Three-dimensional measurement of underwater work sites using structured laser light

Some current operations with remotely operated vehicle (ROVs) require the vehicle to fix itself onto an underwater work site and carry out robotic tasks. For this to occur, a real-time computer model of the work sire and its relationship to the vehicle must be determined, especially if the work site has not been designed for such interventions and no fixed docking structure is available. We described a laser-based system that can be used from an ROV to scan the underwater work site and translate the information into a CAD or virtual reality (VR) model. A single stripe is projected onto the work site and an image acquired by the ROV. An image-processing routine is used to optimize object range and shape information obtainable from this stripe and, when several such stripe positions have been determined, the information is combined. This paper concentrates on the image-processing techniques needed to isolate information from underwater images and the results obtained in trials of the system in a laboratory tank;. The methodology is able to accommodate differing turbidities and the limitations of the system in terms of accuracy are discussed.