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Use of high-resolution sidescan sonar data to quantitatively map and monitor a mid-continental shelf hardbottom 23-mile site, Onslow Bay, NC

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
Matthew Edward Head (Creator)
Institution
The University of North Carolina Wilmington (UNCW )
Web Site: http://library.uncw.edu/
Advisor
Nancy Grindlay

Abstract: This study attempts to constrain the spatial and temporal variations in the seafloor morphology, as well as the relationship between sidescan sonar data and seafloor characteristics at the 23-Mile hardbottom area on the mid-continental shelf of Onslow Bay, NC. The 23-Mile site consists of an upper limestone hardbottom at 29-30m water depth covered in a thin discontinuous veneer of sediments. The lower sand flats at 32- 33m depth consist of concentrated areas of contrasting grain size. A dual-frequency sidescan sonar system was used to repeatedly image the seafloor of a 3.5 km by 2.1 km region at 23-mile site over a period of 2.5 years. Cruises were conducted in December 1999, December 2000, July 2001, and May 2002. Textural analysis of the sidescan sonar imagery was conducted using gray-level co-occurrence matrices. Two textural indices: entropy (acoustic roughness), and homogeneity (level of textural organization) were used in conjunction with gray-level to identify seabed types. The textural analysis successfully identified three of the four chosen seabed types. Groundtruthing by divers indicates that these units show a strong statistical correlation between backscatter intensity and sediment grain size. Subsurface scattering also appears to play a role in the backscatter intensity. A comparison of results between successive surveys shows a significant difference in the spatial orientation of the coarse-grained and fine-grained contacts of the lower sand flats. Significant displacements (>10m) and changes in morphology of contacts in five study areas suggest that the fine-grain sands in this area are highly mobile. The first observation period (1999-2000) represents an entire year of movement, which resulted in a consistent shift in the south to southeast direction in all of the areas except for one. The second observation period (2000-2001) comprised the winter and spring seasons. The contact movement displayed a consistent shift to the north-northwest in all study areas except two. Individual events (nor’easters and fairweather) identified from the near-bottom measurements of waves and currents provide mechanisms for the observed sand movement seen in the sidescan surveys.

Additional Information

Publication
Thesis
A Thesis Submitted to the University of North Carolina at Wilmington in Partial Fulfillment Of the Requirements for the Degree of Master of Science
Language: English
Date: 2009
Keywords
Ocean bottom--North Carolina--Onslow Bay, Onslow Bay (N.C.)--Bathymetric maps, Stochastic processes
Subjects
Ocean bottom -- North Carolina -- Onslow Bay
Stochastic processes
Onslow Bay (N.C.) -- Bathymetric maps