Using acoustic impulses to detect buried objects

Abstract

A detector of nonmetallic buried objects, such as land mines, has been developed using acoustic impulses with a peak energy around 1 kHz. The initial proposal was to compare the difference in signals from two microphones spaced equally on either side of the sound source held a few centimeters above the ground. Ideally, over a uniform surface the difference signal is zero, however, when one microphone is over an object an additional reflected pulse remains after subtraction. In practice, this pulse is small and often obscured by noise. Better results have been obtained if data from a single microphone is subtracted from a ‘‘reference‘‘ waveform. Some problems which have to be overcome are optimizing the alignment of the two pulse trains and deciding whether or not to normalize the signals prior to subtraction. If care is not exercised, either process may distort or mask the required object reflection. This small reflected signal can be further enhanced, compared to the background noise, by correlating with an appropriate known waveform. Mention will also be made of the effects of different media and surface contours.