Detection of buried landmines using seismic waves and microphones

Several non-contact vibrometers have been investigated for use in a seismic landmine-detection system developed at Georgia Tech; however, these non-contact vibrometers are relatively complex and expensive compared to commercially available microphones. This makes the commercial microphones an appealing alternative in applications where reduced surface-standoff distances are permissible (such as small autonomous systems or hand-held mine detectors that exploit seismic techniques). The seismic wave field involves multiple modes of propagation. Among these, the Rayleigh wave has been found to be particularly effective for the interrogation of near-surface soil where landmines are likely to be found. Thus the seismic system currently under development preferentially excites this wave type. The acoustic pressure in the air that results from a Rayleigh wave's surface displacement can only be sensed close to the ground because Rayleigh waves are subsonic in most soils and produce evanescent acoustic fields in the air. Experimental measurements in a laboratory model have shown that buried pressure-fused landmines can be detected by measurement of the acoustic pressure within about five centimeters of the ground's surface. Signal processing efforts including planar near-field acoustic holography, k-space filtering, and mode extraction have been used to amplify the effects of the Rayleigh wave. The signal-to-noise ratio of microphone measurements can also be improved by decreasing the microphone's height above the soil surface or by improving the coupling of the microphone to the evanescent field with a waveguide or a horn. Experimental measurements made with the microphone compare well with direct measurements of surface displacement made using a radar-based non-contact vibrometer that has been described in previous papers.