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X-Ray Computed Tomography of Macroscale Variability in Sediment Physical Properties, Offshore Louisiana
Thomas H. Orsi, Aubrey L. Anderson
Our understanding of the role of benthic environmental processes in generating macrostructure (mm-to-cm scale) and spatial variations in sediment physical properties is limited primarily by the lack of a suitable technique for quantitative analysis at this scale. To remedy this, we used a density-calibrated X-ray computed tomography (CT) scanner to quantify macrostructure-physical property relationships in seafloor sediments from the Louisiana continental shelf, northern Gulf of Mexico. Dominant sources of macroscale variability in the sediments studied are shells and shell debris. Hurricane-driven hydrodynamic forces "homogenize" the upper portions of the seafloor, but they simultaneously generate local variability by creating random shell orientations with macrostructures that can be detected and quantified by CT.
Millimeter-scale vertical profiles of selected CT statistics reveal tiering of sediment properties and an increase in compaction at extremely shallow subbottom depths that are not evident from geotechnical property logs. An observed nonlinear reduction in property variability associated with this compaction is attributed to the presence of pelecypod shells. Concomitant with matrix compaction, shell reorientation in response to increasing overburden pressure results in a rapid decrease in property variability. Once a relatively stable shell configuration is achieved, further compaction of the matrix is slowed, and property variability levels off below this depth.
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