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Woods Hole Coastal and Marine Science Center > Geologic Mapping of the Massachusetts Sea Floor > Data Collection and Interpretation

Geologic Mapping of the Massachusetts Sea Floor


shimData Collection and Interpretation

USGS Woods Hole Coastal and Marine Science Center sea-floor mapping technology

Schematic diagram showing shipboard (and land-based) acquisition of geophysical data. Click image to visit the WHCMSC Sea Floor Mapping Technology page.

Surveys conducted in Massachusetts waters between 2003 and 2013 have provided bathymetric, acoustic backscatter, seismic-reflection profile, sample, and photograph data that are used to create high-resolution geophysical datasets that guide sea floor characterization and the production of interpretive maps. For more information about geophysical data and sample collection capabilities at the Woods Hole Coastal and Marine Science Center visit the WHCMSC Sea Floor Mapping Technology page.

In this project processed geophysical and sample data are integrated in a geospatial framework to address the challenge of mapping geology in heterogeneous glaciated geologic environments like the Massachusetts inner continental shelf. The goal is to provide processed data products and interpretative maps that synthesize the complex geology at a regional scale in readily-usable formats for both scientists and resource managers alike. Each acquired dataset provides unique information about the Massachusetts sea floor environment. Scroll through the images on this page to see the contribution of each data type to geologic mapping and sea floor characterization.

To learn more about the data products in this study, please visit the Geophysical and Sample Data Products or Interpretive Reports page.


Swath Bathymetry

 Map of shaded-relief topography of the sea floor offshore of northeastern Massachusetts between Cape Ann and Salisbury Beach.

Click on image to view the geophysical data report for Cape Ann to Salisbury Beach.

Swath bathymetry and derivative products such as slope, hillshaded relief, and rugosity maps provide information not only on water depth, but also the roughness and smoothness of the sea floor, which correlates with sea floor texture and depositional environment.

The shaded-relief bathymetry image (left) of the sea floor offshore of northeastern Massachusetts between Cape Ann and Salisbury Beach is colored to represent depths in meters, relative to the local mean lower low water (MLLW) datum. Rocky areas are readily distinguishable from sand areas in shaded relief imagery.  Fine-grained deposits on the inner continental shelf  are characterized by smooth contours; for example, the southeastern part of the study area in water depths greater than 40-meters. Rock and coarse grained material are associated with crenulated contours and rugged topography; for example, the sea floor just offshore of the Massachusetts-New Hampshire border.

For more information visit: http://pubs.usgs.gov/of/2007/1373/


Acoustic Backscatter Intensity

Map showing acoustic-backscatter intensity of the sea floor in the Buzzards Bay, Massachusetts.

Click on image to view the geophysical data report for Buzzards Bay.

Acoustic backscatter intensity data are an indication of the relative hardness or softness of the sea floor, which is closely related to sediment texture and cohesion. The map (on the left) shows acoustic backscatter intensity of the sea floor in the Buzzards Bay, Massachusetts. In general, higher values (light tones) represent rock, boulders, cobbles, gravel, shell reefs, and coarse sand. Lower values (dark tones) generally represent fine sand and muddy sediment. Within Buzzards Bay, high acoustic backscatter is most often associated with glacial  deposits, such as moraines and meltwater fans. Low acoustic backscatter in the basins of Buzzards Bay are Holocene marine and estuarine muddy deposits.

For more information visit http://pubs.usgs.gov/of/2012/1002/.


Seismic-Reflection Profiles

Seismic-reflection profile across the inner continental shelf.  Stratified glacial-marine sediment rests on bedrock and is overlain by fluvial and deltaic deposits that formed during coastal regression and sea-level lowstand.

Click on image to view the geophysical data report for Cape Ann to Salisbury Beach.

Seismic-reflection profiles are a critical component to geologic interpretations and add another dimension of analysis by providing a subsurface view of the coastal environment. Seismic data can indicate erosion and deposition, geologic origin, and sediment thickness, all of which provide further insight into defining the regional geologic framework. The seismic-reflection profile (left) across the inner-continental shelf north of Cape Ann is showing stratified glacial-marine sediment resting on bedrock. The glacial-marine sediment is overlain by fluvial and deltaic deposits that formed during a time of lower than present sea level. 

For more information visit: http://pubs.usgs.gov/of/2007/1373/

 


Sediment Samples and Bottom Photographs

) Four sea floor photographs taken by the SEABOSS. Area shown in each photograph is approximately 50 to 70 centimeters (20 to 30 inches) across. Some of the organisms visible in these images are: A, clam shells, B, sand dollars (Echinarachnius parma), C, Atlantic rock crab (Cancer irroratus) and clam shells, and D, organisms attached to angular cobbles: pink crustose (

Click on image to view a Soundwaves article on sample collection.

Sample data and/or bottom photographs are essential to interpretations, because geophysical data alone are a relative measure of sea floor characteristics. Sediment samples and photographs provide a way to validate interpretations and link them to geologic and sedimentological classifications.  USGS uses SEABoss (SEAbed Observation and Sampling System), which is capable of collecting photographs and sediments samples simultaneously. SEABOSS photographs, each approximately 50 to 70 centimeters across, are shown on the left. Some of the organisms visible in these images are: A, clam shells, B, sand dollars (Echinarachnius parma), C, Atlantic rock crab (Cancer irroratus) and clam shells, and D, organisms attached to angular cobbles: pink crustose ("bubblegum") algae, barnacles, and chitons (Tonicella sp.). Organisms (and many more too small to see at this scale) identified courtesy of Adrienne Pappal, Massachusetts Office of Coastal Zone Management. Bottom photographs are important for species identification as well as sediment classification, especially in areas like image D, where large particle size often prevents sample recovery.


Interpretation of Data

Schematic image of data layers combining to generate interpretive maps.

Click image to view the interpretive report for Nahant to Northern Cape Cod Bay.

Individual data layers are combined hierarchically to generate interpretive maps. Polygons are digitized based on changes in acoustic backscatter intensity, relief and slope changes in bathymetric data, surficial geology and stratigraphic interpretations, and then subsequently classified using sample and photograph data. Interpretive datasets include sediment texture maps, which follow the sediment texture classification outlined by Barnhardt and others (1998), physiographic zones, described by Kelley and others (1996), and stratigraphic units modified from earlier work by Oldale and Bick (1978) and Oldale and O’Hara (1990).

Selected References

Barnhardt, W.A., Kelley, J.T., Dickson, S.M., and Belknap, D.F., 1998, Mapping the gulf of Maine with side-scan sonar—A new bottom-type classification for complex seafloors: Journal of Coastal Research, v. 14, no. 2, p. 646–659.

Kelley, J.T., Barnhardt, W.A., Belknap, D.F., Dickson, S.M., and Kelley, A.R., 1996, The seafloor revealed—The geology of the northwestern Gulf of Maine inner continental shelf: Maine Geological Survey Open-File Report 96–6, 55 p.

Oldale, R.N, and Bick, Jennifer, 1987, Maps and seismic profiles showing geology of the inner continental shelf, Massachusetts Bay, Massachusetts: U.S. Geological Survey Miscellaneous Field Studies Map MF–1923, 4 sheets.

Oldale, R.N., and O’Hara, C.J., 1990, Maps showing the geology of the inner continental shelf, Cape Cod Bay, Massachusetts: U.S. Geological Survey Miscellaneous Field Studies Map MF–2118, 4 sheets.

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