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1.
Summary Expressions are obtained for the currents induced inside a conducting sphere placed in the field of an oscillating magnetic dipole. Interesting conclusions can be drawn from an analysis of the formulas for the current induced due to different orientations of the dipole. 相似文献
2.
T. LEE 《Geophysical Prospecting》1983,31(5):766-781
The presence of a conducting environment about a spherical ore body must be considered when calculating the transient electromagnetic response of the ore body due to a step current flowing in a large circular loop at the earth's surface. Failure to do this can easily lead to errors in excess of 10% in numerical calculations. Moreover, there is only a limited time interval in which the response of the spherical conductor is easily seen. In a poorly conducting ground the resonance response of the sphere is the first to be excited. Later, however, the non-resonance or wave-type response is excited. These waves destructively interfere and finally the response of the sphere decays with time as t?7/2. For a range of times and depths the best loop for detecting the sphere has about the same radius as the sphere. 相似文献
3.
S. K. VERMA 《Geophysical Prospecting》1972,20(4):752-770
A technique utilizing the convolution theorem is developed to obtain the time-domain electromagnetic response for a number of input pulses from the given step-function response. For illustration an idealized model of a homogeneously conducting non-permeable sphere placed in a uniform field is considered. The nature of the responses due to different types of pulses and their usefulness in estimating the constitution parameter of the sphere are discussed. To show the applicability of the suggested technique to generalized systems, calculations for a conducting permeable sphere are presented in the appendix. 相似文献
4.
本文采用非齐次麦克斯韦方程组,给出了导电率随时间变化时均匀的和非均匀的导电球,在均匀的和非均匀的外激励场的作用下的电磁响应问题的解析解。并简单讨论了这类电磁响应问题在地球物理学中的可能应用。 相似文献
5.
A polarizable sphere embedded in a conducting half-space can give rise to negative voltage transients in a coincident-loop time-domain electromagnetic system. Such transients have been observed in field situations. Our results are obtained from a model in which the contributions of the host rock, the currents in the sphere, and the interaction between the sphere and the host rock are separated and superposed. This model uses approximations to the integral equation solutions rather than finite-element or finite-difference approximations, and so allows very rapid computation. The theoretical demonstration suggests that interpretation of the negative voltage transients as a polarization response is valid, but more detailed interpretation of polarization properties may not be possible, because the superposition of the polarization response on the normal response depends strongly on the position of the target. 相似文献
6.
S. N. Grover 《Pure and Applied Geophysics》1976,114(4):521-539
Summary The electrostatic force between a charged conducting sphere and a charged dielectric sphere in a uniform, arbitrarily oriented external electric field is obtained and presented in terms of force coefficients which depend only on the relative geometry of the two spheres and the dielectric constants of the dielectric sphere and the medium. When the dielectric constant of the dielectric sphere is allowed to approach infinity, the resulting computed force agrees with that ofDavis (1964) for two conducting spheres, and when the external field and the charge on the dielectric sphere are set to zero, the resulting force agrees with that ofHall andBeard (1975) for a charged conducting sphere near an uncharged dielectric sphere. 相似文献
7.
Summary An initial value induction problem has been solved to examine the response of a massive sulphide ore body under conditions of common occurence. The generalized expression for the induced field due to a step-function dipolar field in the presence of a non-uniformly conducting permeable sphere surrounded by a finitely resistive overburden has been derived. Two cases, when the concentric shell is (i) insulated from and (ii) in galvanic contact with the inner core, have been considered. The analytical results are in computable form to yield the decay-characteristics for application in geoelectromagnetics. Particular cases of the problem treated by the other investigators are discussed.N.G.R.I. Contribution No. 71-234 相似文献
8.
Boundary-value problems in steady-state current flow were solved numerically in bispherical coordinates for a sphere of arbitrary conductivity in a half-space. Solutions for the potential on the surface of the half-space were examined for the cases where both current sources were on the surface, one source on the surface and the second between the surface and the sphere, and one source on the surface and the other in the sphere. Results show a great similarity with the layered case when the buried electrode is placed between the surface and the conducting region. Such a buried electrode configuration makes it possible to obtain an accurate measurement of the depth to the conductor in both cases. A model with the current electrode placed in a conductive sphere is compared with a three-layered model with the source in a conductive intermediate layer, and results indicate that the lateral extent of a finite anomalous zone can be estimated using these limiting curves. The validity of these theoretical calculations for buried spheres was confirmed experimentally by tests conducted on an analog model. 相似文献
9.
F.H. Busse 《Physics of the Earth and Planetary Interiors》1979,20(2-4):152-157
Small-scale motions in the Earth's liquid core are likely to be highly anisotropic because of the effects of rotation. Guided by physical considerations, the “-effect” described by an anisotropic tensor ik is formulated and the corresponding boundary value problem for a sphere is solved for a variety of boundary conditions. A converged solution has been obtained only in the case of the Fermi condition of an infinitely conducting exterior of the sphere. In a second part of the paper, some remarks are made on the hypothetical upper bound on magnetic field strengths in planetary cores originally proposed by Busse (1976). 相似文献
10.
In the inductive method of prospecting, information on conductivity and size of conducting ore-bodies located in non-conducting host rocks, can be conveniently obtained by studying amplitude-frequency relation or amplitude-phase relation of their response with the help of suitable Master-curves. Experiments carried out in the laboratory on the scale-model method show these Master-curves to be different depending on the type of primary excitation and the shape of conductor. Cases of sphere, plate and disc in uniform field, field of line source and dipole field have been studied with an Amplitude-Phase Meter specially constructed in the laboratory for this purpose. 相似文献
11.
In the inductive method of prospecting, information on conductivity and size of conducting ore-bodies located in non-conducting host rocks, can be conveniently obtained by studying amplitude-frequency relation or amplitude-phase relation of their response with the help of suitable Master-curves. Experiments carried out in the laboratory on the scale-model method show these Master-curves to be different depending on the type of primary excitation and the shape of conductor. Cases of sphere, plate and disc in uniform field, field of line source and dipole field have been studied with an Amplitude-Phase Meter specially constructed in the laboratory for this purpose. 相似文献
12.
M.R.E. Proctor 《地球物理与天体物理流体动力学》2013,107(3):235-240
The well-known “toroidal theorem” of Elsasser and Bullard and Gellman rules out dynamo action in a conducting sphere when the velocity field has no poloidal part. It is here shown that for a fixed toroidal velocity field any poloidal velocity must attain a finite size if dynamo action is to be possible. The resulting “anti-dynamo” theorem generalises the earlier result of Childress by giving a bound on the product of the suprema of the toroidal and poloidal velocities. 相似文献
13.
K. MALLICK 《Geophysical Prospecting》1972,20(2):293-303
A frequency-domain analysis is outlined for a conducting sphere in a uniform Input field: inequispaced alternating half-sine wave pulses. The Barringer Input air-borne electromagnetic exploration system uses such source fields. Theoretical profiles of Hρ(t), the horizontal magnetic component over the sphere from different elevations and for various conductivity and geometrical factors are presented. Based on these results some useful features such as penetration and detectibility are discussed. 相似文献
14.
B. N. Satpathy 《Pure and Applied Geophysics》1971,90(1):153-162
Summary Expressions are obtained for the currents induced in a conducting sphere placed in an oscillating uniform field. The induced currents are plane circular loops around the axis of symmetry with respect to the external field. Current profiles are presented at three latitudinal planes inside the sphere for three parametric values of
that are of importance in prospecting. Magnetic fields created by these currents loops at a point on the axis of the sphere are calculated. This study reveals some interesting results like the existence of zones contributing positively and negatively to the signal measured outside the sphere and behaviour of these zones with parameter. 相似文献
15.
Mehdi Eshagh 《Acta Geophysica》2011,59(1):29-54
Solution of the gradiometric boundary value problems leads to three integral formulas. If we are satisfied with obtaining
a smooth solution for the Earth’s gravity field, we can use the formulas in regional gravity field modelling. In such a case,
satellite gradiometric data are integrated on a sphere at satellite level and continued downward to the disturbing potential
(geoid) at sea level simultaneously. This paper investigates the gravity field modelling from a full tensor of gravity at
satellite level. It studies the truncation bias of the integrals as well as the filtering of noise of data. Numerical studies
show that by integrating T
zz
with 1 mE noise and in a cap size of 7°, the geoid can be recovered with an error of 12 cm after the filtering process. Similarly,
the errors of the recovered geoids from T
xz,yz
and T
xx-yy, 2xy
are 13 and 21 cm, respectively. 相似文献
16.
The effect of higher modes on the maximum response of buildings subjected to one horizontal component of earthquake ground motion is discussed with the objective of developing better design formulas for use in building design. Ideal buildings of different numbers of storeys and structural systems are defined; their dynamic properties that define higher mode contribution are identified and are shown to be representative of real buildings. Design formulas that give the required number of modes to be used in a dynamic analysis are developed from parametric studies as a function of the admissible error, the number of storeys and the relation between the fundamental period and the corner spectrum period. The recommendations are simple to use and more rational and accurate than the ones actually in use in most seismic design codes. 相似文献
17.
Carlo Bernasconi 《Pure and Applied Geophysics》1958,41(1):28-32
Riassunto L'Autore elabora formule che servono per rappresentare l'intorno diWeingarten sulla sfera osculatrice, con ciò il problema inverso del trasporto delle coordinate sull'ellissoide viene ricondotto alla risoluzione di un triangolo sferico. Dimostra altresì che le formule della proiezione si potrebbero ottenere, almeno fino al secondo ordine, mediante la teoria dei minimi quadrati.
Summary The Author elaborates the formulas giving the rapresentation ofWeingarten area on the osculatrix sphere; by means of these formulas the inverse problem of coordinate transport on the ellipsoid is reduced to the solution of a spheric triangle. The Author moreover proves that one can refine, at least to the second order, the same formulas making use of the least squares' theory.相似文献
18.
Summary The paper is concerned with the investigation of the magneto-elastic waves and disturbances in initially stressed conducting media. Firstly, (a) the steady state response to moving loads in a magneto-elastic initially stressed conducting medium under an uniaxial tension has been discussed. Secondly, (b) the Lamb's problem corresponding to a line load suddenly applied in a magneto-elastic initially stressed conducting half-space bounded by a plane under uniaxial tension has been investigated. The final results obtained in both cases are in agreement with the corresponding classical problems when the initial tension is zero and the magnetic field is absent. 相似文献
19.
The validity of an analogue model method employing a horizontal magnetic dipole source situated within a conducting layer for the cases of a poorly conducting model earth and a highly conducting model ocean is studied by comparing model magnetic field measurements with theoretical calculations. The model is then used to study one example for each case; the response of a conducting cylinder simulating an ore body embedded in the earth, and the response of a conducting wedge simulating a shelving ocean. 相似文献
20.
K. H. Rädler 《地球物理与天体物理流体动力学》2013,107(3-4):191-211
Abstract This paper builds on a speculation by Moffatt (1979) on an apparent conflict between two results of dynamo theory in the high conductivity limit. Firstly, the finding by Bondi and Gold (1950) on the boundedness of the magnetic dipole moment of a perfectly conducting fluid body is, for a sphere, extended to all magnetic multipole moments. Secondly, a refined version is considered of the simple spherical mean-field dynamo model proposed by Krause and Steenbeck (1967). Some constraints on the mean electromotive force near the boundary of the conducting body are taken into account, which have not been recognized up to now. In the framework of the second order correlation approximation it is shown that it is just these constraints that ensure the boundedness of the magnetic multipole moments in the high conductivity limit. Thus the apparent conflict is resolved. In this context another possible source of error in mean-field dynamo models is pointed out. The present theory also adds insight into dynamo process in cosmical objects, in a way that is briefly discussed. 相似文献