Oerad Tech Ltd articles

Air
Low dielectric (~1), high velocity material i.e. EM waves travel almost at the speed of light. In air the emission pattern of GPR antennas is broader than in the ground. It is recommended to place the GPR as close to the ground as possible to direct emission energy further into a material. Some applications of GPR include emission exclusively in air.

Ground / Surveyed material
Higher dielectric (>1), lower velocity materials than air.