Mark Lawrence and CR Bates

Acoustic ground discrimination techniques for submerged archaeological site investigations

Abstract
Single beam acoustic ground discrimination systems (AGDS) based on standard echo sounders are routinely used for commercial and research applications such as biological, environmental and hydrographic surveys, fishing ground assessment, and military projects. Analysis of the return echo signals using these systems produces indexes of seabed "roughness" and "hardness" which can be used to classify seabed type when correlated to ground truth observations.

The aim of this paper is to assess the potential for using single beam acoustic technologies as a remote surveying method to characterise submerged archaeological sites where particular environmental properties of the sites determine the preservation potential of the cultural material that may be present. The environmental properties include the nature and stability of the seabed sediments within which cultural material lies in addition to the location and level of exposure of the seabed and the quantity and nature of the archaeological material that may be present.

Benefits of characterising sites using remote acoustic properties include, most importantly, the potential for assessing future impacts on the archaeological material based on the assessment of sediment type and stability from the acoustic data. The technology could offer a means by which sites can regularly be monitored for changes or deterioration over time, which might then allow for mitigation strategies to be employed to prevent the loss of cultural material. Further, the presence of buried material beneath the seabed may alter the surface sediment characteristics of the seabed that in itself might add a prospective element to the technology. Intra-site characterisation of exposed archaeological material within sites may even be possible.

AGDS systems have already been shown to differentiate wide-ranging bottom types over large areas of seabed. Initial trials of one particular AGDS for this study indicate that it is possible for very small areas of seabed containing exposed archaeological material to be readily distinguished from the surrounding seabed in terms of the character of the acoustic responses. In addition it is likely that different types of archaeological material will have different acoustic signatures on any individual site.

Further research is necessary to determine if, on a site-to-site basis, relationships can be established between acoustic signature, generic archaeological material and the degree of preservation of the archaeological material. It is postulated that this could lead to the use of remote acoustic systems as an aid for cultural resource management in the future.

The AGDS can be quickly installed on a range of vessels of opportunity thus making the systems ideal tools for use by cultural resource managers or archaeological units. However, while the systems are relatively easy to operate, strict QA/QC and data handling is necessary to obtain consistent results within and between sites.