Friday, 28 October 2011

Maritime Archaeology aboard RV Keary

Since INFOMAR began in 2006, the survey team have regularly supplied data to the Underwater Archaeology Unit (UAU, part of the Department of Arts, Heritage & Gaeltacht) on a variety of shipwrecks that have been surveyed around the Irish coast. The process involves the sharing of detailed sonar data (multibeam and sidescan), which the UAU analyse in order to help ascertain whether a given wreck is known or unknown, whether or not it needs to be protected, and to improve the accuracy of the national shipwrecks database. INFOMAR and the UAU also publish wreck information sheets together, which are available to download at – these provide information on the locations of particular wrecks, their water depths, history and any diving information if available.

At the start of August 2011, this partnership was developed further when RV Keary and RV Geo gave support to the UAU in an archaeological excavation of a potential Spanish Armada wreck near Burtonport, Co. Donegal.

The Rutland Island Wreck, named after a nearby island, had been discovered two years previously by local divers, and had already been investigated by the UAU. However, it had not yet been properly excavated, and so the Keary was provided by the Geological Survey of Ireland as a dive platform to facilitate the team of maritime archaeologists in this endeavour.

RV Keary and UAU vessel moored together above the wreck site

Marine archaeologist wearing commercial diving gear, emerging at the stern of RV Keary

The vessel served very well as a base of operations for the team, providing ample space for their specialised diving equipment – tanks and compressors that provided surface-supplied air, and a communications setup that allowed dialogue between the surface team and the divers below. The vessel’s hydraulic A-frame also played a role in recovering some of the UAU’s equipment from the seabed at the end of the project, and several areas were provided on deck for the analysis of recovered artefacts. One of the Keary's instruments, called a USBL (Ultra-Short Base Line), was also used to great effect in pin-pointing locations on the seabed that were of interest to the archaeologists. This is an acoustic transducer, located on a hydraulic drop-pod between the catamaran's hulls, that can communicate with a set of beacons. The divers assembled a metal grid over the wreck in order to assist with mapping of its various timbers and artefacts, and by placing these beacons on its four corners, the Keary's USBL could be used to give accurate coordinates for the grid's location - and therefore accurate positioning for all discovered artefacts. One beacon was also carried by a diver carrying out metal-detection on the seabed around the wreck, which meant the Keary's survey crew could provide him with coordinates for all objects that were discovered this way.

USBL display showing RV Keary at centre and recorded positions of beacons

The Keary also accommodated a variety of film and radio crews, who documented the excavation for privately-funded documentaries and public service broadcasting. All in all, the vessel played host to a huge variety of scientists, archaeologists and journalists over the three week period.

The INFOMAR, UAU and local dive teams aboard RV Keary

In order to establish a semi-permanent base at the site, the Keary was placed over the wreck and anchored at three points to hold her in position, with the Geo remaining mobile in order to ferry survey crew and archaeologists to and from the base of operations, along with carrying out her own bathymetric survey operations (mapping of water depths).

RV Geo carrying out bathymetric survey

The UAU provided their own RIB (Rigid Inflatable Boat), which tied up next to the Keary each day and excavated the wreck through use of a suction-dredger. The material removed from the site was fed to a sieve aboard the RIB, where a team of archaeologists sifted through the sand and mud to recover any small objects of interest that were removed by the dredger. All recovered objects were then moved to the Keary for analysis and storage.

Archaeologist on surface-supplied air, operating suction dredger (Photo courtesy of the Underwater Archaeology Unit)

Archaeologists aboard the UAU RIB, sieving through material suction-dredged from the wreck below.

Stern of the Rutlant Island Wreck. The metal grid was assembled on the seafloor by divers to assist with mapping of the wreck's timbers and artefacts (Photo courtesy of the Underwater Archaeology Unit)

Diver recording rope and barrel hoops exposed during the excavation

Artefacts recovered from the wreck, clockwise from top: bowl, shot, stone object and walnut shell (Photo courtesy of the Underwater Archaeology Unit)

Independently, RV Geo carried out sidescan sonar surveys of the wreck, along with expanding INFOMAR coverage of the surrounding area’s bathymetry (water depths) – the areas mapped by the Geo included several approach channels that are vital for navigation to and from the island of Arranmore, along with a sector near Lahan Island that had only been partially mapped during the days of the British Admiralty surveys.

The Geo's presence in the area allowed the team to progress INFOMAR seabed mapping while the Keary dedicated itself to the archaeological work, demonstrating once again the flexibility and efficiency that are gained by having both vessels operating together on site. The Geo's interferometer mapping system, which works like a Sidescan sonar, produced valuable imagery of the Rutland Island wreck that will be useful for further archaeological studies.

Sidescan Sonar image of the Rutland Island Wreck. The stern of the vessel is to the right (East). Note the pale shadow cast southwards by the metal grid (colours on Sidescan imagery are inverted here, with pale colours representing sonar shadow)

Total bathymetry mapped by RV Geo during the excavation

Area mapped by Geo near Lahan Island - this area was not completely mapped during the original surveys by the British Admiralty

Bathymetry mapped by RV Geo east and south of Rutland Island, including the wreck site

Shipping channel between Arranmore and Burtonport mapped by RV Geo

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