This grid dataset is part of the 3D hydrostratigraphic model of the Calgary-Lethbridge Corridor (CLC), southwestern Alberta. It represents the structure top of the bedrock topography, which provides a well-constrained unconformity across which Neogene−Quaternary sediments were deposited. This is a significant unconformity surface for shallow modelling studies, as it provides the lowermost surface for modelled Neogene−Quaternary units, and acts as the bounding surface for Upper Cretaceous−Paleogene bedrock strata that Neogene−Quaternary units unconformably overlie.
This dataset supplements Alberta Geological Survey (AGS) Report 91 and INF 150, which include a full description of the process used to produce the bedrock topography grid.
All borehole data from the AWWID were sampled to the 25 m Provincial Digital Elevation Model (DEM) to provide a borehole top elevation (m asl). This process was necessary as the borehole top elevation of AWWID water wells, in many cases, was not available/recorded or verified, and if reported was estimated from a topographic map, field investigation or handheld Global Positional System (GPS) device. Elevation values of moderate- and high-quality data from AGS boreholes and field sites, geophysical logs, and previously published geological and hydrogeological maps were also checked with the DEM to verify elevation accuracy and modified to match the DEM, if necessary.
The vertical accuracy of the 25 m DEM is stated as 5 m @ 90% in the CLC derived from 1:60 000 aerial photographs using photogrammetric methods.
Methodology to create this grid dataset is fully documented in AGS Report 91.
Modelling software used to create grid: ArcMap (grid interpolation) and RockWorks15 (deterministic alterations)
Number of data points available: 17252
Data type: A variety of low- to high-quality borehole data were used including lithologs submitted by water well contractors and compiled by the Alberta Water Well Information Database (AWWID), downhole geophysical logs collected from water wells, AGS borehole logs and field data; stratigraphic picks made from oil and gas geophysical logs by AGS geologists; and the Sustainable Resource Development (SRD; 25 m cell-size) digital elevation model (DEM), where known bedrock outcrop locations were made equal to the elevation value of the DEM.
Cell Size: 400 m
The Geostatistical Analyst ArcMap extension was used to interpolate the elevation value for the CLC bedrock topography gridded data. A regional trend surface was modelled from the CLC bedrock topography data points and removed prior to interpolation. An ordinary kriging algorithm was then used in the ArcMap Geostatistical Analyst extension to interpolate the residual elevation values and create a grid of the structure top of the bedrock surface.
The following Method Report and Prediction Errors provide information about the parameters used to create the grid, and any errors associated with the gridding process.
Method Report:
Method: Kriging
Type: Ordinary
Output type: Prediction
Variogram: Semivariogram
Number of lags: 12
Lag size: 609.04
Nugget: 30.54
Measurement error percentage: 100
Model type: spherical
Range: 5211.08
Anisotropy: Yes
Minor range: 4125.11
Direction: 97.73
Partial sill: 384.32
Prediction Errors:
Total points: 17825 (points include a buffer around the study area to reduce interpolation error at boundaries)
Mean: -0.02
Root mean square error: 11.56
Mean standardized: -0.00
Root mean square error, standardized: 0.99
Average standard error: 10.94
Error exclusion: 0
Number of points excluded: 0
Percent error exclusion: 0
Deterministic Grid Alteration Information:
The regional trend was added back to the residuals to create the final geostatistical layer. The layer file was then extrapolated to cover the full extent of the CLC and then converted into an ArcGIS ASCII grid file. The grid file was truncated where it contacted the DEM (resampled to a 400 m grid cell size).