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

Seafloor bathymetry and acoustic backscatter data provides important information on water depth and on the character of features on the seafloor to improve our understanding of benthic habitat. 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 acoustic backscatter files in this data set are in GeoTIFF image format and cover the 27 Survey Blocks assigned to SoMAS plus a site in deeper water (Barchans). The backscatter images were created using the program QPS FMGT using multibeam track lines that had been edited and processed by CARIS HIPS and SIPS. The program corrects the sonar data to remove cross-track variations in signal levels. Two different multibeam systems were used to collect the data reported for this study and the data values from the two systems were compared in areas where data sets overlapped. The results from the EM3000D survey were offset and scaled so that the backscatter distributions in the overlap areas were similar. 8-bit multibeam backscatter grids were exported as GeoTIFF images. For all the images reported, higher backscatter values are shown in a lighter color and lower backscatter are shown in a darker color. The 2017-2018 SoMAS surveys have few gaps except for 13_JJ (a compilation of transit tracks that overlap the grid 13_DM) and Barchans (lines that image barchan dunes that are migrating across the seafloor). The data is not to be used for navigation. The field surveys were performed in December 2017 to March 2018 (file names DM) and June-July 2018 (file names JJ). The data was collected on the SoMAS Research Vessel Donald W Pritchard using an EM3000D echo-sounder for the surveys from December, 2017 through March, 2018, and using an EM2040cD (dual-head) echo-sounderfor data collected in June and July, 2018. 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. Bathymetry data was processed using CARIS HIPS and SIPS (version 10.4). Data points were edited to remove spurious bed elevations and sound-velocity refraction corrections were applied. Data values were gridded at 1 m horizontal resolution using the Swath Angle gridding algorithm into a series of 26 base maps (resolution is 2 m for the 2018 Barchans file). The 32-bit GeoTIFF gridded base maps are projected in NAD_1983_2011_UTM_Zone_18N and seafloor elevations are reported in meters relative to NAVD-88. Vertical elevations are accurate to about 0.1 to 0.2 m. Elevations may be less accurate at the outer edges of the swath where refraction artifacts can occur. 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).