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Tesmi Jose, National Oceanography Centre (United Kingdom)
Tim Minshull, National Oceanography Centre (United Kingdom)
Graham Westbrook, University of Birmingham (United Kingdom)
Russell Exley, University of Birmingham (United Kingdom)
Herve Nouzé, Ifremer (France)
Stephan Ker, Ifremer (France)
Audrey Gailler, Ifremer (France)
Christian Berndt, National Oceanography Centre (United Kingdom)
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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, partly due to a lack of clear images of the structures that they overlie and through which water and gas flow to the seabed. 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 a ∼220 m wide pockmark named G11 in the region and hence to determine the distribution of gas and gas hydrate in and around the pockmark.
An array of eight 4-component and six 2-component OBS was placed on the top and immediate vicinity of the pockmark with a spacing of ∼100 m. Eight of the instruments recorded at 2500 Hz sampling rate while the other six recorded at 500 Hz. 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. The shots were also recorded on a short near-surface hydrophone streamer to provide near-3D seismic reflection images of the sub-seabed structure. The OBS and reflection data reveal many interesting features of the subsurface geology of the chimney. Several reflectors of high amplitude and reverse polarity are observed on the profiles and clear converted wave arrivals are seen on the records of the horizontal seismometers. A group of bright reflectors underlies the pockmark at a travel time of ∼1.4 s and at deeper depths some of the reflectors show strong attenuation indicating the presence of gas in the sediments. A pipe ascends from a gas charged zone at ∼300 m below the seabed, to where it terminates in the investigated G11 pockmark.
The high resolution nature of the experiment required that the shot positions and OBS positions are as precise as possible (accuracy of ∼1m or less) which were determined using the direct arrival times through the water. The OBS data was strongly contaminated by the ship's noise. This was greatly suppressed by separating the downgoing wavefield from the upgoing wavefield. Five OBS along a line running NE-SW across the pockmark was chosen to carry out an initial 2D tomographic inversion. About 6500 P-wave travel time picks from seven prominent reflectors lying above the gas charged zone were made on the hydrophone component of the OBS records and streamer data. The 2D raytraced forward model shows a gradual increase in velocity between the seafloor and the gas charged zone lying at ∼300 m depth below the seabed. We present some results of the velocity modeling.
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