AMT Sounding Through Conductive Glacial Clays in the Canadian Shield

In the summer of 1981, a scalar audio-magnetotelluric survey was completed in four locations in Northern Ontario. Apparent resistivities, for frequencies from 13Hz to 8, 570Hz, were measured at 101 stations. A survey in Moody Township near Lake Abitibi clearly outlines the clay properties and thickness, and also shows that, at the lower frequencies, we are able to map the electrical resistivity of the basement beneath the clay and/or esker cover. Over a region, tentatively identified as metasediments, the bedrock is seen to be strongly anisotropic, reflecting anisotropy of the sediments themselves. In a second region, Marter Township near Engelhart, two survey profiles mapped a region of thickening clay-rich overburden as well as located high resistivities over a large esker. These profiles were taken in an area where the overburden had been drilled in a program of the Ontario Geological Survey. Our results can be interpreted to be in general agreement. A similar example reported from eastern Manitoba shows that the near-surface apparent resistivity is typically as low as 10ohm-m reflecting the conducting clay. The resistivity then rises sharply to values of 1, 000ohm-m to 10, 000ohm-m reflecting the Precambrian bedrock. Surveys in Bowman Township near Matheson, were conducted on a two-dimensional grid and the data consistently show the presence of four layers, a thin resistive surface layer a few meters thick, followed by a very conductive layer of clay. Bedrock is at a depth of 50-100m and is very resistive. This survey gives a map of the resistivity and the thickness of the clay-rich glaciolacustrine sediments and clearly outlines near surface features such as an esker. In addition to the AMT survey, we have collected clay samples from sites in Larder Lake and in Marter Township to study their electrical properties. Laboratory measurements show that in the Kirkland Lake area, the clays have resistivities on the order of 20ohm-m in agreement with the high frequency AMT data over clay-rich overburden.