U.S. Geological Survey
Geologic Investigations Series Map I-2732-C
By Bradford Butman, Laura Hayes, William W. Danforth, and Page C. Valentine
This 40 x 35 inch map shows backscatter intensity of the sea floor draped over shaded relief, with sea floor depth as topographic contours overprinted in white, at a scale of 1:25,000. It is based on multibeam echo-sounder data collected using a Simrad Subsea EM 1000 Multibeam Echo Sounder (95 kHz) during four cruises conducted between the fall of 1994 and the fall of 1998 aboard the vessel Frederick G. Creed. The map is part of a 3-quadrangle map series showing the area offshore of Boston, Mass., that is companion to the Stellwagen Bank National Marine Sanctuary map series (Valentine and others, 2001, 2003a-c; also see location map). Other maps of Quadrangle 2 depict topographic contours (Butman and others, 2003a), and shaded relief and topographic contours (Butman and others, 2003b).
Backscatter intensity is a measure of surficial sediment texture and bottom roughness. Generally, high backscatter intensity is associated with rock or coarse-grained sediment, and low backscatter intensity characterizes finer grained sediments. Direct observations, using bottom photography or video and sampling techniques such as grab sampling or coring, are needed to verify interpretations of the backscatter intensity. In the image shown here, the backscatter intensity is represented by a suite of eight colors ranging from blue, which represents low intensity (fine-grained sediments), to red, which represents high intensity (rock outcrops and coarse-grained sediments). These data are draped over a shaded relief image created by vertically exaggerating the topography four times and then artificially illuminating the relief by a light source positioned 45 degrees above the horizon from an azimuth of 350 degrees. The resulting image displays light and dark intensities within each color band that result from a feature's position with respect to the light source. For example, north-facing slopes, receiving strong illumination, show as light intensity within a color band, whereas south-facing slopes, being in shadow, show as dark intensity within a color band. The shaded relief image accentuates small features that could not be effectively shown by contours alone at this scale. The bathymetric soundings were gridded at 6 m/pixel resolution and smoothed using a 9-cell by 9-cell median filter; contours having a 5-meter interval were generated from the resulting grid. Blank areas in the image represent places where no data exists. Most areas of no data in this backscatter intensity image are smaller than in the shaded relief image (Butman and others, 2003b) because depth measurements at the outer edge of the swath were sometimes removed as bad data; in many of these cases, the backscatter intensity data were still useable, resulting in greater spatial coverage for backscatter intensity. The two narrow strips of data in the southwest corner of the quadrangle were collected along single ship transits to Boston Harbor. Topographic lows are identified by hachured contours (hachures face deeper water).
Some features in the image are artifacts of data collection and environmental conditions. They include small highs and lows and unnatural-looking features, and patterns oriented parallel or perpendicular to survey tracklines (tracklines were run north-south in the eastern and north-central part of the map area, northwest-southeast in the south-central part, and northeast-southwest in the western part). For example, the wrinkle-like features about 100 m long, oriented east-west and perpendicular to the ship's track between 70°39' W. and 70°42' W., are a result of heave of the vessel during data collection caused by large surface waves; the northeast-southwest-trending lines in the vicinity of 42°21.5' N., 70°47.5' W. are a result of data loss in the far range of individual swaths; and small offsets in features in the southwestern part of the study area, most notable along features that have sharp transitions, are a result of errors in synchronization of the navigation and multibeam clocks during the 1994 survey (a diamond-shaped area with corners at 42°19.5' N., 70°48' W.; 42°23' N., 70°43.5' W.; 42°26' N., 70°48' W.; and 42°23' N., 70°52.5' W.). For example, see the northeast-southwest offsets of the northwest-southeast-trending features near 42°21.94' N., 70°47.88' W., and near 42°23.74' N., 70°48.59' W.
|Thumbnail image of map I-2732-C|
Geologic Investigations Series Map I-2732-C [5-MB PDF file]
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