Gas bubbles in marine mud—How small are they? Free

Free gas in marine mud poses a challenging problem in the realm of ocean acoustics as it readily attenuates (i.e., scatters or absorbs) energy, such that objects lying below the gassy sediment are acoustically masked. Gas‐laden sediments were located in 10‐ to 120‐m water depth adjacent to the South Pass of the Mississippi River in East Bay using a 12‐kHz transducer and the Acoustic Sediment Classification System. Several cores were collected in this region for physical property measurements. Some of the cores were x‐rayed on medical and industrial computed tomography (CT) scanners. Volumetric CT images were used to locate gas bubbles, determine shapes and sizes to within the limits of the CT resolution. Free gas in the East Bay sediments was relegated to worm tubes as well as isolated pockets as was the case in Eckernförde Bay sediments [Abegg and Anderson, Mar. Geol. 137, 137–147 (1997)]. The primary significance of the present work is that gas bubbles have been determined to exist in the tens of μm size range, which is significantly smaller than the smallest bubbles that were previously resolved with medical CT (∼440 μm) with NRL’s HD‐500 micro‐CT System. [Work supported by ONR and NRL.]