ABSTRACT

Proliferation of 3D-seismic in support of hydrocarbon exploration/production has created new data for improved interpretation of sea floor and shallow subsurface geology. Processing of digital seismic data to enhance surface amplitude anomalies produces information for improved assessment of geohazards and identification of sensitive benthic communities protected by environmental regulations. Coupled with high resolution acoustic data and direct observation/sampling using a manned research submersible, surface amplitude maps add critical interpretive information for identification of sea floor types and associated features. Non-reflective zones (acoustic wipeouts) represent the acoustic response of many slope features. Mud diapirs, mud mounds, mud volcanoes, gas-changed sediments, gas hydrates, slump deposits, carbonate hardgrounds, and various types of carbonate mounds are all features that exhibit this common response on high resolution seismic profiles. Surface amplitude data help make specific identifications. Since 1988, submersible data from mid-to-upper slope features (Garden Banks, Green Canyon, and Mississippi Canyon lease block areas) have been analyzed with conventional high resolution acoustic data and 3D-amplitude extraction maps. Areas of rapid venting of sediment and hydrocarbon-charged formation fluids are clearly distinguishable from areas of hard bottom (slow-to-no seepage). Gas hydrates occur as mounds and mounded zones along faults. They are the products of moderate flux rates below 500 m water depths. Gas hydrates function as stored trophic resources that support sensitive chemosynthetic communities. Amplitude extraction maps clearly help identify these and other gas-charged mud-rich areas by a strong low impedance amplitude anomaly. Refinement and "field calibration" of the surface amplitude extraction method may eventually lead to a new standard for evaluating geohazards and the probable locations of sensitive benthic communities on the continental slope.