Geologic Seafloor Mapping Using the ArcMarine Data Model

The U.S. Geological Survey (USGS) Coastal and Marine Geology Program in cooperation with the Massachusetts Office of Coastal Zone Management is conducting geologic mapping of the seafloor to characterize the surface and subsurface geologic framework of near-shore environments in coastal Massachusetts. High-resolution swath bathymetry, side-scan sonar imagery, and seismic reflection profiles were collected over the 128-square-kilometer survey area that covers a portion of the South Essex Ocean Sanctuary between Gloucester and Nahant.

As part of this project, USGS worked with the Esri marine data model working group to apply and tailor the existing model schema to the data collected in the South Essex survey area. This data, including the ArcMarine database, is published as USGS Open-File Report 2005-1293, High-resolution geological mapping of the inner continental shelf, Salem to Gloucester, Massachusetts, 2006, Barnhardt, W. A., Andrews, B. D., and Butman, B. Available at the USGS Web site.

The Swath Bathymetry map shows seafloor topography in shaded relief view, colored by water depth. The shaded relief imagery was created by vertically exaggerating the seafloor topography five times, then artificially illuminating the relief by a light source positioned 35 degrees above the horizon at an azimuth of 045 degrees. Grid cell resolution is five meters.

The Side-scan Backscatter map shows acoustic backscatter intensity from towed side-scan sonar. Backscatter intensity, as recorded with side-scan sonar, is an acoustic measure of the hardness and roughness of the seafloor. In general, higher values (light tones) represent rock, gravel, and coarse sand. Lower values (dark tones) generally represent fine sand and muddy sediment. Grid cell resolution is one meter.

The Hillshaded Pseudocolored Side-scan Backscatter map shows backscatter intensity draped over shaded relief seafloor topography. The intensity of acoustic backscatter is represented by a spectrum of colors, ranging from orange (high backscatter) to blue (low backscatter). High values, depicted by yellow/orange, suggest that the seafloor in those areas is generally covered with coarse sand, gravel, cobbles, boulders, and rock. Moderate values, depicted by green/yellow, indicate sand or muddy sand. Low values, depicted by blue, indicate sandy mud or mud. Accurate interpretation of substrate properties from acoustic backscatter data requires validation by direct sampling, bottom photography, and video.

The Unsupervised Classification Bottom Type map shows three general classes of bottom substrate calculated from multivariate classification using ModelBuilder. The inputs for this unsupervised classification are seafloor depth, substrate type (side-scan backscatter), and seafloor slope (not shown). Grid cell resolution is five meters. Areas of dark blue are ledge. Light blue indicates cobbles and boulders. Yellow areas are sand and silt. Black indicates areas of no data where the three input layers did not overlap.