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A geophysical study of a pockmark in the Nyegga region, Norwegian Sea

A geophysical study of a pockmark in the Nyegga region, Norwegian Sea
A geophysical study of a pockmark in the Nyegga region, Norwegian Sea
Over the last decade pockmarks have proven to be important seabed features that provide
information about fluid flow on continental margins. Their formation and dynamics are still
poorly constrained due to the lack of proper three dimensional imaging of their internal structure.
Numerous fluid escape features provide evidence for an active fluid-flow system on the
Norwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolution
seismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate the
detailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployed
across the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI guns
were also recorded on a near surface hydrophone streamer. Several reflectors of high amplitude
and reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrates
were recovered with gravity cores from less than a meter below the seafloor during the cruise.
Indications of gas at shallow depths in the hydrate stability field show that methane is able to
escape through the water-saturated sediments in the chimney without being entirely converted
into gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a line
running NE-SW across the G11 pockmark shows, a gradual increase in velocity between the
seafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit is
improved if the pockmark is underlain by velocities higher than in the surrounding layer
corresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmark
as seen in the reflection profiles. This could be due to the presence of hydrates or carbonates
within the sediments.
gas hydrates, chimney, high-resolution 3D seismic
5568-[8p]
International Conference on Gas Hydrates
Jose, T.
acdc3a92-b087-488d-9604-c901f246e726
Minshull, T.A.
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Westbrook, G.K.
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Nouze, H.
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Ker, S.
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Gailler, A.
ccb3f498-66af-4ed7-936d-f2989767d9fd
Exley, R.
5cd1ff59-0c9b-4d1b-8866-7f49bf8a455e
Berndt, C.
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Jose, T.
acdc3a92-b087-488d-9604-c901f246e726
Minshull, T.A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Westbrook, G.K.
33ab17e0-7b5a-41d5-afd5-ab7d52d7c4c7
Nouze, H.
6dbfe767-23ae-4136-80a0-dae99f47d8f5
Ker, S.
f7a7a4dd-77ac-4c80-a5d2-356b9ecdfc61
Gailler, A.
ccb3f498-66af-4ed7-936d-f2989767d9fd
Exley, R.
5cd1ff59-0c9b-4d1b-8866-7f49bf8a455e
Berndt, C.
231544d4-f681-44a2-ae6e-74385e588bf6

Jose, T., Minshull, T.A., Westbrook, G.K., Nouze, H., Ker, S., Gailler, A., Exley, R. and Berndt, C. (2008) A geophysical study of a pockmark in the Nyegga region, Norwegian Sea. In Proceedings of the 6th International Conference on Natural Gas Hydrates. International Conference on Gas Hydrates. 5568-[8p] .

Record type: Conference or Workshop Item (Paper)

Abstract

Over the last decade pockmarks have proven to be important seabed features that provide
information about fluid flow on continental margins. Their formation and dynamics are still
poorly constrained due to the lack of proper three dimensional imaging of their internal structure.
Numerous fluid escape features provide evidence for an active fluid-flow system on the
Norwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolution
seismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate the
detailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployed
across the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI guns
were also recorded on a near surface hydrophone streamer. Several reflectors of high amplitude
and reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrates
were recovered with gravity cores from less than a meter below the seafloor during the cruise.
Indications of gas at shallow depths in the hydrate stability field show that methane is able to
escape through the water-saturated sediments in the chimney without being entirely converted
into gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a line
running NE-SW across the G11 pockmark shows, a gradual increase in velocity between the
seafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit is
improved if the pockmark is underlain by velocities higher than in the surrounding layer
corresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmark
as seen in the reflection profiles. This could be due to the presence of hydrates or carbonates
within the sediments.

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More information

Published date: 2008
Venue - Dates: 6th International Conference On Gas Hydrates, Vancouver, Canada, 2008-07-06 - 2008-07-10
Keywords: gas hydrates, chimney, high-resolution 3D seismic

Identifiers

Local EPrints ID: 55303
URI: http://eprints.soton.ac.uk/id/eprint/55303
PURE UUID: a418202d-cf0f-45f4-a0d2-bc8dfccd6dc1
ORCID for T.A. Minshull: ORCID iD orcid.org/0000-0002-8202-1379

Catalogue record

Date deposited: 28 Jul 2008
Last modified: 25 Oct 2023 01:38

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Contributors

Author: T. Jose
Author: T.A. Minshull ORCID iD
Author: G.K. Westbrook
Author: H. Nouze
Author: S. Ker
Author: A. Gailler
Author: R. Exley
Author: C. Berndt

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