The acoustic imaging, reconstruction and characterization of buried archaeological material

Abstract

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

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