Stratigraphic mapping of sedimentary formations in southern Ontario by ground electromagnetic methods

Abstract

Multifrequency and transient electromagnetic (EM) soundings were done at several locations in southern Ontario to evaluate the possibility of stratigraphic mapping of gently dipping sedimentary formations by ground EM techniques. The possibility of detecting structural features such as folds, faults, and grabens in the formations was also explored. The two EM techniques were used to map a buried river valley near the village of Copetown, about 75 km southwest of Toronto. The valley, buried under fairly resistive glacial till and dolomite formations was mapped at depths ranging from 100 to 200 m. The interpreted shape and depth of the valley agreed well with information from a high resolution seismic reflection survey and data from wells penetrating the bedrock. Transient EM (TEM) data, using square transmitter loops with side dimensions comparable to the transmitter‐receiver separation for the multifrequency system, was found to be less affected by the presence of shallow conductors and lateral inhomogeneities than the multifrequency data, and had greater depth of investigation. TEM soundings were made on five additional profiles in the area. Lithologic logs from old oil and gas wells located near the profiles provided information for comparison with EM sounding interpretations. The depths to various dolomite, shale, and limestone formations interpreted from EM data agreed well with drillhole information. The EM soundings also detected structures in the Paleozoic formations such as faults and folds that were not previously known. The soundings confirmed the existence of a graben at one site at a depth of 100 m, which was postulated from logs from a cluster of closely spaced drillholes by geologists. The survey results indicated that EM sounding methods can be used for stratigraphic mapping in areas where detailed geological information is unavailable either because the bedrock is concealed by overburden, or when drillholes are sparsely distributed.