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U.S. Geological Survey Open-File Report 2013–1060

Sea-Floor Geology and Topography Offshore in Northeastern Long Island Sound


Methods

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Click on figures for larger images
Thumbnail image of figure 3 and link to larger figure. Photograph of the Thomas Jefferson.
Figure 3. Port-side view of the National Oceanic and Atmospheric Administration ship Thomas Jefferson at sea.
Thumbnail image of figure 4 and link to larger figure. Photograph of the instrument that collected multibeam bathymetry in the study area.
Figure 4. The RESON 7125 multibeam echosounder hull-mounted to the National Oceanic and Atmospheric Administration ship Thomas Jefferson.
Thumbnail image of figure 5 and link to larger figure. A photograph of the CTD.
Figure 5. Brooke Ocean Technology moving vessel profiler with a Sea-Bird Electronics, Inc. conductivity-temperature-depth profiler used to correct sound velocities for the multibeam data collected aboard the National Oceanic and Atmospheric Administration ship Thomas Jefferson.
Thumbnail image of figure 6 and link to larger figure. Photograph of the Ocean Survey Vessel Bold.
Figure 6. A port-side view of the U.S. Environmental Protection Agency Ocean Survey Vessel Bold that was used to collect bottom photography and sediment samples in the study area offshore in eastern Long Island Sound.
Thumbnail image of figure 7 and link to larger figure. Map of sample locations in the study area.
Figure 7. Locations of stations at which bottom samples and photographs were taken during U.S. Geological Survey cruises 2010-015-FA of Ocean Survey Vessel Bold and 2010-010-FA of Research Vessel Rafael to verify bathymetric and backscatter data.
Thumbnail image of figure 8 and link to larger figure. Photograph of a mid-sized SEABOSS.
Figure 8. The mid-sized Seabed Observation and Sampling System, a modified van Veen grab sampler equipped with still and video photographic systems designed and built by the U.S. Geological Survey.
Thumbnail image of figure 9 and link to larger figure. Chart showing grain-size relationships.
Figure 9. Correlation chart showing the relationships among phi sizes, millimeter diameters, size classifications, and American Society for Testing and Materials and Tyler sieve sizes.
Thumbnail image of figure 10 and link to larger figure. A chart showing grain-size classification.
Figure 10. Sediment-classification scheme from Shepard, as modified by Schlee and Poppe and others.
Multibeam Bathymetry

The NOAA ship Thomas Jefferson (fig. 3) acquired multibeam bathymetric data from the study area over an approximately 52.9-km² area during 2009. The multibeam echosounder (MBES) data were collected with a RESON single frequency (400 kilohertz) 7125 system hull-mounted to the ship (fig. 4). Horizontal positioning was by a Trimble DSM212L differential global positioning system (DGPS); HYPACK 2009 software was used for acquisition-line navigation. Sound-velocity corrections were derived with frequent conductivity-temperature-depth (CTD) profiles using a SeaCat SBE19 CTD profiler and a Brooke Ocean Technology moving vessel profiler (MVP) with an Applied Microsystems smart sound velocity and pressure sensor (fig. 5). Tidal-zone corrections were calculated from data acquired by the National Water Level Observation Network stations at New London (8461490) and New Haven (8465705), Conn. The vertical datum is mean lower low water. Detailed descriptions of the MBES acquisition and processing can be found in the Descriptive Report (National Oceanic and Atmospheric Administration, 2009a) and in the Data Acquisition and Processing Report (National Oceanic and Atmospheric Administration, 2009b).

The bathymetric datasets were acquired and digitally logged with the HYSWEEP package of the HYPACK software and then processed by NOAA using CARIS HIPS hydrographic data processing software. Sound velocity corrections and a zone tidal model were applied to filtered sounding data in CARIS. GeoTIFFs and grids were created from the finalized bathymetric data using CARIS, Fledermaus, and ArcGIS software. The GeoTIFF imagery is vertically exaggerated fivefold and hill-shaded (sun-illuminated) from the northeast at an angle of 45 degrees (°) above the horizon. 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 in the Data Catalog section of this report.

Sampling and Photography

Most of the sediment sampling and bottom photography were conducted aboard the U.S. Environmental Protection Agency Ocean Survey Vessel (OSV) Bold (fig. 6) during cruise 2010-015-FA in May 2010; bottom video was collected at one station aboard the USGS Research Vessel (R/V) Rafael during cruise 2010-010-FA in April 2010. Surficial-sediment samples (0-2 cm below the sediment-water interface) and bottom photography were collected at 21 stations (fig. 7) with a modified van Veen grab sampler equipped with still- and video-photographic systems (fig. 8). 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 and bedrock outcrops where samples could not be collected. A gallery of images collected as part of this project is provided in the Bottom Photography section of this report; 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 micrometers) 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. 9) and the size classifications proposed by Shepard (1954; fig. 10). A detailed discussion of the laboratory methods employed is given in Poppe and others (2005). Because biogenic carbonate shells commonly form in situ but are not hydraulically equivalent to the host sediments, they usually are not considered to be sedimentologically representative of the depositional environment. Therefore, gravel-sized bivalve shells and other biogenic carbonate debris were manually removed and not included in the sediment grain-size data. The grain-size analysis data can be accessed through the Data Catalog section of this report; a data dictionary is available in the Sediment Distribution section.

Interpretations of Geological Features and Sedimentary Environments

To facilitate interpretations of geomorphology and sedimentary environments, the data were supplemented by sediment data from compilations of earlier studies (Poppe and others, 1998a, 2000) and unpublished datasets from the NOAA 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. Interpretations of sedimentary environments are based on the scheme described by Knebel and Poppe (2000); interpretations of sea-floor features are limited by the resolution of the acoustic data and by the spacing of available verification data, but include: rocky seabed, sand waves, megaripples, gravelly pavement, and dredge spoils. The interpretive datasets were digitized at 1:5,000; however, recommended scale for use is no better than 1:24,000. For the purposes of this report, bedforms are defined by morphology and amplitude—sand waves (that is, subaqueous dunes) are higher than 1 m; megaripples are 0.2 to 1 m high; ripples are less than 0.2 m high (Ashley, 1990). Detailed descriptions of the processing steps performed by the USGS to create the interpretive datasets presented herein and definitions of their respective attributes are included in the metadata files in the Data Catalog section of this report. The bathymetric grids and imagery released in this report should not be used for navigation.

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