Effect of Grain Size on the Geophysical Responses of Indigenous Burial Sites

In the search for unmarked graves, we can eliminate methods, for example gravity and seismic, that lack the resolution to “see” the target graves. Electrical methods can work, but may have practical limitations that preclude their use. The most effective combination of techniques for detecting burials uses magnetic field, electromagnetic (EM) and ground penetrating radar (GPR) methods. Surveys of Maori ancestral burial sites, with both marked and unmarked graves, have allowed us to test when and where geophysical surveys are most likely to succeed. Results from five sites in three coastal settings along the east coast of the South Island of New Zealand show that burials in clay and silt (loess) can be identified using geophysical techniques, but burials in sand do not always yield anomalous responses. The differences in responses are likely due to the depositional setting. Clay and loess are usually deposited as layers or massive beds so any disturbance due to burial is relatively clear. In contrast, near-shore, fluvial and dune sands contain sedimentary structures that can be difficult to distinguish from burials, and can mask the geophysical responses of the graves.