USING HIGH-RESOLUTION SIDESCAN SONAR TO MAP AND MONITOR SEDIMENTARY FEATURES AT 23-MILE SITE, ONSLOW BAY, NC

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. Onslow Bay is a sediment starved, ecologically productive area. The 23-Mile site consists of an upper limestone hardbottom at 29-30m depth covered in a thin discontinuous veneer of sediments. The upper hardbottom shows signs of undercutting and reworking as it is bordered by a rubble ramp of relict hardbottom material. The lower sand flats at 32-33m depth consist of concentrated areas of contrasting grain size. These fine-grained to coarse-grained contacts appear very distinctly on the sidescan data. A dual-frequency sidescan sonar system was used to repeatedly image the seafloor morphology and texture 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, and July 2001. During this time period no major tropical storms impacted the shelf, although at least three small (sustained winds <30 kts/hr) nor’easters were recorded in the region of study. Interpretation of the resulting sidescan sonar mosaics was ground-truthed by collection of seabed surface samples. An initial comparison of digital number values of the same pixels in each mosaic shows a significant difference in the spatial orientation of the coarse-grained and fine-grained contacts of the lower sand flats. In some areas sediment movements in excess of 10 m are documented. This movement, however, shows no consistent trend in direction or magnitude. Flow data collected on site by an acoustic doppler profiler moored 2 m above the seabed suggest that mean bottom current velocities are sufficient during small storms to initiate sediment movement. Following documented storm events, medium to coarse-grained sands were retrieved from sediment cups deployed 60cm above the sea bed. Tidally induced mean currents do not appear to play a significant role in shaping the observed sedimentary morphology. Further analysis to document variations in seabed morphology will be done using a GIS. The shape, size or border positions of digitized regions with distinct backscatter characteristics will be compared between mosaics.