On Reconciling Optical and Acoustical Bubble Spectra in the Mixed Layer

Abstract

Acoustically estimated size spectra of bubbles in the upper 15m of the oceanic mixed layer increase logarithmically as diameter decreases, at least to 30 µm (8µm in tap water);numbers fall off slowly with depth. Optically determined estimates, in contrast, show a strong peak above 80 µm; numbers fall off rapidly with depth. Existing theory is insufficiently developed to exclude either of these interpretations. Re-examination of the acoustical and optical responses of small bubbles suggests that classical acoustical theory overestimates small bubble numbers by as much as an order of magnitude, while the optical devices employed are 1000 times less sensitive to small bubbles than they might be.

But these corrections do not bring agreement. The tentative conclusion is that optical and acoustical techniques count fundamentally different objects. Optical methods detect buoyant, specular objects and probably miss the smaller bubbles; acoustical methods detect anything that contains gas, including neutrally buoyant non-specular bubble ‘ghosts’. There is great need for intercomparison studies employing several detection techniques simultaneously.

A new optical method for sizing small bubbles provides 1-μm resolution between 2 and 24μm diameter. (These limits are not absolute, but simply represent those explored in this paper). The technique requires only descriptive estimates of the colour of light scattered at several angles, say, 25, 35, and 45 degrees.