Morphology of Gas Bubbles in Mud: A Microcomputed Tomographic Evaluation
Abstract
Free gas in surficial sediments commonly forms gas bubbles that attenuate and dampen acoustic waves, influence slope stability, and contribute to greenhouse gas concentrations. Therefore, determining the mechanisms that control bubble shape, size, growth, and migration is important to acoustic sediment characterization and other disciplines. Previously, gas bubble shape, size, and distribution was quantified using "low-resolution" (~500 micron) medical computed tomography (CT). Recently, "high-resolution" (to <25 micron) x-ray micro-computed tomography (XMCT) was used to evaluate gas bubbles in mud; reconstituted in the laboratory, collected near the Mississippi River (MR) mouth, and collected from Cole Harbor (CH), Nova Scotia. In the reconstituted mud (mixture of kaolinite clay, bay mud, and sucrose), gas bubbles formed spheroids [surface-area ratio (SAR) of ~.01] with equivalent-bubble radii >30 microns. In several MR cores, gas bubbles formed as vertically oriented oblate spheroids with SARs of ~1.6 (5:1 ratio of length to width), yet in other MR cores, gas bubbles formed elongated fractures that spanned the core width, consequently, a SAR could not be accurately determined. In the CH mud, a gas bubble formed as an oblate spheroid (i.e., coin-shape) with a SAR of ~5.0 (30:1 ratio of length to width) as air was injected incrementally through a capillary tube. It appears that bubble shape (i.e., SAR) and orientation are correlated with sediment physical properties and localized heterogeneity. XMCT images show that gas bubbles grow by fracture mechanics rather than by elastic expansion of the sediments. The images also show that the bubbles exist at sizes that are not resolvable with medical CT and often grow with the principal axis oriented vertically. XMCT has enabled the characterization of gas bubbles that are significantly smaller than those evaluated previously, thus furthering our mechanistic understanding of gas bubble formation and growth.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 01, 2005
- Accession Number
- ADA454883
Entities
People
- Allen H. Reed
- Bernard P. Boudreau
- Chris Algar
- Yoko Furukawa
Organizations
- United States Naval Research Laboratory