Sea floor roughness measured by a laser profiler on a ROV

Abstract

Improvements to a method of seafloor roughness measurement using low-power lasers and a video recorder mounted on a remotely operated vehicle (ROV) are presented. The platform is a Saab Seaeye Falcon observation class ROV. Six green laser beams are spread into illuminated planes using gratings or cylindrical lenses. The intersection of the laser planes with the ocean bottom was imaged on a high-definition digital video recorder at an oblique angle. After correcting for aberrations due to refraction between air and water and other imperfections, the three-dimensional coordinates of the laser profiles relative to the video recorder are computed for each video frame. This is a low-cost device, since the laser modules are commercial line generator modules of moderate power output. Green lasers were chosen for their low attenuation in seawater. The power output of each laser module was chosen to be within the Class 3R designation to avoid the permitting issues of high power lasers, although high power units are being developed. The lasers are housed within an optically clear acrylic tube. In addition to the laser modules, a light-emitting diode (LED) white light source is also deployed within the laser housing to provide background illumination of the seabed around the laser lines. The digital video recorder is a high definition consumer-grade device, housed in a separate tube with a clear acrylic window. Power is provided by the ROV. The signal and controls are carried by one shielded twisted pair cable through the ROV umbilical to the shipboard control unit. Auxiliary data include roll, pitch and heading information are simultaneously recorded. Ultra short baseline (USBL) acoustic positioning information from the ROV is also recorded. In previous experiments, the laser planes were arranged in parallel, but in the current configuration some laser planes are rotated to intersect with the others. The intersections present additional difficulties for the automatic tracing and identifying software, but they provide a number of advantages in calibration and position tracking.