Woods Hole Science Center
Enhanced Sidescan-Sonar Imagery, North-Central Long Island Sound
Sidescan-sonar data were processed by NOAA using the CARIS Sonar Image Processing System (SIPS). Data were multiplexed and filtered, corrected for slant-range distortions, and imported into Chesapeake Technology, Inc., (CTI) SonarWeb to create a mosaic mapped at 1-m/pixel resolution (Marc Moser, NOAA, written commun., 2003). This sidescan-sonar imagery was subsequently improved by using Adobe Photoshop to increase the dynamic range of the data and invert the grayscale and was reprojected from Universal Transverse Mercator onto geographic coordinates. The sidescan-sonar images presented in this report display high backscatter as light tones and low backscatter as dark tones. In general, high backscatter represents coarser grained sediment, while low backscatter represents finer grained sediment. Although sidescan-sonar images from the NOAA surveys were used in prior studies to interpret the sea-floor geology in Long Island Sound (H11043 -- Poppe and others, 2004; H11044 -- McMullen and others, 2005; and H11045 -- Beaulieu and others, 2005), adjacent sidescan-sonar lines in this imagery had not been tonally matched; as a result their interpretation was more complicated and subtle backscatter trends in the surficial geology were partially obscured.
The original XTF sidescan-sonar data files from NOAA surveys H11043, H11044, and H11045 were obtained to reprocess the data and minimize the tonal artifacts. Unfortunately, we were unable both to successfully and efficiently minimize artifacts and to produce a mosaic by using our processing tools (Xsonar and ShowImage; Danforth, 1997) that improved the resolution of backscatter trends representative of the true sea-floor geology. Thus, we enhanced the grayscale range of individual sidescan-sonar lines using Adobe Photoshop. Selected sidescan-sonar lines were stenciled with the Polygon Lasso Tool. The grayscale range of the stenciled line was adjusted to match the grayscale range of adjacent sidescan-sonar lines by using the shadow, midtone, and highlight input sliders in the Levels dialog box. The Spot Healing Brush Tool was also used in several places to eliminate small artifacts, such as wake, within a sonar line.
Enhancing the sidescan-sonar imagery from survey H11043 offshore from Branford, CT, included darkening lines in the north and lightening the southernmost lines to match backscatter tones in adjacent lines (fig. 3). Several segments of dark sonar lines were also lightened and other artifacts of data acquisition were removed. Although there are still artifacts in the sidescan-sonar image that could not be removed without changing the integrity of the image, the overall enhanced imagery has more continuous backscatter tones.
The original sidescan-sonar image of survey H11044 offshore from Millford, CT, included areas of differing backscatter that appear to be related to system settings (possibly gain changes) or the flight position of the tow fish (possibly a slight roll of the tow fish to either port or starboard) and not to differences in the properties of surficial sea-floor sediment (McMullen and others, 2005; fig. 4). To enhance the imagery, the grayscale range of these areas was adjusted, as opposed to modifying individual sidescan-sonar lines. The grayscale range of several isolated sidescan-sonar lines and segments was adjusted individually to match the tones of surrounding lines. The resulting enhanced imagery clearly delineates boundaries of backscatter tones and patterns, such as an area of high backscatter associated with an elongated bathymetric high in the northern part of the survey area (fig. 5).
The original sidescan-sonar image of survey H11045 offshore from Bridgeport, CT, included large areas in which every other line was lighter than surrounding lines, creating a striped appearance (fig. 6). The enhanced image minimized this striping by adjusting grayscale ranges so that they are more consistent with adjacent lines and removed tonal artifacts probably related to system settings in the northern part of the survey. These adjustments made sand waves and trawl marks more apparent (fig. 7).
Enhancing the imagery has minimized and corrected many of the backscatter-tone inconsistencies mostly related to environmental noise and system settings. The enhanced sidescan-sonar imagery will benefit future studies, such as the mapping of habitats and contaminant distributions, that require accurate representation of the true backscatter trends in these areas.