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Many infaunal organisms produce a mucus-like material called extracellular polymeric material. In some marine in- vertebrates, 80% of the animal’s total energy expenditure is accounted for by mucus production (Murray et al., 2002). Soft organic cementation (bonding) between particles re- sulting from infaunal mucus reduces sediment permeability by interstitial pore blockage. Laboratory studies have dem- onstrated that heterotrophic microorganisms can reduce sediment permeability by an order of magnitude, which can have complicated effects on the sound speed and attenuation due to changes in pore fluid mobility. (Meadows and Tufail, 1986). Field measurements have shown that the wet bulk density of sediments decreases and water content increases with increasing organic content, effects that are associated with a decrease in sound speed (Keller, 1982).
The geoacoustic properties of marine sediments have been studied for almost a century, and much is understood about sound propagation through this medium. A large number of measurements have been acquired, empirical relations have been formed, and sophisticated models have been devel- oped. Nevertheless, the field of sediment acoustics remains an interesting and vibrant area of study. Recent work related
Figure 6. Arenicola marina (lugworm) creates large burrows in the upper 100-150 mm of the bed and liquefies and reworks the surface layers, resulting in an increase in bed porosity. Corophium are- narium (a type of small crustacean) builds hollow, U-shaped bur- rows that are open at both ends. They are smaller and occur in much greater densities than those of Arenicola marina. Lanice conchilega (sand mason worm) burrows differ from those of Arenicola marina and Corophium arenarium in their geometry (linear rather than U-shaped) and construction (particle-lined as well as mucus-lined walls). Adapted from Jones and Jago (1993).
to the effects of benthic biology on seabed acoustic proper- ties reveals interactions between sediments and the plants and animals that live in them. It has been shown that biol- ogy can change sediment properties as function of space and time in unexpected ways. For biologically active sediments, understanding geoacoustic properties is a multidisciplinary undertaking, involving both the measurement of acoustic properties and the quantification of biological effects. A complete explanation of the results of these measurements will ultimately lead to new sediment acoustic models that account for the presence of flora and infauna and the chang- es they make to the physical properties of the seabed.
We gratefully acknowledge the support of the Office of Na- val Research and the University of Texas at Austin Applied Research Laboratories Independent Research and Develop- ment Program. We also thank Preston Wilson and Art Pop- per for their helpful comments.
Megan S. Ballard received a BS in ocean engineering from Florida Atlantic Uni- versity in 2005 and a PhD in acoustics from Pennsylvania State University in 2009. She is a research scientist at the Applied Research Laboratories, Univer- sity of Texas at Austin. Dr. Ballard re- ceived the R. Bruce Lindsay Award from
the Acoustical Society of America in 2016, the Postdoctoral Special Research Award from the Office of Naval Research in 2011, and the National Defense Science and Engineering Graduate Fellowship Award in 2006. She currently serves as the chair of the Underwater Acoustics Technical Committee.
Kevin M. Lee, a plasma physicist by train- ing, has been a physical and underwater acoustician at the Applied Research Labo- ratories, University of Texas at Austin, since 2009, where his research interests center on seabed acoustics, the acoustics of bubbles and bubbly media (natural and synthetic), underwater acoustics mea-
surements, and acoustic materials characterization. Dr. Lee is also a cofounder of AdBm Technologies, a developer of novel underwater noise abatement systems, and the holder of several related patents. He is a member of the Acoustical Society of America and currently serves on both the Physical and Under- water Acoustics Technical Committees.
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