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This work is a part of a larger research effort being organized by ONR under the auspices of a Departmental Research Initiative (DRI) on High Frequency Sound Interaction in Ocean Sediments. The DRI addresses high-frequency sound penetration into, propagation within, and scattering from the shallow-water seafloor at a basic research (6.1) level. Both surface roughness and volume heterogeneity have been implicated in the anomalous penetration of high-frequency sound into the bottom. Our thesis is that these characteristics of the seabed are dynamic properties of the bottom environment, with changes that are driven by both biotic and abiotic forces. In an attempt to describe the environment in support of a variety of acoustical measurements to be made within the context of the larger program of research, the principal thrust of our effort is the measurement of the flux of marine animals between the seabed and the water column. This flux of bentho-pelagic organisms (e.g., shrimp, amphipods, and mysids) can modify the surface roughness of the bottomwater column interface and is the source of at least part of the volume heterogeneity that characterizes the seabed. These measurements will be made with several TAPS sensors, two looking up from the bottom and one looking down at the bottom. These instruments operate at frequencies of 265, 420, 700, 1100, 1850, and 3000 kHz.
The proposed work also contains two ancillary elements: measurement of the temporal evolution of scattering at multiple frequencies from a man-made, mine-like target, and a series of measurements from an area of the seabed that will be subjected to bio-manipulations using species endemic to the experimental site. The reasons for the first of these ancillary projects involve the possibilities that acoustically detectable changes may occur in the biological community that develops on and around a foreign object on or in the bottom. These measurements will be made at a series of discrete frequencies between about 4 kHz and 3000 kHz. Sampling of the biota around and on the object placed on the bottom and the bio-manipulation work are cooperative projects with Peter Jumars (UW/Oceanography), his students and staff. We will be examining changes in the reflectivity of several areas of the seabed at 104, 165, 265, 420, 700, 1850, and 3000 kHz. The results of this work will influence directions taken in the development of acoustical technology aimed at improving our capability to detect, enumerate and track benthic organisms in their natural habitat.
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