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. T he locations of some older graves are no longer known, partly through the l oss 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 o ccur elsewhere in the world. The knowledge that Maori graves are traditiona lly set out facing east made survey design easier. Background trends are re moved, and possible anomalous features are identified, despite the complica ting presence of metal fencing. The EM in-phase response, the vertical grad ient of the total magnetic field, and the GPR response were particularly us eful in combination. GPR surveys were carried out using two frequencies: 45 0 and 200 MHz. The higher frequency radar surveys were limited in extent be cause these data had a limited depth of penetration and were prone to signa l "ringing" in the surficial clay. The lower frequency radar surveys were a lso troubled by ringing. Nonetheless, anomalous features, especially diffra ctions, can be recognised. The radar anomalies were enhanced by removal of an average background response. The comparison and correlation of the indiv idual data sets are used to estimate the positions of unmarked graves, usin g marked graves for calibration. Some interpreted positions of graves are a t first surprising, but their locations are consistent with elements of the oral history of the site.