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 Oak Ridges Moraine, southern Ontario, Canada, are processed and interpreted to obtain images of bedrock topography. The moraine has been the subject of a regional hydrogeological study conducted by the GSC to determine the geological framework and to study groundwater flow in the Greater Toronto area.Upward continuation is used to extract a residual gravity anomaly approximating the gravity signal of the bedrock topography. The residual gravity data is then inverted using two techniques. The first inverts for the topography 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 Fourier domain. The second technique is a 3D inversion for the density distribution using an algorithm implementing compactness and smoothness as global constraints.Forward calculations are done on the borehole model of bedrock topography to assess the suitability of the residual gravity anomaly. Profiles on the order of 25 km in length are modelled using a 2.5D modelling program. The results indicate that the quality of interpretation of the gravity signal is highly subject to the choice of residual anomaly, the noise caused by the heterogeneity and surface topography of the region, and the degree of accuracy in the data.Inverted bedrock topographies obtained from both inversion techniques are similar in structure. The results of inversions for density distribution provide more realistic bedrock relief. The gravity interpretation indicates the presence of 10–30-km wide channels trending northwest in the western part of the moraine and northeast east of the Laurentian Channel.