Geophysical mapping of subsurface archaeological features at the Port Arthur Historic Site, Tasmania, Australia

The Port Arthur Historic Site, located in southeastern Tasmania, is a significant heritage-listed site which operated as a major convict settlement between 1830 and 1877. It was largely destroyed in the intervening years due to bushfires and demolition activities. Multi-disciplinary research over the past 30 years has generated a rich contextual framework of knowledge about the 19th century convict and post-penal periods, however information about buried physical evidence of the associated buildings, landscape features and cultural deposits that remain in the subsurface is very limited. A range of geophysical techniques for mapping subsurface cultural features of archaeological value were assessed at three areas within the Port Arthur Historic Site: the Isle of the Dead, Settlement Hill and the Penitentiary Complex. Each of these areas is located within a distinct geological domain, and hosts different archaeological targets and near-surface stratigraphic conditions. Geophysical results were compared with historical documents, including maps and photographs, and oral sources. Archaeological ground-truthing was also employed to explore some features identified at Settlement Hill and the Penitentiary Complex. Research on the Isle of the Dead cemetery aimed to map individual burials, former pathways and other cultural elements, and major stratigraphic interfaces. Prior knowledge of the cemetery layout on the 0.8 hectare island is limited to historic photographs, surface evidence (grave markers and depressions) and incomplete documentation ‚ÄövÑvÆ estimates of the total number of graves range from 1100 to over 1700. Magrtetometry and frequency-domain electro-magnetics were effective in delineating near-surface ferrous cultural features such as former fence lines and paths but were ineffective for detecting burials. Resistivity imaging and seismic refraction tomography helped to resolve the stratigraphic context but were also ineffective for direct detection of burial sites. Ground penetrating radar (GPR) was the most effective technique for locating both individual burials and zones of stratigraphic disturbance. Radar data were very complex and the final interpretation product was a series of maps that depict the density of subsurface ground disturbance rather than more conventional products that illustrate the distribution of hyperbolic responses or amplitude time-slice maps. At Settlement Hill the main aim was to delineate subsurface structural remains in an area with multiple generations of convict era construction. Much of the site is characterised by a thin layer of demolition rubble. Magnetic data in this area is typified by high-amplitude, high-frequency anomalies attributable to natural variations in magnetic dolerite bedrock depth, as well as variations due to penal era cultural features such as terrace excavations, trenches and an aqueduct. Clear rectilinear anomalies in the 500MHz GPR timeslices and apparent resistivity variation map are attributed to very shallow well-preserved structural features, which correlate closely to the building layout available in several historic maps. Surveys were conducted at two sites in the Penitentiary complex: the Sawpits - Tannery Complex and the Parade Ground area. Both sites are characterised by heterogeneous penal and post-penal fill material. At the Sawpits - Tannery Complex, 500 MHz GPR profiling and electrical resistivity tomography successfully map foundation walls, sawpit fill deposits, yard features and reclamation structures. The rubble fill stratigraphy apparent in the GPR data also provides indirect evidence of the Sawpits boundary. Resistivity tomography also clearly defines the interface between reclamation fill and underlying remnant Quaternary beach sands. Integrated interpretation of multiple geophysical datasets from the Penitentiary Parade Ground area enabled the detection and characterisation of a diverse range of penal era archaeological targets, including sections of the parade ground wall, the parade ground gravel surface and remnants of the tramway. The most effective techniques in this complex environment are apparent resistivity and GPR, while the magnetic response is complicated by the presence of magnetic dolerite fill material. A range of geophysical techniques have been successfully applied at the Port Arthur Historic Site for detection and characterisation of a diverse range of subsurface cultural heritage features. However, there is no single technique or recipe for future archaeo-geophysical activities that can be readily applied across the entire site. The choice of the optimal technique or combination of techniques for a particular site should be based on a prior assessment of the local geological conditions and likely target characteristics.