The influence of bubble clouds on acoustic propagation in the surf zone

High-frequency propagation close to an active surf line is explored with 12 - and 100-kHz propagation paths together with measurements of bubble clouds , bubble size distributions, and waves, Breaking waves inject massive bubbl e plumes that are mixed downwards from the roller region by intense turbule nce. If these injections follow one another at intervals less than the time . taken for the bubbles to rise to the surface, acoustic signals will be co ntinuously blocked, forming an acoustical barrier that effectively inhibits any propagation. Occasionally, waves break seaward of this barrier. In thi s case, dense bubble clouds are mixed down beneath the air entrainment zone , but there is sufficient time for them to disappear before succeeding brea kers, allowing intermittent high-frequency propagation recharge the bubble field. The duration and shape of signal dropouts are then determined by the selective removal of bubbles by buoyancy and dissolution. In addition to t urbulence created by the air entrainment process, a lower level of continuo us background turbulence may be generated by interaction of residual curren ts with the wave boundary layer. Our observations illustrate the variable c haracter of acoustic blocking by bubble clouds and serve as a basis for qua ntitative analysis of these effects with a two-dimensional (2-D) propagatio n model coupled to 2-D models of bubble cloud evolution and background turb ulence.