ARCHAEOLOGICAL

METHODOLOGY

The first underwater archaeological investigations started in the early 1950s. Since then, archaeologists have fully developed the necessary techniques to explore and excavate underwater archaeological sites. Given the depth limitation of scuba diving equipment, investigations of underwater archaeological sites have mainly been confined to shallow water. Remotely Operated Vehicles (ROV) originally developed more than half a century ago by the U.S. Navy to locate weapons and ships lost in depths beyond the reach of scuba divers, are now commonly used by world navies and oil and gas companies for deep-water exploration and construction. Since the late 1980s, engineers and archaeologists have advanced from mere visual survey and random removal of artefacts to full-blown robotic excavations of deep water shipwrecks.

Following discovery, a site is surveyed using echo-sounder, high-resolution side-scan sonar, sub-bottom profiler, magnetometer and visual inspection to make a model of the shipwreck site and to develop a pre-disturbance site plan.

REMOTELY OPERATED VEHICLES

(ROV)

MAC will mainly rely on ROVs to conduct the inspection, mapping and excavation of the shipwreck. ROVs are underwater robots connected to a ship on the surface by a long umbilical that enables them to feed real time images back to the control room on the ship. Each ROV can be as large as a small car and is fitted with many propellers to guide it through the water and overcome currents. An ROV pilot in the ship carefully steers and operates the ROV around the wreck site.

MAC’s ROVs are fitted with sonar for mapping, sensors to gather data, manipulator arms to take samples or even to gently pick up and excavate fragile objects. With their precision control and powerful tools, MAC’s ROVs will undertake a variety of precision tasks important for the surveying of the wreck. In the next phase MAC’s ROV team will perform an excavation of the historic artefacts of San José in a controlled and rigorous archaeological manner. The ROVs will be fitted with water jets or small suction hoses to remove sediment from the wreck-site, so that archaeologists can access the objects lying beneath and expose the timbers from the overburden.
During excavation MAC’s ROV systems will collect the same level of information as a land-based excavation using:

• High-definition cameras

• High-resolution stills-cameras

• Lighting

• An acoustic LBL positioning reference system to pinpoint the exact location of the ROV. This is combined with positioning sensors in the manipulator arms to ensure cm-positioning accuracy for each artefact recovered

• 3D photomosaic cameras to document the excavation process

• Archaeological dredges for detailed sediment removal

• Suction pickers for artefact recovery

• All of the data is recorded in an advanced data management
system developed specifically for archaeology projects.

• This ensures that all archaeological data is stored and available for the future. All of MACs project will be published to the highest possible standards.

UNDERWATER

EXCAVATION FRAME

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.

Positioning systems

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.

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.