MAC intends to adopt the concept of using an excavation support frame to assist in delivering the highest degree of accuracy in operations. This technique was pioneered for an eighteenth century wreck located in the North Sea and MAC intends to use the same equipment and methodology in the San José excavation. The excavation frame acts as a guide for the ROVs to comprehensively remove historic artefacts with precision and care, minimising the risk of damage at the same time as recording their exact location in the wreck.
MAC’s methodology for the San José archaeology calls for the ROV to dock onto the excavation frame to reduce the risks of the ROV free swimming close to fragile items, and to reduce the impact of current on it. Even deep water locations can have strong or unpredictable currents
The ROV sits only a few centimetres above the wreck site and picks up artefacts using its sensitive manipulator arms. The ROV pilot on the excavation vessel then uses a joystick to place them in baskets on the seabed for future recovery. The ROV docking platform can move in all directions on the frame through the use of motorized cogwheels, so that the ROV can access the whole area of the wreck that is within the frame. Our excavation frame thus provides excellent positioning control for the ROV and it will allow archaeologists to excavate the San José site with great precision, so that the maximum amount of research data can be extracted at the same time as ensuring the careful handling of the artefacts to be recovered to the surface. Sediments are removed with a specially developed marine archaeology tools, either suction tools, or jetting with seawater.
In order to conduct a precision operation on the seabed interfaced with the vessel’s dynamic positioning system and the ROVs, MAC will use state of the art acoustic positioning tools. Transponders on the ROVs and on the seabed ping sound signals to each other to enable MAC’s surveyors to track the exact location of the ROVs at the wreck site, and to record the precise locations of objects from the wreck. The surface vessel is fitted with Differential Global Positioning Systems (DGPS) which provide much greater accuracy than the standard GPS signals.
To achieve the accuracies needed for quality archaeological records, which is as detailed as a few centimetres, MAC will therefore use DGPS data to position the ROVs and excavation tools, coupled with positioning input from the excavation frame, manipulator arms and 3D photogrammetry.
Photogrammetry is a 3-dimensional coordinate measuring technique that uses photographs to build up a model of the features of an underwater landscape, or in the case of San José, a ship wreck. By taking multiple photographs from the ROVs, MAC’s photogrammetry package can produce a full 3D model of wreck-site. The models can be used to compare the progress of the excavation over time between the different models and enable scientists to understand the construction and remains of the wreck more clearly.