3D-Seismic Amplitude Analysis of the Sea Flooc An Important Interpretive Method for Improved Geohazards Evaluations
Authors: Roberts H.H. , Doyle E.H. , Booth J.R. , Clark B.J. , Kaluza M.J. , Hartsook A. 
Published: October 2008

Evaluation of geohazards on the Louisiana continental slope using a combination of high-resolution acoustic data (standard geohazards survey data), 3D-seismic amplitude maps of the sea floor, and direct observation/sampling by a manned submersible reinforces the virtue of 3D-seismic amplitude data for feature identification. Amplitude extraction data from surface and near-surface horizons are valuable for establishing the links between high-resolution seismic signature and actual sea floor response, particularly in settings characterized by various types and rates of hydrocarbon venting/seepage.

It was found that amplitude extraction data could accurately define the areas, configurations, and relative rates of hydrocarbon seepage (from anomaly strength and target size). in areas evaluated with 3D-seismicamplitude extraction data, this procedure provided a rapid method of identifying sites of hydrocarbon venting/seepage, their relative activities, and the likelihood of encountering sensitive chemosynthetic communities and other features such as mud vents, gas hydrate mounds, hard grounds, and sizable buildups of authigenic carbonates. Results of this study support the value of using 3D-seismic amplitude extraction data for improving our Understanding and predict ability of the slope's surface geology and seep-related benthic habitats.

Introduction

Since beginning of the Cenozoic has been the site of a massive the northern Gulf of Mexico influx of sediments derived from the continental interior and delivered by rivers trending northwest-southeast to roughly north-south. The net result has been to prograde the shelf margin southward over older geologic units including thick and already deformed Jurassic salt, The interaction between input of large volumes of sediment in rather localized areas, deformed 1, over relatively short intervals of geologic time has resulted in a complex present configuration of the prograde continental slope. The complexity of the northern Gulf slope is perhaps unmatched by other continental slopes within today's ocean basins. Although the northern Gulf can be technically classified as a passive margin, the dynamic movements of sediments and salt make this setting anything but passive. The engineering problems presented by the slope's dynamic environments are substantial, and their solutions depend on developing methods of determining the slope's geologic and geotechnical characteristics.

Geologic Framework.

Topography of the Louisiana-Texas continental slope, especially the middle to upper slope, is dominated by halokinesis of allochthonous salt, resulting in a network of domes and basins. The domes represent the presence of salt in the shallow subsurface. Adjacent basins contain thick sedimentary sequences that have formed as both a product of substantial input of sediments to the slope province and salt withdrawal. Ref. 2 indicates the presence of more than 105 intraslope basins with relief in excess of 150 m, 28 mounds, and 8 submarine canyons in the northwestern Gulf of Mexico alone. The lower slope is characterized by intraslope-interlobal basins formed by the coalescing of salt canopies and supralobal basins formed by down building into a salt nappe. Therefore, the surficial geology and topography of the northern Gulf of Mexico's continental slope are forced by the interplay between rapid sediment input accommodated by deep-cutting faults and compensating salt deformation.