Sub-bottom profiling for large-scale maritime archaeological survey An experience-based approach

The development of cost-effective management strategies, including mapping of the submerged cultural heritage - potentially extensive even at quite great depths - is becoming ever more important, given increasing commercial activity in marine areas which may cause damage or prevent further investigation of the sea floor. This commercial activity can take the form of aggregate extraction, fishing, installation of facilities such as windmills, cables or pipelines and the construction of bridges, harbours etc.Non-invasive acoustic survey methods play a significant role in the mapping of the submerged cultural heritage. Elements such as archaeological wreck sites exposed on the sea floor are mapped using side-scan and multi-beam techniques. These can also provide information on bathymetric patterns representing potential Stone Age settlements, whereas the detection of such archaeological sites and wrecks partially or wholly embedded in the sea-floor sediments demands the application of highresolution sub-bottom profilers. This paper presents a strategy for the cost-effective large-scale mapping of unknown sedimentembedded sites such as submerged Stone Age settlements or wrecks, based on sub-bottom profiling with chirp systems. The mapping strategy described includes: a) definition of line spacing depending on the target; b) interactive sailing, i.e. when potential archaeological anomalies are located, their character is immediately investigated in more detail by a denser pattern of sub-bottom survey lines; c) on-site interpretation while acquiring data; d) recognition of anomalies not due to geology. Consequently, this strategy differs from those employed in detailed studies of known wreck sites (eg. Plets et al. 2009) and from the way in which geologists map the sea floor and the geological column beneath it. The strategy has been developed on the basis of extensive practical experience gained from the use of an off-the-shelf 2D chirp system and, given the present sta- e of the technology, appears well-suited to large-scale maritime archaeological mapping.