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