Imaging of buried objects by laser-induced acoustic detection

We report here on the use of acoustic pulses generated by a pulsed-laser incident on the ground surface for the depth- and shape-resolution of buried objects. The laser-induced acoustic wave has considerable advantages over other acoustic wave generation techniques for landmine detection applications. (1) It is efficient because the sound is generated directly in the ground. (2) The acoustic source can be precisely positioned or scanned by optical redirection of the laser spot. (3) Remote, non-ground-contact detection can be accomplished with a receiving microphone in the air or by using optical vibrometry of the soil surface for detection. Research has been focused on the data acquisition and signal processing applicable to de-mining scenarios. A de-convolution method using a Wiener filter is introduced to the processing of data. By scanning the laser position and filtering the time-trace of the reflected acoustic pulse, we have obtained 3D images of the underground objects. The images give us the clear discrimination of the shapes of underground objects. The quality of the images depends on the mismatch of acoustic impedance of buried objects, the bandwidth of acoustic sensor, and the selection of filter function.