Woods Hole Science Center
Sea-Floor Character and Sedimentary Processes of Great Round Shoal Channel, Offshore Massachusetts
Two 29-foot launches deployed from the NOAA Ship THOMAS JEFFERSON were used to acquire multibeam bathymetric data from the study area over an approximately 33.9 km² area during 2004 (figs. 8, 9, and 10). The bathymetric data were collected with hull-mounted 455-kHz Reson 8125 and 240-kHz Reson 8101 systems. These systems measure two-way sound travel time across a 150° swath with each swath consisting of 111 individually formed 2° beams (figs. 11 and 12). The sidescan-sonar data were acquired by the THOMAS JEFFERSON with a hull-mounted Klein 5000 system operating at 100 kHz (fig. 13) and processed by NOAA using the Caris Sonar Image Processing System. The sidescan-sonar data, which were collected over an approximately 17.1 km² area along roughly 106 km of survey lines, were only acquired over the eastern part of the survey area due to time and water-depth limitations. Original horizontal resolution of the multibeam bathymetric data varied with water depth but ranged from 0.5 to 1.0 m. However, these base surfaces were combined and gridded to 3 m to eliminate data gaps. Horizontal resolution of the sidescan-sonar data is 1 m. Vertical resolution is about 0.5% of the water depth.
The survey lines were generally run parallel to the bathymetry contours at a line spacing three to five times the water depth. Navigation was by differential GPS-assisted inertial navigation systems; Hypack MAX was used for acquisition line navigation. Sound velocity corrections were derived using frequent SEACAT CTD (conductivity-temperature-depth) profiles (fig. 14). Typically, a CTD cast was conducted every four to six hours of multibeam acquisition. Tidal zone corrections were calculated from data acquired at the Nantucket tidal guage. Vertical datum is mean lower low water.
The multibeam and sidescan-sonar data were acquired in XTF (extended Triton data format), recorded digitally through an ISIS data acquisition system, and processed using CARIS SIPS/HIPS (Sidescan Image Processing System / Hydrographic Image Processing System) software for quality control, to incorporate sound velocity and tidal corrections, and to produce the continuous digital terrain model (DTM) and sidescan-sonar mosaic.
A hill-shaded surface was generated from the interpolated grids using ArcGIS 9 (Arc Catalog) with illumination from 0° and from 45° above the horizon. Although choosing a direction parallel to most of the ship's tracks (270°) would minimize artifacts, a direction perpendicular to the ship's tracks was chosen to accentuate geologic features. The geotiff imagery was created with the ArcView Image Conversion Georeferencing extension grid2image and a 10x vertical exaggeration was applied. Detailed descriptions of the processing performed to create the datasets presented in this report are included in the metadata files. These files can be accessed through the Data Catalog section of the report.
Analog high-resolution seismic-reflection subbottom-profile data from a previous USGS cruise (O'Hara and Oldale, 1987) aided in the geologic interpretation of the multibeam and sidescan-sonar data. These data were obtained with a surface-towed EG&G Unit Pulse Boomer having an energy output of 300 joules at frequencies ranging from 400 Hz to 8 kHz.
To verify the acoustic data, surficial sediments (0-2 cm below the sediment-water interface) and (or) bottom photography were collected at 36 stations (fig. 15) during June 2006 aboard the Research Vessel RAFAEL (fig. 16). The samples and photography were collected with a modified Van Veen grab sampler equipped with still- and video-camera systems (fig. 17). The photographic data were used to appraise intra-station 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 Photographs 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. 18), the inclusive graphics statistical method of Folk (1974) and the size classifications proposed by Shepard (1954; fig. 19). 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 Sediment Data 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, 2003) 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, on tonal changes in backscatter on the imagery, and on the bathymetry. For the purposes of this paper, bedforms are defined by morphology and amplitude. Sand waves are >1 m high; megaripples are 0.2-1 m high; ripples are < 0.2 m. The bathymetric grids and imagery released in this report should not be used for navigation.
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Title | Introduction | Setting | Methods | Overview | Discussion | Interpretation | Data Catalog | Acknowledgments | References | Figures | Bottom Photographs | Sediment Data | Contacts