Abstract 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.

Abstract 2D and 3D seismic data from the mid-Norwegian margin show that polygonal fault systems are widespread within the fine-grained, Miocene sediments of the Kai Formation that overlie the Mesozoic/Early Cenozoic rift basins. Outcropping polygonal faults show that de-watering and development of polygonal faults commenced shortly after burial. On the other hand, the polygonal fault system's stratigraphic setting, upward decreasing fault throw and the association with fluid flow features that are attributed to de-watering of the polygonal fault systems shows that polygonal faulting and fluid expulsion is an ongoing process since Miocene times.