Journal of geomagnetism and geoelectricity
Online ISSN : 2185-5765
Print ISSN : 0022-1392
ISSN-L : 0022-1392
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Volume 35, Issue 7
Displaying 1-3 of 3 articles from this issue
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  • An E. M. Method for Earth Resistivity Measurements Using Power Line Harmonic Fields
    D. C. MCCOLLOR, T. WATANABE, W. F. SLAWSON, R. M. SHIER
    1983 Volume 35 Issue 7 Pages 221-243
    Published: 1983
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.35.221
    JOURNAL FREE ACCESS
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    An experiment was carried out to infer ground conductivity by making use of magnetic fields created by a power line carrying electric currents as source signals for E. M. exploration. The experiment was done in an area, 10km×10km approximately, which is under an overwhelming influence of a major 3 phase a. c. power line. It runs generally east-west and south of the area. The magnetic field measurement showed that the 60Hz, 180Hz, 300Hz, and 420Hz signals were the four strongest harmonic components. A plot of the field measurements along a northsouth line, normalized to the value at a reference station, shows that the vertical magnetic field strength decreases inversely with the distance from the power line in the vicinity of the source and to the cubic inverse of the distance further from the source. The crossover distance (between the 1/r relation and 1/r3 relation) is dependent on frequency. Extension of an existing theory predicts this trend with the crossover distance √2 times greater than the skin depth, providing that the Earth is considered a uniform conductor. The effective resistivity is inferred to be of the order of 104Ωm from the crossover distance for 300Hz and 420Hz. No crossover is seen for 60Hz, and for 180Hz, the fall-off ∝ 1/r3 begins to take place near the edge of the survey area (-10km in distance from the power line). A deviation from the general trend occurs at the same location for all four frequencies. A change in geologic material in the subsurface is mapped in this locality.
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  • The Effect of the Depth of the Non-Conducting Layer on the Induced Magnetic Field at the Peruvian Dip Equator
    A. M. OSELLA, S. DUHAU
    1983 Volume 35 Issue 7 Pages 245-253
    Published: 1983
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.35.245
    JOURNAL FREE ACCESS
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    The internal part of the geomagnetic daily variations at the Peruvian dip equator shows the presence of an anomaly in the earth conductivity. This anomaly is further investigated in the present work using the external and internal parts of those variations, found in a previous work. The field induced at the ground by the current system determined from the external part is computed assuming a simple conductivity model. The depth of the non-conducting layer, p, is then found by fitting the results with the internal part. This procedure makes it possible to remove the usual assumption that the quotient between the internal and external parts of the planetary field is 0.4 and to find its actual value which, in the model, is a function of p.
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  • The Model of the Geomagnetic Non-Dipole Field
    Tan-Wen WANG, Gui-Zhong QI
    1983 Volume 35 Issue 7 Pages 255-280
    Published: 1983
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.35.255
    JOURNAL FREE ACCESS
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    Yukutake's model separates the geomagnetic field into drifting and standing parts, not only fits well to the observed field for the past several hundred years, but also emphasizig the regularities of the westward drift of the geomagnetic non-dipole field. However, there are still some problems to be reinvestigated:
    a) In Yukutake's model the geomagnetic secular variation consists of only drifting parts, which is not consistent with the observed results;
    b) The drift velocity depends on the order m of spherical functions. It is very difficult to understand its physical significance, or to imagine the configuration of its source field.
    In this paper we make some modifications to Yukutake's model, having the standing part dependent on time, and adopting a uniform drift velocity ν=0.295°/yr.
    The modified model is much more fitting to the observed fields than that of Yukutake's. The root-mean-square value of the differences between the main fields of the computed and the observed decreases by 50%, and that between the secular variations decreases even by 10%.
    The more important fact is that the modified model can successfully minimize the above mentioned uncertainties of Yukutake's model.
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