Soil-plate oscillator: A model for nonlinear acoustic landmine detection

A soil-plate oscillator (SPO) apparatus involves a cylindrical column of granular medium (sand, soil, and pebbles) supported by a thin circular elastic plate (acrylic) that is rigidly clamped to the bottom of a thick walled aluminum tube. The plate is air-backed. The soil column is driven from above by two subwoofers electrically connected to an amplified swept sinusoidal slowly varying chirp. In nonlinear tuning curve experiments, the resonant frequency decreases with increased amplitude—representing a softening in the nonlinear system. Hysteresis effects can be observed along with slow dynamic behavior (associated with mesoscopic nano-scale nonlinear elasticity). In two tone tests, numerous combination frequencies are observed in the surface vibration of the soil along with the sum frequency (typically 200 Hz) and second and higher harmonics. Soil surface vibrations and plate vibrations show similar effects when the soil layer is even 30 times the plate thickness. SPO results compare well when an inert VS 1.6 anti-tank mine is buried in a large concrete soil box. Nonlinear hysteresis models of the soil alone and for the soil-plate interaction are useful in understanding these results, since off-the mine vs. on-the mine tuning curves shapes behave significantly different, indicating some false alarms can be eliminated.