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Abstract: Bubble Characteristics in Gassy Aqueous Sediments

Thomas H. Orsi, Aubrey L. Anderson

Although gas bubbles are known to dramatically alter the biogeochemical, geotechnical, and geoacoustic characteristics of bottom sediments in numerous marine, estuarine, and lacustrine sediments worldwide, little is known about their size, shape, or spatial distribution within the sediments. In this study, we describe an approach for quantifying gas bubble characteristics in gassy aqueous sediments using x-ray computed tomography (CT or CAT scanning). By calibrating the CT scanner using a specially machined Plexiglas disk with preformed spheres of different diameters, the proper thresholding technique can be identified for segmenting the CT images into matrix (sediment) and bubble (free gas). To illustrate the approach, we examine a naturally gassy sediment from a shallow-water lake n ar College Station, Texas. Free gas within the sediments occurred as large bubbles with diameters >2 mm, resulting in volume fractions ranging from essentially zero to a high in excess of 0.08. The distinct vertical segregation of bubbles correlated well with sediment structure and water content, but had a nominal effect overall on sediment bulk density. Bubble size and shape were related as bubble shape progressed from spherical to elliptical to amorphous (blobs) in form with increasing size. These results suggest that the assumption of spherical bubbles distributed uniformly throughout a volume of aqueous sediments, as commonly invoked for modeling purposes, may be inaccurate.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994