Borehole geophysical log signatures and stratigraphic assessment in a glacial basin, southern Ontario

Abstract

Over the past two decades, the Geological Survey of Canada has used a standardized suite of slim-hole geophysical tools to log 57 polyvinyl chloride cased boreholes drilled in the glacial sediments of southern Ontario. This article documents downhole tool responses (natural gamma, apparent conductivity, magnetic susceptibility, and seismic velocity) in the context of mineralogical characteristics of the region and grain-size data from 28 of the 57 boreholes. Characteristic geophysical properties and (or) patterns are identified within the units of a regional hydrostratigraphic framework in southern Ontario. The importance of a calibrated suite of tools is emphasized, as stratigraphic units may have variable response from site to site. The use of a high-sensitivity magnetic susceptibility induction probe is shown to be an important tool in the log suite for lithostratigraphic interpretation, and more broadly, for provenance studies of source rock across the region. Ranges of compressional (P) and shear (S) wave velocities and their ratios are provided for each of the hydrostratigraphic units. Case studies are presented to demonstrate how logs may assist in the interpretation of glacial processes at lithological boundaries.