Efficiency of marine acoustic techniques for the archaeological prospection of intertidal areas: Results from a test study on the Belgian coast (Ostend-Raversijde)

Sub-bottom acoustic studies of intertidal areas along the North Sea coast pose major technological challenges due to the shallow water depth, wave action, strong currents and large tidal range. Moreover in Belgium these areas are often marked by the presence of shallow gas which limits the acoustic penetration. As a result these areas are rarely investigated in a structured way, which is very unfortunate since they have always been a zone of human activity and are known to be rich in archaeological remnants. In recent years several investigations have been carried out along the Belgian coast at Ostend-Raversijde in order to test the efficiency of marine acoustic techniques for archaeological prospection. The site is known for its artefacts and structures dating from Roman and medieval times, including old coastal defence structures (e.g. dykes), remnants of a drowned fishing village, and intensive peat/salt exploitation. Due to sand suppletion works in the '70ies the archaeological remains are now buried a few m below the beach. An additional focus of the study was to map the old coastal palaolandscape marked by creeks, tidal channels and small islands/peninsulas. Very high resolution 2D subbottom profiling using a parametric echosounder evidenced a highly complex system of palaeogullies, some of which could be linked to past islands and/or dyke structures. Regular patterns of strong, interrupted reflectors could be related to trench systems and peat digging. The observed pattern correlated well with electromagnetic (EMI) data obtained over the same area at low tide. 3D seismic test measurements allowed to image the peat/salt excavation pattern in the highest detail, setting new standards for marine archaeological research. So far no remnants of the drowned medieval fishing village were found. The results show that the integrated use of complementary methods forms a good strategy for a better understanding of the shallow (intertidal) sedimentary environment and its archaeological potential.