Characterization of the two-dimensional roughness of wave-rippled sea floors using digital photogrammetry

Surface roughness is a fundamental sea-floor property affecting a variety of physical phenomena including sediment transport and the interaction of acoustic energy with the sea floor. Characterization of sea-floor surface roughness and its dynamics is therefore essential for understanding and quantifying the influence of sediment microtopography. Field measurements have been taken recently with an "end-to-end" digital photogrammetry system providing quantitative, two-dimensional sea-floor 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 high-frequency sea-floor acoustic scatter. For these kinds of surfaces, simple roughness parameters such as rms height or the power law exponent and spectrum strength parameter 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 sea floors with peaked forms, it is the phase information in the spatial frequency domain that is required, as this controls the shape characteristics of a surface. Characterization of sea-floor roughness based on these ideas will be presented using results from the digital photogrammetry system.