HR: 1340h
AN: B43E-1653    [Abstracts]
TI: A Geophysical Study of a Pockmark in the Nyegga Region, Norwegian Sea
AU: * Jose, T
EM: tjose@noc.soton.ac.uk
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, United Kingdom
AU: Minshull, T
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, United Kingdom
AU: Westbrook, G
AF: University of Birmingham, School of Geography, Earth and Environmental Sciences, Edgbaston, Birmingham, B15 2TT, United Kingdom
AU: Berndt, C
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, United Kingdom
AB: Pockmarks are seabed expressions of gas/fluid escape chimneys which are thought to be of global significance as they are pathways for the escape of methane from beneath continental margins to the atmosphere and provide habitats for chemosynthetic communities of biota. Their formation and dynamics are 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 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 two pockmarks (named CN03 and G11) in the region. The G11 pockmark is ~220 m wide and has a rugged topography with irregular ridges divided by a central sediment basin and carbonate piles. An array of eight 4-component and six 2-component OBS was deployed across the pockmark with a spacing of ~100 m and 1 m location accuracy. The source consisted of 13/35 and 24/24 cubic inch mini GI guns and the data were acquired on a grid of lines of minimum length 5000 m at 50 m and 100 m line- spacing corresponding to shot intervals of 4s and 6s. A pattern of circular lines was also shot to cover a full range of offsets and azimuths for the OBS array. The shots were also recorded on a short near-surface hydrophone streamer. Several reflectors of high amplitude and reverse polarity are observed on the profiles indicating the presence of gas. A pipe ascends from a gas charged zone at ~300 m below the seabed, to where it terminates in the investigated G11 pockmark at the seabed. 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 the gas charged zone. The traveltime fit is improved if the pockmark is underlain by velocities lower than in the surrounding layer. 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. Different possible geological processes will be examined to explain this phenomenon.
DE: 0416 Biogeophysics
DE: 0999 General or miscellaneous
DE: 3004 Gas and hydrate systems
SC: Biogeosciences [B]
MN: 2007 Fall Meeting