Seafloor Calibration of High Resolution Acoustic Data and Amplitude Rendering of the "Diapiric Hill," Garden Banks 427
Authors: Roberts H.H. , Doyle E.H. 
Published: October 2008

Often analysis of the best remotely sensed high resolution acoustic data (echo sounder profiles, scismic, and side-scan sonar), 3D-seismic attribute data, and seafloor renderings still leaves fundamental questions unanswered concerning surficial geology and biology. Such questions existed after initial geohazards 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 if extremely high backscatter on the upper flanks and top of the feature. In addition, the cone demonstrates linear patterns radiating away from the crest areas. Highly reflective surfaces on the cone flanks correlate to areas of seafloor lithification. 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 13C-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 (Calyptogena 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 3-Dseismic attribute data. A phase reversal across the top of this feature on 3D-seismic profiles suggests gas-charged near sufaces 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 seafloor renderings with the best ground truth observations via manned submersibles 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 are useful in support engineering site surveys and compliance with federal regulations concerning protected biological communities.

INTRODUCTION

In the process of evaluating seafloor tracks for potential geohazards, surface-tow high resolution acoustic data (both seismic and side-scan sonar) is the standard in shelf-depth waters. However, as water depths increase across the shelf break, surface-tow acoustic sources (boomers, water gun, air guns, spakers, and other)＿ become less accurate in representing the seafloor and cable lengths necessary for collecting side-scan sonar data become unmanageable. These reliable shallow and intermediate water depth instruments are systematically being replaced with higher quality deep-tow subbottom profilers (3.5 kHz) and improved resolutions side-scan systems (up to 500 Hz) for deepwater use. In addition, the deepwater oil and gas province of the northern Gulf of Mexico now has large areas covered by exploration-scale 3D-seicmic which represents another data type that is becoming more important for deepwater site investigations. Although these new 3D data sets lack the resolution of their higher frequency counterparts, 3D seismic can provide unique insight into seafloor conditions.