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

Abstract

A collaborative, multi-institute experiment called the Scripps Pier Experiment was conducted in the vicinity of the Scripps pier in La Jolla, California, in March 1997 to study the fate of bubbles in the surf zone and the effects of these bubbles on acoustic propagation. This paper discusses data gathered by the Applied Physics Laboratory, University of Washington, using a set of four upward-looking sonars (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles at four locations. The transport of bubbles via rip currents emerged as an important, 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 between 5 and 10 min caused by the passage of bubble clouds over the sonar. Time lags for the onset of increased scattering at the four locations are consistent 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 inferred to be in the range 50–100 m. The influence of the incoming surface wave field is also discussed.