Lithological and hydrogeological properties from a downhole geophysical dataset in the Vars-Winchester esker, Ontario

Abstract

A study examining the relationship between hydraulic and geophysical properties is underway in the South Nation Conservation Area, southeast of Ottawa, Ontario. Groundwater resources for the local community are drawn from a sand and gravel esker, which is capped by a thick sequence of post-glacial massive to laminated mud sediment. Surface geophysical techniques (high-resolution seismic and electrical resistivity) identified the depth, shape, and general stratigraphy of the esker within the 2-km study area. Four boreholes were then drilled into the center and flanks of the esker to install fully screened PVC casings to conduct hydraulic packer testing and downhole geophysical logging. The geophysical log suite provided a number of insights into the esker stratigraphy and overlying Champlain Sea mud. Gamma, conductivity, magnetic susceptibility, and density logs refined the depths of lithological boundaries and the nature of these contacts (abrupt vs transitional). Bulk conductivity profiles revealed differences in pore water conductivities in the mud sediments overlying and lateral to the esker. Magnetic susceptibility logs revealed the presence of stratigraphic layering within the esker, which nuclear magnetic resonance (NMR) logs identified as having variable porosity.

Hydraulic Conductivity from Nuclear Magnetic Resonance Logs in Sediments with Elevated Magnetic Susceptibilities

“…A reference dataset of downhole geophysical logs was collected in the wells consisting of natural gamma, gamma‐gamma density, bulk conductivity, MS, compressional and shear wave velocities, and fluid temperature/conductivity (Crow et al 2020b). This was followed by NMR logging in the three wells intersecting the esker using a Javelin JP238 logging tool (Vista Clara Instruments) in stationary readings at 0.5 m increments.…”

Downhole nuclear magnetic resonance logging in glaciomarine sediments near Ottawa, Ontario, Canada

“…Henceforth, porosities are referred to as either NMR or core porosities when comparing the two data sets. Pore waters were centrifuged In situ: bulk conductivity, magnetic susceptibility, density Crow et al (2020) from core samples at sites 1 and 2 to analyse pore water conductivity, where values ranged from 180 to 38,510 µS/cm (site 1) and 280 to 45,120 µS/cm (site 2). Elevated conductivities, along with measurements of ionic concentrations of Na, Cl and Br (site 1), indicate a high proportion of relic sea water is preserved in pore waters at both sites.…”

“…Geophysical logs (continuous traces) and core measurements (discrete bars and points) for the boreholes at site 3 (upper panel: GSC-SN2-17-1, lower panel: GSC-SN2-17-2). Lithological unit boundaries and geophysical logs adapted fromCrow et al (2020).…”