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Hydrocarbon Seeps of the Louisiana Continental Slope: Seismic Amplitude Signature and Sea Floor Response
Harry H. Roberts (*), Douglas J. Cook (**), Mark K. Sheedlo (**)
Processing of high quality 3-D seismic data from 3 lease blocks in the Green Canyon Area (Blocks 53, 184, 185, 272) provided a data base for selecting hydrocarbon seeps to be further evaluated using a highly maneuverable research submersible. The digital seismic data were processed to enhance amplitude anomalies at the sea floor and in both the shallow and deep subsurface. Amplitude extraction maps of the surface and shallow subsurface were extremely useful for identifying seep areas on the basis of the strength, size, and general configuration of the amplitude anomaly zone. These methods provided valuable insight into the relative importance and structural/depositional setting for active sea floor seeps. Although high resolution acoustic data were more useful for establishing details of sea floor features, the relative activities of seeps and distributions of subtle sea floor features were reflected in the amplitude anomaly data.
On high resolution seismic data seeps are commonly correlated with "acoustic wipe-out zones" which are accepted as a response to gas-charged sediments, hydrates, authigenic carbonates, or combinations of these conditions. Three-dimensional digital seismic data commonly image beneath these zones and provide information on underlying structure and lower limits of gas hydrate stability (a strong low impedance seismic event). In addition, surface and near-surface amplitude data enhance structural patterns and suggest a hierarchy of seep activity which can be tested by direct observation and sampling using a research submersible (e.g. Johnson Sea-Link).
In general, the amplitude anomaly data reflected seep activity level. Low to moderate relative amplitude anomalies often correlated with mounded carbonates with evidence of microseepage or with patches of bacterial mats (Beggiatoa sp.) over dark, reducing sediment. Cores through the latter sites frequently indicated the presence of hydrocarbon gases, brine, and some traces of crude oil. Strong (low impedance) amplitude anomalies were characterized by thick and more extensive bacterial mats, hydrates, abundant macroscopic chemosynthetic communities of tube worms (Lamellibranchia sp.) and mussels (Bathymodiolus sp.), widespread authigenic carbonates, and evidence of gas/mud venting. These seeps were also frequently rich in crude oil beneath the biologic membrane established by microbial mats. All extremely active seep sites were linked to well-defined fault-related pathways to the deep subsurface.
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