Phase inversion and two-frequency interaction in nonlinear time-reversal acoustic method of land mine detection

Abstract

Seismic waves are known to generate a nonlinear response in soil, but data show that the presence of a mine increases the nonlinearities due to interactions of the mine and soil system. The nonlinear effects were investigated by two methods. The first, phase inversion, uses two short, broadband TRA focused signals with opposite signs. These are broadcast sequentially and the responses are added in post processing. This results in cancellation of the linear response leaving the nonlinear system components. The second method relies on two-frequency intermodulation. In this case time-reversed seismic energy at 280 and 350 Hz was used to excite the system while the response was analyzed at the sum frequency, 630 Hz. The experiments demonstrate that the presence of the mine increases nonlinearity by 12 dB for the phase-inversion method and by 6 dB for two-frequency interactions in comparison with intact sand. These results support the hypothesis that the interface between mine body and surrounding sand significantly increases the nonlinear response as compared to sand alone. Work was supported by U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate.