Geophysical Imaging of an Early Nineteenth Century Colonial Defensive Blockhouse: Applications of EM Directionality and Multi-parameter Imaging

Abstract

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.