Nonlinear acoustic techniques for landmine detection: Experiments and theory

A nonlinear acoustic technique for detecting buried landmines has been suggested by Donskoy [SPIE Proc. 3392, 211 (1998); 3710, 239 (1999)]. Airborne sound at two primary frequencies f1 and f2 undergo acoustic‐to‐seismic coupling and a superimposed ‘‘slow’’ compressional wave interacts with the compliant mine and soil. The nonlinear mechanism involves a simple model of the top surface of the mine–soil planar surface separating two elastic surfaces. During the compression phase of the wave, the surfaces stay together and then separate under the tensile phase due to a relatively high compliance of the mine. This ‘‘bouncing‘‘ soil–mine interface is thought to be a bimodular oscillator that is inherently nonlinear. Geophone measurements scanning the soil’s surface (at the difference frequency) profile the mine, but off the mine some nonlinearity exits. Amplitude‐dependent frequency response curves for a harmonically driven mass‐soil oscillator are used to find the nonlinearity of the soil acting as a ‘‘soft’’ spring. Finally, a layer of soil in a large tube, terminated by a thin elastic plate, is used to model the bimodular nonlinearity. [Work supported by the United States Army Communications‐Electronics Command Night Vision and Electronic Sensors Directorate.]