The drift thickness map represents the thickness of overburden between the land surface and the bedrock surface. Petrophysical logs were the primary source of information used for constructing the bedrock topography. A suite of the common well logs (gamma, resistivity, spontaneous potential, density, neutron, sonic and calliper) were useful in making the pick for top of bedrock; however, the gamma and resistivity logs proved to be the most useful. The drift typically displays a lower gamma response and higher resistivity response than the underlying bedrock. Other sources of data were water well lithologs, mineral exploration drillholes and outcrop information.
Mapping the bedrock surface was difficult in some areas where data were sparse. Many of the log traces were absent from the upper part of the hole because of surface casing. The depth of surface casing set in bedrock was used for an estimate of maximum drift thickness in places with few data. Conversely, many water wells did not penetrate deep enough to intersect the bedrock, so only a minimum drift thickness value could be determined.
The drift thickness interpretation was made after completion of the bedrock topography contouring. The bedrock topography contours were initially generated from bedrock surface picks using a computer-contouring program with some subsequent modifications by hand. The bedrock topography surface, in a digital grid format, was subtracted from a digital elevation model of the present-day surface. The resulting grid was then contoured to form an isopach map of the drift. Preliminary versions of this map were released as Alberta Geological Survey (AGS) publications by Pawlowicz and Fenton (2002) and Andriashek et al. (2001).
The drift thickness and structure contour of the bedrock surface maps were determined from bedrock stratigraphic picks made from three main sources: oil and gas wells, water wells and outcrops. In oil and gas wells, the bedrock picks were made using geophysical logs. From water wells and outcrop, lithologic decriptions were used to determine the depth to bedrock pick. Picks data were tabulated and imported into a contour-mapping software (MacCad v6.0) using a Macintosh desktop computer. The data were then gridded by triangulation using the Delaunay tuple method. The resultant grid was then contoured using the mapping software. The contour lines were then manually edited on the computer using the MacCad software. Next, the final contour lines were gridded again using 400 x 400 grid matrix to produce infill colour rendition. The drift thickness map was finally created by subtracting the bedrock surface grid from a grid of the ground surface topography. The resultant grid was then contoured to create the drift thickness map.
Final map preparation was as folows: the bedrock contour and gridded data were exported from MacCad in .dxf format and imported into ArcInfo v8.3. The attributes were deleted. The coverages were transformed from page units to real world coordinates (RMS error +/- 20 m) and projected to UTM12 NAD83. The contour lines were smoothed, tagged with the corresponding elevation/drift thickness values and exported to shapefiles. Map compilation was done using ArcGis v8.3.