Presence Of Buried Objects Research Articles

Principles of surface-phase-resolved shearography

There is a need to remotely measure the full phase and amplitude information of small-scale acousto-seismic vibrations in order to detect the presence of buried objects (e.g., tunnels, etc.), or for other purposes. This remote sensing information may need to be collected with a large area coverage rate and at a safe standoff distance. To accomplish this, we have implemented a shearographic imaging system that incorporates phase stepping in a novel way, automatically separating random speckle noise from surface motion, without requiring an intermediate unwrapping step. This method, which we call surface-phase-resolved shearography, is especially effective for very low-amplitude motions that generate less than one light-wavelength of phase change. In laboratory studies, we have demonstrated sensitivity of two nanometers RMS with 532-nm-wavelength light.

Influence of buried objects on the dynamic nonlinearity of inhomogeneous materials

Donskoy et al. [J. Acoust. Soc. Am. 111, 2705–2714 (2002)] and Korman et al. [J. Acoust. Soc. Am. 116, 3354–3369 (2004)] have shown that the nearby presence of compliant buried objects results in a significant nonlinear response in the spectrum of surface vibration induced by acoustic excitation. The latter have suggested that there is a strong connection with the inherent Nonlinear Mesoscopic Elasticity (NME) of inhomogeneous materials. Laboratory experiments are reported that investigate the comparative NME of granular and fibrous materials and the variation observed between several states of a single material (sand) subject to high sound pressure levels in the frequency range 100 Hz to 2 kHz. Fiberglass and gravel are shown to have strong inherent nonlinearity whereas wet compacted sand has relatively little. In both two-tone and single tone experiments, it is shown that, the presence of buried objects may enhance or reduce the inherent NME of the embedding medium. In cases where the NME is enhanced, compliant buried objects have a greater effect than relatively stiff objects. [Work supported by DSTL, UK.]