<title>Linear and nonlinear acoustic velocity profiles over buried land mines</title>

Abstract

Acoustic-to-seismic coupling has proven to be an extremely accurate technology for locating buried landmines. Most of the research to date has focused on linear acoustic techniques in which sound couples into the ground, interacts with the buried mine, and causes increased vibration of the ground above the mine. However, Donskoy' has suggested that nonlinear acoustic techniques may be applicable to acoustic mine detection. This technique has recently been used with success in field tests at the University of Mississippi and US Army mine lanes. In the nonlinear acoustic technique, airborne sound is produced at two primary frequencies which couple into the ground and a superimposed compressional wave interacts with the mine and the soil. Because the mine is compliant, contact between the soil and the mine is maintained during the compression phase of the wave, but they separate during the tensile phase. This creates a bimodular oscillator that is inherently non-linear. This effect has been demonstrated on inert landmines at the University of Mississippi and at US Army test lanes. Results of these tests indicate that nonlinear measurements over buried landmines have more sensitivity than linear measurements. Non-compliant objects such as concrete disks do not exhibit nonlinear phenomena but can be located using linear techniques.

Resonance vibrations of buried landmines

“…It has been shown Sabatier et al, 2002;Scott et al, 2003;Van den Heuvel et al, 2003 ) that impedance contrast between a buried mine and surrounding soil allows for detection of both antipersonal and antitank mines.…”

“…This difference permits discrimination of the buried landmine from the surrounding soil. It is known that response of the buried mine is frequency dependent , Sabatier et al, 2002 and the mine could exhibit either a resonant (Figure 1a) or non-resonant (Figure 1b) behavior. The latter does not necessarily mean that the mine could not be detected with the seismo-acoustic method.…”

Comparision of measured versus predicted buried mine resonant behavior

“…The natural frequency of an undamped SDOF mechanical system is given by equation (2). m m n M K (2) At resonance (i.e. when = n ), substitution of equation (2) into (1) results in the cancellation of the imaginary term in the impedance expression.…”

“…The mechanical impedance of a single degree of freedom system is given by the following expression: where is angular frequency and R m , M m, and K m are respectively the damping, mass, and stiffness coefficients which describe the top diaphragm of the mine. The natural frequency of an undamped SDOF mechanical system is given by equation (2). m m n M K (2) At resonance (i.e.…”

“…The dynamic mass is calculated by evaluating expression (3) at a frequency in the vicinity of resonance ( n ). The remaining unknown, K m , can then simply be determined by substituting the dynamic mass into equation(2).…”