Abstract

Audio frequency magnetotelluric soundings in the frequency range of 10 Hz to 10 kHz have been carried out over a section of the Superior Province of the Precambrian Shield in northwestern Ontario. Measurements were made in the English River sub-province, which is mainly a gneissic belt. Resistivity values over gneissic rocks and granitic intrusives were found to be generally very high. Resistivity values over greenstone belts were lower. In addition, there was a distinct conductive layering to the crust that was independent of the surface rock type.At the surface there is a thin conducting layer due to water in the surface soils and fractured rocks. Below a few tens of metres, the resistivity rises to values typically between several thousand and 100 000 ohm∙m. This is undoubtedly due to the presence of massive, unfractured rock with little water content. Beyond a depth of 7 km the resistivity drops sharply to values of 100–1000 ohm∙m. One explanation of this drop is the presence of small amounts of trapped pore-space fluids.

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