The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information 14. ABSTRACT The mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance. We offer definitive evidence on the mode of bubble formation in the form of X-ray computed tomographic images and comparison with theory. Natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in response to growth stresses. We also present bubble-rise results from gelatin, a mechanically similar but transparent medium, that suggest that initial rise is also accomplished ABSTRACT idization, e.g., gravity flows, during some natThe mechanics of uncemented soft sediments during bubble growth are not widely ural disturbances have suggested that such understood and no rheological model has found wide acceptance. We offer definitive ev-sediments can act fluidly or plastically in reidence on the mode of bubble formation in the form of X-ray computed tomographic sponse to stress. Past mechanical models of images and comparison with theory. Natural and injected bubbles in muddy cohesive bubbles in these sediments have visualized the sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by bubbles as essentially spherical (e.g., Wheeler, fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft 1988; Sills et al., 1991), with the implication, sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in intentional or not, that the surrounding mediresponse to growth stresses. We also present bubble-rise results from gelatin, a mechan-um reacts fluidly or plastically to their growth ically similar but transparent medium, that suggest that initial rise is also accomplished and rise. Scientists and engineers have develby fracture. Given that muddy sediments are elastic and yield by fracture, it becomes oped an impressive understanding of bubble much easier to explain physically related phenomena such as seafloor pockmark forma-growth in fluids, and a vast literature covers tion, animal burrowing, and gas buildup during methane hydrate melting.the topic (e.g., Clift et al., 1978; Lohse, 2003). However, we show here that muddy sediment Keywords: bubbles, mud, fracture, methane.does not respond mechanically either a...
