The generation and measurement of CO2 laser-induced acoustic pulses in sand

Abstract

A pulsed CO2 laser (1-μs pulse width, 10-J pulse energy) when focused to a 1-cm2 spot on the surface of sand generated an acoustic field in the sand. The sound pressure level (SPL) measured with a hydrophone 1 cm below the surface was 173 dB re 1μPa with a frequency spectrum <3 kHz. Focusing this same laser on a water surface yields equivalent values in water of 200 dB re 1μPa and 25–30 kHz, respectively. Sound velocities in sand ranged between 145–184 m sec−1 depending on the dryness of the sand. When the acoustic pulses were generated in a few cm of water covering the sand, the sound velocities measured in the sand increased to 610–750 m s−1 while the spectrum was composed of both the high frequency water spectrum and the low-frequency sand spectrum. Acoustic pulse measurements were made to sand depths of 40 cm. In addition, pulses were detected after reflection off a metal plate at a depth of 45 cm. A discussion will be given of possible remote sensing applications. [Research supported by ONR, Code 422CS.]