Abstract |
Nonlinear Acoustic Techniques for
Landmine Detection
M.S.Korman (Physics Department, United
States Naval Academy, Annapolis, Maryland, USA); J.M.Sabatier (National Center
for Physical Acoustics, University of Mississippi, Mississippi, USA)
e-mail:
korman@nadn.navy.mil
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 f1 and f2 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.
Section
: 11