Observation and modelling of bubble growth and rise in muddy sediments.

Just like in bread, micro-organisms in nearshore and marsh sediments ferment (dead) organic matter to produce free gas that forms bubbles. These natural bubbles are primarily made of methane, and they constitute the most important way this gas enters the atmosphere from marine and freshwater sediments. These bubbles make it almost impossible to image the bottom with sound (sonar), and can cause instabilities in the sediment which threaten structures on the seafloor. My research aims to understand and predict how these bubbles grow, what size they can reach, what numbers can exist in a volume of sediment, and how and when they rise and leave the sediment. The reason it is difficult to know these things is because we cannot see what happens in sediments. We have discovered some very odd things about these bubbles. Using an X-ray CT scanner, like those found in hospitals, we have found that these bubbles are not spheres, but thin disks. And we can predict the shape and size if we assume that the sediment acts like an elastic material (like a sponge) that tears apart to make room for a new hole filled with gas, which is a bubble.