High resolution acoustic imagery from a shallow archaeological site: The Grace Dieu - a case study

Shallow water (< 5 m) archaeological shipwreck sites present unique challenges to geophysical investigation. Retrieval of reliable data is very difficult due to issues of water column bubble turbulence and the restricted acoustic geometry of the system. This paper will present an acquisition method that tackles these problems through the combination of a high resolution sub-bottom system, DGPS navigation and a non-motorised deployment method. The test site for this system was the Grace Dieu (1418), the ‘great ship’ of Henry V’s fleet, which was scuttled at its berth in the Hamble River (UK). The site is typically covered by 2-5 m of water, and is partially buried within muddy inter-tidal sediments. At exceptionally low tides, during the spring equinox, a few of the marginal timbers are exposed. Close survey line spacing (< 2.5 m), accurate navigation and decimetre resolution acoustic data enable a full 2D and pseudo-3D interpretation, including amplitude analysis, of the site to be undertaken. This data has identified the true plan form and dimensions of the remaining segments of this historic vessel supporting the assertion that it was the most significant naval design for over two centuries. It has also been possible to identify the presence of a horizon of incoherent timbers associated with the scuttling of the vessel. Through an archaeological case study the potential of imaging buried objects in extremely shallow environments has been effectively demonstrated.