Measurement of Seabed Roughness with Laser Scanning System

Knowing the temporal change of the seabed, we can understand the nature of the undersea environment in more details. The scale of the features on the seabed varies from meters for large sandwaves, and down to less than one millimeter for the prints left by marine creatures. So far, there is not a single instrument that can cover the whole range and still preserve the resolution. For measuring small-scale roughness of seabed, laser scanning is an alternative. In this work, we report the integration of Seabed Laser Scanner (SLS), developed by Institute of Undersea Technology, Sun Yat-sen University, on the linear stage of In Situ Measurement of Porosity 2 (IMP2), developed by Applied Physics Laboratory, University of Washington to carry out deep sea sediment 2D roughness measurement and comparison. IMP2 consists of a 4 meter long scaffold and a linear stage. SLS is mounted on the linear stage to carry out the scanning. To simplify the integration and avoid the possible failure coming from the additional underwater connections, SLS is designed to be a self-contained system. It runs on a PC104 with Windows XP and feeds on its own battery pack. The only interaction between the two systems is achieved by the proximity of a set of magnets and a relay. As the linear stage starts, the first magnet triggers SLS to power up and standby. As the stage comes to the end of the rack, the second magnet triggers the image acquisition. The integrated system was deployed three times during the Shallow Acoustics Experiment 2006, about eighty miles off the coast of New Jersey. We successfully retrieved data from SLS for the first two trials; the third trial failed due to the aborted mission of IMP2. A 300 cm times 30 cm and a 350 cm times 25 cm seafloor were mapped at 80-meter water depth. From the reconstructed 3D surfaces, we found that the seafloor is full of shell debris and covered by mud-like sediment. The 2D spectra were estimated from the 3D surface. The results indicated that the seafloor roughness follows a power-law spectrum and isotropic. These spectra estimated provide the boundary condition for modeling the acoustic propagation and scattering.