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 locations, which result in errors in either X, Y, or both.
- incorrectly recorded ground or KB elevations, which result in errors in the Z dimension.
- incorrect stratigraphic top pick elevation values, or incorrectly identified formations result in errors 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.
The vertical positional error in the Precambrian top picks is unknown. There is potential for the introduction of error due to inaccurate KB elevations, and mistakes made during selection of the stratigraphic pick.
Potential errors in KB elevation are likely the greatest source of vertical error in this dataset. The KB values are derived by adding the height of the drilling platform to the surveyed ground elevation. If this is not done correctly, it can introduce vertical error in the KB elevation, which is then propagated into error in the measured depth to the pick and therefore also impacts sub-sea pick depth.
Uncertainty in the vertical positional accuracy can also be caused by human error resulting from incorrect placement of the pick on the well logs.
The occurrence and magnitude of these errors are difficult to identify, however these types of errors were mitigated through comparison with existing published pick datasets and through geostatistical evaluation to identify potential outliers.
Step 1 (compiling): 8416 stratigraphic picks of the top of the Precambrian were compiled from an unpublished model dataset and from new Precambrian top data from AGS geologists.
Step 2 (spatial filtering): The Precambrian top picks are based on well log interpretation and represent the top of the Precambrian. The top also exists in the area of Cordilleran deformation, however picks within this area were removed.
Step 3: (geostatistical analysis): The points were geostatistically analyzed for the presence of outliers, defined as points with an elevation value greater than 20 metres from the modelled surface. Any collocated data points were also identified and removed. 2197 data points were identified as outliers or collocated data, four additional data points were added to improve the data coverage, leaving a total of 6223 points for geostatistical interpolation and modelling.