ELECTROMAGNETIC METHODS FOR MAPPING SHALLOW PERMAFROST

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Abstract
Two electromagnetic techniques, radiohm and magnetic induction, were evaluated for their ability to delineate and measure the thickness of shallow permafrost. Radiohm instruments in the VLF (15–30 kHz) and LF (200–400 kHz) band, and a magnetic induction instrument using horizontal coplanar coils, separated by a distance of 3.66 m and operating at a frequency of 39.2 kHz, were tested. Along test sites in the Mackenzie Valley, N.W.T. and in northern Alberta, boreholes were placed to verify the interpretation of the geophysical data. The results showed that both LF and the magnetic induction instrument accurately delineated permafrost. The effective depth of exploration of VLF radiation was, in general, too large for delineating boundaries of shallow permafrost. The resistivity layering over permafrost in the winter could be represented by a two‐layer model for computing the thickness of frozen ground.

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