Constraining an I-data model within 1 dipping stratigraphic unit

[Adrien]

I'm having issues trying to constrain a modeled contaminant plume within 1 stratigraphic unit. To set the stage: the strat unit is dipping 40 deg to the west and 90% of the contamination is found within this unit. The water table is the upper limit, however (issue #1) when I create the water table surface it plots 4000ft below land surface (I tried plugging in depth bgs as well as elevations for water table and neither will plot correctly). Is there a way to create a planar surface to act as the water table at a specific elevation thus acting as my upper surface data filter (spatial (xyz) filtering)? Next, I've constrained the I-data model in the strat unit (dipping 40 deg) via upper and lower model constraints (top and bottom grids of my strat unit), but when I run the I-data model the whole strat unit becomes "contaminated" not where I know it to be (near the surface exposure to a depth of ~100 ft) (I have high fidelity and Logarithmic selected), the high concentration areas are in the correct location, but according to the model the entire unit has 100ppb. Lastly, my stratigraphy model looks o.k. near the surface (bedrock dipping correctly, appropriate thicknesses, etc.) but at depth everything becomes planar. How do I tell the software that beds that dip at the surface continue to dip in the subsurface?

[Alison]

Adrian:

1. Regarding the problem you are having with the water table. If you are trying to create a grid representing the water level, you should be able to do this by entering your data into the Water Levels table in the Borehole Manager, and then going to Aquifers  Model to create the grid file. As with all data in the Borehole Manager database, you must enter your data based on depth below the collar elevation (the program uses the depth information and the collar elevation for the hole to calculate elevation). If you are having problems with this, and for some reason want to create just a flat surface at a constant elevation (or at a constant dip), you can do this through the Grid  Initialize tool in the RockWorks Utilities part of the program. To find this, go to Grid  Initialize, and enter a reference point through which the grid passes, as well as a constant elevation or a strike and dip for the planar grid.

2. Do you have plenty of low or non-detect values in your dataset? I have seen some datasets where people only entered data that was above detection limits. If you do not have 0 values or very low “non-detect” values in the dataset in areas where you know that there is not contamination, the program will not know how to constrain the plume. If this is not the problem, then I may need to take a look at your data to determine what the problem is.

3. This is a tough one – other than using the Grid  Initialize tool to create a planar surface with a constant dip, there is no way to “force” a grid to have a user-specified dip. You might consider adding some additional, deeper control points to your dataset to force the surfaces to extend down into the subsurface. If this doesn’t work, I have one other idea about how you could use the Grid  Initialize tools, along with the grid residuals tools in the Utilities to get the surface that your want. This would probably be another topic in the forum. Please let me know if you are interested and I will make a posting.

You are welcome to contact me directly with additional questions: alison@rockware.com