Mapping of Quaternary sediments in northeastern Ontario using ground electromagnetic methods

Abstract

Since 1985, the Geological Survey of Canada has been evaluating the suitability of electromagnetic (EM) methods for Quaternary geologic mapping. Initially, a variety of ground EM instruments were tested at the Val Gagné, Ontario, site. The multifrequency horizontal‐loop equipment (APEX MaxMin I) was selected for transect surveys in the Kapuskasing‐Timmins area. After interpretation of ground EM results, 70 holes were drilled in the area using Rotasonic equipment. A correlation of drilling logs and geophysical data shows that clay, till, and sand have distinct EM responses. A constrained inversion of the ground EM data, in which drilling information was used to fix the layer thicknesses, yielded resistivity estimates for sediments encountered in boreholes. The following average resistivities were obtained: clay, 47 Ω⋅m; till, 123 Ω⋅m; sand, 251 Ω⋅m; the respective standard deviations were 7, 35, and 70 Ω⋅m. Resistivities of clays and tills were also determined in the laboratory on drill core samples. The results of the study indicate that resistivities of Quaternary sediments in northeastern Ontario are sufficiently stable to justify using EM methods for their identification. When layering is not too complex, inversion of EM data can be used to determine overburden thickness. For some applications, for which only qualitative scanning of bedrock topography is required (detection of bedrock valleys), ground EM techniques are a cost‐effective alternative to more accurate, but more expensive, seismic methods.