Electromagnetic soundings in the sedimentary basin of southern Ontario—A case history

Abstract

The results of electrical and electromagnetic (EM) soundings conducted in the sedimentary basin of southern Ontario are presented. The sounding sites are located strategically to take advantage of the dipping nature of the sediments, a progressive study being undertaken from shallow to deep sections. The vertical magnetic field transfer function, estimated with a pseudonoise source EM system, and conventional Schlumberger apparent resistivity are jointly inverted. For each site, the layered earth model containing the minimum number of layers is fitted to the data. The joint inversion enables up to eight distinct layers to be identified at some sites. The nonuniquenesses or ambiguities in each model, for example S (conductivity‐thickness product) equivalence of a deep thin conductive layer, are revealed through an eigensolution analysis. The intrinsic ambiguities of the models of the deeper sections are resolved by a systematic, progressive site‐to‐site correlation of electrical units across the basin. For example, the conductivity of a conductive unit is measurable when it is at a shallow depth. In a deeper section, if the unit appears as a thin layer, it is assigned this value of conductivity removing the S‐equivalence ambiguity. The individual inversions combined with the site‐to‐site correlation produce an overall electrical model of the basin consistent with the known geologic section.