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.
Highlights
Regional groundwater studies are often faced with limited data to resolve hydrostratigraphy and aquifer heterogeneity
Using the suite of geophysical tools presented here, log response within the glacial stratigraphy across southern Ontario can assist in placing logged intervals of boreholes within a hydrogeological context
Over the past three decades, a suite of geophysical logs has been collected by the Geological Survey of Canada (GSC) in 57 polyvinyl chloride (PVC)-cased wells drilled in the glacial sediments of southern Ontario
Summary
Regional groundwater studies are often faced with limited data to resolve hydrostratigraphy and aquifer heterogeneity. Downhole geophysical logging (wireline logging) has a long history of applications in the petroleum industry (e.g., Cant 1984) for characterization of basin stratigraphy, stratigraphic and formation correlation, characterization of scale and nature of reservoir heterogeneity, and estimation of reservoir properties It has been applied for over 35 years to similar problems in groundwater studies (e.g., Paillett and Crowder 1996; Keys, 1997; Paillett and Reese 2000). Prior to the 1990s, geophysical logging was conducted using analogue tools, but manufacturers have since migrated to digital data collection and storage This greatly enhances the value of the logging data, and offers an opportunity for quantitative (re)analysis, and integration of datasets for regional analyses. This increased level of interpretation, raises the significant issue of tool calibration and collection protocols
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