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Constraints on fluid flow pathways from shear-wave splitting in and around an active fluid-escape structure: Scanner Pockmark, North Sea

Constraints on fluid flow pathways from shear-wave splitting in and around an active fluid-escape structure: Scanner Pockmark, North Sea
Constraints on fluid flow pathways from shear-wave splitting in and around an active fluid-escape structure: Scanner Pockmark, North Sea

Vertical fluid-escape structures observed in seismic reflection data represent an important class of potentially active fluid flow pathways. An understanding of the mechanism of fluid flow in these types of structures is needed to assess the risk of natural gas venting from potential subsurface carbon dioxide storage operations. The Scanner Pockmark Complex is a 22 m deep, 900 × 450 m seabed depression in the North Sea, which actively vents methane, and is underlain by a seismic chimney structure with horizontal dimensions of ∼300 × 600 m. Gas accumulation is evidenced by the presence of bright reflectors at the top of this seismic chimney, at a depth of ∼50 m below the seabed. Here, we analyse seismic anisotropy in these shallow sediments using shear wave splitting observed on ocean bottom seismographs (OBS). Anisotropy varies spatially, with a strength of ∼1-4 per cent, on several OBS located in and around the pockmark complex. By correlating these observations with calculated subsurface P- and S-wave velocities, we show that there is anisotropy present throughout the sediments through which the chimney passes, which are interpreted as relating to syn- and post-depositional glaciomarine processes. However, within the chimney itself the orientation of the fast direction is different to that outside the chimney and the degree of anisotropy is lower. We attribute this difference as indicating that the anisotropy observed within the chimney is associated with the formation and continued presence of the gas migration system, which overprints the background depositional anisotropy.

Controlled source seismology, Fracture and flow, Gas and hydrate systems, Seismic anisotropy
0956-540X
1164-1195
Robinson, Adam
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Bayrakci, Gaye
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Macdonald, Calum
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Minshull, Timothy
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Bull, Jonathan
974037fd-544b-458f-98cc-ce8eca89e3c8
Chapman, Mark
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Henstock, Timothy
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Callow, Ben
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Robinson, Adam
f5aea908-d884-465b-bfc1-ed67ca16ed85
Bayrakci, Gaye
8717575b-4aa2-4046-9b6b-39b955c5a742
Macdonald, Calum
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Minshull, Timothy
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Bull, Jonathan
974037fd-544b-458f-98cc-ce8eca89e3c8
Chapman, Mark
4f8c8192-9dd1-49f1-8e50-379357ccb1b0
Henstock, Timothy
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Callow, Ben
15166203-d3e6-4b28-8369-e99e1bd00240

Robinson, Adam, Bayrakci, Gaye, Macdonald, Calum, Minshull, Timothy, Bull, Jonathan, Chapman, Mark, Henstock, Timothy and Callow, Ben (2022) Constraints on fluid flow pathways from shear-wave splitting in and around an active fluid-escape structure: Scanner Pockmark, North Sea. Geophysical Journal International, 231 (2), 1164-1195. (doi:10.1093/gji/ggac197).

Record type: Article

Abstract

Vertical fluid-escape structures observed in seismic reflection data represent an important class of potentially active fluid flow pathways. An understanding of the mechanism of fluid flow in these types of structures is needed to assess the risk of natural gas venting from potential subsurface carbon dioxide storage operations. The Scanner Pockmark Complex is a 22 m deep, 900 × 450 m seabed depression in the North Sea, which actively vents methane, and is underlain by a seismic chimney structure with horizontal dimensions of ∼300 × 600 m. Gas accumulation is evidenced by the presence of bright reflectors at the top of this seismic chimney, at a depth of ∼50 m below the seabed. Here, we analyse seismic anisotropy in these shallow sediments using shear wave splitting observed on ocean bottom seismographs (OBS). Anisotropy varies spatially, with a strength of ∼1-4 per cent, on several OBS located in and around the pockmark complex. By correlating these observations with calculated subsurface P- and S-wave velocities, we show that there is anisotropy present throughout the sediments through which the chimney passes, which are interpreted as relating to syn- and post-depositional glaciomarine processes. However, within the chimney itself the orientation of the fast direction is different to that outside the chimney and the degree of anisotropy is lower. We attribute this difference as indicating that the anisotropy observed within the chimney is associated with the formation and continued presence of the gas migration system, which overprints the background depositional anisotropy.

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Accepted/In Press date: 24 May 2022
Published date: 1 November 2022
Additional Information: Funding Information: This work was funded by the Natural Environment Research Council (CHIMNEY; NERC Highlight Topic; NE/N016130/1 and NE/N015762/1) and the European Union s Horizon 2020 research and innovation programme under grant agreement No. 654462 (STEMM-CCS). We would like to thank all those involved in the planning and acquisition of data during RRS James Cook cruise JC152, including the officers, engineers and crews, the scientific parties, and all seagoing technicians and engineers. OBSs were supplied by the UK Ocean Bottom Instrumentation Facility (Minshull et al. 2005).We are also grateful for the support of Applied Acoustics Ltd during sparker data acquisition. We thank Ross Haacke (CGG) for advice on the design of the CHIMNEY seismic surveys and for providing codes for rotation of OBS data and Giuseppe Provenzano for work processing of the seismic reflection data. We thankMartha Savage, Sunny Singhroha and editor Jenny Collier for their helpful reviews. The final accepted version of this manuscript is available through ePrints Soton (eprints.soton.ac.uk). Publisher Copyright: © 2022 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.
Keywords: Controlled source seismology, Fracture and flow, Gas and hydrate systems, Seismic anisotropy

Identifiers

Local EPrints ID: 457738
URI: http://eprints.soton.ac.uk/id/eprint/457738
ISSN: 0956-540X
PURE UUID: 5aa87a63-40c6-45c6-affb-3d178b16bdde
ORCID for Adam Robinson: ORCID iD orcid.org/0000-0003-3447-870X
ORCID for Timothy Minshull: ORCID iD orcid.org/0000-0002-8202-1379
ORCID for Jonathan Bull: ORCID iD orcid.org/0000-0003-3373-5807
ORCID for Timothy Henstock: ORCID iD orcid.org/0000-0002-2132-2514

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Date deposited: 16 Jun 2022 00:28
Last modified: 11 Oct 2022 01:59

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Contributors

Author: Adam Robinson ORCID iD
Author: Gaye Bayrakci
Author: Calum Macdonald
Author: Jonathan Bull ORCID iD
Author: Mark Chapman
Author: Ben Callow

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