Geophysical surveys of burial sites: A case study of the Oaro urupa

A Maori family burial site was surveyed using shallow electromagnetic (EM), magnetometer/gradiometer, and ground-penetrating radar (GPR) techniques. The locations of some older graves are no longer known, partly through the loss of markers and partly through the loss of the oral record. There are at least 150 such burial sites throughout New Zealand, and similar problems occur elsewhere in the world. The knowledge that Maori graves are traditionally set out facing east made survey design easier. Background trends are removed, and possible anomalous features are identified, despite the complicating presence of metal fencing. The EM in-phase response, the vertical gradient of the total magnetic field, and the GPR response were particularly useful in combination. GPR surveys were carried out using two frequencies: 450 and 200 MHz. The higher frequency radar surveys were limited in extent because these data had a limited depth of penetration and were prone to signal "ringing" in the surficial clay. The lower frequency radar surveys were also troubled by ringing. Nonetheless, anomalous features, especially diffractions, can be recognised. The radar anomalies were enhanced by removal of an average background response. The comparison and correlation of the individual data sets are used to estimate the positions of unmarked graves, using marked graves for calibration. Some interpreted positions of graves are at first surprising, but their locations are consistent with elements of the oral history of the site.