Using soil and groundwater to understand resistivity surveys over a simulated clandestine grave.

Bulk ground resistance geophysical surveys have been used in a number of attempts to locate clandestine shallow graves, despite only a limited understanding of how the graves may be detected with this technique. In this study, soil and groundwater samples have been obtained from a simulated grave in an urban environment using a pig cadaver, in order to better understand how the presence of a grave may influence the soil bulk electrical properties and hence explain observations based on repeated (time-lapse) resistance survey data collected over another simulated clandestine grave at the same site over a six-month period. An area of low resistance, compared to background values, was observed at the grave location between 4 and 20 weeks postburial. Before and after this ‘time window’, the grave was more difficult to identify. This low resistance grave anomaly appeared to be caused by highly conductive fluids released by the actively decomposing carcass and was consistent with the relatively short timescale during which the grave was detectable. Because of this, it is suggested that the best time to use bulk ground resistance surveys in a search for an urban grave is within the first few months after burial, when a cadaver is most likely to be undergoing active decomposition. However, other authors have observed low resistance anomalies over much older graves that suggests other factors may contribute to longer lasting bulk electrical property affects of grave soil in different environments. Hence, further work is required to supplement this initial study.