Working with 3d DXF, how can I export both top and bottom as individual surfaces or points

Hello,
I have been provided with 3d rock strata layers that have been created using Maptek Vulcan software.
The layers were provided as 3d DXFs and contain a 3d object file.
I wish to be able to split the top surface from the bottom surface so that I have two separate surfaces.
I am open to exporting the files as a point cloud/vector data or as another 3d DXF

Can anyone help me out? 
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