Seismic data collected within basins along the outer Bering Sea shelf often exhibit a distinct change in seismic character between 1.0 and 2.0 sec two-way time. This change appears on seismic sections as a reflector or as an increase or decrease in amplitude. The feature is of regional extent.
This seismic character change is a manifestation of what has been called in other basins a bottom simulating reflector (BSR). BSRs are reflectors that are (1) subparallel with sea-floor topography, (2) discordant with stratigraphy where the sea floor dictates, and (3) do not demonstrate the characteristics of a multiple.
Two causes of BSRs are generally accepted. One involves an ice-like mixture of water and gas, termed “gas hydrate,” in which gas molecules are trapped within a framework of water molecules. The other cause involves the diagenetic alteration of biogenic opal-A to opal-CT in diatomaceous sediments.
BSRs were penetrated at three locations in the Bering Sea in water depths greater than 6,000 ft (1,829 m) on Leg 19 of the Deep Sea Drilling Program (DSDP). The BSRs at these locations were attributed to the diagenetic alteration of opal-A. This same diagenesis of opal-A to opal-CT is interpreted to be the cause of seismic character changes noted in basins on the Bering Sea shelf. This interpretation implies that limits can be placed on lithologic interpretation of the stratigraphic section in these basins.
Pitfalls in seismic interpretation may be encountered where this reflector intersects other reflectors at an observable angle. The BSR may look like a sequence boundary with prograding clinoforms above it. This would place constraints on a seismic stratigraphic analysis in terms of predicted paleoenvironments and lithologies. Where this reflector crosses an anticlinal structure, it may appear to be a fluid contact and thus a direct hydrocarbon indicator.
Recognition of the presence of this seismic character change is of two-fold importance to explorationists: (1) understanding the geology of the Bering Sea shelf, and (2) avoiding seismic interpretational pitfalls.