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U.S. Geological Survey Open-File Report 2009-1001

Geological Interpretation of the Sea Floor Offshore of Edgartown, Massachusetts
Sidescan Sonar Title             PageList of             FiguresConversion             FactorsAbstractIntroductionSettingMethodsBathymetrySidescan             Sonar Seismic             Profiles SedimentsBottom             Photography Data             CatalogSummaryAcknowledgmentsReferences             Cited Contacts
Click on figures for larger           images.
Figure 20. Sidescan-sonar imagery produced             from data collected during National Oceanic and Atmospheric             Administration survey H11346. Figure 21. Interpretation of the sea floor             surveyed in National Oceanic and Atmospheric Administration survey             H11346.Figure 22. Detailed planar view of the             seaward extension of Middle Flats from the sidescan-sonar mosaic             produced during National Oceanic and Atmospheric Administration             survey H11346.Figure 23. Detailed planar view of the             sidescan-sonar mosaic produced during National Oceanic and             Atmospheric Administration survey H11346.Figure 24. Detailed planar view of the             sidescan-sonar mosaic produced during National Oceanic and             Atmospheric Administration survey H11346.
Distinctive tonal patterns revealed on the sidescan-sonar mosaic (fig. 20) include (1) complex patches of high and low       backscatter with individual high-backscatter targets (objects), (2)       alternating bands of high and low backscatter in a tiger-stripe pattern,       (3) areas of relatively high backscatter (light tones), and (4) areas of       relatively low backscatter (dark tones). Boundaries between patterns are       commonly gradational, and backscatter is not uniform throughout these       areas. Water-column phenomena, such as turbulence around obstructions on       the bottom, are also evident on the image.

The complex patches of high and low backscatter with individual       high-backscatter targets delineate the rocky, bouldery areas (figs. 21, 22). Bouldery areas on the mosaic are limited to the tip       of the seaward extension of Middle Flats.

Areas characterized by alternating bands of high and low backscatter       include much of the western part of the study area between Oak Bluffs (fig.       2) and Middle Flats and small patches off Cape Poge and the seaward       extension of Middle Flats (fig. 21). This tiger-stripe pattern (fig. 23), which is produced by sand waves and       megaripples, results from a combination of topographic changes affecting       the angle of incidence of the sidescan sonar and the differences in       sediment texture commonly present between crests and troughs of sand waves       (Reineck and Singh, 1980). Although parts of the image exhibit a pattern       similar to that of sand waves, the direction alternates from line to line,       creating a herringbone effect, and the pattern does not continue from line       to line (for example, near 41°26.0'N, 70°26.75'W). In these instances, the       pattern is caused by acoustic noise from surface-wave conditions in the       shallow water.

Areas characterized by relatively high backscatter are concentrated       along shorelines, on the crests and flanks of bathymetric highs, and in       depressions east and west of the seaward extension of Middle Flats (fig. 20). Most of the areas of higher backscatter tend       to be produced by coarser grained sediments, typically coarse sand and       gravelly sand, and indicate higher energy environments. The areas of high       backscatter east and west of the seaward extension of Middle Flats,       however, are caused by shell beds that produce strong acoustic backscatter       (figs. 21, 24). Patches characterized by relatively low backscatter       are in the inner part of Edgartown Harbor and offshore in the deeper parts       of the study area. These areas coincide with lower energy environments       that prevail in areas protected from strong storm-driven currents. The       lower backscatter is produced by finer grained Holocene marine sediment.       These sediments are clayey silt in the inner harbor and fine sand in       deeper water. 

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