Geophysical data acquisition for hydro-stratigraphic mapping in Southern Ontario

In two collaborative projects with the OGS in the Southern Ontario, Canada, the GSC has acquired high-resolution reflection seismic and passive seismic data combined with downhole geophysics data. We have acquired innovative high-resolution compressional (P-) and shear (S-) wave reflection sections using a vibratory source and 3-component (3-C) landstreamer system during field programs carried out in 2013 and 2015. The work was part of a strategy to improve the knowledge and understanding of groundwater resources within the glacial and post-glacial sediments in Southern Ontario. The >80 line-km of seismic profiling is an excellent example of an hydrogeophysics data set, providing detailed information on the depth to bedrock, the architecture and stratigraphy, and physical properties of the overlying sediments on both the regional and local scales. Complex sedimentary features observed include deep bedrock valleys, deltaic and till deposits interconnected by erosive metre to multi-kilometre size structures for which observation and measurements are essential for hydro-stratigraphic mapping. Geophysical logging in deep boreholes was undertaken to assist with the calibration of the seismic sections. We also made an attempt to compare shear wave seismic reflection with passive resonance H/V analysis, the first results are encouraging but need further calibration and modeling over existing borehole data to be conclusive. The seismic surveys were performed using an IVI "Minivib 1" source with a "landstreamer" three-component geophone array built by the GSC. The landstreamer consists of 72 or 48 - 3 kg metal sleds spaced at 1.5 m towed using low-stretch belts. The source vibrates in in-line (H1 or H2) horizontal mode, using a 7 second nonlinear logarithmic sweep of -2 DB/Oct from 20 to 300 Hz to increase the time spent in the low end of the sweep to enhance shear body wave energy. Data were recorded using six to nine 24-channel Geometrics Geode engineering seismographs operated in the cab of the Minivib. Uncorrelated records are collected to allow pre-whitening of the data and careful choice of the correlating function is the first step in the data processing sequence. P-wave sections are derived from data acquired on the vertical geophones, while S-wave sections are produced using the in-line, H1 or the cross-line, H2, component. Seismic sections are then correlated with borehole geophysical data. Interpretation of the equivalent compressional (P-) wave section permits delineation of seismic facies sequences. The shear wave data produce remarkably detailed sections over buried valleys from the surface down to 150 m or more. In the Niagara region, near the seismic lines 6 boreholes have been logged for natural gamma, apparent conductivity, density and fluid temperature, P-wave and S-wave velocities were measured. One hole is situated on a seismic line and was successfully used for seismic depth calibration.