Geophysical Imaging of an Early Nineteenth Century Colonial Defensive Blockhouse: Applications of EM Directionality and Multi-parameter Imaging
In 1845, the French navy built three blockhouses as part of their defence of French settlers in Akaroa, located on Banks Peninsula, near Christchurch, New Zealand. In the late 1860s, the blockhouses were removed and the timber used for other purposes. Two of the blockhouses were situated at either end of Akaroa township; their locations are well known and documented. The position of the third, in the village of Takamatua, near Akaroa, was not well known, but was thought to have been situated in what became a public reserve, first known as the Blockhouse Domain and more recently as the Takamatua Domain.
To support local archaeological studies, non-invasive, non-destructive geophysical imaging was carried out across the Takamatua Domain. We used a complementary combination of horizontal loop electromagnetic (HLEM), total field magnetic, ground penetrating radar (GPR), and tomographic electrical imaging (EI) techniques. We expected that little if any of the blockhouse itself would remain. However, the nature of the construction was such that we expected to find the defensive trench that enclosed the blockhouse.
Like many geophysical techniques, HLEM methods exhibit directionality in their responses, i.e. the response depends on the orientation of the instrumentation relative to the target orientation. This directionality can be used to enhance the detection of linear features. When we combined our suite of complementary geophysical methods, we identified coincident linear anomalous responses. The clear and unequivocal results of the geophysical surveys indicate that the blockhouse and its surrounding trench or moat have been found.
DCN thanks his field assistants, dubbed “Team Radar” by one of its members: Richard Cooksey and Jon Lapwood. The entire survey team wish to thank the Akaroa Museum and the Takamatua community for their support and hospitality, as well as Gill and Trevor Bedford, who provided beds and meals, John Roe, who also assisted in the field, and Jean and Neville Rogatski, who supplied us with many cups of tea and trays of biscuits.
- Field G, Leonard G, Nobes DC (2001) Where is Percy Rutherford’s grave? In Jones M, Sheppard P (eds) Australasian connections and new directions: Proceedings of the 7th Australasian Archæometry Conference. Research in anthropology and linguistics, vol 5. University of Auckland, pp 123–140Google Scholar
- Jacobson WEM (1940) Akaroa and Banks Peninsula, 1840–1940: Story of French Colonising venture and early whaling activities. Akaroa Mail, AkaroaGoogle Scholar
- Kearey P, Brooks M (1991) An introduction to geophysical exploration, 2nd edn. Blackwell Scientific Publications, MaldenGoogle Scholar
- Maling PB (1981) Early sketches and charts of banks Peninsula, 1770–1850. Reed, WellingtonGoogle Scholar
- McNeill JD (1980) Electromagnetic terrain conductivity measurement at low induction numbers. Technical Note TN-6, Geonics Ltd, MississaugaGoogle Scholar
- Nobes DC (2007) Detecting linear features using the directionality of the HLEM response. In: Proceedings of Near Surface 2007: 13th European Meeting of Environmental and Engineering Geophysics, Paper A23, IstanbulGoogle Scholar
- Reynolds JM (1997) Applied and environmental geophysics. Wiley, ChichesterGoogle Scholar
- Tremewan P (1990) French Akaroa. Canterbury University Press, ChristchurchGoogle Scholar
- Tremewan P (2010) French Akaroa: an attempt to colonise Southern New Zealand, 2nd edn. Canterbury University Press, ChristchurchGoogle Scholar
- Walton A (2003) New Zealand Redoubts, Stockades and Blockhouses, 1840–1848. Department of Conservation Science Internal Series No. 122, WellingtonGoogle Scholar