Investigation of microphones as near-ground sensors for seismic detection of buried landmines.

Commercially available microphones were investigated as near-ground sensors to measure the acoustic pressure and the vertical pressure gradient of evanescent air-acoustic waves associated with audio-frequency seismic waves. Measurements in close proximity to the surface and the use of waveguides were found to improve the microphone signal's quality, the comparison of its seismic sensitivity to its sensitivity to propagating sound (ambient acoustic noise and nonseismic reverberation). Landmine images formed using microphone data collected in a laboratory experimental model clearly locate buried inert landmines but exhibit more clutter than images of the same objects formed with seismic displacement data collected using other techniques.

Spectrum analysis of seismic surface waves and its applications in seismic landmine detection.

In geophysics, spectrum analysis of surface waves (SASW) refers to a noninvasive method for soil characterization. However, the term spectrum analysis can be used in a wider sense to mean a method for determining and identifying various modes of seismic surface waves and their properties such as velocity, polarization, etc. Surface waves travel along the free boundary of a medium and can be easily detected with a transducer placed on the free surface of the boundary. A new method based on vector processing of space-time data obtained from an array of triaxial sensors is proposed to produce high-resolution, multimodal spectra from surface waves. Then individual modes can be identified in the spectrum and reconstructed in the space-time domain; also, reflected waves can be separated easily from forward waves in the spectrum domain. This new SASW method can be used for detecting and locating landmines by analyzing the reflected waves for resonance. Processing examples are presented for numerically generated data, experimental data collected in a laboratory setting, and field data.