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Previous investigations related to acoustic propagation through seagrass have shown that sound waves are sensitive to the gas contained within seagrass tissue as well as free bubbles produced by photosynthesis. The acoustical effects include reduced low-frequency sound speed and increased sound attenuation. The effects are more pronounced when the density of vegetation is higher as well as during daylight hours when sunlight-driven photosynthesis occurs. However, the application of mathematical models to describe these phenomena have been limited to effective medium models for water containing spherical gas bubbles. These approaches neglect both the effects of elastic properties of seagrass tissue as well as the shape of the aspherical gas bodies constrained within the plant. In this work, a finite element model of an acoustic resonator containing seawater and seagrass blades was developed to explain previously published measurements. The model utilizes independently measured values of the elastic properties of seagrass tissue and microscopic cross-section imagery of the gas volumes contained within the seagrass tissue. These results represent a step towards defining an appropriate effective medium model for acoustic propagation through seagrass tissue. [ABSTRACT FROM AUTHOR]

This talk presents results from an acoustic propagation experiment conducted in the Lower Laguna Madre to characterize the acoustical properties of a seagrass meadow. At the location of the experiment, the water was one meter deep, and the seabed was covered by a dense growth of Thalassia testudinum, a type of seagrass that grows from a long, jointed rhizome buried 5 cm to 10 cm below the seafloor. The biological processes and physical characteristics associated with seagrass are known to affect acoustic propagation due to bubble production, which results in dispersion, absorption and scattering of sound. During the experiment, a Combustive Sound Source was used to produce broadband signals at ranges of 20 m to 1000 m from the receiver location. Three sensors were positioned at the receiver location: two hydrophones located within and above the seagrass canopy, and a single-axis geophone. The data were analyzed for the purposes of predicting acoustic propagation in seagrass meadows and for estimating environmental parameters in very shallow, biologically active environments. Additionally, seagrasses are a vital part of the global carbon cycle and this work explores the feasibility of using of acoustic signals to estimate carbon stores. [ABSTRACT FROM AUTHOR]