Comparisons of magnetic and electrical resistivity surveys over simulated clandestine graves in contrasting burial environments

摘要

Determining the effectiveness and limitations of near-surface, non-invasive geophysical techniques is imperative when attempting to locate clandestine burials. Unlike in archaeology, there has been limited forensic research with regard to optimum methodologies, with most emphasis to date being on metal detectors and ground-penetrating radar. However, these techniques may not be suitable in certain soil types (e.g., conductive or highly magnetic) or for certain non-metallic targets. Therefore, in this study, magnetic and electrical resistivity detection techniques have been utilized over different aged (0.25-1 year) simulated clandestine burials with no buried metal, in contrasting depositional environments. These environments included semi-rural, urban, woodland and a parkland medieval grave site acting as an archaeological analogue. The magnetic surveys showed mixed success in detecting clandestine burials. Elevated magnetic gradient readings, with respect to background values, were observed over very shallow burials, whereas deeper burials displayed a reduction in gradient and/or no associated magnetic anomalies. Magnetic anomalies were observed over surface-burials and validated by simple 2D forward modelling. Magnetic anomalies were also observed in the control data set. Electrical resistivity surveys produced anomalies over all the simulated burial positions, including surface burials but did not produce anomalies at the archaeological analogue site. Laboratory analysis of fluid retrieved from simulated graves showed an overall increase in iron levels over a year post-burial, which may account for the observed magnetic anomaly variation. There was also a corresponding increase in grave 'fluid' conductivity, which was interpreted to be the cause of the observed low resistivity anomalies. This research suggests that, as a technique for locating clandestine burials, bulk ground resistivity is more successful than the tested magnetic methods. Moreover, magnetic techniques are more effective when used as part of a multi-technique study over rural and semi-rural sites that are relatively low in magnetic and electrical 'noise'. These results have important implications for the use of geophysical techniques when searching for clandestine burials. We emphasize that local depositional environment, soil type, likely style of burial and search area size should all be considered when choosing forensic geophysical detection techniques. We also provide evidence to show that geophysical data can assist in locating a primary deposition site even when no physical evidence remains.