Constraining topmost unit without BH data

I wonder if anyone could suggest a method(s) to constrain the topmost geological unit where there is no borehole data.
My problem:
The model includes made ground (fill) overlying a sedimentary unit. I know from site observation that made ground does not extent past a certain boundary on site. However because the ground surface rises above the nearby borehole collar elevations (which include made ground as topmost unit), LF is extending the made ground volume above what is reality. I've attached a couple of images to illustrate.

I've attempted to use points and polylines (and their tangents) added as contacts to the surface. So far that hasn't really worked. 
Is this the best way of limiting this type of deposit? In which case I'll persevere. If anyone has better suggestions, I'd be grateful to hear them.



  • I have a similar issue when modelling bauxite sometimes. One simple method I use is to place polylines as control points 'up in the air' i.e. well above the topo surface and then snap the lower contact of that unit to them. This acts to drag the lower surface up above the topographic surface. Also extend some control points beyond the extent of your model so the surface doesn't drop back down again. You just need to be careful that it doesn't warp the surface which can potentially artificially thicken the volume where it starts to rise up. 
  • Hi Michael, another alternative approach is use your polyline as a fault to cheat Leapfrog. You could cut your fault against a base reference surface to fault only layers above this reference surface. This way the surface do not pass to other side of fault. I think that it is could work for you.