Bubble characterization in the surf zone

Abstract

Breaking waves entrain bubbles ranging in radius from tens of microns to centimeters. In the surf zone, these bubbles are evident above the ocean’s surface as patches of foam that persist for a few seconds after the passage of a breaking wave. Beneath the surface, the bubbles are organized into dense plumes with void fractions of air exceeding 0.1. In the absence of additional breaking, the void fraction of air decreases to 1e-6 or less in a minute or so as bubbles rise to the surface, dissolve, and disperse. Thus, a full characterization of bubbles in the surf zone requires instruments that operate over a wide range of void fractions and bubble radii. In practice, a variety of techniques is used, and the application and results from a number of optical and acoustical instruments, including photographic techniques, travel time sonars, Doppler sonars, and acoustical resonators will be discussed. Some of the physical acoustical effects associated with the large populations of bubbles entrained by breaking surf, such as acoustic hot spots in surf noise and acoustic transmission dropouts, will also be presented.