A COMPARISON OF 4 METHODS FOR BUBBLE-SIZE AND VOID FRACTION MEASUREMENTS

We compare the performance of four different bubble-sensing techniques in a range of environments from the surf zone to the open ocean: a re mote sensing method using high-frequency backscatter, two in situ meth ods using an acoustical resonator and a pulse propagation sensor, and a bulk method using electrical conductivity. Comparisons between the t echniques show general consistency within the appropriate operational bubble density ranges, although spatial variability in bubble clouds i ntroduces substantial variance. Each technique has its strengths and l imitations. Our acoustical resonator is suitable for bubble concentrat ions with air fractions greater than approximately 10(-9) and the puls e propagation sonar for air fractions from 10(-6); the upper limit for both is constrained by attenuation and the validity of the Foldy scat tering approximation. Both sensors can be implemented to encompass a w ide frequency range with high resolution, corresponding to resonant bu bble radii of 10 similar to 1200 mu m. Far air fractions higher than s imilar to 5x10(-4), bulk measurement using electrical conductivity pro vides a measure of air fraction. Sufficient overlap in operational air -fraction range exists between in situ acoustical techniques and condu ctivity measurement to permit comparison and demonstrate consistency i n the measurement, Single- and multifrequency backscatter sonars may b e used for low air fractions (<1x10(-5)) and provide a continuous vert ical profile from a deployment beneath the active surface zone, but ar e subject to masking by dense bubble clouds and are unable to resolve high air fractions close to the surface. This study suggests that the best approach is to use a combination of sensors to probe the bubble f ield.