Abstract:
The earliest attempts to determine ocean bubble densities were optical, because everyone knows that ``seeing is believing.'' Unfortunately, bubbles smaller than 60-(mu)m radius were not always recognized in photographs of the dirty, detritus-contaminated, bubbles. And bubbles greater than 150-(mu)m radius were so infrequently observed that the average values were quite unreliable. Acoustical ``counting'' by inversion of the linear processes of backscatter, excess attenuation, and dispersion has great advantages over optical measurements. Primarily this is because the acoustic cross sections of a resonating bubble are orders of magnitude greater than optical values for the same radius. The ability to gather data rapidly, the ease of obtaining correlations with other oceanographic variables, and the simple inverse relation between sound frequency and resonant bubble radius have given acoustics great advantages over optical bubble-counting at sea. For 30 years, at several locations, under various ocean conditions, active in-situ pulse echo systems and resonators using broadband transducers have yielded a rich harvest of well-resolved data for bubble radii from 15 to 300 (mu)m. More recently, by passively listening to newly created damped oscillating bubbles, acousticians have provided meteorologists and climatologists with accurate fundamental data of ocean surface gas transfer and hydrosol production due to breaking waves and rainfall. Techniques and results of active and passive linear acoustical monitoring are reviewed. [Research supported by ONR.]