Introduction

This 27 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 1 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, northfacing 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; data coverage begins 6 to 7 km offshore. 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 shoreline was extracted from the National Oceanic and Atmospheric Administration Medium Resolution Digital Vector Shoreline (data downloaded from http://seaserver.nos.noaa.gov/projects/shoreline/shoreline.html, Segment EC80_05, digitized from Chart 13267 (National Ocean Service, 1997)). 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 run northwest-southeast). For example, wrinkle-like features in the northeastern part of the quadrangle, which are about 100 to 150 m long and are oriented northeast-southwest and perpendicular to the ship's track, are a result of heave of the vessel during data collection caused by large surface waves. The northwest-southeast-trending dark lines, especially apparent in the high-backscatter areas of the image, are a result of noisy backscatter return directly beneath the ship. Other northwest-southeast-trending lines, especially noticeable along the shallow southwestern part of the mapped area of the quadrangle (for example, the linear feature extending between 42°16.37' N., 70°40.79' W. and 42°15.63' N., 70°39.80' W.), are a result of marginal data overlap between swaths where survey lines were spaced too far apart.