Application of time‐frequency analysis to landmine detection using a laser Doppler vibrometer

The feasibility of detecting landmines acoustically was recently demonstrated by measuring insonified ground surface velocity using a laser Doppler vibrometer (LDV), since the presence of a mine enhances ground matrix velocity [J. M. Sabatier and N. Xiang, J. Acoust. Soc. Am. 106, No. 4, Pt. 2 (1999)]. Although mines were initially detected using discrete LDV measurements, they were detected for this work by spatially continuous measurement, obtained by sweeping the beam onto the ground surface with a constant velocity in order to hasten the detection process. The ground was excited by single‐tone acoustic waves. After LDV signal demodulation, a sine wave was obtained which was amplitude‐modulated due to mine presence and corrupted by speckle noise ‘‘bursts’’ whose energy grew with the sweeping beam velocity. The signal was time‐frequency analyzed by means of the smoothed Wigner distribution that provides straightforward physical interpretation since time is proportional to beam position. The mine was detected when the signal energy was enhanced at the excitation frequency. The wideband nature of speckle ‘‘bursts’’ allowed corrupted signal sections to be discarded. Results are provided for different mine depths, types of ground, and sweeping velocities. The limits of the analysis are investigated.