LIS Phase II seafloor mapping: Sidescan images (GeoTIFF format), eastern Long Island Sound (2017-18)

Seafloor sidescan data provides important information on water depth and on the character of features on the seafloor to improve our understanding of benthic habitat. Sidescan and multibeam sonar data was collected in 2017-2018 during field surveys by SoMAS, Stony Brook University, in eastern Long Island Sound as part of the Phase 2 of the Long Island Sound Cable Fund Benthic Habitat Mapping Project. The sidescan files in this data set are in GeoTIFF image format and cover 10 of the 27 Survey Blocks assigned to SoMAS. The values of sidescan acoustic backscatter reported in these images depends somewhat on the equipment used and the computer processing that is done in creating the backscatter images. Acoustic backscatter data recorded depends on the azimuth of the sonar track so the final images tried to keep acoustic data collected in a similar orientation. The sidescan sonar data was collected with a pole-mounted system and there was some small across-track amplitude variation due to the instrument roll and there are some residual along-track amplitude variations related to the sonar beam pattern. Although no quantitative analysis was done, the sidescan backscatter data reported during this project is qualitatively similar to that reported by NOAA in NCEI Dataset 0167531 when the image DN values are appropriately stretched. The data is not to be used for navigation. The sidescan field surveys were performed in December 2017 to March 2018 (file names DM). The data was collected on the SoMAS Research Vessel Donald W Pritchard using a pole-mounted EdgeTech 272-TD system which was operated at 100 kHz. All field programs used an Applanix motion sensor system for vessel motion and heading, GPS positions corrected using data from the ACORN RTN (UConn) system, vessel elevations determined using GPS RTN elevations supplemented by scaled and offset land-based water-level measurements, and sound velocity profiles collected using an AML BASEX probe. The sidescan data was recorded in xtf format and processed using programs in the SwathEd program suite developed at the University of New Brunswick (UNB) with modifications done at Stony Brook University. The sonar position, altitude and orientation (heading, roll and pitch) were merged with the sonar data using values recorded by the multimbeam system operating at the same time. The xtf files where re-written with updated sonar navigation, altitude, heading, roll and pitch and those files have been archived. A slant-range correction was made assuming a flat sea floor and the sonar values were plotted on a 1 m grid but only the data closest to a ship track was plotted. The ship tracks used in the mosaic were selected to have consistent orientations and to avoid crossing lines. A correction was applied to the sidescan sonar data based on fish height and slant range to correct signal levels for sonar beam pattern, attenuation and angle-of-incidence but some along-track signal anomalies exist in the map grids. The final maps were made using the corrected signal levels. No correction was made for instrument roll and there is some striping in the sonar mosaics because of the sonar roll. The 8-bit sonar mosaics were imported to ArcMap as tif files and exported as georeferenced GeoTIFFs. Horizontal positions are accurate to about 1 m. The data files show the conditions at the time of the survey in 2017-2018. The full suite of SoMAS mapping comprises four components: multibeam bathymetry (26 maps), bathymetry hillshade (26 maps), multibeam backscatter (26 maps), and sidescan sonar backscatter (9 maps), for a total of 87 maps. The multibeam bathymetry and backscatter data were integrated by NOAA into project-wide maps that can be found at NCEI (Dataset 0167531 version 2.2, https://doi.org/10.25921/tgp1-w632). Funding for this study was provided by the Long Island Sound Seafloor Mapping Fund administered cooperatively by the EPA Long Island Sound Study and the Connecticut Department of Energy and Environmental Protection (DEEP).