The bedrock topography of Alberta is the surface of the top of Upper Cretaceous and Paleogene bedrock, and reveals geomorphic features created by Paleogene to Recent river systems and Quaternary glaciation.
This grid is a computer-generated geostatistical model of the bedrock topography of Alberta using previously published information from Alberta Geological Survey (AGS) maps and reports, as well as new data. The quality of this data has been assessed and then a quality-weighting approach was applied to the dataset prior to interpolation. The surface was modelled using a 500m x 500m grid cell spacing, and as such should not be used for local scale studies. This grid represents the surface portrayed in AGS Map 602, Bedrock Topography of Alberta, and this grid will continue to be updated as part of the Alberta Geological Framework project.
Explanatory notes, references, and data sources appear on a supplementary page to accompany Map 602. These notes describe the distribution of physiographic terrain elements of the bedrock surface across Alberta, and the data sources and geostatistical methods used to interpolate this surface.
A stratigraphic pick in a well is a point defined in three dimensions (X, Y and Z).The accuracy of the pick, either in measured depth from the kelly bushing (KB), or with respect to sea level, is difficult to quantify. Sources of error include:
- incorrectly recorded well surface or bottom hole location, which results in an error in either X, Y, or both.
- incorrectly recorded ground or KB elevation, which results in an error in the Z dimension.
- incorrect stratigraphic top pick elevation values, or an incorrectly identified formation result in error in the Z dimension.
- errors in data entry or data transfer, resulting in errors in X, Y and/or Z values.
Any errors in the initial data points were reduced in the final data set by geostatistical evaluation to identify and remove outliers.
Step 1: (Input) The grid representing the elevation at which the uppermost bedrock unit was encountered and was modelled using pick data compiled from a variety of data sources. These data were considered to be of variable quality depending on the source from which they were acquired, and were quality-weighted during interpolation of the bedrock topography surface. This was accomplished by spatially filtering the data based on data quality. The medium quality data were filtered to remove all data from this subset that were within 1000m (2 grid cells) of any high quality data point. The low quality data were filtered to remove any data that was within 2500m (5 grid cells) of any high or medium quality data point. The quality filtered dataset data were assigned a quality code of high, medium, or low, and this information is included in the data table for the point dataset. For areas where there was no bedrock top pick available within 10 kilometers, a control point was inserted to help constrain the surface during interpolation. For additional information on the data used to model the bedrock topography, please see page 2 of AGS Map 602.
Step 2: (Modelling the surface) A regional trend surface was modelled from the 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 bedrock topography 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: 14
Lag size: 900
Nugget: 0.8
Measurement error percentage: %100
Model type: Spherical
Range: 9,500
Anisotropy: Yes
Minor range: 6,000
Direction: 27
Partial sill: 634
Prediction Errors:
Total points: 174980
Mean: 0.02
Root mean square error: 7.41
Mean standardized: 0.00015
Root mean square error, standardized: 1.02
Average standard error: 9.93
Step 3: (Creating the grid)
The regional trend was added back to the residual elevation values to create the final geostatistical layer. The layer file was then extrapolated to cover the full extent of the province and then converted into an ArcGIS ASCII grid file.