During 1970-71, electromagnetic methods were used to determine permafrost thickness on the North Slope of Alaska between Barrow and Prudhoe Bay. In the measurements, the electromagnetic coupling between two loops of wire lying on the ground was measured over a frequency range from 20 to 8,000 Hz. An initial interpretation was done using graphical curve matching, but the data have been subsequently reinterpreted using a computer-based inversion approach. The results indicate that permafrost thickness, as indicated by high electrical resistivity in near surface rocks, is highly variable, ranging from 0 to 2,200 ft (0-660 m). The frozen rock has the least thickness and lowest resistivity in the deltaic areas beneath northward-flowing rivers. The effect may possibly be explai ed by the warming effect of the water flow in the rivers or by a change to finer grain size in the sediments in the deltaic areas.

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