Impulsive vibration of the ground can generate seismic interface waves of the Rayleigh type, which decay exponentially with depth into the soil, and spread cylindrically with lateral range. At useful frequencies around 100 Hz, they typically travel at speeds around 100 m/sec, with wavelengths around a meter. Rayleigh waves can be made to propagate in sonar-like pulses to buried targets, reflect, and return to the sonar for reception and signal processing, providing range, bearing, and information as to target type. We have conducted new experiments and analyses with seismic sonar in a clay soil. A focused array of 10 sources and 8 receivers (tri-axial seismometers) were deployed at a range of 4.5 m to examine a 20 lb. landmine as well as a clump of rocks, and other false targets. After vector polarization processing, the amplitude of the mine target echo was 28 dB above the environmental backscatter. Mine-like target confirmation was provided by cross-Wigner-Ville transformation applied to polarized echoes. A first attempt to discriminate man-made from natural targets by identifying energy patterns on this evolutional cross-spectrum approach is presented. The potential for target detection as well as a level of target type classification, at relatively long ranges, was demonstrated.
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