Geophysical Imaging of an Early 19th Century Colonial Defensive Blockhouse

In 1845, the French navy built three blockhouses as part of their defence of French settlers in the Akaroa area, located on Banks Peninsula, near Christchurch, New Zealand. In the 1860’s, the blockhouses were removed and the timber used for other purposes. Two of the blockhouses were situated at either end of the Akaroa township; the locations are well known and documented. The position of the third, in the neighbouring village of Takamatua, is not well known, but is thought to have been sited in what became a public reserve, first known as the Blockhouse Domain and more recently as the Takamatua Domain. To aid local archaeological studies, non-invasive, non-destructive geophysical imaging was carried out across the Takamatua Domain. We expected that little if any of the blockhouse itself remained. However, the nature of the construction was such that we expected to find the defensive trench that enclosed the blockhouse. Using horizontal loop electromagnetic (HLEM), total field magnetic, ground penetrating radar (GPR), and electrical resistivity tomography (ERT) techniques, we identified coincident linear anomalous responses. The clear and unequivocal results of the geophysical surveys suggest that we have indeed found the blockhouse and its surrounding trench or moat.