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Seismic anisotropy within an active fluid flow structure: Scanner Pockmark, North Sea

Seismic anisotropy within an active fluid flow structure: Scanner Pockmark, North Sea
Seismic anisotropy within an active fluid flow structure: Scanner Pockmark, North Sea
Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2 storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.
S-wave splitting, azimuthal anisotropy, ocean bottom seismometer, scanner pockmark, wide-angle seismic
626416
Bayrakci, G.
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Callow, B.
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Bull, J. M.
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Minshull, T. A.
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Provenzano, G.
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North, L. J.
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Macdonald, C.
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Robinson, A. H.
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Henstock, T.
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Chapman, M.
dc7e7859-cd34-4bc5-bc51-e4dddd15a0b4
Bayrakci, G.
e0b89aa5-d514-4ecb-91b1-8ed8bd472eda
Callow, B.
15166203-d3e6-4b28-8369-e99e1bd00240
Bull, J. M.
974037fd-544b-458f-98cc-ce8eca89e3c8
Minshull, T. A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Provenzano, G.
076fb0cd-74db-4b62-bc0b-afffbe442cc4
North, L. J.
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Macdonald, C.
f634abbd-f26a-4d92-847a-7f95b0ba2507
Robinson, A. H.
42a3e980-2bd6-4d1c-8a36-000931e90ba2
Henstock, T.
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Chapman, M.
dc7e7859-cd34-4bc5-bc51-e4dddd15a0b4

Bayrakci, G., Callow, B., Bull, J. M., Minshull, T. A., Provenzano, G., North, L. J., Macdonald, C., Robinson, A. H., Henstock, T. and Chapman, M. (2021) Seismic anisotropy within an active fluid flow structure: Scanner Pockmark, North Sea. Frontiers in Earth Science, 9, 626416, [626416]. (doi:10.3389/feart.2021.626416).

Record type: Article

Abstract

Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2 storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

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Accepted/In Press date: 4 May 2021
Published date: 27 May 2021
Keywords: S-wave splitting, azimuthal anisotropy, ocean bottom seismometer, scanner pockmark, wide-angle seismic

Identifiers

Local EPrints ID: 449623
URI: http://eprints.soton.ac.uk/id/eprint/449623
PURE UUID: db1caa5c-49c8-4d73-bc16-2eff7d588c76
ORCID for T. A. Minshull: ORCID iD orcid.org/0000-0002-8202-1379
ORCID for T. Henstock: ORCID iD orcid.org/0000-0002-2132-2514

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Date deposited: 09 Jun 2021 16:31
Last modified: 15 Sep 2021 01:44

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