QUESTION: How can I cut one surface with another? For example, below I would like it if the projected cross-section image (texture_walls) was cut off at the ground surface (geologic_surface). 

You can cut any object with a surface using the surf_cut module.  We normally use surf_cut to cut volumes with surfaces, but it can cut surfaces also.

the surface to be cut will go into the left input port and the cutting surface into the right port.  You would use plume (or intersection if you needed to do other subsetting operations) to do the actual cutting.

There is an example of using surf_cut to cut surfaces in the Studio Projects in the “Salt Caverns for Oil and Gas Storage” folder.  The application is “salt-dome-caverns.intermediate.evs”.