Seafloor roughness measurement from a ROV

Abstract

A method of seafloor roughness measurement using low-power lasers and a camcorder mounted on a ROV was demonstrated in the EVA (Experiments for Validation of Acoustics modeling techniques) sea trials, conducted near Marciana Marina, Isola d'Elba, Italy. Six red laser beams were spread by cylindrical lenses into parallel planes. The intersection of the laser planes with the ocean bottom was imaged on a digital camcorder, pitched down at a nominal angle of 45 degrees. NTSC video recordings were collected and analyzed to extract the laser profiles. After correcting for aberrations due to refraction between air and water and other imperfections, taking into account the scanning and interlacing effects of the NTSC video, and given the relative positions of the camcorder and lasers, the 3-D coordinates of the laser profiles relative to the camcorder were computed. Images of the seafloor were used in an image registration algorithm to track the trajectory of the camcorder, and stack the laser profiles to reconstruct the seafloor surface. The measurements were made in conjunction with acoustic reflection measurements to determine the effects of incoherent scattering on the mean value of the reflection coefficient over a wide range of angles.