Downhole Seismic Logging Techniques In A Regional Hydrogeology Study, Oak Ridges Moraine, Southern Ontario

Abstract

Downhole seismic logging surveys have been carried out in plastic-cased, continuously-logged boreholes drilled in the Oak Ridges Moraine (ORM) area in support of a regional hydrogeology study. Compressional-wave logs were obtained using a multi-channel hydrophone array with 0.5 m detector spacings and a surface in-hole shotgun source. These data provide detailed and accurate velocity profiles down the hole. In this area, high velocities (>2500 m/s) are associated with the Newmarket Till, a regionally significant aquitard. These velocities are significantly higher than those associated with adjacent stratigraphic units, and more clearly delineate the Newmarket Till than other borehole logs (natural gamma, conductivity or magnetic susceptibility). These data show that the Newmarket Till is a thick (10’s of metres) unit that can be traced regionally beneath the Oak Ridges Moraine. The downhole seismic data also provide a direct correlation between reflection events and depths which improves the calibration and interpretation of surface seismic reflection data. Shear-wave logs have been obtained in one ORM borehole using a compass-oriented, S-component, well-locked geophone. In this borehole, preliminary correlations have been made between seismic velocities (both compressional and shear) and porosity measurements obtained from the core.