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Explanation of Information Shown on Surficial Geology of the Maine Inner Continental Shelf Maps:
SURFICIAL GEOLOGY LEGEND
This map series shows the geology of the surface of the ocean floor. These maps of Maine's inner continental shelf are based on geophysical data, bottom samples, National Ocean Service provisional bathymetric maps and published nautical charts. These data were supplemented with bottom photographs and direct observations from submersibles. Experience with these data, together with side-scan sonar images (the underwater equivalent of aerial photographs), permitted generalized mapping of the inner continental shelf.
The map areas shown by the four colors below were not directly imaged with side-scan sonar. Contacts between these geologic units were inferred, based on bathymetry and other information (see Features and Data Source Map).
The bright colors on the map and in the Interpretation of Side-Scan Sonar Images legend below show areas of seafloor imaged by sonar. The linear colored swaths on the maps follow ship tracklines and have a width that represents the sonar swath to each side of the vessel.
INTERPRETATION OF SIDE-SCAN SONAR IMAGES
On side-scan sonar images, rock, gravel, sand, and mud reflect acoustic energy differently and appear as various shades of gray printed by the instrument's recorder. The classification scheme above is unique and based on the acoustic reflectivity of the Maine inner continental shelf. The dominant "end member" (Rock, Gravel, Sand, or Mud) is abbreviated with a capitalized first letter. A less abundant, subordinate seafloor type is represented with a lower case letter (r, g, s, or m). For example, a predominantly rocky seabed with gravel infilling fractures is designated Rg. The sixteen combinations of seafloor types shown above are used for areas where side-scan sonar coverage exists and appear as bright colors on the map. In areas beyond the scan range only four generalized units were used (see the Surficial Geology Legend and the Sidebar for details).
When individual units of rock, gravel, sand, and mud were greater than 10,000 square meters in area (about the size of 3 football fields), they were mapped as separate features. In many places, however, a heterogeneous seabed composed of numerous small features required composite map units. In areas where no single seafloor type exceeded 10,000 square meters, a composite map unit was used. The selection of map units to describe this complexity involves a compromise between providing detailed information where it exists, and generalizing where data are scarce or absent. In many places the seabed is composed of numerous small features, none exceeding the minimum area of 10,000 square meters. Consequently, not all details in the sonar records could be presented on this map. It should be realized that spatial heterogeneity exists at all scales, even down to areas less than a square meter (ten square feet).
Rock yields a strong, dark, acoustic return. In areas with steep bathymetric relief and fractures, light acoustic shadows are visible within the dark areas of rock (see adjacent panels A, C, and D). Gravel deposits also produce a relatively strong acoustic return (black to dark gray), and are often closely associated with rock, but lack relief (A, B, C, D). Sand produces a much weaker acoustic return (light to dark gray) than either gravel or rock, and usually lacks local relief (B). Mud yields a very weak surface return (light gray to white) and, except where it accumulates on steep slopes or near gas-escape pockmarks, it is associated with a smooth seabed (D). The Surficial Geology section in the far right column describes the distribution and abundance of these areas on Maine's inner continental shelf.
FEATURES AND DATA SOURCES
Last updated on April 25, 2012
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