USGS Open-File Report 2005-1162, Sidescan-Sonar Imagery and Surficial Geologic Interpretation of the Sea Floor off Bridgeport, Connecticut
The sidescan-sonar imagery contained in this report was produced from data collected during 2003 as part of NOAA survey H11045 aboard the NOAA Ship RUDE. The data for this survey were acquired and stored in XTF (eXtended Triton Format) using an Klein 5500 sidescan-sonar tow fish (fig. 3) set to sweep 100 m to either side of the ship's track. The tow fish emits two 50°-wide beams tilted down 20° from horizontal, and transmits at 100 kHz. The sidescan sonar were digitally recorded in XTF format using an ISIS data acquisition software package. Once digitally recorded, the XTF data were processed using CARIS SIPS.
The sidescan-sonar data were multiplexed and filtered to convert the data into the desired processing format and to remove speckle noise and to correct for slant-range distortions. The data were imported into SonarWeb, an application that creates mosaics from raw XTF sidescan-sonar data. The mosaic includes the following standards: 1-m resolution, image 'shine through' to account for areas of overlap, and auto-contrast adjustment. Tonal artifacts caused by inaccurate across-track normalization are present on the sidescan-sonar imagery. This mosaic was then displayed in geographic coordinates. Contrast enhancement based on the dynamic range of the data was applied to the mosaic to produce the final enhanced image. More information regarding sidescan-sonar acquisition and processing is accessible the USGS Woods Hole Science Center's Sea-Floor Mapping Technology Web site.
Lighter tones in the sidescan-sonar imagery suggest relatively stronger acoustic returns, coarser sediments and higher energy environments. Darker tones in the sidescan-sonar imagery suggest relatively weaker acoustic returns, finer sediments, and lower energy environments.
For verification, bathymetric (DiGiacomo-Cohen and others, 1998), photographic, and surficial sediment data (Poppe and Polloni, 1998, Poppe and others, 2000a) were used in the interpretion the sidescan-sonar imagery. The bathymetric contours were digitized from NOAA charts; vertical datum is mean lower low water. Most of the surficial sediment samples were collected with a modified Van Veen grab sampler (fig. 4) by Poppe and others (2000a). The photographic data were used to appraise intra-station bottom variability, faunal communities, and sedimentary processes and to observe boulder fields where samples could not be collected.
The sediment descriptions are based on the nomenclature proposed by Wentworth (1922), the inclusive graphics statistical method of Folk (1974) and the size classifications proposed by Shepard (1954) (fig. 5). A detailed discussion of the laboratory methods employed is given in Poppe and others (2000b). Because biogenic carbonate shells
The interpretations of sea-floor features (fig. 6), surficial sediment distributions (fig. 7), and sedimentary environments (fig. 8) presented in this report are based on data from the sediment sampling and bottom photograph stations, tonal changes in backscatter, and bathymetry.