The acoustic imaging, reconstruction and characterization of buried archaeological material

Maritime archaeologists are increasingly required to investigate important sites in a nondestructive<br/>manner. Therefore, this thesis aimed to adapt and apply sub-bottom acoustic<br/>techniques to shallowly buried shipwrecks and submerged palaeo-landsurfaces.<br/>The imaging of archaeological sites requires a geophysical source with high resolving power. A<br/>model was developed to determine the vertical resolution of the Chirp sub-bottom profiler, the<br/>source used throughout this thesis. The horizontal resolution, on the other hand, was determined by<br/>modelling the directivity pattern of an array of Chirp transducers. The results were then used to<br/>choose a suitable Chirp sweep to image shallowly buried objects. Three sites were selected to test<br/>the ability of the system to image, reconstruct and characterize archaeological material. The first<br/>site comprised of a shipwreck buried within muddy riverine sediments: the Grace Dieu (1418).<br/>The survey utilized both a 2D- and 3D-Chirp system. In both cases, the source was pushed over the<br/>site by divers. Close survey line spacing, accurate navigation and decimeter-scale resolution data<br/>enabled the construction of a pseudo- and full 3D-image of the site. In the second site, the<br/>Yarmouth Roads Wreck (16th century), an acoustic blanking zone, a consequence of the highly<br/>attenuating character of degraded wood, was detected and used to create maps of the buried<br/>remains. The final site concentrated on the detection of peat layers in the English Channel, which<br/>are crux to palaeo-landscape reconstructions. Results suggest that peat buried in fine to medium<br/>grained sediments has an easily identifiable acoustic signature. The seismic data acquired for the<br/>aforementioned case studies was subsequently used for quantitative studies of the buried material.<br/>Firstly, reflection coefficients were calculated and used to determine the degradation state of the<br/>hull remains. Secondly, the Quality factor was calculated for the Yarmouth Roads Wreck site to<br/>objectively demonstrate the highly attenuating character of degraded wood buried in sediments in<br/>comparison to the surrounding sediments.<br/>Overall, this PhD has demonstrated the potential of the Chirp sub-bottom system for<br/>archaeological purposes, by creating (pseudo-) 3D reconstructions and making object<br/>characterization possible.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Plets, R.M.K.