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Laser-induced acoustic detection of shallow-buried objects

A Laser-induced acoustic technique has been demonstrated for the potential application of underground object detection. The high acoustic impedance mismatch between granular soil and buried mine (usually landmines are made from plastic, rubber and metal) gives good contrast for detection. We obtained 3-D images of underground objects by scanning a pulsed laser across the surface of the ground and keeping the acoustic detector at relatively fixed position with respect to the laser spots. A signal processing method is introduced by using de-convolution filter. When a laser pulse impinges on the surface of ground, a laser-induced acoustic pulse will be excited after a sudden increase of surface temperature followed a sudden volume expansion. We used a laser vibrometer as acoustic receiver and found that the acoustic pulse width depends on laser pulse width. Using 100 ns CO2 laser, we obtained 4 to approximately 5 microsecond acoustic sound pulses.