Abstract:

Propagation of elastic waves in damp, compacted sand involves pressure, shear, and Rayleigh waves. The associated seismic surface displacements can be detected by sensing the acoustic pressure immediately above the surface. Propagation speeds are very low in sand. The high wave numbers of seismic displacements are, therefore, evanescent in air. Thus, the acoustic pressure can only be measured well within a seismic wavelength of the surface. Planar near-field acoustic holography techniques can then be used to back-propagate these signals and calculate surface displacements. Measurements have been made using a laboratory experimental model to investigate the potential of using this technique to detect buried land mines. The experimental model utilizes a surface-coupled transducer to generate elastic waves in a sand-filled tank, which simulates the earth. The microphone and a radar system were used to independently measure the surface displacements. Data taken with both sensors compare well and exhibit the signature of a buried inert antipersonnel mine. For a 100--800-Hz incident pulse, the mine signature can be seen in the raw microphone data when the height of the microphone is less than 3 cm. Holographic signal-processing techniques will be investigated to increase the allowable height for the microphone. [Work supported by ARO.]