Nonlinear time reversal acoustic method of land mine detection: Experiment and modeling

Abstract

The nonlinear vibration response of land mines is an effective detection clue. This response can be measured by application of two frequency acoustic waves producing a vibration spectrum above the mine that is different from its surrounding. As an alternative to this approach, this work investigates time reversal techniques as a method of acoustically exciting nonlinear mine vibrations over a broad frequency range. The phase-inversion method used here requires two short broadband time-reversed focused signals with opposite sign, i.e. phase inverted. The focused signal and the inverted focused signal are broadcast sequentially and the responses are summed in post processing. This cancels the linear response leaving the nonlinear component. A theoretical model has been developed that treats the mine stiffness as a nonlinear spring. It predicts that the dominant nonlinear frequencies correspond to the second harmonics of the mine’s primary resonance. Experiments were conducted using six loudspeakers in a box placed directly over a mine. The measured responses confirm most of the model predictions, and illustrate differences between the soil and mine nonlinearities. Work was supported by the U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate