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Shaded relief map of Boston Harbor

  MAP OF BACKSCATTER INTENSITY AND SHADED RELIEF OF THE REGION OFFSHORE OF BOSTON, MASSACHUSETTS

     Shaded relief map with pseudo-colored backscatter intensity draped over the topography. 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 a light intensity within a color band, whereas south-facing slopes, being in shadow, show as a 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. Some features are artifacts of data collection and environmental conditions. They include small highs and lows and unnatural looking features or patterns oriented parallel or perpendicular to survey tracklines. The orientation of the tracklines are identified by the faint parallel stripes in the image. White areas are areas of no data. In some areas, the backscatter intensity shows stripes parallel to the survey tracklines. The effect is particularly noticeable in the Stellwagen Bank region (for example see the area around 42o 26.5' N., 70o 19.0' W.). This striping is a result of a critical angle effect, where the reflectance of sound from the sea floor varies with the angle of incidence (Hughes Clarke and others, 1997). This produces high backscatter intensity in the inner part of the swath and a rapid change to reduced intensity at the outer part. Some backscatter intensity is the result of bad weather, for example the single high intensity strip that runs northwest-southeast across the southern part of Stellwagen Bank.

     The image illustrates the complex and wide variety of sedimentary environments in this region of the coastal ocean. The transitions between sediment types is often very sharp. Topographic features observed here were formed for the most part by glacial processes. Ice containing rock debris moved across the region, sculpting its surface and depositing sediment to form basins, knoll, banks and other features. Today, the sea floor is mainly modified by storm currents and waves from the northeast. These currents erode sand and mud from the shallow banks and transport them into the basins. Stellwagen Bank and Jefferys Ledge are shallow banks (20-40 m water depth) covered with sand and gravel. Stellwagen Basin (100 m water depth) is floored with mud. In deeper water to the northeast (water depth 85-140 m), the spaghetti-like pattern on the sea floor are gouges (of order 5 m deep and up to 120 m wide) in the muddy sand caused by icebergs that grounded here at the close of the last period of glaciation. Present and past disposal sites are characterized by high backscatter material and are especially distinct when the background material is fine grained, such as in Stellwagen Basin (yellow arrow indicates Massachusetts Bay Disposal Site (MBDS)). The yellow line is the location of the new ocean outfall that began discharging treated sewage effluent from the Boston metropolitan area into Massachusetts Bay on September 6, 2000. The red line outlines the Stellwagen Bank National Marine Sanctuary.

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