Preserving Stratigraphic Thickness Across Fault Zones

[Chris Lyche]

I am using the stratigraphy section of the borehole manager to generate models which include a faults table to offset the the units. I have been having difficulty preserving the thickness of the units either side of the fault, which also relates a notable shift in thickness when I run either IDW or Kriging  model runs. I have found underlap to preserve the fault sense more accurately but in scenarios with limited data at depth (LHS of the attached strat model) on the east of the linked site map how would rockworks best preserve the trend/dip and thicknesses.

 

Chris

[Brianna]

Thanks for this question Chris.

 

If you want to have uniform thickness across faults, you have to add in some control. RockWorks needs to create a grid across the entire project area. This includes above ground where the units may have eroded away. You need to approach your project with a couple of things in mind: your geology is a result of multiple events and you will need to construct the stratigraphy model in the same way. 

 

I would recommend creating a grid surface of your alluvium on its own first. This gives you a known thickness of alluvium across the site. The bottom of the alluvium will then be the constraining surface to the underlying layers. You can save the alluvium for later as it's modeled by turning off that unit in the stratigraphy list so it's not re-interpolated.  The stratigraphy model is then constructed below including layers that would have intersected the alluvium prior to erosion. Do this by adding stratigraphy above the ground (you can have strat units with negative values indicating that they are above the ground). This gives RockWorks the ability to interpolate the grids across the entire site. When you model this, use the alluvium base as the constraining surface so it cuts the units above that grid surface. You may also need to get your units below your data to extend the beds that may not have been seen in the deeper holes. 

 

Ideally, if you run the stratigraphy model without onlap or overlap (using none)  you shouldn't see any overlapping layers because you are forcing an average thickness of the units where you don't have the data. We are working on an update to RockWorks that will give you the ability to truncate your striplogs in the sections so it doesn't show the data above 0 and below your TD so all of the inferred units in your logs will not display. This allows you to keep single boreholes and not duplicate holes with the extra data. 

 

If you have any more questions please contact me directly.