U.S. Geological Survey Open-File Report 2010–1150
Sea-Floor Geology and Topography Offshore in Eastern Long Island Sound
During 2008, the National Oceanic and Atmospheric Administration (NOAA) Ship Thomas Jefferson acquired multibeam bathymetric data from the study area covering approximately 133.7–km² (figs. 2, 3). Multibeam echosounder (MBES) data were collected with a hull-mounted RESON dual frequency (200–kHz/400–kHz) 7125 system. Original horizontal resolution of the multibeam bathymetric data was 0.5 meters (m) for water depths less than 20 m and 2 m for the deeper parts of the survey area. The bathymetric data were acquired in extended Triton format (XTF), recorded digitally through an ISIS data acquisition system, and processed by NOAA using CARIS HIPS (Hydrographic Image Processing System) software for quality control, to incorporate sound velocity and tidal corrections, and combined into a 2–m grid to produce the final digital terrain model. A more detailed description of the acquisition parameters and processing steps performed by NOAA and the USGS to create the datasets presented in this report is included in the metadata files. These files can be accessed through the Data Catalog section of this report. Vertically exaggerated (5x), hill-shaded (illuminated from 0° north at an angle of 45°) imagery was created using CARIS HIPS software. The raster grids allow users to query and manipulate the bathymetric data; the color, sun-illuminated imagery (geoTIFFs) accentuate small topographic features that cannot be shown by contours at this scale.
Horizontal positioning was by differential global positioning systems (DGPS); Hypack MAX was used for acquisition-line navigation. Sound-velocity corrections were derived with frequent conductivity-temperature-depth (CTD) profiles using a Sea-Bird Electronics, Inc. SEACAT CTD; casts were made with a Brooke Ocean Technology Moving Vessel Profiler 100 (fig. 4). Tidal corrections were calculated from data acquired by the National Water Level Observation Network stations at New London and New Haven, Conn. For this study, bathymetric soundings were referenced to mean lower low water (MLLW). Detailed descriptions of the MBES acquisition and processing can be found in the descriptive report (National Oceanic and Atmospheric Administration, 2008).
Sampling and Photography
The sediment sampling and bottom photography were conducted during May 2010 aboard the Environmental Protection Agency (EPA) Ocean Survey Vessel (OSV) Bold during USGS cruise 2010-015-FA (fig. 5). Bottom photography was collected at 28 stations with a modified Van Veen grab sampler equipped with still- and video-camera systems; surficial sediment samples (0-2 cm below the sediment-water interface) were collected at 19 of the stations (figs. 6, 7). The photographic data were used to appraise intrastation bottom variability, faunal communities, and sedimentary structures (indicative of geological and biological processes) and to observe boulder fields, where samples could not be collected. A gallery of images collected as part of this project is provided in the Bottom Photography section; locations of the bottom photographs can be accessed through the Data Catalog section.
In the laboratory, the sediment samples were disaggregated and wet sieved to separate the coarse and fine fractions. The fine fraction (less than 62 microns) was analyzed by Coulter Counter; the coarse fraction was analyzed by sieving; and the data were corrected for salt content. Sediment descriptions are based on the nomenclature proposed by Wentworth (1922; fig. 8) and the size classifications proposed by Shepard (1954; fig. 9). A detailed discussion of the laboratory methods employed is given in Poppe and others (2005). Because biogenic carbonate shells commonly form in situ, they usually are not considered to be sedimentologically representative of the depositional environment. Therefore, gravel-sized bivalve shells and other biogenic carbonate debris were ignored. The grain-size analysis data can be accessed through the Data Catalog section of this report and in the Sediment Distribution section.
To facilitate interpretations of the distributions of surficial sediment and sedimentary environments, these data were supplemented by sediment data from compilations of earlier studies (Poppe and others, 1998a; Poppe and others, 2000) and unpublished datasets from the National Geophysical Data Center. The interpretations of sea-floor features, surficial-sediment distributions, and sedimentary environments presented herein are based on data from the sediment-sampling and bottom-photography stations and on the bathymetry. For the purposes of this paper, bedforms are defined by morphology and amplitude. Sand waves are higher than 1 m; megaripples are 0.2 to 1 m high; ripples are less than 0.2 m high (Ashley, 1990). The bathymetric grids and imagery released in this report should not be used for navigation.