criteria to assign physical properties at each nodes of a 3d model

Hi, I'm building a 3d geological model (i.e. two layers). I would assign at each node of the model a physical property (e.g. thermal conductivity as I'm building a thermal model) taking into account the two different lithologies. The boundary between the bottom of the first layer and the top of the second one is defined by a surface (e.g. a plane). First, I defined the volume of the model:

[X,Y,Z] = meshgrid(287800:50:297800.000000,4167350:50:4177750,0:50:5000);

Second, I defined the layer boundary (in this example I supposed it's a plane but it will be a irregular surface described by its x,y,z coordinates):

[xi,yi] = meshgrid(287800:50:297800.000000,4167350:50:4177750);
zi = xi .* 0.9e-03 + yi .* 0.3e-03;

To assign at each node of the first layer a selected preperty I think I need to use index operations of the type

for i = 1:209
   for j = 1:201
       for k = 1:zi
       th(i,j,k) = 2.5
       end
   end
end

but written like that, it's wrong. Any suggestion?

Thanks

Gianluca