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We have investigated the use of acoustic energy produced by a pulsed CO2 laser to detect objects underwater or buried in sand. The CO2 laser produced 1 50 mJ pulses ofduration 1 00 ns. The resulting acoustic pulses were detected with an audio microphonewith a response to 1 5 kHz or a PZT transducer with a resonant frequency at 28 kHz. With the laser incident on the surface ofa water-filled tank, acoustic echos were observed from the tank walls and from objects in the tank. For objects buried in sand,changes in the acoustic lineshape related to the presence of subsurface objects were observed. Analysis of the data to extractclear signatures ofthe mine are in progress.Keywords: laser-induced, acoustic, land mine detection, photoacoustic, optoacoustic 1. INTRODUCTION The large acoustic mismatch between granular soil and solid subsurface objects suggests that acoustic methods have considerablepromise for land mine detection. Sound velocities in sand or granular soil can be on the order of or less than the velocity in air(330 mIs)' while in metals or plastics characteristic ofland mine casing or fillers, sound velocities are typically greater than 2000rn/s. The velocity and/or density mismatch between soil and mine creates a large acoustic reflection from the mine.In common with other anomaly detection techniques, the challenge of acoustic detection is to distinguish the effect of a buriedmine from stones, roots, and other clutter in the ground. Two separate approaches to acoustic discrimination depend on theidentification ofacoustic resonances ofthe target object, or the resolution ofthe target shape. While mine-like objects may haveidentifiable acoustical resonances from a few kilohertz to hundreds ofkilohertz, these resonances are strongly damped when theobject is in soil. Target identification through shape resolution, on the other hand, requires multiple acoustic sources or receiversto correlate the received signal with the position or angle ofthe source. Coupling acoustic energy from the air into the soil ormatching of separate movable ground-contact transducers makes spatial resolution with acoustic transducers difficult.Optical methods for the detection or generation of sound have advantages for remote position-sensitive acoustic detection.Optical laser vibrometry is a commercially available tool for thedetection ofsound at a solid surface which has been applied for the |
