‘Split’ strata-bounded gas hydrate BSR below deposits of the Storegga Slide and at the southern edge of the Vøring Plateau

Following the investigations of the 8th Training Through Research (TTR8) cruise (1998), the Vøring Plateau/Storegga Slide area was revisited during the TTR10 expedition of RV Professor Logachev in August 2000 using a single-channel seismic system. Bottom simulating reflections (BSRs) were observed in undisturbed sediments of the Vøring Plateau and below the deposits of the Storegga Slide. The BSR is expressed on the seismic records either as a strong reflection, often crosscutting local stratigraphy, or as a facies change between high-amplitude (enhanced) reflections below and normal amplitude reflections above it. Analysis of the seismic signature of the BSRs and adjacent enhanced reflections showed that the anomalous reflections demonstrate reversed polarity and cause amplitude and frequency shadows below, indicating the presence of free gas trapped below hydrate-bearing sediments. For each of the lines where BSR was observed, the theoretical base of the local gas hydrate stability field (GHSF) was calculated. The resulting calculated depths of the theoretical GHSF fit well with observed BSRs, and confirm the gas-hydrate nature of the reflections. On most of the profiles the BSR is ‘split’ into several isolated zones. The reflection appears at specific stratigraphic layers, between strata where the BSR is not observed. Four ‘BSR-demonstrating’ layers were identified, continuing upslope (below the GHSF) as enhanced reflections. These layers must possess some physical or petrographical property, that makes them most favourable for gas hydrate formation and/or accumulation of free gas. All these layers lie within the thick (>1 s TWT) upper Pliocene–Quaternary glacial/interglacial sedimentary sequence. It is assumed that they correspond to some strata of coarser terrigenic material, which would feature higher permeability relative to surrounding sediments and/or to undercompacted layers with increased water content.