Nonlinear acoustic techniques for landmine detection

Abstract

When airborne sound couples into the ground seismic waves can interact with a target buried in the soil and effect the vibration velocity of the surface. Acoustic-to-seismic coupling (using linear acoustic techniques) has proven to be an extremely accurate technology for locating buried land mines [J. M. Sabatier and N. Xiang, IEEE Trans. Geoscience and Remote Sensing 39, 1146-1154 (2001)] . Donskoy [SPIE Proceedings 3392, 211-217 (1998); 3710, 239-246 (1999)] has suggested a nonlinear technique that can detect an acoustically compliant buried mine that is insensitive to relatively noncompliant targets. (Utilizing both techniques could eliminate certain types of false alarms.) Airborne sound at two primary frequencies f 1 and f 2 undergo acoustic-to-seismic coupling and a superimposed seismic wave interacts with the compliant mine and soil to generate a difference frequency component that can effect the vibration velocity at the surface. Geophone measurements scanning the soil's surface at the difference frequency (chosen at a resonance) profile the mine with more relative sensitivity than the linear profiles -but off the mine some nonlinearity exists. Amplitude dependent frequency response curves for a harmonically driven mass-soil oscillator are used to find the nonlinearity of the soil acting as a "soft" spring. Donskoy's nonlinear mechanism (over the mine) involves a simple model of the top surface of the mine-soil planar surface separating two elastic surfaces. During the compression phase of the wave, the surfaces stay together and then separate under the tensile phase due to a relatively high compliance of the mine. This "bouncing" soil-mine interface is thought to be a bi-modular oscillator that is inherently nonlinear.

Granular layers on vibrating plates: Effective bending stiffness and particle-size effects

“…1 and is based on that of Korman and Sabatier. 9 The plate-sand system is created usa͒ Author to whom correspondence should be addressed; electronic mail: jaturner@unl.edu ing a circular brass plate ͑mass= 26.4 g, thickness = 0.25 mm, radius= 50.8 mm͒ clamped at the outer edge. Acoustic waves are generated by a loudspeaker positioned below the plate.…”

“…Thus, a clear understanding of the response of a platesoil system is important for increasing the probability of detection and for reducing false alarms. The influence of a soil layer on the frequency response of a plate was recently examined experimentally by Korman and Sabatier 9 for loess soil and by Zagrai et al 10 for moist sand. In all cases, it has been observed that the resonance decreases as the layer thickness increases, reaches a minimum and then increases.…”

Resonance and Nonlinear Seismo-Acoustic Land Mine Detection

“…We are not discounting the hysteretic and relaxation nature of the soil dynamics. In fact, such nonlinearity is common for grainy media and could contribute to the nonlinearity of the soil-mine system, as pointed out by Korman & Sabatier, 2004. Regardless of its nature, the cubic nonlinearity could have appreciable effects in a resonance system, such as soilmine.…”

“…When this mechanism is dominant, the separation is taken place even at relatively low levels of the exerted dynamic force. In addition to the interface nonlinearity, soil itself can contribute to the overall nonlinear dynamics of the soil-mine system, as suggested by Korman & Sabatier, 2004. However, in the foregoing discussion, we will focus only on the interface nonlinearity, following Donskoy, et al, 2004.…”

“…These studies, supported by numerous laboratory and field tests, demonstrated high potential of the nonlinear technique for landmine detection (Donskoy, et.al. 2002, Korman & Sabatier, 2004. Specifically, the nonlinear technique demonstrated very high (up to 40dB) detection contrast and low false alarm rate due to low clutter sensitivity.…”