Ceramics do not inherently affect Ground Penetrating RADAR in a different way than other hard building materials like concrete. What can happen in some cases is that a wire mesh will be placed under the tiles in the grout layer and this creates a faraday cage that will block the signal from above. This effects both GPR and X-ray. However, if the slab is elevated then it can be scanned from the underside. This will show the technician the contents of the slab up to the wire mesh mat layer that is under the tiles. The only case that would not able to be scanned is tile that has the wire mesh layer in the grout and is slab-on-grade or elevated slab with metal pan.