Page 18 - 2018Fall

The Acoustics of Marine Sediments
Figure 5. a: Depth dependence of acoustic and physical properties above and in a Thalassia testudinum (turtlegrass) bed (solid green lines) compared with the same measurements at a nearby site with no seagrass, only bare sediment (solid black lines). b: Red dotted line indicates the increase in attenuation in the seagrass bed relative to the bare sediment site. Negative values of depth indicate locations in the water column and positive values indicate locations within the sediment. The dashed horizontal lines at zero depth in each plot indicate the water-sediment in- terface. The dotted horizontal lines at a depth of −8 cm (above the water-sediment interface) indicate the approximate height of the seagrass canopy. The biomass for the bare-sediment case is multiplied by a factor of 10 so that it can be seen in the same scale as the values for the seagrass bed. c: Green labels on x-axis correspond to the biomass in the seagrass case and the black labels correspond to the bare sedi- ment case. Adapted from Lee et al. (2017).
attenuation takes on a maximum value of 340 dB/m. The large change in the acoustic sediment properties was attributed to the seagrass tissue, associated gas volumes within the tissue, and the diffusion of gas into the sediment.
Benthic Infauna and Bioturbation
Bioturbation, which refers to the churning, stir- ring, mixing, or reworking of sediments by organ- isms during such activities as feeding, locomo- tion, or home building, alters sediment physical properties including grain size and sorting, po- rosity, bulk density, permeability, packing, tortu- osity, and consolidation behavior (Jackson and Richardson, 2007). Most infaunal organisms are found in the upper 25 cm of sediment, known as the benthic boundary layer. Changes in the sedi- ment acoustic properties resulting from infau- nal activity have been predicted using empirical formulas based on measurements made in both disturbed and nondisturbed sediments (Rich- ardson and Young, 1980). Laboratory and field measurements have demonstrated complicated relationships between infauna, bioturbation, and geoacoustic properties (Richardson et al., 2002; Lee at al., 2016b).
One of the most conclusive studies on this topic relates trends in the measured physical proper- ties of the sediment to the relative abundance of three species of burrowing organisms (Jones and Jago, 1993). The drawings in Figure 6 show each organism’s burrow. Correlations between the number of organisms and the electrical re-
acquired using acoustic probes deployed in the water above the seagrass canopy, inside the seagrass canopy, and at sev- eral depths in the sediment below the seagrass. For compari- son, measurements were also obtained at the same depths at a nearby site with bare sediment. Sediment cores were col- lected at both sites and analyzed for plant biomass, sediment bulk density, and mean grain size. The depth dependence of the sound-speed and attenuation profiles are markedly dif- ferent between the two measurement locations. The great- est difference is observed for the shallowest acoustic mea- surements below the seafloor where the plant rhizome and root systems exist. In the bare sediment patch, the sound- to-speed ratio is greater than unity, consistent for fine-sand sediments. However, in the seagrass-bearing sediment, the sound–to-speed ratio is reduced to a value of 0.37 and the
16 | Acoustics Today | Fall 2017
sistivity and shear-wave speed revealed how the organisms’ activities affected the geoacoustic properties of the seabed. Arenicola marina (lugworm) and Corophium arenarium (a type of small crustacean) decreased the sediment’s rigidity by creating open burrows, observed as a decrease in shear- wave speed with increasing organism abundance. However, Lanice conchilega (sand mason worm) increased rigidity by building shell-lined tubes, indicated by an increase in shear- wave speed when more organisms were present. All three species produced a reduction in the electrical resistivity, in- dicative of changes in bulk porosity/tortuosity, and hence changes to the sound speed. The process of burrow con- struction, involving the selection of grains by size and shape, also produced changes in sediment texture and properties between the burrows.