Nonlinear acoustic experiments involving landmine detection: Status and future

Abstract

In past experiments [J. Acoust. Soc. Am. 116, 3354–3369 (2004)], airborne sound at two primary tones, f1 and f2 (closely spaced near resonance), undergo acoustic-to-seismic coupling. Due to interactions with the buried plastic landmine, a scattered surface profile can be measured. Particle velocity profiles at f1, f2, and f1−(f2−f1) exhibit single peaks; those at 2f1−(f2−f1), f1+f2, and 2f2+(f2−f1) involve higher-order mode shapes. For some combination frequencies the ‘‘on target’’ to ‘‘off target’’ contrast ratio can improve by over 20 dB. Near resonance, the bending (softening) of a family of increasing amplitude tuning curves, involving the vibration over the landmine, exhibits a linear relationship between the peak particle velocity and corresponding frequency. Hysteresis effects and slow dynamic behavior have also been observed. The interaction between the top-plate interface of a buried landmine and the soil above it appears to exhibit many characteristics of the mesoscopic/nanoscale nonlinear effects that are observed in geomaterials like rocks (sandstone). In linear detection schemes the natural inhomogeneous soil layering can generate some false alarms. Recent ‘‘off target’’ versus. ‘‘on target’’ tuning curve measurements show a substantial increase in nonlinearity, suggesting this to be a good false alarm discriminator. [Work supported by ONR.]