Observing through the water surface with a green-wavelength terrestrial laser scanner

Abstract

Laser scanning technique is an upcoming technique for many engineering purposes. Terrestrial laser scanners have become widely accessible for companies, universities and research centres in the last decade. A green-wavelength terrestrial laser scanning device is available for scan projects at the Delft University of Technology. Green-wavelength terrestrial laser scanners have successfully been used in many scan projects. Specifically, because its laser light is green and able to scan through the water surface, the scanner may also be used in through water scan applications. In this graduation thesis the possibilities and limitations of a green-wavelength terrestrial laser scanner in underwater scan applications are explored. Recommendations of the laboratory experiments will lead to experimental set ups of field experiments. The latter will provide recommendations for future research goals in the field. In this research project various targets have been scanned through the water. Basic laboratory experiments have been set up to discover the required and preferred scan settings. Various scan targets have been scanned both dry and under water to determine the scan prospects. Also a design for the field experiments was created based on the experiments that simulated outdoor environments. The underwater targets were chosen specifically for their presence in the outdoor environment. Two case studies were set up that represented real life situations. The first case study concerned the scanning of mooring poles for marine structure inspection purposes. The second case study was set up for both bathymetry and ecological studies as it consisted of mapping the bottom of a water body and the underwater vegetation. Even though the terrestrial laser scanner in question, the Leica C10 ScanStation, is not especially designed for underwater scan applications, the scanner is able to detect underwater objects. The main limitation is induced by the laser scanning device itself. For the most part, scans of underwater targets by a terrestrial laser scanner require a downward scan angle. The field of view limitation that applies to the vertical downward scan angle of the Leica C10 makes adjustment to the experimental set up necessary. The scan possibilities are defined by three dominating factors: the scan target, the water conditions and the power and wavelength of the light that the laser scanning device omits. Experience learns that under certain water conditions some scan targets will provide sufficient returns and others may not. As the characteristics of the laser scanning device are fixed, the combination of the reflectance spectra of the target and water depth and turbidity will determine whether the scan is successful. Before scanning in the field the surroundings and target should be investigated thoroughly before conducting the experiment. After the environment is explored and the project goals are clear it is advised to make an informed choice for the type of laser scanning device and to draw an experimental design. One of the deliverables of this thesis is a 'how to scan with the Leica C10' manual that is attached as an appendix. With this manual a new user should be able to operate the Leica C10 in a variety of scan projects. A mind map is created to support the conclusions of this research project. The mind map summarises all of the findings and can be used as a guideline for further research studies.