# '''Terrain Surfaces:''' These blocks are used to provide one or more impenetrable, ground surfaces for the propagation scene. Rays simply bounce off terrain objects. The global ground acts as a flat super-terrain that covers the bottom of the entire computational domain.
EM.Cube's [[Propagation Module]] allows you to define block groups of each of the above three types. Each block group has the same color or texture and its members share the same material properties: permittivity eε<sub>r</sub> and conductivity sσ. Also, all the penetrable surfaces belonging to the same block group have the same wall thickness. You can define many different block groups with certain properties and underneath each introduce many member objects with different geometrical shapes and dimensions. The table below summarizes the characteristics of each block type:
{| class="wikitable"