Methods for determining gas flux within the water column

Abstract

The passage of greenhouse gases, from both natural and anthropogenic sources, through the upper sedimentary succession and into overlying aquatic systems is a poorly understood process. Our understanding of it however conditions our ability to detect potential leaks from Carbon Capture and Storage sites (CCS) as well as our overall knowledge of the global carbon cycle. In this thesis repeated high-resolution seismic reflection surveys are used to image carbon dioxide (CO2) gas released into shallow sub-surface sediments above a potential CCS storage site in the North Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the bulk permeability of sediment were used to develop a four-stage model of the evolution of gas migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration, and Pathway Closure. Variations in ebullition rates from natural methane (CH4) seeps in Lake Constance (central Europe) are observed over a 9-month period using physical and acoustic measurement techniques, demonstrating a significant negative correlation between gas flux and in-situ pressure. The water level (hydrostatic pressure) dictates flux rates on monthly timescales, while atmospheric pressure causes minor fluctuations on daily to weekly periods, the effect of land-lake breeze cycles are observed for the first time. Exploiting this relationship, we find that long-term ebullition rates are best estimated by quantifying the relationship between in-situ pressure and gas flux, and then using this relationship to predict gas flux from more easily measurable in-situ pressure data. Finally, the use of passive acoustic flux inversion techniques is refined by measuring the initial amplitude of a bubble’s excitation when released from sediment, a previously poorly constrained parameter, demonstrating a strong correlation with the bubble equilibrium radius. We demonstrate the use of this refined acoustic inversion technique by measuring the flux from a volcanic CO2 seep in offshore Panarea (Italy), seeing a significant increase in precision with estimates consistent with optical and physical flux measurements. These findings have enhanced our understanding of gas migration in the near surface and improved our ability to measure gas emissions.

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Identifiers

ORCID for Ben Roche: orcid.org/0000-0002-0730-2947

ORCID for Jonathan Bull: orcid.org/0000-0003-3373-5807

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