ABSTRACT

High quality 3-D seismic data from the Green Canyon area (Blocks 53, 185, 272) were used in conjunction with direct sea floor observations/data collection by research submersible to help establish the links between seismic signature and sea floor response in a setting characterized by hydrocarbon seepage. Amplitude extraction maps of surface and near-surface horizons were compiled from digital 3-D seismic data. They were useful in horizons showing the area, configuration, and relative anomaly strength which provided a target field on which to select and prioritize submersible dives. In areas where 3-D seismic data were acquired, the amplitude extraction method offered a quick indicator of potential seeps, their relative activities, and the probability of encountering chemosynthetic communities and other features commonly associated with seeps. The 3-D seismic data also offered more complete surface and near-surface coverage of a seep area than conventional high resolution acoustic data used for shallow geohazard evaluations.

Sea floor expressions of the sites selected for direct observation/sampling ranged from localized areas of gas-rich sediment in a reduced state which were covered with bacterial (Beggiatoa) mats (both white and orange) to fields of fused hydrate mounds containing crude oil and inhabited by dense chemosynthetic communities of tube worms (Lamellibranchia sp.) and mussels (Bathymodiolus sp.). In areas where sea floor evidence of active seepage was most compelling, local mud flows, gas-related craters, fields of mussel shells, and distinct hardgrounds/buildups of authigenic carbonate were common features. These sites also correlated well with the strongest zones of surface to near-surface amplitude anomalies. The amplitude anomalies are thought to correspond to slow-velocity gas-charged sediments rather than fast-velocity authigenic carbonates which tend to form poor-reflecting, discontinuous boulder fields and irregular mounds. At sites with substantial surface evidence of hydrocarbon seepage, clear fault-related pathways for migration of fluids and gases to the surface were observed on the 3-D seismic data.

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