Wide-bandwidth time-reversal techniques for landmine detection

Abstract

Detection of landmines using acoustic to seismic wave propagation has shown that accurate landmine localization is possible by sensing vibrations of a soil surface above the mine. Time-reversal acoustic (TRA) focusing allows concentration of elastic energy at a location in the soil that increases landmine vibration and improves signal/noise levels. Previous experiments with TRA landmine detection utilized narrow-band signals, but in the most recent experiments wide-bandwidth signals provide clear excitation of the mine resonant frequencies. A loudspeaker array broadcasts these signals and the resulting soil vibration is measured by a laser Doppler vibrometer. The experiment utilizes linear frequency sweeps and orthogonal signals with frequency content between 50 and 500 Hz. Using the orthogonal signals, the suggested TRA procedure can be implemented simultaneously with multiple transmitters to increase the scanning speed. The spatial distribution of energy on and off mine demonstrates that the TRA system concentrates elastic wave energy in the desired area with the focal spot dimension comparable to a wavelength. The spectral density of the TRA focused signal increases in the area near the resonant frequency of the landmine.