Using acoustic impulses to identify a buried nonmetallic object

Abstract

Metal detectors are ineffective in locating shallowly buried plastic objects such as water pipes or land mines. However, the acoustic reflection from such objects has the potential to locate and identify the body. If an impulse is utilized, the ground reflection is followed by one from the object, the delay depending on its depth below the surface. A newly developed probe compares signals from two microphones located on either side of a very reproducible impulse source. When one microphone is over a buried object and the other over a homogeneous matrix, there are subtle differences between the reflected signals, as the probe is effectively an edge detector. Variations in the surface flatness and the matrix homogeneity cause unwanted noise in the difference signal, which can be partially eliminated by correlation techniques. The size and location of the object is determined by traversing the microphone probe over the surface while its depth be deduced by setting different time windows. Trial results have distinguished between plastic strips, disks, and rocks buried between 4 and 15 cm into a matrix of small pebbles. Advantages of extending the technique to three or four microphones will be described.