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

Abstract

In nonlinear acoustic detection schemes, airborne sound at two primary tones, f1, f2 (closely spaced near an 80-Hz resonance) excites the soil surface over a buried landmine. Due to soil wave interactions with the landmine, a scattered surface profile can be measured by a geophone. Profiles at f1, f2, f1−(f2−f1) and f2+(f2−f1) exhibit single peaks; those at 2f1−(f2−f1), f1+f2 and 2f2+(f2−f1) involve higher order mode shapes for a VS 2.2 plastic, inert, anti-tank landmine, buried at 3.6 cm in sifted loess soil [J. Acoust. Soc. Am. 116, 3354–3369 (2004)]. 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. Results are similar to nonlinear mesoscopic/nanoscale effects that are observed in granular solids like Berea sandstone. New experiments show that first sweeping up through resonance and then immediately sweeping back down result in different tuning curve behavior that might be explained by ‘‘slow dynamics’’ where an effective modulus reduction persists following periods of high strain. Results are similar to those described by TenCate et al. [Phys. Rev. Lett. 85, 1020–1023 (2000)]. [Work supported by U.S. Army RDECOM CERDEC, NVESD.]