The Ground Penetrating Radar technology (GPR) uses radio waves to extract information from the subsurface. The GPR has a very simple concept of emission of short radio waves (pulses) to the underground and receiving their “echo” after their reflection in different subsurface features.
This is exactly the same physical effect of the acoustic “echo” phenomenon when we speak at a certain distance from a wall. Radio waves (like the well-known WiFi technology) are able to penetrate into the ground. Coming up to the surface, they contain important information about the subsurface geometry.