Multicomponent Seismic Studies of the Gas-hydrate System at the Storegga Slide

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.