Nonlinear acoustic experiments involving landmine detection: Connections with mesoscopic elasticity and slow dynamics in geomaterials, Part II

Abstract

In nonlinear acoustic detection, airborne sound at two primary tones, f1, f2 (chosen several Hz apart from resonance) insonifies the soil surface over a buried landmine, and due to soil wave interactions with the landmine, a scattered surface profile can be measured by an LDV. Profiles at f1, f2, f1−(f2−f1) and f2+(f2−f1) exhibit a single peak while profiles at 2f1−(f2−f1), f1+f2 and 2f2+(f2−f1) are attributed to higher order mode shapes. The lowest resonant frequency for a VS 1.6 plastic, inert, anti-tank landmine, buried at 3.6 cm deep is ∼125 Hz. The ‘‘on target’’ to ‘‘off target’’ contrast ratio, for some of the nonlinearly generated combination tones, is roughly 15–20 dB higher compared to either primary component. Near resonance, the bending (softening) of a family of increasing amplitude tuning curves, involving the surface vibration over the landmine, exhibits a linear relationship between the peak particle velocity and corresponding frequency. The tuning curves exhibit hysteresis effects. Landmine-soil vibrations exhibit similar characteristics to nonlinear mesoscopic/nanoscale effects that are observed in geomaterials like rocks or granular materials. Nonlinear mechanisms of soil and the soil interacting with the top-plate of the mine case are compared. [Work supported by U.S. Army RDECOM CERDEC, NVESD.]