This underground heating oil tank was detected under a concrete floor using a ground penetrating radar (GPR) unit (See figure above). If only a metal detector was used, at best it would be suspected as a potential candidate for an oil tank. Exploratory digging would be difficult due to the concrete floor. However, Since the oil tank footprint occupied almost the whole concrete floor and there were steel stairs/fences and brick walls nearby, the metal detector responses could be easily disregarded as interferences from aboveground objects and reinforcement in the concrete. As a result, the oil tank could be missed if only a metal detector was used. Fortunately, we always use a GPR with a metal detector for oil tank…