The use of a combination frequency technique to measure the surf zone bubble population

Abstract

There are great benefits to sizing bubbles using a two frequency technique, which examines the appearance of sum-and-difference signals generated by the interaction between a resonant bubble pulsation and a much higher frequency imaging beam. This paper presents the results from using the technique to size bubbles in the ocean surf zone, and details the pulsation model used to calibrate the returned data such that the height of the bubble scattered signal can be related to the number of resonant bubbles of that size. It also shows how ambiguities and inaccuracies ͑brought on through turbulence and the substantial off-resonance nature of the signal͒ which affected earlier oceanic tests using the same method can be identified in the returned signal or removed from the estimate during the data processing.

Near-surface measurements of sea spray aerosol production over whitecaps in the open ocean

“…The other open ocean spectra shown here are similarly time averages; the surf zone spectrum of Phelps et al (1997) is also a time average, but that of Dean and Stokes (1999) is restricted to the active portion of breaking waves. Within the near-surface layer (depth < 1 m), populations of smaller bubbles have been found to be persistent, varying little over time (Farmer and Vagil, 1989).…”

“…Figure 5 shows the mean bubble spectra from each deployment. Also shown for comparison are three other open ocean spectra Phelps and Leighton, 1998; and measurements from DOGEE summarised by Brooks et al, 2009a), two spectra measured in the surf zone (Phelps et al, 1997;Deane and Stokes, 1999), and the MN03 laboratory spectra at water temperatures of 5 • C and 15 • C (measured with the same instrument used here). For completion we have also included bubble spectra from three additional buoy deployments (4, 5 April and 28 March) where there are no corresponding aerosol data available, but were obtained at wind speeds of 10-11 m s −1 , lying between those of the other deployments.…”

Passive Acoustic Determination of Wave-Breaking Events and Their Severity across the Spectrum

“…If the number of bubbles is assumed infinite, continuum approximations based on (1) permit overall acoustic properties of a bubbly cloud to be calculated (e.g., Commander and Prosperetti 1989;Duraiswami et al 1998). The acoustic properties of bubbles have been the basis of several oceanographic instruments (e.g., Phelps et al 1996;Terrill and Melville 2000) as well as industrial instruments (Duraiswami et al 1998;Manasseh et al 2001;Boyd and Varley 2001) although none are in widespread use. Most systems measure bubble-size distributions, relying on an active principle.…”

Modelling acoustic scattering, sound speed, and attenuation in gassy soft marine sediments

“…The model presented in Section I is suitable for implementation with short high amplitude pulses (as required for range resolution [58]), and for nonlinear inversions (e.g. when the high amplitudes required to overcome sediment attenuation produce harmonics, ultraharmonics and subharmonics in the bubble wall displacements [54] or produce nonlinear mixing of two insonification frequencies [59][60][61], a technique which is receiving increasing interest in sediment studies [16,17,62,63]). …”