Date of Award

Fall 2022

Project Type

Dissertation

Program or Major

Ocean Engineering

Degree Name

Doctor of Philosophy

First Advisor

Thomas C Weber

Second Advisor

Nivedita Gupta

Third Advisor

Larry Mayer

Abstract

The importance of methane as a greenhouse gas has led to widespread research focused on understanding the sinks and sources for atmospheric methane. One source of atmospheric methane could include methane bubbles released from the ocean’s seafloor. These gas bubbles transport methane from the seafloor through the ocean’s water column, and the methane released during their ascent affects water column geochemistry and microbial populations. Under certain physical and chemical conditions, methane transported by gas bubbles can reach the upper water column and potentially leak across the sea-air interface. The dynamics of gas bubble emission, ascent, and dissolution in the water column have been studied using a combination of in-situ instruments (e.g., optical systems), numerical transport models, and acoustic systems. These methods often rely on assumptions related to physical bubble properties (e.g., shape, size, surface coating) that are violated in nature; therefore, there is a need to understand how deviations from these assumptions affect our understanding of the fate and transport of methane from emission at the seafloor through the water column.

This thesis aims to understand how external physical processes (e.g., tides, atmospheric pressure, etc.) and variations in bubble physical properties impact our capability to observe, measure, and quantify methane transport through the ocean and potentially to the atmosphere. Several controlled laboratory experiments were conducted to 1) quantify effects that deviations in bubble size and shape have on the acoustic scattering properties of gas bubbles in liquids, given that measurements of acoustic scattering are commonly used to detect and quantify methane transport in the water column, and 2) quantify effects that bubble shape, size, and hydrate coating on the bubble surface have on the rise velocity of gas bubbles. In addition, a field experiment that collected a long-term time series of acoustic backscatter measurements was conducted within the Coal Oil Point seep field near Santa Barbara, California, to investigate long-term variability of methane gas flow within a shallow water column.

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