Multicomponent ocean bottom cable data in gas hydrate investigation offshore of Norway

Abstract

A multicomponent seismic ocean bottom cable (OBC) line from the northern flank of the Storegga Slide offshore of Norway is the first OBC line ever acquired over a gas hydrate‐related bottom simulating reflector (BSR). The seismic data are of very good quality, and display strong P ‐ S converted wave reflections for the whole sediment column. All P ‐ S reflections follow the sediment layers and are therefore interpreted to be caused by increased shear modulus and density due to sediment compaction. No P ‐ S reflections are associated with the BSR along this line, indicating that the gas hydrate‐bearing sediments at the base of the hydrate stability zone are not stiff enough to increase the shear modulus of the sediments to produce P ‐ S converted wave reflections. Seismic interpretation and seismic stratigraphic modeling suggest that the BSR in this area is a contrast in compressional wave velocity only, mainly caused by small amounts of free gas confined to dipping sediment layers beneath the BSR. Gas hydrate saturation above the BSR, tentatively estimated to be about 1–1.5% of the sediment pore space, is most likely too low to distinguish between different models for microscale gas hydrate distribution.