Instrumentation for the Measurement of Sound Speed near the Ocean Surface

Abstract

Air bubbles entrained by breaking waves in the ocean surface layer can dramatically alter the velocity and attenuation of acoustic waves. The development of an effective technique for directly measuring the sound speed near the ocean surface is reported, The method makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm. Phase distortions caused by acoustic reflections from the surface or from nearby buoy structural elements are separated in time from the direct path signal. A DSP-based data processing system was implemented to cross correlate the transmitted and received acoustic pulses and thus yield sound-speed measurements in real time. Perhaps the most significant novelty of the present measurement technique is its ability to make simultaneous measurements of the sound speed at several depths, starting as close as 0.5 m to the surface, at frequencies down to 5 kHz, and at a sample rate of 4 Hz per channel. Furthermore, the technique is direct and thus avoids the many difficulties involved with inferring the sound speed from in situ bubble population measurements. Results from controlled tests in the laboratory and in a lake are presented. The results confirm the validity of the technique and establish basic performance criteria. Data from the field that demonstrate the operation of the instrument in an ocean environment are also presented.