Bubble clouds and their transport within the surf zone as measured with a distributed array of upward-looking sonars

A collaborative, multi-institute experiment called the Scripps Pier Experim ent was conducted in the vicinity of the Scripps pier in La Jolla, Californ ia, in March 1997 to study the fate of bubbles in the surf zone and the eff ects of these bubbles on acoustic propagation. This paper discusses data ga thered by the Applied Physics Laboratory, University of Washington, using a set of four upward-looking sonars (frequency 240 kHz), which simultaneousl y measured vertical profiles of acoustic volume scattering from bubbles at four locations. The transport of bubbles via rip currents emerged as an imp ortant, though episodic and localized, feature of the acoustic environment in the surf zone. Images of volumetric backscattering strength vs time and depth reveal the episodic events (of increased scattering level) lasting be tween 5 and 10 min caused by the passage of bubble clouds over the sonar. T ime lags for the onset of increased scattering at the four locations are co nsistent with a seaward velocity of the bubble clouds of order 10 cm/s, and the length scales of these bubble clouds in the seaward direction are infe rred to be in the range 50-100 m. The influence of the incoming surface wav e field is also discussed. (C) 2001 Acoltstical Society of America. [DOI: 1 0.1121/1.1331108].