Evaluation of parametric array technology for acoustic landmine detection

There has been interest in using the parametric array for obtaining a highly directional low frequency source in acoustic landmine detection [M. S. Korman, J. Acoust. Soc. Am. 122 (2007)]. Earlier experiments used a 0.5 m commercial parametric array made up of 70 elements located 1.5 m directly over the target. The array was driven by a 110–1100 Hz swept sine audio modulated 65 kHz tone. A VS 1.6 inert plastic antitank landmine was buried 2.5 cm deep in dry sifted masonry sand in a concrete box. The laser Doppler vibrometer to microphone rms response was sufficient to measure the “on target” to “off target” contrast ratios of 20 and 3 for peaks near 850 and 1050 Hz, respectively (upon signal averaging) but missed the largest peak near 150 Hz (where the SPL was 40 dB per 20 μPa) among others. Recent “forward looking” experiments aligned the array beam axis at 30° from grazing. Sweeping from 110–210 Hz (1600 points) at 16 s/sweep and signal averaging 20 sweeps produced a contrast ratio of 10 for resonance at 148 Hz. Considerable improvement in SPL is necessary in order to make this technology practical for low frequency applications. [Work supported by ARL.]