Nonlinear acoustic landmine detection: Comparison of ‘‘off target’’ soil background and ‘‘on target’’ soil-mine nonlinear effects

Abstract

When airborne sound at two primary tones, f1, f2 (closely spaced near a resonance) excites the soil surface over a buried landmine, soil wave motion interacts with the landmine generating a scattered surface profile which can be measured over the ‘‘target.’’ Profiles at f1, f2, and f1−(f2−f1), f2+(f2−f1), 2f1−(f2−f1), f1+f2 and 2f2+(f2−f1) (among others) are measured for a VS 1.6 plastic, inert, anti-tank landmine, buried at 3.6 cm in sifted loess soil. It is observed that the ‘‘on target’’ to ‘‘off target’’ contrast ratio for the sum frequency component can be ∼20 dB higher than for either primary. The vibration interaction between the top-plate interface of a buried plastic landmine and the soil above it appears to exhibit many characteristics of the mesoscopic/nanoscale nonlinear effects that are observed in geomaterials like sandstone. 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. Tuning curve experiments along with two-tone tests are performed both on and off the mine in an effort to understand the nonlinearities in each case. [Work supported by U.S. Army RDECOM CERDEC, NVESD.]