently modulated both climate and magnetism for at least the past 2,000,000 years. A speculative hypothesis regarding the mechanism which may be responsible for a relationship between the eccentricity of the earth's orbit, geomagnetism, and climate is suggested. 相似文献
15.
A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) an initial inversion model is computed from surface impedance measurements, (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of \(\sim \) 300–1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and electric field sensors to be developed in a future project. Our results suggest that three-component SQUID and fluxgate magnetometers should be developed to facilitate borehole MT measurements at signal frequencies above and below 1 Hz, respectively. 相似文献
16.
Elimination of the water-layer response from multi-component source and receiver marine electromagnetic data 总被引：1，自引：0，他引：1
Janniche Iren Nordskag Lasse Amundsen Lars Løseth Egil Holvik 《Geophysical Prospecting》2009,57(5):897-918
This paper presents the theory to eliminate from the recorded multi‐component source, multi‐component receiver marine electromagnetic measurements the effect of the physical source radiation pattern and the scattering response of the water‐layer. The multi‐component sources are assumed to be orthogonally aligned above the receivers at the seabottom. Other than the position of the sources, no source characteristics are required. The integral equation method, which for short is denoted by Lorentz water‐layer elimination, follows from Lorentz' reciprocity theorem. It requires information only of the electromagnetic parameters at the receiver level to decompose the electromagnetic measurements into upgoing and downgoing constituents. Lorentz water‐layer elimination replaces the water layer with a homogeneous half‐space with properties equal to those of the sea‐bed. The source is redatumed to the receiver depth. When the subsurface is arbitrary anisotropic but horizontally layered, the Lorentz water‐layer elimination scheme greatly simplifies and can be implemented as deterministic multi‐component source, multi‐component receiver multidimensional deconvolution of common source gathers. The Lorentz deconvolved data can be further decomposed into scattering responses that would be recorded from idealized transverse electric and transverse magnetic mode sources and receivers. This combined electromagnetic field decomposition on the source and receiver side gives data equivalent to data from a hypothetical survey with the water‐layer absent, with idealized single component transverse electric and transverse magnetic mode sources and idealized single component transverse electric and transverse magnetic mode receivers. When the subsurface is isotropic or transverse isotropic and horizontally layered, the Lorentz deconvolution decouples into pure transverse electric and transverse magnetic mode data processing problems, where a scalar field formulation of the multidimensional Lorentz deconvolution is sufficient. In this case single‐component source data are sufficient to eliminate the water‐layer effect. We demonstrate the Lorentz deconvolution by using numerically modeled data over a simple isotropic layered model illustrating controlled‐source electromagnetic hydrocarbon exploration. In shallow water there is a decrease in controlled‐source electromagnetic sensitivity to thin resistors at depth. The Lorentz deconvolution scheme is designed to overcome this effect by eliminating the water‐layer scattering, including the field's interaction with air. 相似文献
17.
M. S. Zhdanov 《Surveys in Geophysics》1980,4(1-2):115-136
The main results in the theory of the interpretation of geopotential fields are generalized to the case of arbitrary variable electromagnetic fields by means of elaborating electrodynamic analogues for the integral of the Cauchy type.The generalized Kertz method for separating a variable electromagnetic field into parts related to the sources located in different regions of space is elaborated on the basis of this technique. The generalized Kertz method allows the selection of external and internal, normal and anomalous parts of the geomagnetic field, as well as the separation of geomagnetic anomalies into the surface and deep components caused by conductivity inhomogeneities in the Earth's crust and upper mantle.The theory of analytical continuation of variable electromagnetic fields in a conducting medium is also developed in the present work using the technique of analogues for the integral of the Cauchy type. It is shown that analytical continuation of a field downwards permits the determination of the location and form of deep geoelectric inhomogeneities according to the configuration of the isolines of flux functions for magnetic and electric fields. 相似文献
18.
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
19.
B. V. Borovskii L. M. Bogomolov A. S. Zakupin V. A. Mubassarova 《Izvestiya Physics of the Solid Earth》2011,47(10):926-936
The combined impacts of the electromagnetic fields generated by two independent sources on the process of crack formation
in loaded heterogeneous materials (rocks) are studied. The interest in such experiments lies in the fact that in natural conditions,
the combination of several fields—the potential triggers of seismic activity—is rather the rule than the exception. It is
shown by examples that in the conditions of crossed electric and magnetic fields (CrEMFs) the enhancement in acoustic activity
can be triggered with a lower strength electric field than in case of the electric impact alone. 相似文献
20.
深水环境下进行大地电磁数据采集,信号幅值微弱,高频分量被严重衰减;仪器承载的环境压力巨大,且伴随底流推曳、淤泥吸附等不利因素;海上作业情况复杂多变,带给设备投放与回收诸多困难.针对这些特殊问题,从理论上分析了大地电磁场在海水及其以下层状介质中的传播规律,计算了电场与磁场随不同水深的衰变比值,研讨了与仪器上浮速度相关的物理因素;从技术上阐述海底大地电磁仪的工作原理,包括信号传感器、数据采集器、声控释放单元以及机械组装部件等等.整套仪器信号分辨率达到nV级,频带宽度为10~0.3×10-3 Hz,最大工作水深4000 m.该仪器搭载“海洋六号”科学考察船,在我国南海中沙群岛海域进行了国内首次深水环境下的大地电磁数据采集试验.结果显示,所研发的仪器性能指标吻合先前的理论推算值,达到设计要求.我国的海底大地电磁探测技术已完全享有自主的知识产权. 相似文献