Sound speed and attenuation in seagrass from the water column into the seabed

Abstract

Biological processes and physical characteristics associated with seagrass can greatly affect acoustic propagation in coastal regions. An important acoustical effect is due to bubble production by the plants, which can have significant impact on both object detection and bottom mapping sonars by increasing clutter through reflection, absorption, and scattering of sound. In addition to photosynthesis bubbles and gas-bearing leaf tissue in the water column, the plant rhizomes also contain aerenchyma (gas-filled channels), which allow for diffusion of oxygen into the surrounding sediment. To study these effects, in situ acoustic measurements were conducted in a bed of Thalassia testudium in east Corpus Christi Bay, TX. Direct measurements of sound speed and attenuation were obtained in the water column above the seagrass canopy, inside the seagrass canopy, and at discrete depths within the sediment. A complimentary set of measurements were obtained in a bare sediment region located a few meters away. In addition to standard measurements of geoacoustic properties (sediment density, grain size, etc.), biomass was also estimated from cores collected at each site. The sediment beneath the seagrass bed had significantly lower wave speed and higher attenuation compared to the bare sediment. [Work supported by ARL:UT IR&D program and ONR.]