Journal of geomagnetism and geoelectricity
Online ISSN : 2185-5765
Print ISSN : 0022-1392
ISSN-L : 0022-1392
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Volume 36, Issue 3
Displaying 1-3 of 3 articles from this issue
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  • Precision of Geomagnetic Field Measurements in a Tectonically Active Region
    M. J. S. JOHNSTON, R. J. MUELLER, R. H. WARE, P. M. DAVIS
    1984 Volume 36 Issue 3 Pages 83-95
    Published: 1984
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.36.83
    JOURNAL FREE ACCESS
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    A linear regression on standard deviations of differential proton magnetometer measurements at distances from a few meters to 50 kilometers indicates a standard deviation of hourly means that varies with site separation as σ=a+bd where a=0.07±0.08nT, b=0.01±0.003nT/km and d is the site separation in kilometers. At a few meters separation, for sites with low cultural noise in both seismically inactive and active regions, the standard deviation of hourly mean data has a mean of 0.12±0.03nT for instruments with 0.25nT sensitivity and 0.07±0.01nT for instruments with 0.125nT sensitivity. The least-count noise contributions are expected to be less than 0.06nT and 0.03nT, while undetermined instrument noise appears to contribute 0.10 and 0.06nT respectively. Instrument temperature sensitivity does not exceed 0.001nT/°C over a range from-6°C to 21°C. For typical site separations of 10 to 15km throughout the San Andreas fault, estimates of σ for hourly mean data range from 0.15 to 0.3nT depending on local magnetization characteristics. Spectral density estimates indicate difference field noise power decreases with increasing frequency at about 3 db/octave. Dominant spectral peaks occur at diurnal harmonics and at the tidal M2 frequency. Geomagnetic difference-field noise limits measurement capability at all frequencies below about 2 c.p.h. Where instrument precision starts to limit detection capability, instruments of higher sensitivity may be useful.
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  • A Profile Study of Geomagnetic Variations in the Nigerian Equatorial Electrojet Region
    P. O. EZEMA, C. A. ONWUMECHILI
    1984 Volume 36 Issue 3 Pages 97-111
    Published: 1984
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.36.97
    JOURNAL FREE ACCESS
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    The profile study involved the evaluation of the effect of the equatorial electrojet on magnetic variations for stations lying between geographic latitudes 14°N and 5°N or dip latitude 4°N and 8°S; and longitude 8°E. An attempt was made to use the strong non-uniform source field of the jet for day effects to make a sounding of average electrical conductivity as a function of depth. Results obtained from the analysis of data show consistency with FORBUSH and CASAVERDOE (1961) and with ONWUMECHILI and OGBUEHL (1967). A model based on the observed Sq shows that the depth of penetration is about 600km while a model based on the day time fast fluctuations shows that at periods of about one hour the depth of penetration lies between 400km and 600km.
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  • Depth of the Non-Conducting Layer in Central-Africa
    S. DUHAU, A. M. OSELLA
    1984 Volume 36 Issue 3 Pages 113-120
    Published: 1984
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.36.113
    JOURNAL FREE ACCESS
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    The geomagnetic daily variations in Central Africa are separated into their parts of external and internal origin and from these parts the depth of the non-conducting layer in that zone is found using methods previously discussed by the authors. A sharp latitudinal gradient in this depth is found in the interval (-400km, 600km) around the dip equator.
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