3-D MAPPING IN THICK DRIFT AREAS, SOUTHERN ONTARIO, CANADA: RECENT ADVANCES IN MODELLING

The Ontario Geological Survey (OGS) is undertaking regional 3-D mapping of surficial sediments as part of a broader initiative designed to provide geoscience information for the identification, protection and sustainable use of the provincial groundwater resource. Groundwater Resource Studies summarizing the geology and the properties of the modelled hydrostratigraphic units as well as companion digital data sets including grid files of modelled surfaces and abridged versions of the subsurface databases used for the construction of the 3-D surfaces are key program deliverables. A cross-section viewer and a .kmz file that portrays fence diagrams, transparent overlays of structural contours, isopachs and aquifer recharge/susceptibility maps as well as borehole location and lithologic information in a web-based (Google™ Earth) environment allows for enhanced user interaction with the spatial data.

Integrating variable quality data is an on-going challenge for 3-D modellers. Recent changes to the OGS system of interpolation allows user-defined strata picks to be attributed with a quality of high, medium or low based on confidence in the data source. During the estimation process, a minimum of 1 high, 2 medium or 3 medium or low quality picks are required within user-defined search radii to assign an elevation to a given surface. This method results in a surface that both preferentially honours high quality data and may be more continuous where the data distribution is sparse.

Methods for accurately modelling large and complex valley systems have also been developed. Scripted rules ensure that the stratigraphic integrity of the model is maintained where individual strata are absent whether due to non-deposition or erosion, but do not fully address problems resulting from large and rapid changes in elevation. Changes to the modelling software’s customizable interface allow user-selected groups of strata to be modelled. Using this function, the surface elevations of the ‘upland’ and ‘valley’ strata are modelled separately so that the interpolated elevations of the upland strata reflect their pre-eroded state. A 3-D clipping surface is used to cut the valleys out of the upland strata and trim the excess away from the valley strata. The two sets of strata are then merged into a single model.