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Sidescan-Sonar Imagery, Multibeam Bathymetry, and Surficial Geologic Interpretations of the Sea Floor in Rhode Island Sound, off Sakonnet Point, Rhode Island

Data Acquisition and Processing

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Data Acquisition and Processing

Click on figure for larger image and full caption.

Figure 2. Location of figures, bathymetric profiles, historic seismic-reflection profiles, and sand and rocky sediment samples within the study area.
Figure 2. Location of figures, bathymetric profiles, historic seismic-reflection profiles, and sand and rocky sediment samples within the study area.

Figure 5. Photo of NOAA Ship RUDE (Courtesy of NOAA).
Figure 5. Photo of NOAA Ship RUDE (Courtesy of NOAA).

Sidescan-sonar and multibeam bathymetric data were acquired by NOAA in 2004, during Survey H11320, aboard the NOAA Ship RUDE (fig. 5). A Klein 5500 towfish transmitting at 455-kHz and Triton-Elics ISIS software were used to acquire sidescan-sonar data and to archive the data in extended Triton format (XTF). All sidescan-sonar data were processed using USGS sidescan-sonar processing software packages XSonar and ShowImage. A median filtering routine was applied to the sidescan-sonar data to remove speckle noise, resulting in a 0.06-m pixel size. Time-varied gain was applied to sonar data collected on Julian Day 118 and 133 (year 2004), as the dynamic range of these data were low. Sonar data were further processed to correct for slant-range and radiometric distortion inherent in the sonar data. All processed data were mosaicked using XSonar, resulting in an enhanced, geographically correct, sidescan-sonar mosaic with 1 meter/pixel resolution. The mosaic was converted to a TIFF image, which was imported into Adobe Photoshop CS2 in order to apply a 'stretch' (i.e. increase the dynamic range of the data) to the image. Within the sidescan-sonar imagery light tones represent strong acoustic reflectivity (generally coarser grained sediment) and dark tones represent weak acoustic reflectivity (generally finer grained sediment).

A Reson SeaBat 8125, which operates at 455-kHz, was used to acquire multibeam bathymetric data in areas with possible navigational hazards and along several tie-lines. Multibeam data were processed using CARIS HIPS/SIPS software. Four large areas of complete multibeam bathymetry coverage, totaling 20 km², were separated from the tie-lines and single-beam bathymetric data that were included in the survey's grid, as the single-beam data were vertically offset from the multibeam data. The four areas of multibeam data were gridded at 4-m resolution. Bathymetry from the Coastal Relief Model created by the National Geophysical Data Center (NGDC) is used in the study area to provide rough bathymetry on which the higher resolution multibeam bathymetry can be overlaid (Divins and Metzger, n.d.). The NGDC Coastal Relief Model bathymetry data from around the study area were downloaded as ASCII raster grid and converted to a GeoTIFF and Generic Mapping Tools (GMT) grid using FWTools. The GMT grid was resampled at 4 meters and converted back to an ASCII grid and finally a GeoTIFF. Sidescan-sonar and bathymetry data were converted to geographic coordinates.

O'Hara and Oldale (1980) collected 670 km of seismic-reflection data in eastern Rhode Island Sound and Vineyard Sound with an EG&G Unit Pulse Boomer during 1974 and 1975, about 30 km of which are within this study area (fig. 2). The transducer was triggered every 0.5 second and the sweep rate was 0.25 second. Seismic-reflection data has 1-1.5 m resolution.

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