High-resolution geologic mapping of the inner continental shelf: Boston Harbor and approaches, Massachusetts

U.S. Geological Survey Open-File Report 2006-1008 Map Sheet 5 : Shaded-relief topography, slope, backscatter and descriptive map units of the seafloor. Introduction A series of five map sheets shows the sea floor topography and geology of Boston Harbor and Approaches. Sheets 1-4 are at a scale of 1:25,000; Sheet 5 is at a scale of 1:60,000. Sheet 1 shows sea floor topography in shaded-relief, colored by water depth. Sheet 2 shows shaded-relief topography in grayscale with data from highresolution multibeam surveys superimposed and colored by water depth. Sheet 3 shows sidescan-sonar backscatter intensity in grayscale with high backscatter displayed as light tones and low backscatter as dark tones within the imagery. Sheet 4 shows shaded-relief topography colored by backscatter intensity with red tones representing high backscatter and blue tones representing low backscatter. Sheet 5 shows shaded relief, backscatter intensity and sample locations, sea floor slope, and an interpretive map of sea floor environments. The location of sampling sites referenced in the text below can be found on the sediment texture map on sheet 5. These maps are produced as part of a cooperative effort by the U.S. Geological Survey (USGS), the Massachusetts Office of Coastal Zone Management (CZM) and the National Oceanic and Atmospheric Administration (NOAA) to systematically map the sea floor geology offshore of Massachusetts and provide geologic framework information for resource management, scientific research, industry and the public. These map sheets are part of an USGS Open File Report (Ackerman and others, 2006) describing data collection, processing, and analysis of geophysical and sample data. The report (on DVD-ROM) also includes all of the data in GIS format and as part of an ESRI ArcMap project. Data and Methods The bathymetric and sidescan-sonar data used to generate these maps were collected as part of hydrographic surveys of the navigable areas within Boston Harbor and its approaches carried out by NOAA in 2000 and 2001 (surveys H10990, H01991, H10992, and H10994) by the NOAA ship Whiting and its launches. These cruises acquired sidescan-sonar data over an area of 155 km2 and single-beam bathymetric data over an area of approximately 170 km2. In addition, multibeam echosounder data were acquired over 65 km2 (approximately 37% of the survey area). The multibeam echosounder data were collected in navigation channels and at approximately 450 site-specific locations that were identified as potential hazards to navigation. The most dense multibeam echosounder coverage is within the navigation channels (President Roads, North Channel, South Channel, and Nantasket Roads), Boston Inner Harbor, and east of the outer Boston Harbor Islands. The multibeam echosounder data were reduced to Mean Lower-Low Water (MLLW) using data from tide station 8443970, Boston, MA. Bottom photographs, video, and grab samples were collected by CZM and the USGS in 2004 in order to guide the interpretation of the geophysical data. See Ackerman and others (2006) for a detailed description of the field program and data processing. A composite bathymetry grid was created from single-beam and multibeam echosounder data and was used to create the shaded-relief image shown on sheet 1. Multibeam echosounder data were exported at a 2-meter grid interval for each of the four survey areas and single-beam echosounder data from surveys H10990, H10992, and H10994 were exported at a 5-meter grid interval. The single-beam echosounder data from survey H01991 were provided by NOAA as a separate xyz file. Generic Mapping Tools (GMT; http://gmt.soest.hawaii.edu/) was used to create an interpolated bathymetric grid using the “surface” routine with a grid cell size of 30 m and a tension parameter of 0.2. The hydrographic surveys were designed for target identification and therefore NOAA collects overlapping sidescan-sonar data to ensure complete coverage of the sea floor. An Edgetech model 272-T (100 kHz) and a Klein T-5500 (455 kHz) sonar were used for the sidescan-sonar surveys. System and vessel configurations varied between and within individual surveys. The sidescan-sonar data were processed to correct for radiometric and geometric distortions inherent in sonar data. The sidescan sonar data were mosaicked using PCI Geomatics and exported as georeferenced TIFF image files at 1 meter/pixel resolution. Tone-matching was applied in order to correct for the variations in dynamic range of the sidescan-sonar data collected within individual surveys. Map Sheets The shaded-relief bathymetric maps (sheets 1 and 2) were created by vertically exaggerating the sea floor topography ten times and artificially illuminating the relief by a light source positioned 45° above the horizon from an azimuth of 0° (due north). Topographic features, such as channel boundaries or submarine ridges, are enhanced by strong illumination on north-facing slopes and by shadows cast on south-facing slopes. The shadedrelief image accentuates small features that could not be effectively shown by contours alone at this scale. The water depth, using a color scale from red (shallow) to blue (deep), is superimposed on the sun-illuminated topography. Smoothed topographic contours at 5-m intervals are shown. The shaded relief imagery is derived from 2-m multibeam echosounder data merged with the composite 30-m bathymetry described above. The locations of the multibeam echosounder data are shown in sheet 2. Gridded single-beam echosounder data display smoothed topography due to wide-line spacing and interpolation. Multibeam echosounder data reveal more detailed sea floor topography. Boundaries between datasets are delineated by sharp transitions from smooth to detailed topography (sheet 2). The channels northeast of George’s Island (multibeam echosounder) and Gallop’s Island (single-beam echosounder) exemplify the rough and smooth sea floor, respectively. Sheets 3 and 4 display sidescan-sonar backscatter intensity. Backscatter intensity is a relative measure of the reflectivity of the material on the sea floor. The intensity of acoustic backscatter is represented by 256 shades of gray, ranging from lighter shades (representing high backscatter values) to darker shades (representing low backscatter values). Shadows appearing in the sonar imagery can be useful in the identification of features and objects on the sea floor. Direct sampling of the sea floor sediments, bottom photography and video are needed to accurately interpret sidescan-sonar backscatter intensity. In general high backscatter corresponds to areas of coarse sand, gravel, cobbles, boulders and rock within Massachusetts Bay. Moderate backscatter corresponds to sand or muddy sand. Low backscatter is associated with sandy mud or mud. Some artifacts are present within the data. These include small highs and lows, and unnatural-looking patterns oriented parallel or perpendicular to survey tracklines. Artifacts may be due to environmental conditions or result from data collection and processing. Tracklines were generally run parallel to the major channels in Boston Harbor and around the Harbor Islands. They were predominantly north-south in the harbor approaches. Slight mismatches in the grayscale tones in the sidescan-sonar (sheet 3 and 4) are also artifacts of data collection and processing. These occur where acquisition parameters in one swath are different from the adjacent swath, making it difficult to match the grayscale tone along the entire length of those lines. Areas that could not be surveyed because they were too shallow (typically less than a few meters deep) are shown in light gray. Additional data are included on all sheets to show the regional topography in areas adjacent to the survey. To the east, offshore of the new survey area, sea floor topography in shaded-relief view is shown at a resolution of 6 m/pixel (Butman and others, 2004). Inshore of the survey area and to the east, sea floor topography generated from the NOAA/NOS estuarine bathymetry database (NOAA, 1998) is shown in shaded-relief view at a resolution of 30 m/pixel. Onshore topography was extracted from Massachusetts Geographic Information System (MassGIS, 2005) displayed at a resolution of 20 m/pixel. Features This study encompasses Boston Inner Harbor, Boston Outer Harbor, the northern approaches to Boston Harbor (Broad Sound; north of the Harbor Islands to Nahant), and the southern approaches to Boston Harbor (outer Harbor Islands and nearshore east of Nantasket Beach). The bathymetry and sidescan-sonar data show natural features and sea floor modification from anthropogenic activities. Dredging and other anthropogenic activities are generally focused in the shipping channels. Evidence of dredging is visible within the imagery as straight-sided channels, unnatural-appearing roughness and/or linear features on the sea floor that are typically oriented parallel to a channel. Disposal of dredged material is clearly displayed within the multibeam echosounder data as rounded mounds; often occurring in a straight line. Some have a central high and a surrounding moat thought to be created as the material was deposited on the sea floor. The mounds sometimes are identified in the sidescansonar by high backscatter intensity, but are not always resolved. Other anthropogenic features on the sea floor include wrecks of small boats and barges, pipelines, and piles of debris. Almost all of the Inner Harbor from Castle Island to Long Wharf was mapped by multibeam echosounder. In the Outer Harbor and the Harbor Approaches, the 2-m resolution multibeam echosounder data are displayed with the 30-m resolution single-beam echosounder data; interpretation of features and their spatial extent is limited by these mixed observations. The seafloor landscape varies from gently sloping sub-tidal flats to areas of rugged elevation exhibiting as much as 7 m of local relief (sheet 1). The acoustic