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.
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Macdonald, C.
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Robinson, A. H.
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Henstock, T.
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Chapman, M.
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27 May 2021
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.
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Macdonald, C.
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Robinson, A. H.
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Henstock, T.
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Chapman, M.
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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].
(doi:10.3389/feart.2021.626416).
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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feart-09-626416
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More information
Accepted/In Press date: 4 May 2021
Published date: 27 May 2021
Additional Information:
Funding Information:
This work received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No.654462 (STEMM-CCS) and the Natural Environment Research Council (CHIMNEY project: grants NE/N016130/1, NE/N016041/2 and NE/N015762/1).
Funding Information:
We would like to thank all those involved in the planning and acquisition of data during research cruise JC152, including the officers, engineers and crews, the scientific parties, and all seagoing technicians and engineers. The NERC Ocean-Bottom Instrumentation Facility (Minshull et al., 2005) provided the OBSs and their technical support at sea during JC152. We are also grateful for the support of Applied Acoustics Ltd. during Sparker data acquisition. We acknowledge PGS for the use of their dataset. We are grateful to Schlumberger Ltd. for the donation of Petrel software to the University of Southampton.
Publisher Copyright:
© Copyright © 2021 Bayrakci, Callow, Bull, Minshull, Provenzano, North, Macdonald, Robinson, Henstock and Chapman.
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
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Date deposited: 09 Jun 2021 16:31
Last modified: 30 Nov 2024 02:38
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Contributors
Author:
G. Bayrakci
Author:
B. Callow
Author:
G. Provenzano
Author:
L. J. North
Author:
C. Macdonald
Author:
M. Chapman
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