Influence of displacement currents on the response of helicopter electromagnetic systems

Geophysics ◽  
2005 ◽  
Vol 70 (4) ◽  
pp. G95-G100 ◽  
Author(s):  
Changchun Yin ◽  
Greg Hodges
Keyword(s):  
Phase Shift ◽  
Dielectric Permittivity ◽  
Half Space ◽  
Free Space ◽  
Phase Changes ◽  
Displacement Current ◽  
Speed Of Light ◽  
Finite Speed ◽  
The Earth ◽  
Full Solution

For the purpose of shallow-earth geophysical mapping, progressively higher frequencies have been developed for helicopter electromagnetic (HEM) systems. However, concern has been expressed about the vulnerability of high-frequency EM signals to the influence of the displacement current, especially the phase shift of the HEM signal resulting from the finite speed of light that describes the propagation of the EM wave in free space. In this paper we investigate the influence of the displacement current and the finite speed of light on HEM responses, based on a full solution of the EM field for a conductive, magnetically, and dielectrically polarizable earth half-space and an overlying half-space of air with free-space dielectric permittivity. We calculate the amplitude change and the phase shift of the HEM signal and the change in the apparent resistivity. We find that the displacement current, when both the air and the earth half-space assume the free-space dielectric permittivity, has a small influence on the HEM signal, while substantial influence may occur when the earth is dielectrically polarizable. The finite speed of the EM propagation in free space does not result in significant phase changes in the HEM signal.

Mapping of the resistivity, susceptibility, and permittivity of the earth using a helicopter‐borne electromagnetic system

Geophysics ◽  
2001 ◽  
Vol 66 (1) ◽  
pp. 148-157 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Dielectric Properties ◽  
Dielectric Permittivity ◽  
Half Space ◽  
Magnetic Permeability ◽  
Free Space ◽  
Electromagnetic System ◽  
Space Model ◽  
The Earth

Interpretation of helicopter‐borne electromagnetic (EM) data is commonly based on the mapping of resistivity (or conductivity) under the assumption that the magnetic permeability is that of free space and dielectric permittivity can be ignored. However, the data obtained from a multifrequency EM system may contain information about the magnetic permeability and dielectric permittivity as well as the conductivity. Our previous work has shown how helicopter EM data may be transformed to yield the resistivity and magnetic permeability or, alternatively, the resistivity and dielectric permittivity. A method has now been developed to recover the resistivity, magnetic permeability, and dielectric permittivity together from the transformation of helicopter EM data based on a half‐space model. A field example is presented from an area which exhibits both permeable and dielectric properties. This example shows that the mapping of resistivity, magnetic permeability, and dielectric permittivity together yields more credible results than if the permeability or permittivity is ignored.

scholarly journals Navigation, world mapping and astrometry with Galileo's moons

2010 ◽  
Vol 6 (S269) ◽  
pp. 42-48 ◽  
Author(s):  
Kaare Aksnes
Keyword(s):  
Light Curves ◽  
Speed Of Light ◽  
Finite Speed ◽  
The Earth

AbstractGalileo realized that the four moons he discovered, besides supporting the heliocentric system, could also serve as a clock in the sky for longitude determination. Navigation at sea by this method did not prove practical but G. Cassini used it to improve land mapping. O. Rømer discovered that the interval between eclipses of the moons by Jupiter increased or decreased according to whether the Earth moved away from or toward Jupiter. He attributed this to the finite speed of light which he in 1676 determined with an error of about 25%. Timings of the eclipses by Jupiter have served to compute accurate orbits of the moons, notably by means of R. A. Sampson's theory of 1921. Beginning in 1973, light curves of mutual eclipses and occultations between pairs of moons have been made regularly at six years intervals. From these observations very accurate radii and positions of the moons have been obtained.

Dielectric permittivity and resistivity mapping using high‐frequency, helicopter‐borne EM data

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 727-738 ◽  
Author(s):  
Haoping Huang ◽  
Douglas C. Fraser
Keyword(s):  
Dielectric Permittivity ◽  
Half Space ◽  
High Frequency ◽  
Free Space ◽  
Apparent Resistivity ◽  
Phase Component ◽  
High Frequencies ◽  
Space Model ◽  
Homogeneous Half Space ◽  
Displacement Currents

The interpretation of helicopter‐borne electromagnetic (EM) data is commonly based on the transformation of the data to the apparent resistivity under the assumption that the dielectric permittivity is that of free space and so displacement currents may be ignored. While this is an acceptable approach for many applications, it may not yield a reliable value for the apparent resistivity in resistive areas at the high frequencies now available commercially for some helicopter EM systems. We analyze the feasibility of mapping spatial variations in the dielectric permittivity and resistivity using a high‐frequency helicopter‐borne EM system. The effect of the dielectric permittivity on the EM data is to decrease the in‐phase component and increase the quadrature component. This results in an unwarranted increase in the apparent resistivity (when permittivity is neglected) for the pseudolayer half‐space model, or a decrease in the apparent resistivity for the homogeneous half‐space model. To avoid this problem, we use the in‐phase and quadrature responses at the highest frequency to estimate the apparent dielectric permittivity because this maximizes the response of displacement currents. Having an estimate of the apparent dielectric permittivity then allows the apparent resistivity to be computed for all frequencies. A field example shows that the permittivity can be well resolved in a resistive environment when using high‐frequency helicopter EM data.

Diffusion of electromagnetic fields into the earth from a line source of current

Geophysics ◽  
1982 ◽  
Vol 47 (11) ◽  
pp. 1585-1592 ◽  
Author(s):  
M. L. Oristaglio
Keyword(s):  
Half Space ◽  
Line Source ◽  
Scattering Problems ◽  
The Earth ◽  
Steady Current ◽  
Transient Solutions ◽  
Em Induction ◽  
Time Domain Electromagnetic ◽  
Full Solution ◽  
Smoke Ring

The popularity of time‐domain electromagnetic (EM) systems in geophysical exploration has stimulated much interest in transient solutions of Maxwell’s equations. Although the fields at the surface of the earth are of the most practical value, the patterns of current flow in the ground can also be very enlightening, as demonstrated by Nabighian (1979) who showed the development and diffusion of the now famous “smoke ring” of current in the earth after shut‐off of a steady current in a loop at the surface. Nabighian’s result, which is based on Weaver’s (1970) general theory of EM induction in a half‐space, is an approximate image solution involving a single current filament; however, calculation of the full solution by numerical integration of the appropriate Hankel transforms showed that the real currents in the earth follow a similar smoke‐ring pattern (Nabighian, 1979; Lewis and Lee, 1978). Since the two‐dimensional (2-D) approximation is often used in solving EM scattering problems, it is interesting to consider the currents generated by a line source at the surface of a half‐space.

Neubestimmung der natürlichen irdischen Tritiumzerfallsrate und die Frage der Herkunft des „natürlichen“ Tritium

1959 ◽  
Vol 14 (4) ◽  
pp. 334-342 ◽  
Author(s):  
F. Begemann
Keyword(s):  
Phase Shift ◽  
Decay Rate ◽  
Production Rate ◽  
Residence Time ◽  
Cosmic Radiation ◽  
Sunspot Cycle ◽  
Energy Component ◽  
Tritium Content ◽  
The Earth ◽  
Snow Samples

The terrestrial decay rate of “natural” tritium has been re-determined from measurements of the tritium content of old snow samples from Greenland. The finding by CRAIG and BEGEMANN and LIBBY has been confirmed that the tritium decay rate is about 10 times higher than was anticipated previously.Two mechanisms to explain the discrepancy are discussed,a) production by the low energy component of the cosmic radiation andb) the accretion of solar tritium by the earth, as suggested by FELD and ARNOLD.It is shown that in case all the tritium is produced by cosmic radiation the tropospheric production rate may be expected to vary in antiphase with the sunspot cycle, whereas in case of accretion of solar tritium by the earth the variation should be in phase with the sunspot cycle. In both cases a phase shift between the stratospheric production rate and the amount of tropospheric tritium is to be expected because of the residence time of tritium in the stratosphere. A measurement of the phase shift should allow to determine this residence time.The data obtained on the Greenland samples appear to show such a variation of the production rate. The results can be explained best by assuming that all the tritium is produced by cosmic radiation. This result, however, is only preliminary. More systematic measurements are required to decide between the two possibilities.

On modeling a well casing for resistivity and induced polarization

Geophysics ◽  
1984 ◽  
Vol 49 (11) ◽  
pp. 2061-2063 ◽  
Author(s):  
James R. Wait
Keyword(s):  
Electrical Properties ◽  
Electric Dipole ◽  
Free Space ◽  
Eddy Currents ◽  
Interfacial Polarization ◽  
Induced Polarization ◽  
Mutual Impedance ◽  
The Earth ◽  
Horizontal Electric Dipole ◽  
Displacement Currents

In a previous communication I proposed an analytical model to simulate the electromagnetic (EM) and induced polarization (IP) response of a metal well casing (Wait, 1983). To facilitate the analysis, the earth was idealized as a homogeneous conducting half‐space of electrical properties (σ, ε, μ). The well casing was represented as a filamental vertical conductor of semiinfinite length that was characterized by a series axial impedance to account for eddy currents and interfacial polarization. A further basic simplification was to neglect displacement currents in the air; this was justified when all significant distances were small compared with the free‐space wavelength. Initially, the source was taken to be a horizontal electric dipole or current element I ds on the air‐earth interface. By integration of the results, the mutual impedance between two grounded circuits could be ascertained. In the absence of the vertical conductor (i.e., the well casing) the results reduced to those given by Sunde (1968) and Ward (1967).

scholarly journals SUBLUMINAL OPERA NEUTRINOS

2012 ◽  
Vol 27 (07) ◽  
pp. 1250033 ◽  
Author(s):  
ICHIRO ODA
Keyword(s):  
Special Relativity ◽  
Recent Work ◽  
Causal Structure ◽  
Pion Decay ◽  
Speed Of Light ◽  
The Earth

The OPERA Collaboration has announced to have observed superluminal neutrinos with a mean energy 17.5 GeV, but afterward the superluminal interpretation of the OPERA results has been refuted theoretically by Cherenkov-like radiation and pion decay. In a recent work, we have proposed a kinematical resolution to this problem. A key idea in our resolution is that the OPERA neutrinos are not superluminal but subluminal since they travel faster than the observed speed of light in vacuum on the earth while they do slower than the true speed of light in vacuum determining the causal structure of events. In this paper, we dwell upon our ideas and present some concrete models, which realize our ideas, based on spin 0, 1 and 2 bosonic fields. We also discuss that the principle of invariant speed of light in special relativity can be replaced with the principle of a universal limiting speed.

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