An investigation of time-reversal techniques in seismic landmine detection

A system is under development at the Georgia Institute of Technology that utilizes a seismic source to propagate Rayleigh waves through a medium such as soil. Non-surface-contacting electromagnetic sensors are used to detect the displacement of the medium created by interaction of the Rayleigh waves with a target, such as a landmine. The system has been tested in a relatively uncluttered medium and has yielded encouraging results, demonstrating that the system is effective for the detection of targets buried just below the surface. The system performs well in an uncluttered medium. However, when the medium is filled with a large number of scattering objects, the Rayleigh wave will be broken up by the scatterers in the medium to the point that the wave front no longer interacts with the target as it would in an uncluttered medium. This causes detection of a target to be uncertain or impossible. In an effort to extend the application of this system to a highly cluttered medium, the time reversal method is applied to the seismic system, and evaluated for focusing Rayleigh wave fronts at a desired location. Numerical and experimental results are presented for a propagation medium with no scatterers present, and with multiple scatterers present. Time-reverse focusing results are also compared to uniform excitation and time-delay beamforming methods.