Movement of large gas bubbles in unsaturated fine‐grained sediments

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Abstract
Where undissolved gas occurs within fine‐grained marine sediments it usually takes the form of discrete bubbles that are much larger than the normal void spaces. The possibility of buoyancy‐induced movement of these relatively large bubbles must be included when considering the transport of gas through marine sediments. A theoretical analysis shows that, under static loading conditions, bubbles larger than a critical size should have sufficient buoyancy to move upward through a fine‐grained sediment stratum, whereas bubbles smaller than the critical size should remain fixed in position. The critical radius is directly proportional to sediment shear strength, and bubbles of a realistic size should move upward only in extremely weak sediments. Further theoretical analysis shows that the critical bubble size is reduced under cyclic loading conditions, but movement of typical‐sized bubbles should still be restricted to sediments of low shear strength. A simple laboratory experiment provides support for the conclusions of the theoretical analysis. The results indicate that buoyancy‐induced movement of relatively large gas bubbles in fine‐grained sediments is most likely to occur under storm loading conditions and is unlikely to occur at depths greater than a few meters below the seabed.