INSTRUMENTATION FOR THE MEASUREMENT OF SOUND SPEED NEAR THE OCEAN SURFACE

Air bubbles entrained by breaking waves in the ocean surface layer can dramatically alter the velocity and attenuation of acoustic waves. Th e development of an effective technique for directly measuring the sou nd speed near the ocean surface is reported, The method makes use of t he travel time of short acoustic pulses between a transmitter and a re ceiver separated by 40 cm. Phase distortions caused by acoustic reflec tions from the surface or from nearby buoy structural elements are sep arated in time from the direct path signal. A DSP-based data processin g system was implemented to cross correlate the transmitted and receiv ed acoustic pulses and thus yield sound-speed measurements in real tim e. Perhaps the most significant novelty of the present measurement tec hnique is its ability to make simultaneous measurements of the sound s peed 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. F urthermore, the technique is direct and thus avoids the many difficult ies involved with inferring the sound speed from in situ bubble popula tion measurements. Results from controlled tests in the laboratory and in a lake are presented. The results confirm the validity of the tech nique and establish basic performance criteria. Data from the field th at demonstrate the operation of the instrument in an ocean environment are also presented.