Dynamics of nearshore sedimentary environments revealed through the analysis of multibeam sonar data.

摘要

Nearshore environments are a dynamic and complex component of the coastal boundary where the physical processes of the ocean surface and bottom boundary layer converge determining the fate of the terrestrial sediments delivered to the ocean. Multibeam sonar technology provides the opportunity to investigate shallow water environments by revealing, for the first time, detailed information about the morphology and texture of the seabed. This research focuses on the analysis of high-frequency multibeam sonar (300 kHz) data that was collected in a variety of nearshore environments.; Observations of seabed morphology and texture greatly improve our interpretation of seabed features. Rippled Scour Depressions (RSDs) have been described in many nearshore environments as large shore-normal linear depressions associated with high backscatter. The superposition of RSDs on a large-scale subtle bathymetric undulation indicates that they are associated with physical processes acting over a larger area than indicated by the RSDs alone. Analysis of physical oceanographic data reveals that RSDs are transverse features related to the combined effects of longshore tidal currents and waves.; Small-scale details of the seabed can be quantified with detailed analysis of backscatter data. A ripple detection algorithm was developed to identify the wavelength and orientation of ripples large enough to be imaged but too small to be evident in backscatter mosaics. An image segmentation method that relies on Principal Component Analysis (PCA) was developed to seek patterns in the backscatter signal as a function of angle of insonification that relate to the combined effects of roughness and sediment type. The geoacoustic facies identified were related to observations of the seabed from stereo-photographs and grab samples. The most significant variables for backscatter prediction were median grain size, the magnitude of roughness, and the regularity of roughness, although the relative importance of these variables is likely to be different at other sites. An empirical relationship was also derived to estimate the magnitude of roughness from backscatter data and grain size information.