This dataset is part of the 3D stratigraphic model of Paleogene−Quaternary sediments in west-central Alberta (WCAB). It represents the modelled top of the informal stratigraphic unit (SU) 1, which contains coarse-grained fluvial materials lying directly on bedrock and is constrained to uplands surfaces in the Paleogene−Quaternary succession of WCAB.
This dataset supplements Alberta Geological Survey (AGS) Report 93, which includes a full description of steps taken to produce the modelled grid.
Numerous water wells from the AWWID were used in the study to broadly define stratigraphic units in the Paleogene−Quaternary succession. The AWWID has abundant, relatively low-quality data records, which include location inaccuracies resulting in up to 10 wells with duplicate locations. In cases of collocated data, water-well records were evaluated on cross-section to consider surrounding data and the accuracy of lithology information contained within the record. This was a manual process to identify the most correct duplicate well litholog to ensure a laterally extensive stratigraphic unit could be assigned to subsurface information. Water-well records that were not considered to be correct were maintained in a final database, but not used as input for the modelled surface.
The original input data were classified using unpublished stratigraphic picks to create a laterally extensive and mappable geobody. Classification of lithological data into stratigraphic units may incorporate assumptions and human error when simplifying complex geological data, as unit geometry, extent, and description are a product of the author’s conceptual understanding. The SU1 grid is for use in regional-scale studies, as these mappable stratigraphic units may be more complex or absent at the scale of a local investigation.
This grid represents all areas within the geographic boundary of SU1 and has been exported to ASCII grid format.
All borehole data from the AWWID were sampled to the 12 m provincial digital elevation model (DEM) to provide a borehole top elevation (m asl) to correlate stratigraphic units between boreholes to create stratigraphic grids. 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: RockWorks17™ (grid interpolation) and Viewlog™ (deterministic alterations)
Number of data points available: 844
Data sources: A variety of low- to high-quality multisource data were used including borehole and field data, geospatial datasets in GIS format, and published gridded information. 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.
Cross-sections of these data sources were plotted in Viewlog™ to generate stratigraphic picks of SU1.
Cell Size: 200 m
Grid Interpolation Information:
Stratigraphic picks of SU1 were interpolated using RockWorks17™. Prior to interpolation the ‘Fill in Missing Stratigraphy Intervals’ RockWork17™ tool was invoked to automatically assign a 0 m thickness to eroded or not deposited stratigraphic units along a borehole path. This function allows the interpolation procedure to define stratigraphic pinch-outs between data points and creates a continuous modelled surface across the model domain.
The following parameters were used to create the modelled grid of SU1.
Algorithm used to interpolate data points: Inverse distance weighting (IDW2)
Minimum search radius: 8 points
Sector Based Searching: 90 degrees (4 sections)
Cutoff Distance: 15%
Following interpolation, the full stratigraphic model with all units, was built in RockWorks17™ from the base up to ensure that onlapping of units occurred with no unit cross-overs.
Deterministic Grid Alteration Information:
All stratigraphic gridded data used to form the WCAB model (SU 1, 3, 4a, 4b, and 5) as well as the WCAB bedrock topography were exported as ASCII grids from RockWorks17™ and imported into Viewlog™.
A number of grid math procedures were performed in Viewlog™ which include:
1) Constraining the top and base of SU1 by the bounding unconformable surfaces of the surface topography (i.e., the DEM; resampled to a 200 m cell size) and bedrock topography.
2) Calculating isopach grids from the modelled top and base surfaces of SU1.
3) Running the isopach grid through a boolean grid filter to determine where the unit exists (the value ‘1’ denotes SU1 is present and has a modelled thickness greater than 0 m, and the value ‘0’ which indicates the calculated isopach grid is equal to 0 m, and SU1 is absent).
4) Converting the boolean grids to a value of ‘0’ where no SU1 stratigraphic picks were available to support the presence of SU1.
5) Multiplying the boolean grid of SU1 to the calculated isopach grid of SU1 and adding the resultant grid to the underlying bedrock topography to achieve an elevation (m asl) surface for the top of SU1.
Root mean square error (after deterministic grid alterations): 3.15 m
The RMSE compares how similar interpolated grid values are to the original input data. This is a global estimate of error for the SU1 grid and provides neither indication of how error is distributed spatially in the model domain nor the accuracy of the input data.