A broadband acoustic technique for measuring bubble size distributions: Laboratory and shallow water measurements

The development of a broadband sound velocimeter that allows the simultaneo us measurement of sound speed and attenuation over a wide range of frequenc ies is described. The velocimeter measures the attenuation and dispersion o f a broadband acoustic pulse over frequencies ranging from 4 to 100 kHz acr oss a fixed pathlength using a two-transducer system. The resulting data ar e inverted to arrive at bubble size distributions over bubble radii in the range 30-800 mu m. The instrument was tested in the large wave channel at the Hydraulics Labor atory of Scripps Institution of Oceanography. The channel can generate brea king waves of 0(1 m) height using a hydraulically driven wave generator, gi ving bubble size distributions similar to those found in the field. The pre sence of the bubbles significantly changes the acoustical properties of the water. internal consistency checks of the acoustic data and measurements o f bubbles using an independent optical sizing technique support the accurac y of the acoustic system in measuring bubble size distributions. A field test of the system was performed off Scripps Pier in water of appro ximately 6-m depth. Observations demonstrate that bubble transport events w ith significant temporal and spatial variability are associated with rip cu rrents and introduce significant vertical gradients in the acoustical prope rties of the water. The performance of the system in the field was found to be comparable to that found in the laboratory.