Laser-acoustic measurements for remotely determining bathymetry in shallow turbid waters

Abstract

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.