GPR scanning uses the contrasting properties of materials to view layers or objects located inside of those layers. When waves transition from one material to another and a portion of the waves reflect. The amount that reflects is based on the change in dielectric constant. An example of this is an air-filled pvc pipe vs a water-filled pvc pipe. The concrete has a dielectric constant of around 9 - 12 and air being 1 would give a contrast of around 10 and water being 81 would give a contrast of around 71. Both would be visible to the GPR technician however the water-filled pipe would show 7 times brighter on the GPR screen. Metal like that in electrical conduits, PT cable, and rebar have a contrast of 100, this means 100% of the signal is reflected back to the GPR receiver creating very dark pecks. This makes locating objects in a concrete slab a perfect case for GPR.