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Woods Hole Coastal and Marine Science Center

Geologic Interpretation and Multibeam Bathymetry of the Sea Floor in Southeastern Long Island Sound


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In 2004, a 95-kHz Simrad EM1002 multibeam echosounder system mounted on the hull of NOAA Ship Thomas Jefferson was used to acquire bathymetry over 754 km of survey lines from within the study area (fig. 2). This system measures two-way sound travel time of 111 individually formed 2° beams across a 150° swath. Horizontal resolution of the data varies with water depth, but averages about 2 m. Vertical resolution is about 0.5% of the water depth.

The multibeam 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, using a Trimble DSM212L receiver; Hypack MAX was used for acquisition line navigation. Sound velocity corrections were derived using frequent CTD (conductivity-temperature-depth) profiles (fig. 3). Typically, a CTD cast was conducted for every four to six hours of multibeam acquisition. Tidal zone corrections were calculated from data acquired at primary and secondary tide-gauge stations using acoustic stilling-well gauges. Vertical datum is mean lower low water.

The multibeam data were acquired in XTF (extended Triton data format), recorded digitally through an ISIS data acquisition system, and processed using CARIS HIPS (Hydrographic Image Processing System) software for quality control, beam editing, to incorporate sound velocity and tidal corrections, and to produce a continuous digital terrain model (DTM) shown in figure 4.

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.

Diagonal 150-m wide strips that criss cross some of the DTMs are artifacts of "tie lines", which are accentuated by the hill shading (i.e. sun illumination), and result from verification lines run by the ship to check the bathymetric data.

A dual-frequency Knudsen 320BR 2-7 kHz Chirp and 200-kHz echosounder system collected high-resolution seismic-reflection subbottom profile data. This system used hull-mounted transducers, and recorded in SEG-Y digital format. Boomer data from two previous State of Connecticut-USGS seismic cruises (Lewis and Needell, 1987; Poppe and others, 2002a) aided in the geologic interpretation of the multibeam data.

The sediment descriptions are based on the nomenclature proposed by Wentworth (1922), the inclusive graphics statistical method of Folk (1974) and the size classifications proposed by Shepard (1954). A detailed discussion of the laboratory methods employed are 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 excluded.

The bathymetric data released in this report should not be used for navigation.

Click on figures for larger images.

Figure 2. NOAA Ship Thomas Jefferson.
Figure 2. NOAA Ship Thomas Jefferson.

Figure 3. CTD profiler.
Figure 3. CTD profiler.

Figure 4. Digital terrain model (DTM) from survey H11255.
Figure 4. Digital terrain model (DTM) from survey H11255.

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