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Chris Jones, Senior Engineer
Ocean Acoustics Department
Applied Physics Laboratory
University of Washington
Office of Naval Research
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The sonar was successfully deployed in four geographicaly distinct areas with many hours of sampling. The first deployment was aboard the NOAA R/V Freeman at Kodiak Island in 2006. The goal for the first field season was primarily testing, although pollock aggrgations were detected and sampled. Subsequent field tests focused on the development of sampling and processing techniques (e.g., different ways to deploy the array, how to integrate the sonar in surveys, improved signal processing). Two field deployments were performed off the New Jersey coast and in the Chesapeake Bay as part of joint experiment on the R/V Sharp in 2006 and 2007. The final experiments were in the Puget Sound in 2008 using the R/V Robertson. A large amount of data was collected in a variety of configurations. Examples of data from two locations illustrate two different spatial scales of waveguide imagery. The first example (Figure 4) shows images of small fish schools in very shallow water (10 m) at ranges on the order of 100 meters. The second example shows an image of a larger school in typical continental shelf water depth (100 m) at longer range of the order 1 km. |
CHESAPEAKE BAY Preliminary analysis of the PIMS acoustic data recorded in the Chesapeake Bay show imaging of bay anchovy aggregations (Figure 4) corresponding to observations made with downward looking multibeam sonar and groundtruthing using a variey of trawls. Figure 4 illustrates one example of a circular horizontal image with several aggregations. Each aggregation in the image is approximately 5 to 10 m in diameter. In this example aggregations were imaged at a range of 100 m in water depths of approximately 10–20 m, illustrating nicely the waveguide scattering geometry in very shallow water. PUGET SOUND Data collected during NOPP field experiments in the Puget Sound of Washington State better illustrate the PIMS system data in more typical shallow water environments. Figure 5 illustrates a 1 km circular image of area near Middle Bank in the Puget Sound. In the area of the image, shoaling fish would rise during the night from daytime bottom layers. During the day, the near-bottom layers were not visible in the images or were indistinguishable from background reverberation. At night, identifiable areas of increased backscatter were observed and associated with aggregations of fish rising to mid-water, as observed separately with a down-looking EK60 (Figure 6). These fish shoals (believed to be smelt) are relatively weak targets, comparable in level to scattering and reverberation associated with topography. The strong scattering at the upper right of the image is backscatter from the rising slope of Middle Bank. |
