This grid dataset is part of the 3D stratigraphic model of west-central Alberta (WCAB). It represents the structure top of the bedrock topography, which provides a well-constrained unconformity across which Paleogene−Quaternary sediments were deposited. This is a significant unconformity surface for shallow modelling studies, as it provides the lowermost surface for modelled Paleogene−Quaternary units, and acts as the bounding surface for Upper Cretaceous−Paleogene bedrock strata that Paleogene−Quaternary units unconformably overlie.
This dataset supplements Alberta Geological Survey (AGS) Report 93, which includes a full description of the process used to produce the bedrock topography grid.
All borehole data, interpreted contours, and synthetic data were sampled to the 12 m provincial digital elevation model (DEM) to provide a borehole top elevation (m asl) to correlate the bedrock strata between boreholes to create the bedrock topography grid. This process was necessary as the 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. GPS devices have accuracies reported by the AWWID of ~1-30 m, but are of varying device quality and cannot be assessed for accuracy between devices. Elevation values of moderate-and high-quality data were also checked with the DEM to verify elevation accuracy and modified to match the DEM, if necessary.
Original elevation information for subsurface information that was not sourced from the AWWID (i.e., AGS boreholes and field data, and geophysical logs), was deemed high quality; comparisons were made between the elevation information and the DEM to ensure the correctness of the original elevation information. If the original elevation data from a borehole or field site was approximately the same as the DEM the original elevation information was used. Where original elevation information deviated from the DEM, the DEM value was used at that location.
The vertical accuracy of the 12 m DEM is stated as 5 m @ 90% in the WCAB derived from 1:60 000 aerial photographs using photogrammetric methods.
Methodology to create this grid dataset is fully documented in AGS Report 93.
Modelling software used to create grid: ESRI® ArcMap™ 10.1 (grid interpolation) and Viewlog™ (deterministic alterations)
Number of data points available: 17997
Data sources: A variety of low- to high-quality multisource data were used including borehole and field data, geospatial datasets in GIS format, and interpreted contour and point data from published gridded models. Borehole data consists of lithologs submitted by water well contractors and compiled by the AWWID, downhole geophysical logs collected from water wells, AGS drillholes and field data, as well as industry information.
Interpreted contour and point data were subsampled from the provincial scale bedrock topography, and after being evaluated and compared on cross-section with available borehole data, the contour data were used along with borehole and field data to generate a new bedrock topography for WCAB. Subsampled data include: contour data from published reports, observed bedrock outcrops or bedrock mapped at surface by surficial mapping projects, sediment veneers overlying bedrock, and synthetic control points. Synthetic points were created to constrain the bedrock topography where data was sparse or the complexity of the bedrock topography required further control points to influence the geometry of the modelled surface.
Cross-sections of these data sources were plotted in Viewlog™ to generate stratigraphic picks of the bedrock topography.
Cell Size: 200 m
The Geostatistical Analyst ArcMap extension was used to interpolate the elevation value for the WCAB bedrock topography gridded data. A regional trend surface was modelled from the WCAB 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: 321.48
Nugget: 18.00
Measurement error percentage: 100
Model type: spherical
Range: 2708.16
Anisotropy: No
Partial sill: 374.82
Prediction Errors:
Mean: 0.01
Root mean square error: 11.32
Mean standardized: 0.00
Root mean square error, standardized: 1.10
Average standard error: 10.90
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 WCAB model domain and then converted into an ArcGIS ASCII grid file. The grid file was truncated where it contacted the DEM (resampled to a 200 m grid cell size).