Characterization of small-scale seafloor roughness using close-range digital photogrammetry

Abstract

Surface roughness is a fundamental property affecting a variety of physical phenomena including sediment transport and the interaction of acoustic energy with the seafloor. Characterization of bottom roughness and its dynamics is therefore essential for understanding and quantifying the influence of the sediment microtopography. Extensive field measurements of bottom roughness have been taken recently with an end-to-end digital photogrammetry system providing quantitative, two-dimensional surface roughness measurements on spatial scales of approximately a millimeter to a meter. Results of these measurements have shown that sediment surfaces in shallow water are often anisotropic and/or exhibit non-Gaussian height distributions, both of which have the potential to strongly affect seafloor acoustic scatter. For these kinds of surfaces, simple roughness parameters such as rms height or the slope and offset of a power-law representation of the power spectra will not give a sufficiently complete description. Two-dimensional statistical models are needed to capture the anisotropic nature of sediments with oriented features, while for seafloors with peaked forms, it is the phase information in the frequency domain that is required, as this controls the shape characteristics of a surface. Characterization of seafloor roughness based on these ideas will be presented using results from the digital photogrammetry system.