My graduate research involves the detection and classification of buried object using high pressure waterjets. The idea being that a pulse from a high pressure waterjet will cause a buried object to make a characteristic sound. For a basic understanding of the underlying concept you can observe a different sound from just putting first a plastic soda bottle and then a metal can (both preferably empty) in from of the nozzle of a garden house. Of course buried object underground do not produce nearly as distinctive differences.......