The acoustic imaging, reconstruction and characterization of buried archaeological material

Maritime archaeologists are increasingly required to investigate important sites in a nondestructivemanner. Therefore, this thesis aimed to adapt and apply sub-bottom acoustictechniques to shallowly buried shipwrecks and submerged palaeo-landsurfaces.The imaging of archaeological sites requires a geophysical source with high resolving power. Amodel was developed to determine the vertical resolution of the Chirp sub-bottom profiler, thesource used throughout this thesis. The horizontal resolution, on the other hand, was determined bymodelling the directivity pattern of an array of Chirp transducers. The results were then used tochoose a suitable Chirp sweep to image shallowly buried objects. Three sites were selected to testthe ability of the system to image, reconstruct and characterize archaeological material. The firstsite comprised of a shipwreck buried within muddy riverine sediments: the Grace Dieu (1418).The survey utilized both a 2D- and 3D-Chirp system. In both cases, the source was pushed over thesite by divers. Close survey line spacing, accurate navigation and decimeter-scale resolution dataenabled the construction of a pseudo- and full 3D-image of the site. In the second site, theYarmouth Roads Wreck (16th century), an acoustic blanking zone, a consequence of the highlyattenuating character of degraded wood, was detected and used to create maps of the buriedremains. The final site concentrated on the detection of peat layers in the English Channel, whichare crux to palaeo-landscape reconstructions. Results suggest that peat buried in fine to mediumgrained sediments has an easily identifiable acoustic signature. The seismic data acquired for theaforementioned case studies was subsequently used for quantitative studies of the buried material.Firstly, reflection coefficients were calculated and used to determine the degradation state of thehull remains. Secondly, the Quality factor was calculated for the Yarmouth Roads Wreck site toobjectively demonstrate the highly attenuating character of degraded wood buried in sediments incomparison to the surrounding sediments.Overall, this PhD has demonstrated the potential of the Chirp sub-bottom system forarchaeological purposes, by creating (pseudo-) 3D reconstructions and making objectcharacterization possible