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Electrical conductivity inhomogeneities in the earth's upper mantle Hyndman, Roy David


This study was undertaken in order to investigate the possible occurrence of horizontal variations in the electrical conductivity of the Earth’s upper mantle in southwestern Canada, using a series of simultaneous magnetic variograph recordings. Until recently no variation in the conductivity in a horizontal direction had been anticipated nor observed. During the past five years Parkinson in Australia, Rikitake in Japan, Schmucker in Germany and others have observed marked differences in the magnetograms at closely spaced stations. Secondary magnetic fields produced by induction in high conductivity regions in the Earth’s upper mantle have been suggested as the cause of these differences. The profile described in this thesis indicates conductivity inhomogeneities in southwestern British Columbia and southwestern Alberta. The vertical magnetic fields produced by induction in these inhomogeneities for magnetic variations with periods from 10 to 120 minutes have magnitudes of about 30 to 60 percent of the normal horizontal component. The normal vertical component is about 20 percent of the horizontal. These regions appear to be essentially two dimensional with anomalous induction resulting only from that component of the incident magnetic variations which is perpendicular to their strike. This strike and the intensity of the induced field have been estimated at each station. A pronounced difference has also been found between the vertical component of the diurnal geomagnetic variations at a station In the Rocky Mountains and those at the rest of the stations along the profile.

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