Gravity interpretation of bedrock topography: the case of the Oak Ridges Moraine, southern Ontario, Canada

Gravity data collected by the Geological Survey of Canada (GSC) over the Oa k Ridges Moraine, southern Ontario, Canada, are processed and interpreted t o obtain images of bedrock topography. The moraine has been the subject of a regional hydrogeological study conducted by the GSC to determine the geol ogical framework and to study groundwater flow in the Greater Toronto area. Upward continuation is used to extract a residual gravity anomaly approxima ting the gravity signal of the bedrock topography. The residual gravity dat a is then inverted using two techniques. The first inverts for the topograp hy of a slab of infinite lateral extent having a flat bottom and a uniform density contrast with the material above. The calculation is done in the Fo urier domain. The second technique is a 3D inversion for the density distri bution using an algorithm implementing compactness and smoothness as global constraints. Forward calculations are done on the borehole model of bedrock topography t o assess the suitability of the residual gravity anomaly. Profiles on the o rder of 25 km in length are modelled using a 2.5D modelling program. The re sults indicate that the quality of interpretation of the gravity signal is highly subject to the choice of residual anomaly, the noise caused by the h eterogeneity and surface topography of the region, and the degree of accura cy in the data. Inverted bedrock topographies obtained from both inversion techniques are s imilar in structure. The results of inversions for density distribution pro vide more realistic bedrock relief. The gravity interpretation indicates th e presence of 10-30-km wide channels trending northwest in the western part of the moraine and northeast east of the Laurentian Channel. (C) 2001 Publ ished by Elsevier Science B.V.