ABSTRACT

We describe an approach to quantifying bubbles in gassy sediments using X-ray computed tomography (CT or CAT scanning). By calibrating the CT scanner using a specially machined plexiglass disk (phantom) with spheres of different diameters, a proper thresholding technique can be established to segment the CT images into matrix (sediment) and bubble (free gas). The contoured thresholds permit assessment of the geometry of the bubbles, and the volume fraction of free gas can be determined from the segmented images as the ratio of segmented "bubble" voxels (three-dimensional pixels) to the total number of voxels examined.

To illustrate this approach, we examine a naturally gassy sediment from a shallow-water lake near College Station, Texas. Free gas within these sediments occurs as large bubbles with diameters generally greater than 2 mm and free gas volume fractions ranging from essentially zero to in excess of 0.08. The vertical segregation of bubbles correlates well with sediment structure and water content but has only a nominal effect on overall sediment bulk density. Bubble size and shape are related as bubble shape progresses from spherical to elliptical to amorphous "blobs" in form with increasing size. These results suggest that the assumption of tiny spherical bubbles within pores distributed uniformly throughout a volume of aqueous sediments, as commonly invoked for modeling purposes, may be inaccurate.