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3-component HR seismic reflection, a new paradigm for near surface exploration of aquifers and aquitards

The recent development of landstreamers towed by vibroseis sources at the Geological Survey of Canada and the resulting typical acquisition of up to 6 km of seismic line per day, has greatly enhanced our capacity to provide regional data for groundwater modeling. In a single pass of
acquisition, the recording of 3-component (3-C) data allows us to observe and process P-waves, S-waves and converted PS-waves. The testing of this technique over various types of unconsolidated sediment has yielded very meaningful observations. We have observed in general that the P-wave is mostly
vertically polarised, while the PS wave can be seen in the vertical and the in-line component, and the S-wave can be seen on all receiver-components with a polarisation that may evolve between vertical and horizontal depending on the geological materials encountered. The polarisation of S-wave data
is observed whether the seismic source position is in a vertical or in a horizontal mode. The use of common-midpoint, normal-moveout velocity analyses, calibrated with geophysical logs and resistivity profiling data, has shown us that seismic velocities can be used to characterise the lithologies
present in the seismic sections. In the St Lawrence Lowlands in eastern Canada, aquifers in the form of tunnel-shaped eskers and coarse sediment layers buried by up to 100 m of glacio-marine silts have been imaged very successfully using this innovative seismic technique. Shear wave seismic sections
acquired in this environment provide subsurface resolution on a scale previously only observed using waterborne seismic methods. The sections show erosional surfaces and vertical structures interpreted to be gas and/or water escape features present in the silt deposits. These high-resolution data
are providing critical new insights into the structural geometry of this type of aquitard. In central and western Canada our seismic system has been used for regional mapping of buried valley aquifers protected and sealed by thick till aquitards. In southeastern Manitoba, a 3-C seismic survey
combined with geophysical dowhole logs has provided critical depth calibrations for a 1000 square-km airborne EM data that has highlighted the presence of a three dimensional network of buried valleys. The high rate of production of 3-C high resolution S-wave and P-wave seismic reflection combined
with other geophysical techniques provides very reliable depths, structural evidence and a better characterisation of the physical property of aquifers and aquitards present in unconsolidated sediments.

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