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Methods for determining gas flux within the water column

Methods for determining gas flux within the water column
Methods for determining gas flux within the water column
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.
University of Southampton
Roche, Ben
2746ee9e-1b87-4d2f-b4e1-dcdc0ca7a719
Roche, Ben
2746ee9e-1b87-4d2f-b4e1-dcdc0ca7a719
Bull, Jonathan
974037fd-544b-458f-98cc-ce8eca89e3c8

Roche, Ben (2021) Methods for determining gas flux within the water column. University of Southampton, Doctoral Thesis, 247pp.

Record type: Thesis (Doctoral)

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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Roche, Ben_PhD_Thesis_Sep_2021 - Author's Original
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Available under License University of Southampton Thesis Licence.

More information

Published date: 21 September 2021

Identifiers

Local EPrints ID: 451572
URI: http://eprints.soton.ac.uk/id/eprint/451572
PURE UUID: 4ae1a9cd-6695-4a97-ad37-cec3e6f5e8b2

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Date deposited: 11 Oct 2021 16:33
Last modified: 11 Oct 2021 16:33

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Contributors

Author: Ben Roche
Thesis advisor: Jonathan Bull

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