Electromagnetic Induction in Non-Uniform Conductors, and the Determination of the Conductivity of the Earth from Terrestrial Magnetic Variations

Spherical harmonic analysis coefficients of the external and internal parts of the quiet-day geomagnetic field variations (Sq) separated for the seven continental regions of the observatories have been used to determine conductivity profiles to depths of about 600km by the Schmucker equivalent substitute conductor method. The profiles give evidence of increases in conductivity between about 150 and 350 km depth, then a general increase in conductivity thereafter. For South America we found a high conductivity at shallow depths. The European profile showed a highly conducting layer near 125km. At the greater depths, Europe, Australia and South America had the lowest values of conductivity. North America and east Asia had intermediate values whereas the African and central Asian profiles both showed the conductivities rising rapidly beyond 450km depth. The regional differences indicate that there may be considerable lateral heterogeneity of electrical conductivity in the Earth's upper mantle.

Section: Discussionsupporting

confidence: 65%

Upper mantle electrical conductivity for seven subcontinental regions of the earth.

Campbell

Schiffmacher²

1988

“…Some published estimates of the Earth conductivity exist for the depth range of our report. The five that summarize such estimates are : LAHIRI and PRICE , 1939;BANKS, 1969 ;LARSEN, 1975 ;ANDERSSEN et al, 1979 ;and SCHMUCKER , 1979. They are shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

Conductivity of the subcontinental upper mantle: an analysis using quiet-day geomagnetic records of North America.

Campbell

Anderssen

1983

Electrical conductivity properties of the upper mantle for a North American sector of the Earth have been determined using the 24-, 12-, 8-, and 6-hr spectral components of the quiet-day geomagnetic field variations. Spherical harmonic coefficients obtained from an analysis of the three components of the quiet daily variation (Sq) field for the solar-quiet year of 1965 were applied to a modeling procedure that was modified from SCHMUCKER'S (1970) publication. From depths of about 140 km to about 540 km, the conductivity, 6(ohm-meter) -1, may be represented by where d is the depth in kilometers.Small perturbations of conductivity indicating some layering at 140 to 220, 220 to 400, and 400 to 600 km correspond to the similar behavior of the Earth's density in these regions. From temperature-depth models we infer that the multiphase bulk properties of the expected silicates in these regions behave approximately as where T is the temperature in degrees kelvin. The constants of this equation do not seem to be very model dependent.

“…There have not been many previous estimates of resistivity at depths between 10 and 100km. Lahiri and Price (1939) …”

Section: Discussionmentioning

confidence: 99%

Determination of the Resistivity Distribution at Meanook, Alberta, Canada, by the Magnetotelluric Method

Srivastava

Jacobs

1964

An attempt is made to interpret the magnetotelluric sounding curve at Meanook, Alberta, Canada. Records obtained during field operations in August 1961 and magnetograms and tellurograms recorded at the same time at the Dominion Observatory Meanook are analysed by a visual correlation method to obtain apparent resistivity values for periods between 10 and 3000 seconds. Several theoretical layered earth models are considered in order to fit the data thus obtained. A resistivity distribution, obtained from one of the models which gives an adequate fit to the observed data, agrees fairly well with previous resistivity estimates obtained by other investigators. A probable decrease in the resistivity between the depths 94-140km is obtained by this technique. The lack in resolving power of the magnetotelluric method to delineate various layers with moderate resistivity contrasts such as at the 'Moho' and at a depth of 90km is demonstrated with the help of a few resistivity models.