Remote sensing of sandy sediment ripples using high-frequency backscatter

Abstract

The presence of bottom ripples changes the acoustic interaction with sediments. Two important ripple parameters are the height and spatial wavelength. In previous work, in situ measurements of ripple were made using stereophotography and electrical conductivity. Although these methods are accurate and dependable, they can only be carried out over limited areas (the order of 15 meters on the side). Using acoustic backscattering to remotely measure the ripple field, on the other hand, is fast and can cover much larger areas (the order of 50 m by 50 m). Data from a 300 kHz system with two closely spaced receivers was used to estimate ripple wavelength and height. Data analysis and numerical simulation show that, as long as the wavelength of the sound wave is much shorter than the ripple wavelength, it is straightforward to resolve the wavelength of the ripple field. To measure the ripple height, however, is more difficult. The best results were obtained when the horizontal projection of the sound beam was neither parallel nor perpendicular to the ripples. In simulations a composite model was used, where the large scale ripples and small scale roughness are treated as Kirchhoff and Lambertian scatterers, respectively. [Work supported by ONR-USA.]