Near surface hydro-geophysics in southern Ontario

Abstract

The Ontario Geological Survey (OGS) and Geological Survey of Canada (GSC) have been collaborating on groundwater projects in the southern and central parts of the County of Simcoe, south of Georgian Bay and in the Niagara Peninsula region of Southern Ontario. Geophysical tools used in these projects consisted of geophysical surveys using active and passive seismic, downhole geophysical logging, gravity, airborne EM and magnetic surveys. The main research goal is to better characterise the subsurface by defining geophysical properties of lithological units, defining their spatial distribution, and sedimentary architecture. The ultimate goal is to improve our understanding of regional hydrostratigraphic units, and locally aquifers and aquitards. More than 100 km of high resolution seismic reflection data have been acquired along public roads. The work was performed using a reconfigured IVI "Minivib 1" source with a "landstreamer" three-component geophone array built by the GSC. The landstreamer length is adapted to the depth of imaging using 48 or 72 geophones, mounted on 3 kg metal sleds spaced at 1.5 m towed using low-stretch belts. Data were acquired with shot points every 4.5 m. The Minivib source vibrates a 140 kg mass in cross-line (H2) horizontal mode, using a 7 second nonlinear logarithmic sweep of -2 DB/Oct from 20-280 Hz. This type of sweep increases the sweep time in the low end to increase low frequency energy of shear body waves. Data were recorded using Geometrics Geode engineering seismographs operated in the cab of the Minivib. P-wave sections are processed using the vertical geophones data, while S-wave sections are produced using the cross-line, H2, component data. Seismic sections are then correlated and interpreted with borehole geophysical data. The method has allowed us to measure thicknesses of aquitards and aquifers down to the resolution limits of seismic reflection which is approximately of 1.5 m for s-wave and 3 m for p-wave reflection data. Passive seismic data have been acquired using Tromino Eng-Geo seismographs for further research and comparisons along seismic sections; more research is required to identify the potential use in groundwater exploration of this method. Downhole geophysical logging was collected in 20 OGS boreholes; most of the holes have been logged using natural gamma, apparent conductivity, magnetic susceptibility, gamma-gamma density, fluid temperature and seismic P and S-velocity. This data set provides a high vertical resolution for characterisation of formation properties, allowing an assessment of discontinuities, variation in grain size and mineral composition, porosities and compaction for further interpretation on geophysical data. These new geophysical and borehole data sets have led us to identify unknown and potentially significant groundwater resource targets in both areas of investigations. Aquifers have been highlighted in the form of gravel sheets and eskers. Results constrain the extent and architecture of a newly discovered deep gravel groundwater reservoir that may be a supply for municipal use in the Stayner - Wasaga Beach area. Up to 20 km long seismic sections assist in depicting the architecture of regionally significant stratigraphic units, as well as the extents of sedimentary lobes associated with meltwater related sand, gravel, and mud and diamicton that are very accurately mapped using shear wave reflection data. The seismic section with the borehole data and local water well analysis demonstrate that in the Georgian Bay area, several erosion surfaces truncate aquifers and aquitards, creating a complex sedimentary architecture. Dome-shaped structures up to several kilometres wide have been profiled by seismic reflection in the Niagara region, near Welland, data is still being processed. Additional features awaiting processing and interpretation are potential buried valleys, notably one north of Port Dover, This project highlights the valuable role geophysics can play for groundwater reservoir exploration.