Removing data below a polysurface threshold

I have a matrix (XYZ) with which I want to remove data above a certain threshold. The threshold is set by a polysurface. How can I use a polysurface as this threshold?

I don't think this is possible using logical indexing (from by newbie knowledge!) I'm thinking of creating a new matrix (A) by selecting data below the polysurface using something similar to:

if (z(i) < y(%polysurface function%)
   then z(i) = A

Is there a better way to do this? Currently I'm still working on a code that's workable!

Thanks!