In situ acoustic measurements of bubble populations in coastal ocean waters

Abstract

A portable pulse-echo, pulse-scatter acoustical system has been used to make in situ measurements of excess attenuation and scatter over the frequency range 20 to 200 kHz, and thereby to infer numbers of bubbles of radius approximately 180 to 18 microns at sea. The study was made at various depths to 50 feet in isothermal coastal waters, at sea states one and two, over 24-hour periods. At sea state one densities of bubbles of the order of 1000/m3 are identified in 1-micron bands of incremental radius; these microbubbles, with decreasing numbers as radius is increased (proportional to R−4), are postulated to be entrained by aerosols as they fall into the ocean. Densities of the order of 100/m3 in a 1-micron band are found for bubbles of radii greater than 40 microns; for the largest bubbles the numbers vary approximately as R−2. Numbers of bubbles of radius 60 microns or larger have a depth dependence Z−1/2, whereas numbers of smaller bubbles follow the exponential law e−Z/L with L approximately 7 meters. Analysis in the Lagrangian frame suggests that the bubbles originate at the surface, with the smaller bubbles, particularly, losing mass as they are carried downward by convection and diffusion.