共查询到20条相似文献，搜索用时 31 毫秒
1.
The inductive response of a conducting horizontal cylinder embedded in a uniform earth is studied using numerical results obtained for an analytical solution for the problem of a conducting cylinder buried in a homogeneous earth for the case of a uniform inducing field. A check of the validity of the numerical results is made by a comparison with analogue model measurements for a number of cases. Numerical results for a range of cylinder radii (a = 1–10 km), depths of burial (d= 0–4 km), conductivity contrasts (σ2= 10?2-10 Sm?1), and source frequencies (f= 10?1-10?4 Hz) of interest in the interpretation of magnetotelluric field measurements are presented. The results indicate that for a uniform inducing field the conductivity and depth of burial of a horizontal cylindrical inhomogeneity are best determined through a measurement of the amplitudes Hy, Hz and Ex and the phases φy and Ψx. 相似文献
2.
INDUCTIVE SOUNDING OF A STRATIFIED EARTH WITH TRANSITION LAYER RESTING ON DIPPING ANISOTROPIC BEDS *
The generalised three layer boundary value problem with a transition layer sand-witched between an isotropic overburden and dipping anisotropic substratum is discussed assuming that plane electro-magnetic waves are incident normally over the air-earth inter-face. The tangential electric (Ey) and magnetic (Hx) fields and the expression for surface-impedance (Ey/Hx) have been evaluated at the earth's surface. Through numerical analysis it is shown that changes in the values of the parameters m (coefficient of anisotropy), h (thickness of the transition layer), α (angle of inclination of the dipping beds), and b (conductivity ratio between substratum and upper layer) modify the amplitude and phase-variation curves (with skindepth) significantly. 相似文献
3.
H.W. Dosso W. Nienaber W.D. Parkinson 《Physics of the Earth and Planetary Interiors》1985,39(2):118-133
Laboratory analogue model magnetic measurements are carried out for a model of the region including Tasmania, Bass Strait with its highly conductive deep sedimentary basins, and the south coast of mainland Australia. The model source frequencies used simulate naturally occurring geomagnetic variations of periods 5–120 min. In-phase and quadrature magnetic Hx, Hy and Hz field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E-polarization (electric field of the source in the N-S direction) and for H-polarization (electric field of the source in the E-W direction). Large anomalous in-phase and quadrature model magnetic fields are observed over Bass Strait and the coastal regions at short periods for both E- and H-polarization, but with increasing period, the field anomalies decrease more rapidly for E-polarization, than for H-polarization. The difference in response with polarization for the Bass Strait region is attributed to current induced in the deep ocean, for all periods, being channelled through Bass Strait for H-polarization but not for E-polarization. The persistent large coastal field anomalies elsewhere, for H-polarization, can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea-floor and channelled through Bass Strait and around the southern coast of Tasmania. The phenomena of current deflection and channelling for H-polarization for the geometry of the southern Australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range.The coastal horizontal Hx and Hy field anomalies, present for E-polarization at short periods and for H-polarization at all periods, do not extend far inland, and thus, for inland station sites somewhat removed from the coast, should not present serious problems for magnetic soundings in field work. The sharp vertical field (Hz) gradient over Tasmania at short periods, which is predominantly in the E-W direction for E-polarization and the N-S direction for H-polarization, is strongly frequency dependent, becoming almost undetectable at 60 min. The behaviour of the Hz field gradients, however, are very similar from traverse to traverse over inland Tasmania, and thus, the effects of the ocean should not present too serious a problem in the interpretation of field station studies. The discrepancies between model and field station results should be useful in mapping geological boundaries in the region. 相似文献
4.
M. Poddar 《Pure and Applied Geophysics》1981,119(5):1024-1036
FollowingDmitriev (1960) a rigorous theoretical solution for the problem of scattering by a perfectly conducting inclined half-plane buried in a uniform conductive half-space has been obtained for plane wave excitation. The resultant integral equation for the Laplace transform of scattering current in the half-plane is solved numerically by the method of successive approximation. The scattered fields at the surface of the half-space are found by integrating the half-space Green's function over the transform of the scattering current.The effects of depth of burial and inclination, of the half-plane on the scattered fields are studied in detail. An increase in the depth of burial leads to attenuation of the fields. Inclination introduces asymmetry in the field profiles beside affecting its magnitude. Depth of exploration is greater for quadrature component. An interpretation scheme based on a phasor diagram is presented for the VLF-EM method of exploration for rich vein deposits in a conductive terrain.List of symbols
x, y, z
Space co-ordinates
-
Half-space conductivity
- 0
Free-space permeability
-
Excitation frequency (angular)
-
T
Time
-
h
Depth of the half-plane
-
a
Inclination of the half-plane
-
E
x
x-Directed total electric field
-
E
x
p
x-Directed primary electric field
-
E
xo
p
x-Directed primary electric field atz=0 directly over the half-plane
-
H
y
y-Component of total magnetic field
-
H
y
p
y-Component of primary magnetic field
-
H
y0
p
y-Component of primary magnetic field atz=0 directly over the half-plane
-
H
z
z-Component of total magnetic field
-
H
z
p
z-Component of primary magnetic field
-
J
x
Surface density ofx-directed scattering current
-
G
Green's function
-
k
0,K
Wave numbers
-
u,u
0,u
1,u
2
Functions
-
Space co-ordinate
-
s
Variable in transform domain
-
Variable of integration
-
Normalized scattering current
-
Laplace transform of
-
N
Normalized
- , 0, 1, 2
Functions
-
t
Variable of integration
-
Skin depth
-
H
Total magnetic field
-
H
p
Primary magnetic field
-
H
0
p
Primary magnetic field atz=0 directly over the half-plane
-
M,Q,R,S,U,V
Functions
-
N
1,N
2
Functions 相似文献
5.
G.J. Heard H.W. Dosso W. Nienaber J.E. Lokken 《Physics of the Earth and Planetary Interiors》1985,39(3):178-181
An oscillating vertical line current source was used in the laboratory to simulate the naturally occurring 8 Hz Schumann Resonance field due to lightning strokes. Vertical to horizontal magnetic field ratios (coast effect), and magnetotelluric ratios, for traverses over a laboratory model of the Assistance Bay region of Cornwallis Island (N.W.T. Canada) for the vertical line current source field are compared with those for an overhead uniform source field. The E- and H-polarizations of the source fields used are defined as the cases of the horizontal magnetic field of the source roughly perpendicular and parallel to the general coastline, respectively.The magnetic field ratios are essentially identical for the two field sources for E-polarization, but differ by a factor of approximately two for H-polarization, with the ratios greater for the overhead source than for the vertical line current source. The magnetotelluric ratios are found to be identical for the two field sources of each of the E- and H-polarizations. 相似文献
6.
7.
8.
9.
The response of a horizontal conducting cylinder embedded in a uniform conducting earth is studied using mathematical models of uniform and line current source excitation for the period range 10 to 104 s. The line current source is located at heights ranging from 100–750 km above the surface of the earth. From the calculated results, it is shown that for periods greater than 103 s the ratioE
x
/H
y
at the surface of the earth for localized fields, such as the auroral and equatorial electrojet normally situated at heights of about 100 km, is considerably different from that for a uniform source. The results presented also show that the magneto-telluric method of geophysical prospecting for ore bodies in regions of the electrojet may not be very practicable for periods exceeding 103 s. 相似文献
10.
Dependence of geomagnetic activity during magnetic storms on the solar wind parameters for different types of streams 总被引：1，自引：0，他引：1
The dependence of the maximal values of the |Dst| and AE geomagnetic indices observed during magnetic storms on the value of the interplanetary electric field (E
y
) was studied based on the catalog of the large-scale solar wind types created using the OMNI database for 1976–2000 [Yermolaev
et al., 2009]. An analysis was performed for eight categories of magnetic storms caused by different types of solar wind streams:
corotating interaction regions (CIR, 86 storms); magnetic clouds (MC, 43); Sheath before MCs (ShMC, 8); Ejecta (95); Sheath (ShE, 56); all ICME events (MC + Ejecta, 138); all compression regions Sheaths before MCs and Ejecta (ShMC + ShE, 64); and an indeterminate type of storm (IND, 75). It was shown that the |Dst| index value increases with increasing electric field E
y
for all eight types of streams. When electric fields are strong (E
y
> 11 mV m−1), the |Dst| index value becomes saturated within magnetic clouds MCs and possibly within all ICMEs (MC + Ejecta). The AE index value during magnetic storms is independent of the electric field value E
y
for almost all streams except magnetic clouds MCs and possibly the compressed (Sheath) region before them (ShMC). The AE index linearly increases within MC at small values of the electric field (E
y
< 11 mV m−1) and decrease when these fields are strong (E
y
> 11 mV m−1). Since the dynamic pressure (Pd) and IMF fluctuations (σB) correlate with the E
y
value in all solar wind types, both geomagnetic indices (|Dst| and AE) do not show an additional dependence on Pd and IMF δB. The nonlinear relationship between the intensities of the |Dst| and AE indices and the electric field E
y
component, observed within MCs and possibly all ICMEs during strong electric fields E
y
, agrees with modeling the magnetospheric-ionospheric current system of zone 1 under the conditions of the polar cap potential
saturation. 相似文献
11.
Alice Courvoisier 《地球物理与天体物理流体动力学》2013,107(2):217-243
We look at the large-scale dynamo properties of spatially periodic, time dependent, helical 2D flows of the form u(x, t)?=?(? y ?ψ?(x, y, t), ?? x ?ψ?(x, y, t), ?ψ (x, y, t). These flows act as kinematic fast dynamos and are able to generate a mean magnetic field uniform and constant in the xy-plane but whose direction varies periodically along z with wavenumber k. Using Mean Field Electrodynamics, the generation mechanism can be understood in terms of a k-dependent α-effect, which depends on the magnetic Reynolds number, R m . We calculate this effect for different motions and investigate how its limit as k?→?0 depends on R m and on the properties of the flows such as their spatial structure or correlation time. This work generalises earlier studies based on 2D steady flows to motions with time dependence. 相似文献
12.
In the theory of E-region plasma instabilities, the ambient electric field and electron density gradient are both included in the same dispersion relation as the key parameters that provide the energy for the generation and growth of electrostatic plasma waves. While there exist numerous measurements of ionospheric electric fields, there are very few measurements and limited knowledge about the ambient electron density gradients, Ne, in the E-region plasma. In this work, we took advantage of the EISCAT CP1 data base and studied statistically the vertical electron density gradient length, Lz = Ne/(dNe/dz), at auroral E-region heights during both eastward and westward electrojet conditions and different ambient electric field levels. Overall, the prevailing electron density gradients, with Lz ranging from 4 to 7 km, are found to be located below 100 km, but to move steadily up in altitude as the electric field level increases. The steepest density gradients, with Lz possibly less than 3 km, occur near 110 km mostly in the eastward electrojet during times of strong electric fields. The results and their implications are examined and discussed in the frame of the linear gradient drift instability theory. Finally, it would be interesting to test the implications of the present results with a vertical radar interferometer. 相似文献
13.
The theme of the 2003 EAGE/SEG imaging workshop concerned the contrast between different philosophies of ‘model building’: whether an explicit, user‐determined model should be imposed throughout the processing, with user updates at each step; or alternatively, whether user intervention should be kept to a minimum so as to avoid preconceived bias, and instead to allow the data itself to guide some heuristic process to converge to an optimal solution. Here we consider a North Sea study where our initial approach was to build the subsurface model using interpreted horizons as a guide to the velocity update. This is common practice in the North Sea, where the geology ‘lends itself’ to a layer‐based model representation. In other words, we encourage preconceived bias, as we consider it to be a meaningful geological constraint on the solution. However, in this instance we had a thick chalk sequence, wherein the vertical compaction gradient changed subtly, in a way not readily discernible from the seismic reflection data. As a consequence, imposing the explicit top and bottom chalk horizons, with an intervening vertical compaction gradient (of the form v(x, y, z) =v0(x, y) +k(x, y).z), led to a misrepresentation of the subsurface. To address this issue, a gridded model building approach was also tried. This relied on dense continuous automatic picking of residual moveout in common‐reflection point gathers at each iteration of the model update, followed by gridded tomography, resulting in a smoothly varying velocity field which was able to reveal the underlying local changes within the chalk. 相似文献
14.
3D joint inversion of magnetotelluric and airborne tipper data: a case study from the Morrison porphyry Cu–Au–Mo deposit,British Columbia,Canada 
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下载免费PDF全文 Benjamin M. Lee Martyn J. Unsworth Juliane Hübert Jeremy P. Richards Jean M. Legault 《Geophysical Prospecting》2018,66(2):397-421
Z‐axis tipper electromagnetic and broadband magnetotelluric data were used to determine three‐dimensional electrical resistivity models of the Morrison porphyry Cu–Au–Mo deposit in British Columbia. Z‐axis tipper electromagnetic data are collected with a helicopter, thus allowing rapid surveys with uniform spatial sampling. Ground‐based magnetotelluric surveys can achieve a greater exploration depth than Z‐axis tipper electromagnetic surveys, but data collection is slower and can be limited by difficult terrain. The airborne Z‐axis tipper electromagnetic tipper data and the ground magnetotelluric tipper data show good agreement at the Morrison deposit despite differences in the data collection method, spatial sampling, and collection date. Resistivity models derived from individual inversions of the Z‐axis tipper electromagnetic tipper data and magnetotelluric impedance data contain some similar features, but the Z‐axis tipper electromagnetic model appears to lack resolution below a depth of 1 km, and the magnetotelluric model suffers from non‐uniform and relatively sparse spatial sampling. The joint Z‐axis tipper electromagnetic inversion solves these issues by combining the dense spatial sampling of the airborne Z‐axis tipper electromagnetic technique and the deeper penetration of the lower frequency magnetotelluric data. The resulting joint resistivity model correlates well with the known geology and distribution of alteration at the Morrison deposit. Higher resistivity is associated with the potassic alteration zone and volcanic country rocks, whereas areas of lower resistivity agree with known faults and sedimentary units. The pyrite halo and ≥0.3% Cu zone have the moderate resistivity that is expected of disseminated sulphides. The joint Z‐axis tipper electromagnetic inversion provides an improved resistivity model by enhancing the lateral and depth resolution of resistivity features compared with the individual Z‐axis tipper electromagnetic and magnetotelluric inversions. This case study shows that a joint Z‐axis tipper electromagnetic–magnetotelluric approach effectively images the interpreted mineralised zone at the Morrison deposit and could be beneficial in exploration for disseminated sulphides at other porphyry deposits. 相似文献
15.
The mean tangential stresses at a corrugated interface between a solid, electrically insulating mantle and a liquid core of magnetic diffusivity λ are calculated for uniform rotation of both mantle and core at an angular velocity Ω in the presence of a corotating magnetic field B. The core and mantle are assumed to extend indefinitely in the horizontal plane. The interface has the form z = η(x, y), where z is the upward vertical distance and x, y are the zonal and latitudinal distances respectively. The function η(x, y) has a planetary horizontal length scale (i.e. of the order of the radius of the Earth) and small amplitude and vertical gradient. The liquid core flows with uniform mean zonal velocity U0 relative to the mantle. Ω and B possess vertical and horizontal components.The vertical (poloidal) component Bp is uniform and has a value of 5 G while the horizontal (toroidal) field BT = Bpαz, where α is a constant. When |α| ? 1, the mean horizontal stresses are found to have the same order of magnitude (10?2 N m?2) as those inferred from variations in the decade fluctuations in the length of the day, although the exact numerical values depend on the orientation of Ω as well as on the wavenumbers in the zonal and latitudinal directions.The influence of the steepness (as measured by α) of the toroidal field on the stresses is investigated to examine whether the constraint that the mean horizontal stresses at the core-mantle interface be of the order of 10?2 N m?2 might provide a selection mechanism for the behaviour of the toroidal field in the upper reaches of the outer core of the Earth. The results indicate that the restriction imposed on α is related to the value assigned to the toroidal field deep into the core. For example, if |α| ? 1 then the tangential stresses are of the right order of magnitude only if the toroidal field is comparable with the poloidal field deep in the core. 相似文献
16.
Evaluation of electric parameters of anisotropic sandy-shaly Miocene formations on the basis of resistivity logs 总被引：1，自引：1，他引：0
Maria Bała 《Acta Geophysica》2011,59(5):954-966
In this paper, electrical properties of an anisotropic rock formation are discussed. Electrical anisotropy parameters, i.e., horizontal resistivity, R
H
, and vertical resistivity, R
V
, and anisotropy coefficient in the sandyshaly Miocene formation, are determined on the basis of induction logs and laterologs.
Graphs of R
V
= f(R
H
) calculated for different resistivities of sandstones and shales and variable volumes of those two components allow us to
conclude about the correctness of the applied method for determining the resistivity parameters of autochthonous Miocene formations. 相似文献
17.
Since its introduction by Svensson in 1959, the power law curve y = axb (where x and y are horizontal and vertical direction, respectively) has been widely used in morphological analysis of glacial trough cross-profiles. The numerical constants a and b are obtained by a linear regression analysis of the logarithmic form of the power law curve (ln y = ln a + b ln x). The value b then gives a measure for the form of the cross-section. However, over the years this form of the power law has endured a lot of criticism. This criticism is well founded, since this particular form of the power law is not suitable for curve fitting in morphological analyses. In this paper a general power law is proposed, of the form y − y0 = a|x − x0|b (where x0, y0 are the coordinates of the origin of the cross-profile). A unique and unbiased solution for this equation is obtained with a general least-squares method, thereby minimizing the error between the cross-profile data and the curve, and not between the logarithmic transform of the data and its regression line. This provides a robust way to characterize trough cross-section forms. © 1998 John Wiley & Sons, Ltd. 相似文献
18.
Ground-based ionosonde and magnetic-field observations on the equatorial station Huancayo, ESRO4 neutral-composition measurements, and theoretical model calculations were used to analyze disturbed E×B vertical plasma drift during the phase of solar minimum in 1973. Vertical drifts calculated for disturbed days do not show the systematic decrease often mentioned in publications, and demonstrate strong dependence on IMF-Bz changes. It is confirmed with the help of our drift calculations that Bz turnings to a northward direction result in a decrease (up to reversal) of normal Sq (eastward during daytime and westward at nighttime) in the zonal component of electric field. Southward Bz excursions enhance normal Ey both in daytime and nighttime hours. Model predictions of Ey’s reaction to IMF-Bz changes are discussed. 相似文献
19.
Summary The finite element method, with triangular elements, is used to study the effect of a two-dimensional sloping contact on the surface electromagnetic fields. It is found in the case ofH-polarization and small slopes that the electric field and the apparent resistivity near the contact, on the conductive side, are higher than their asymptotic values. In the case ofE-polarization the apparent resistivity and phase values on the conductive side fall off less rapidly to their asymptotic values with decreasing slope resulting in higher apparent resistivity and phase values on the conductive side, than those expected for a vertical contact. The peak in the amplitude and phase of the normalized vertical magnetic field shifts from the resistive side for a vertical contact to the conductive side for a sloping contact. Far from the sloping contact, on the conductive side, higher values are observed for the normalized vertical magnetic field than in the case of a vertical contact. 相似文献
20.
The dependence of the zonal geomagnetic indices (AE, Ap, Kp, Kn, and Dst) on the solar wind parameters (the electric field E y component, dynamic pressure P d and IMF irregularity σB) has been studied for two types of events: magnetic clouds and high-speed streams. Based on the empirical relationships, it has been established that the AE, Ap, Kp, and Kn indices are directly proportional to the E y value at E y < 12 mV m?1 and are inversely proportional to this value at E y > 12 mV m?1 for the first-type events. On the contrary, the dependence of Dst on E y is monotonous nonlinear. A linear dependence of all geomagnetic indices on E y is typical of the second-type events. It has been indicated that the specific features of geoeffectiveness of magnetic clouds and high-speed solar wind streams are caused by the dependence of the electric field potential across the polar cap on the electric field, solar wind dynamic pressure, and IMF fluctuations. 相似文献