Attach the 3D lattice as input "A" and a GRD file with the topography as input "B" to the input of the Math module.
Note: The GRD file must be imported as a uniform lattice.
Use an IF function with the following syntax:
IF Z > B, 0, A Or Z > B? 0:A
With a GRD file, the elevation data is imported as component 1.
Both statements say to set output value to 0 if the Z value is greater than the elevation data in the GRD file (or "B"). If the Z value is less than the elevation value in B, use the data value in A. In this example, a value of 0 is used, but any value less than the Z minimum of the data can be used.
Connect a VolRender module to the output of the Math module, and set the 0 values to be transparent in the Colormap Editor accessed via the ellipsis button [...] to the right of the Colormap example in the VolRender Property Manager.
Blank a lattice above a GRD surface.
The same principle can be applied to blanking to the right of an X value in a BLN file. Remove the BLN header, save to a DAT file in Surfer, and grid the DAT file using the X column for the Z value. In Voxler, attach the Gridder module and GRD file to a Math module as above, and change the reference in the Math equation from Z to X:
X > B? 0 : A
Blank a lattice to the right of an X value in a BLN file.