Laser-induced acoustic generation for buried object detection

Mechanisms for the production of acoustic energy in soil by pulsed CO2 laser excitation of the surface are reported. When the laser pulse in unfocused with a spot size about 1 cm in diameter, a single narrow acoustic pulse is observed with a spectral content near the detector limit of 100 kHz and a velocity of 255 m/s, close to the speed of sound in air. Whenthe laser is focused to a spot size on the order of 1 mm diameter, the audible acoustic intensity in greatly increased and we observe a second broad acoustic feature. This feature has a much lower frequency and velocity. We have tentatively identified the fast mode as a normal compressive mode and the slow mode as a Biot slow-wave. A study of visible light emission when the focused CO2 laser beam strikes the sand surface indicates ionized nitrogen, oxygen, and silicon are present. This implies that the mechanism for sound production with the focused beam involves ionization by the optical electric field, expansion, and subsequence collapse of the air. The mechanisms for sound production by the unfocused beam, which produces better imaging of underground objects, appears to be quite different.