NE Bathymetry and Backscatter Compilation

Western Gulf of Maine, Southern New England and Long Island

CONTACTS: Paul Johnson and Larry Ward

DISCLAIMER: All information provided here is for displaying seafloor properties and cannot be used for navigation purposes.

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Overview

The U.S. northeast (NE) continental shelf seafloor from Maine to Long Island was shaped by interactions of riverine sediment inputs, glaciations, and sea-level fluctuations. To the north, the Western Gulf of Maine (WGOM) is characterized by extensive bedrock outcrops, marine modified glacial deposits, numerous bathymetric highs (e.g., Jeffreys Ledge, Stellwagen Bank), and deep ocean basins (e.g., Jeffreys Basin, Wilkinson Basin). Further south from Cape Cod to Long Island, the shelf is composed of wide sandy plains and marine modified glacial deposits. Here, the bottom can change rapidly due to storms and other high energy events. As a result, the seafloor is extremely complex and can vary significantly over short distances or have extensive homogeneous regions. Fortunately, there have been a number of high-resolution multibeam echosounder (MBES) surveys in the WGOM off southern Maine, New Hampshire, and Massachusetts that provide excellent bathymetric coverage of the seafloor. The same is true for much of Long Island Sound.

The “Northeast Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England and Long Island“ (hereafter referred to as the “Compilation”) brings together the MBES surveys for the NE with the primary goal of presenting a synthesis of all of the high-resolution bathymetry in a single gridded surface and presenting backscatter where available and of good quality (Figure 1). Also, a lower resolution regional synthesis is presented. The high-resolution bathymetry synthesis primarily consists of MBES surveys gridded at 4 m. The regional bathymetry surface consists of single beam echosounder (SBES) and MBES surveys gridded at 16 m. The backscatter includes compilations for the inner shelf off New Hampshire (NH) by the University of New Hampshire (UNH) Center for Coastal and Ocean Mapping and Joint Hydrographic Center (CCOM/JHC) and for the Massachusetts coast north of Cape Cod by the United States Geological Survey (USGS). In addition, there are a number of individual MBES surveys presented.

The “Compilation” is a major expansion of the “Western Gulf of Maine Bathymetry and Backscatter Synthesis” published by CCOM/JHC in 2016 (Ward et al., 2016). More details of the “Northeast Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England and Long Island” can be found in Ward et al. (2021).

Satellite image of New England coast with blocks of color gradients showing ocean depth.
Figure 1. High-resolution bathymetry footprints displayed over the regional bathymetry map (blue ramp) from the Northeast Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England and Long Island.

Bathymetry and Backscatter Sources

One hundred and ninety-six surveys conducted by multiple agencies were used to construct the bathymetry compilations. The major agencies and groups include: National Ocean and Atmospheric Administration (NOAA) National Ocean Survey (NOS); United States Geological Survey (USGS); UNH CCOM/JHC; Gulf of Maine Mapping Initiative (GOMMI); United States Army Corps of Engineers (USACE); Maine Coastal Mapping Initiative; and private organizations (e.g., Science Applications International Corporation, Inc., SAIC). The survey source, original gridding, and other relevant information are included in the metadata for each survey which can be accessed directly from the gridded surfaces by displaying the “Footprints” and moving the cursor over the survey area.

High Resolution Bathymetry Map

The high-resolution bathymetry synthesis covers two regions: the Western Gulf of Maine (WGOM) and Southern New England and Long Island. The Western Gulf of Maine (WGOM) map extends from southern Maine to Massachusetts Bay (Figure 2). The high-resolution bathymetry synthesis was not extended further north into Maine due to the lack of accessible MBES surveys. However, relatively good coverage was obtained from southern Maine to Cape Cod and seaward to Jeffreys Ledge. The Southern New England and Long Island bathymetry synthesis extends from Cape Cod south to New York Harbor and includes the inner shelf off Long Island and Long Island Sound  (Figure 3). The MBES coverage in Long Island Sound is relatively complete. However, the high-resolution MBES bathymetry coverage along the Atlantic Coast from south of Cape Cod to New York is sparse leaving wide gaps in coverage. The high-resolution bathymetry syntheses are primarily composed of MBES surveys enhanced by lidar in very shallow water.

Satellite image of New England coast with blocks of color gradients showing ocean depth.
Figure 2. High-resolution bathymetry synthesis of the Western Gulf of Maine.
 
Satellite image of New England coast with blocks of color gradients showing ocean depth.
Figure 3. High-resolution bathymetry coverage of Southern New England and Long Island.
 

Regional Bathymetry Map

The regional bathymetry compilation extends from northern Maine to New York City and extends over much of the Gulf of Maine, southern New England, and Long Island Sound (Figure 1). The regional bathymetry map was built using the same procedure and bathymetry described previously but also includes older surveys of a much lower resolution or existing regional syntheses. This included the Global Multi-Resolution Topography (GMRT) Synthesis that is maintained as a multi-resolution gridded global Digital Elevation Model (DEM) that includes cleaned, processed ship-based multibeam sonar data at their full spatial resolution (~100 m in the deep sea) (https://www.gmrt.org/about). Global Multi-Resolution Topography (GMRT) grids were combined with high-resolution multibeam data available from CCOM and NOS and gridded at 16 m.

JHC/CCOM High Resolution Bathymetry and Backscatter Surveys

The UNH CCOM/JHC Hydrographic Field Course MBES surveys primarily conducted on the continental shelf off NH feature very high-resolution MBES bathymetry with co-registered backscatter. Backscatter or reflectivity is the strength of the acoustic signal that returns to the transponder and is strongly affected by complex interactions with the seafloor properties such as bottom sediment texture, roughness, or biota. Although MBES backscatter can be problematic due to calibration issues and differences in instrumentation used in surveys (e.g., frequency), it often provides valuable information concerning the seafloor bottom sediment or texture. The bathymetry from the Hydrographic Field Course is gridded between 0.5 to 2.0 m. The backscatter is gridded at the same or finer resolution. The reflectivity mosaics use decibel units allowing quantitative analysis. The Field Course surveys are presented separately and include the bathymetry and the backscatter (Figure 4).

Satellite image of Long Island with blocks of gradient colors depicting ocean depth.
Figure 4. UNH JHC/CCOM Hydrographic Field Course MBES backscatter surveys from the NH shelf.


Multibeam Echosounder Backscatter Mosaics

Two grayscale backscatter mosaics incorporating multiple backscatter surveys are included in the Compilation that cover the inner shelf off New Hampshire and Massachusetts Bay and are referred to here as the USGS Backscatter Composite and the JHC/CCOM WGOM Backscatter Synthesis. The USGS Composite includes MBES and side scan sonar surveys gridded at 10 m and covers the continental shelf from the NH border to Massachusetts Bay and includes parts of Jeffreys Ledge and Stellwagen Bank. Details and metadata for the USGS synthesis are given in Pendleton et al. (2013).

In order to develop a composite of MBES backscatter for the WGOM, a subset of available surveys was assembled that were available from NOS and UNH JHC/CCOM that covered the inner shelf off New Hampshire (Figure 5). The MBES backscatter synthesis was processed using QPS Fledermaus FMGT, then mosaicked in ArcGIS. Due to the varying frequencies, different systems, and dynamic range of relative backscatter intensity values from combining multiple backscatter surveys, the range of pixel intensity values were normalized and standardized to create a seamless backscatter mosaic image. The backscatter layer gridded at 2.0 m. The JHC/CCOM WGOM Backscatter Synthesis was described in more detail in Ward et al. (2016) and Ward et al. (2021).

Sattelite image of the western Gulf of Maine with a grayscale backscatter layer showing.
Figure 5. Western Gulf of Maine Backscatter Synthesis.


Additional Bathymetry Resources

Other bathymetry compilations exist for the Northeast US. Notable is a comprehensive bathymetry created by the National Ocean Survey that compiles all available bathymetry resources as they become available into the “The National Bathymetry Source” (NBS). The Compilation presented here focuses on high resolution bathymetry and where possible includes backscatter surveys. For more information about the “The National Bathymetry Source” go to https://nauticalcharts.noaa.gov/updates/building-the-national-bathymetry. For questions contact ocs.nbs@noaa.gov.

References

Pendleton, E.A., Baldwin, W.E., Barnhardt, W.A., Ackerman, S.D., Foster, D.S., Andrews, B.D., and Schwab, W.C. 2013, Shallow geology, seafloor texture, and physiographic zones of the Inner Continental Shelf from Nahant to northern Cape Cod Bay, Massachusetts: U.S. Geological Survey Open-File Report 2012-1157, http://pubs.usgs.gov/of/2012/1157.

Ward, L.G., Johnson, P., Bogonko, M., and McAvoy, Z.S., 2021, Northeast Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England and Long Island Sound: BOEM/New Hampshire Cooperative Agreement (Contract M14ACOOO10) Technical Report, Department of Interior, Bureau of Ocean Energy Management, Marine Minerals Division, 45600 Woodland Road, Sterling, VA, 20166, 23 pp.

Ward, L.G., Johnson, P., Nagel, E., McAvoy, Z.S., and Vallee-Anziani, M., 2016, Western Gulf of Maine bathymetry and backscatter synthesis: BOEM/New Hampshire Cooperative Agreement (Contract M14ACOOO10) Technical Report, BOEM Marine Minerals Branch, 45600 Woodland Road, Sterling, VA, 20166, 18 pp.

Use of Compilations

The information provided here is freely available for use. However, the University of New Hampshire (UNH) Joint Hydrographic Center/Center for Coastal and Ocean Mapping (JHC/CCOM) should be acknowledged for the bathymetry syntheses and WGOM backscatter mosaics. The USGS should be acknowledged for the backscatter composite extending from the NH border to Massachusetts Bay. The agencies that conducted individual surveys are given in the metadata displayed in the footprints.

Recommended Citation

Ward, L.G., P., M. Bogonko, and P. Johnson. 2020. Northeastern U.S. Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England and Long Island. University of New Hampshire Center for Coastal and Ocean Mapping and Joint Hydrographic Center, Durham. https://maps.ccom.unh.edu/portal/apps/webappviewer/index.html?id=5d314116ad094afebbd02ffc185164f6.

Acknowledgements

Development of the “Northeast Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England and Long Island“ was supported by UNH/NOAA Joint Hydrographic Center Award NA10NOS4000073 and the Bureau of Ocean and Energy Management (BOEM) Marine Minerals Program Award M14AC00010. Michael Bogonko, Erin Nagel, and Zachary McAvoy aided with the analysis and development of the syntheses.