Shallow geophysics in a hydrogeological investigation of the Oak Ridges Moraine, Ontario

In 1993, the Geological Survey of Canada launched a three year program to investigate the hydrogeology of the Oak Ridges Moraine. This feature is a major recharge/aquifer system affecting over 3 million residents in the greater Toronto area. The moraine is on the order of 160 km in length, covers approximately 1500 square kilometres, and in places the thickness of overburden exceeds 200 metres. Because of the size of the feature, three type areas have been selected for investigation. Extensive use of shallow geophysical surveys is planned before deciding on the most strategic locations for drilling a number of boreholes. Geophysics will play a prominent role in this project's goal of improving the understanding of the complex regional three-dimensional stratigraphy and structure of the moraine, as input for hydrogeological modelling studies. To a large extent, shallow seismic reflection surveys will be used to delineate subsurface stratigraphy. This past summer, approximately 15 line-km of 1Zfold CDP reflection data were collected. As well, over a hundred single spread test shots were recorded to obtain a regional picture of the subsurface structure of major units. The shallow seismic program was aimed particularly at investigating subsurface valleys that may extend beneath the Oak Ridges Moraine. Other geophysical surveys carried out during the field season included shallow electromagnetic (EM-34) surveys to characterize the variability of near-surface materials and evaluate the regional applicability of infiltration test results; deep electromagnetic soundings (EM-47) to determine the depth to bedrock; ground probing radar profiling to delineate the near-surface stratigraphy and map the water table; borehole logging (natural gamma, conductivity and magnetic susceptibility) for lithological interpretations; and high resolution temperature logging for indications of groundwater flow. This paper presents and discusses preliminary results from these shallow geophysical surveys, and evaluates their contributions to the hydrogeological study of the Oak Ridges Moraine.