Concrete Scanning is a non-destructive method of finding the location of steel reinforcing bars, post-tension cables, power cables, water pipes and voids within a concrete slab.
The way it works is that the Ground Penetrating Radar, (which is what a concrete scanner is, only it uses a much higher frequency antenna compared to what we would use out in the street to locate water and gas pipes) transmits radio waves from the unit into the subsurface.
If the radio wave hits a target, part of the energy from the radio wave will reflect and bounce back to the ground penetrating radar unit. The other parts of the energy from that radio wave will get absorbed by the target and some of it will pass through the target on the way to look for more targets that might be deeper.
It is obviously a lot more technical than this, but basically, if you have something that is within a slab of concrete and you need to know its position, then this is the method of locating you would use.
This method of locating is ideal for those that are drilling core holes or concrete chasing.