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Bunz, S., J. Mienert, and K. Andreassen,
Multicomponent Seismic Studies of the Gas-hydrate System at the Storegga Slide
Stefan Bunz,1 Jurgen Mienert,2 Karin Andreassen3
1Department of Geology, University of Tromso, Tromso, Norway
2Department of Geology, University of Tromso, Tromso, Norway
3Department of Geology, University of Tromso, Tromso, Norway
We are grateful to Petroleum GeoServices (PGS) for providing the ocean-bottom cable data. The University of Tromso acknowledges support by Landmark Graphics via the Landmark University Grant Program. Norsk Hydro is acknowledged for providing some of the seismic reflection data and for the geotechnical information. The GMT software (Wessel and Smith, 1991) was used to create maps in Figures 2 and 4. The work is based on funding by the Norwegian Research Council grants 128164/432 and 158733/V30 (EUROMARGINS) and EU funding by grant HPRN-CT-2002-00212 (EURODOM).
A multicomponent seismic technology is able to broaden our knowledge of the gas-hydrate reservoir. In the marine environment, shear waves (S waves) can be generated by conversion from a downward-propagating compressional wave (P wave) on reflection at a sedimentary interface. The upward-propagating S wave can be recorded at the ocean floor using horizontal geophones. S waves can be useful in addition to P-wave data because the S-wave velocity is slower than P-wave velocity and S waves are less affected by the pore fill of porous rocks. This clearly gives a distinct improvement because (1) seismic resolution using S waves increases, (2) targets of gas or of poor P-wave reflectivity are imaged well, (3) pore fluids and lithology can be discriminated, and (4) the enhanced ability exists to estimate gas-hydrate concentrations. On the mid-Norwegian margin, multicomponent seismic data have enabled us to choose a proper rock-physical model for the hydrate-bearing sediments. We are able to constrain seismic velocities from ocean-bottom seismic data. This allows us to obtain more accurate estimations of gas-hydrate and free-gas concentrations and to assess the occurrence of overpressures within the gas-bearing sediments underneath the hydrates. Improved acoustic images look through the zone underneath the hydrate-bearing sediments, which is obscured on the P-wave data because of the occurrence of gas.
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