Automated Processing of Oceanic Bubble Images for Measuring Bubble Size Distributions underneath Breaking Waves

Raied S Al-Lashi,Helen Czerski,Steve R Gunn

doi:10.1175/jtech-d-15-0222.1

Abstract

AbstractAccurate in situ measurements of oceanic bubble size distributions beneath breaking waves are needed for a better understanding of air–sea gas transfer and aerosol production processes. To achieve this goal, a novel high-resolution optical instrument for imaging oceanic bubbles was designed and built in 2013 for the High Wind Gas Exchange Study (HiWinGS) campaign in the North Atlantic Ocean. The instrument is able to operate autonomously and can continuously capture high-resolution images at 15 frames per second over an 8-h deployment. The large number of images means that it is essential to use an automated processing algorithm to process these images. This paper describes an automated algorithm for processing oceanic images based on a robust feature extraction technique. The main advantages of this robust algorithm are it is significantly less sensitive to the noise and insusceptible to the background changes in illumination, can extract circular bubbles as small as one pixel (approximately 20 μm) in radius accurately, has low computing time (approximately 5 seconds per image), and is simple to implement. The algorithm was successfully used to analyze a large number of images (850 000 images) from deployment in the North Atlantic Ocean as part of the HiWinGS campaign in 2013.

Highlights

Bubble plumes entrained by breaking waves in the open ocean have a significant influence on various oceanographic phenomena, including air–sea gas transfer (Farmer et al 1993; Wanninkhof et al 2009), marine aerosol production (Fuentes et al 2010), and scavenging of surfactants (Zhou et al 1998)
This paper describes a robust automated algorithm for analyzing oceanic images based on the Hough transform
The paper is organized as follows: a brief description of the imaging instrument is given in section 2; the deployment and measurement in the ocean is illustrated in section 3; section 4 describes the automated bubble extraction algorithm, its implementation, and evaluation; section 5 presents bubble size distributions obtained from applying the algorithm on a sequence of images; the discussion of the results is in section 6; and we conclude with section 7

Summary

Introduction

Bubble plumes entrained by breaking waves in the open ocean have a significant influence on various oceanographic phenomena, including air–sea gas transfer (Farmer et al 1993; Wanninkhof et al 2009), marine aerosol production (Fuentes et al 2010), and scavenging of surfactants (Zhou et al 1998). Stokes and Deane (1999) developed an optical instrument to study the bubbles within breaking waves Their image processing algorithm included two preprocessing operations: correcting nonuniform illumination and thresholding. Taboada et al (2006) presented a semiautomated image analysis algorithm to count bubble sizes and oil droplets in complex dispersions occurring in bioreactors Their algorithm consists of two stages: preprocessing to obtain a binary image and postprocessing to extract circular bubbles and droplets using the Hough transform. The paper is organized as follows: a brief description of the imaging instrument is given in section 2; the deployment and measurement in the ocean is illustrated in section 3; section 4 describes the automated bubble extraction algorithm, its implementation, and evaluation; section 5 presents bubble size distributions obtained from applying the algorithm on a sequence of images; the discussion of the results is in section 6; and we conclude with section 7

Optical instrument for imaging bubbles

Deployments and measurements in the open sea

Bubble size distributions

Findings

Discussion

Conclusions