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.