Measurements of passive and active microbubbles in the sea

Abstract

Around 1960 an oceanographer at the Navy Electronics Laboratory dared to question physics laboratory measurements when he wrote an internal memo titled ‘‘Do Invisible Microbubbles Exist at Sea?.’’ Indeed they do! From 1964 to 1974 a series of M.S. students at the Naval Postgraduate School published the first microbubble density distributions at sea. These were measured by several in-situ ocean techniques including: change of transducer impedance; photography; excess attenuation, backscatter and sound-speed dispersion in a small pulse-echo system: Fourier transform of a sawtooth signal propagating between two separated hydrophones. An ‘‘unbelievable’’ immense number of coastal bubbles of radius 15 to 300 μm were found. The 1990s have seen a renaissance in the field. The spatial and temporal variation of bubble numbers have been studied, and acoustical oceanographers now use bubbles as tracers to determine ocean processes near the ocean surface. Sea state noise and rain noise have both been definitively ascribed to the radiation from huge numbers of infant microbubbles. Indeed, the ‘‘noises’’ have now become ‘‘signals.’’ The underwater sound of breaking waves has been inverted to yield the spectrum of ocean wave height and recently the underwater sound of rainfall has been inverted to reveal the real-time rainfall drop size distribution. The distinctive spectrum of underwater sound during rainfall even allows one to describe the clouds from which the rain has fallen. [Work supported by ONR.]