A laser technique is described that has the potential for being developed into an airborne remote sensing system for bathymetric mapping of shallow-turbid waters. This hybrid system consists of an infrared CO2 laser transmitter and a highly sensitive microphone receiver located in the air. The interaction of the laser beam with water generates acoustic signals in water having pulse widths of 20–30 μs and a frequency spectrum peaking between 15–25 kHz. Measurements have been made from both a floating laboratory and an operating ship. These measurements were made using a 5 to 15-J CO2 laser to generate a sound pressure level (SPL) of 185–195 dB re: 1 μPa in water at a depth of 1 m. Measurements made in quiescent waters recorded water depths of 20±0.1 m, while the maximum water depth obtained aboard ship was 13±1 m. The two major problem areas encountered were environmental noise and the angular dependence of the bottom sediment echoes as they penetrated a ‘‘wavy’’ water surface.

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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.]