Seafloor Calibration of High Resolution Acoustic Data and Amplitude Rendering of the 'Diapiric Hill,' Garden Banks 427

摘要

Often analysis of the best remotely sensed high resolution acoustic data (echo sounder profiles, seismic, and side-scan sonar), 3D-seismic attribute data, and seafloor renderings still leaves fundamental questions unanswered concerning surlicial geology and biology. Such questions existed after initial gcoha/.ards evaluations for the diapiric hill in the Auger Prospect Area. This paper describes the results of two manned submersible dives on this cone-shaped feature and the correlation of surficial ground truth (collected from the base of the feature to its crest) to acoustic data. High resolution acoustic data and 3D-seismic renderings indicate areas of extremely high backscatter on the upper flanks and top of the feature. In addition, the cone demonstrates linear patterns radiating away from the crest area. Highly reflective surfaces on the cone flanks correlate to areas of seafloor lithifi-cation. These hard-bottom areas occur on topographically higher flanks of gully-like linear depressions that trend down-slope away from the feature crest. The products of lithification are ~(13)C-depleted carbonates and are therefore formed as by-products from microbial degradation of hydrocarbons. The gullies appear to be transport pathways for fluids expelled from the feature crest. Perhaps some of these gullies are fault-controlled as suggest by seismic data. Complex hydrocarbon seep communities are absent from this feature. However, disarticulate clam shells (Calyplogena sp) and carbonate clasts are scattered around depressions that appears to inactive mud vents near the feature crest. These crestal areas also correspond to high backscatter on side-scan data and high amplitudes on 3D-scismic attribute data. A phase reversal across the top of this feature on 3D-seismic profiles suggests gas-charged near-surface sediments, an observation supported by the presence of small bacterial mats, clam shells and authigenic carbonates. The process of calibrating high quality acoustic data and scafloor renderings with the best ground truth observations via mannedsubmersibles leads to more reliable use of remotely sensed data for interpreting seafloor conditions.Application of results from this investigation helps upgrade our ability to interpret seafloor geology, geohazards, and to a lesser extent the presence or absence of chemosynthetic communities. These data arc useful in support of engineering site surveys and compliance with federal regulations concerning protected biological communities.