Finding rebar, pipes, and structural members within concrete can be a necessary but difficult task.  Classic techniques involve magnetometers and coring.  Magnetometers are very useful but provide limited information on the exact location and depth of the target and only work with metallic objects.  Coring is of course very invasive and often destroys the target you are trying to preserve.

Ground penetrating Radar (GPR) can be a very useful non-destructive geophysical tool for locating metallic and non-metallic targets.  The two radargrams shown here are examples of locating pipes and rebar within concrete floors.  The technique is relatively quick and can give location and depth information to centimeter accuracy.