Applications Of Shallow Geophysics In A Regional Geological And Hydrogeological Investigation, Oak Ridges Moraine, Southern Ontario

Geophysics has played a critical role in a multi-disciplinary research program in the Oak Ridges<br>Moraine area, southern Ontario, designed to improve understanding of the regional geological and<br>hydrogeological framework of a significant Quaternary aquifer complex. Geophysics was particularly<br>important because of the large study area, complex glacial stratigraphy, and thickness of overburden<br>sediments (up to 200 m). Borehole, surface and airborne geophysical surveys have provided information on<br>the subsurface at scales of investigation varying from sub-metre to kilometre. A suite of geophysical logs<br>were obtained in eleven, deep (90-190 m) stratigraphic boreholes. These data were particularly important<br>in identifying downhole stratigraphic relationships and effecting regional correlation. Ground probing radar<br>was used extensively in glaciofluvial sand and gravel deposits of the ORM to obtain depth to water table and<br>very-near-surface structural and sedimentological information. Electromagnetic techniques were tested, but<br>not used extensively during the project. Over 50 line-km of land-based, shallow seismic reflection profiles<br>provided a means of investigating the subsurface architecture and stratigraphic relationships of the complete<br>sequence of unconsolidated sediments. These surveys were instrumental in identifying the regionally<br>extensive and eroded nature of the Newmarket Till beneath the ORM deposits. Data from a regional gravity<br>survey consisting of over 5500 stations were inverted to obtain an interpretation of bedrock topography.<br>Though as yet unproven, high-resolution airborne magnetic data have identified anomalies which may be<br>related to channel features within 200 m of the surface. Overall, the combined geophysical data have<br>provided high-quality control for a large set of archival data, and have allowed an assessment of the lateral<br>continuity of major hydrostratigraphic units. This paper outlines the objectives and applications of these<br>geophysical surveys in the Oak Ridges Moraine study, and assesses their potential applications in other<br>regional hydrogeological investigations.