Spiritwood Buried Valley 3D geological modelling - part of a multidisciplinary aquifer characterization workflow

Abstract

A three dimensional (3D) digital geological model of the Spiritwood buried-valley aquifer complex in southwestern Manitoba is developed to support quantitative hydrogeological modelling. The model maps the bedrock surface and delineates the various hydrostratigraphic units in three-dimensions. The model is based on borehole logs, surficial geological maps, a helicopter time-domain electromagnetic (HTEM) survey covering 1062 km2 as well as 63.5 line-km of high-resolution seismic reflection (HRSR) profiles. A 3360 km2 model area was selected within a larger data capture area of ~12000 km2. The methodologies used for preparing and interpreting various subsurface datasets to construct the geological model are presented. The 3D geological surfaces were modelled using Leapfrog Hydro software. Multiple episodes of glacial erosion and sedimentation superimposed on a broad pre-glacial bedrock valley have resulted in a complex configuration of buried channels and valleys. The bedrock surface shows major erosional features: i) the broad Spiritwood buried valley, ii) deep valleys incised into shale bedrock within the broad Spiritwood buried valley, iii) narrow, steep-sided valleys both within and outside the broad buried valley, and iv) the modern Souris and Pembina river valleys. This bedrock surface detail is only possible because of the spatial continuity of the geophysical data sets and could not have been practically obtained from borehole records alone. The geological model includes 13 hydrostratigraphic units including sandstone, unfractured and fractured shale bedrock, 3 coarse sediment aquifers, 4 till units and 3 thin, near-surface units. Coarse sediment aquifers include the deep buried-valley aquifer, deep sands and gravels both within and outside the broad buried valley and inter-till aquifers at variable depths throughout the model. The model indicates potential hydraulic connections from surface recharge to the deep buried-valley aquifer and for discharge from the deep buried-valley aquifer to streams. Two digital versions of the model are provided - a 'view-only' format accessible with the freely available Leapfrog Viewer and a Drawing Exchange Format (DXF) that can be imported to many 3D modelling applications.