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Open-File Report 2014–1018

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### Sea-Floor Geology in Northwestern Block Island Sound, Rhode Island 

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## Sidescan Sonar

| Title PageList of FiguresConversion FactorsAbstractIntroductionSettingMethodsBathymetry**Sidescan Sonar**SedimentsBottom PhotographyGIS Data CatalogSummaryAcknowledgmentsReferencesContactsIndexClick on figures for larger images**Figure 16.**Sidescan-sonar image of National Oceanic and  Atmospheric Administration survey H12296.**Figure 17.**Detailed sidescan-sonar image of high-backscatter  scour depressions and low-backscatter erosional outliers of modern marine  sediment.**Figure 18.**Detailed sidescan-sonar image of megaripples and  boulders in the northern part of the study area.
The sidescan-sonar mosaic shows areas of high and low  backscatter (fig. 16). High-backscatter areas (light tones) indicate strong  acoustic reflectivity, hard sea floor, and generally coarser grained sediment; low-backscatter  areas (dark tones) indicate weak acoustic reflectivity, soft sea floor, and  generally finer grained sediment or acoustic shadows. The data show that there  is generally high backscatter in the eastern part of the study area, and low  backscatter along the shore and in areas of the central and western parts of  the study area. Sidescan-sonar data were not collected in the southern part of  the study area.

 Scour depressions are easily visible as areas of high  backscatter, which reflects their coarser grained sediment, whereas the  surrounding modern marine sediments and erosional outliers within the  depressions are denoted by a sharp contrast to dark-toned, low backscatter  areas that reflects their finer grained sediment (fig. 17). Ripples or small  megaripples of gravelly sediment with east-west oriented (roughly parallel to  shore) crests can be seen in the sidescan-sonar data in some of the scour  depressions (fig. 17). Although these ripples are smaller than the resolution  of the bathymetric data, they are visible in the high resolution (1-m) sidescan-sonar  data as parallel lines of alternating high and low backscatter. The ripple  orientation suggests a sediment bedload transport direction within these  features that is perpendicular to shore, most likely in an offshore direction  if they are associated with downwelling. This agrees with modeling by Gutierrez  and others (2005), which suggests cross-shore bedload transport in similar  depressions (also called sorted bedforms) off the coast of North Carolina;  however, they believe these features are also associated with alongshore suspended-sediment  transport. Thus, suspended fine material is likely moved parallel to the coast  from one area of modern marine sediment to the next, unable to fall out of  suspension in the scour  depressions as a  result of the increased turbulence (Murray and Thieler, 2004).

 Two other features distinguished in the sidescan sonar,  boulders and bedforms (sand waves and megaripples) were previously discussed in  detail in the bathymetry section of this report (fig. 18). Boulders are visible  as high-backscatter targets with low-backscatter shadows. Sand waves and  megaripples are visible in the mosaic as tiger-striped, high and low  backscatter. 

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U.S. Department of the Interior |            U.S. Geological Survey                                                URL: http://pubs.usgs.gov/of/2014/1018/html/sidescan.html            Page Contact Information: GS Pubs Web Contact                        Page Last Modified: Friday, February 21, 2014, 12:29:03 PM |
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