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

Abstract

It can be crucial to know the effectiveness of particular geophysical detection techniques when trying to locate clandestine burials of murder victims. . Unlike in archaeology, there has been limited forensic research with regard to optimum methodologies, with most emphasis to-date on metal detectors and Ground Penetrating Radar by forensic search teams. This may not be suitable in certain soil types, non-metal targets or in significant search areas. Therefore in this study, magnetic and electrical resistivity detection techniques have been utilised 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 parkland, the parkland Medieval grave site acting as a control.The magnetic surveys showed mixed success of 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 dataset. Electrical resistivity surveys produced anomalies over all the simulated burial positions, including surface burials, but it did not produce anomalies at the control site.Laboratory analysis of simulated grave 'fluid' showed an overall increase in iron levels over a year post-burial (from xx to xx) which may account for the observed 3 magnetic anomaly variation. There was also a corresponding increase in grave 'fluid' conductivity which was interpreted to cause the observed resistivity anomalies.Study results have important implications for use of geophysical techniques when searching for clandestine burials. Local depositional environment, soil type, likely style of burial and search area size should all be considered when choosing forensic geophysical detection techniques. Geophysical data could locate a primary deposition site even though no physical evidence remains.4

Preliminary results of sequential monitoring of simulated clandestine graves in Colombia, South America, using ground penetrating radar and botany

“…For each grave, the overlying vegetation was removed and c. 2 m x 2 m holes were dug in a regular pattern (Fig. 2a) as other researchers have undertaken [46][47][48][49][50][51][52]. Soil from both the top and main andisol soil were taken from all graves for analysis.…”

“…Recent geophysical research using simulated clandestine graves have found optimal detection methods and configurations are highly variable, depending upon a host of factors, the most important deemed to be time since burial, burial style, local soil type, vegetation and climate [35,[43][44][45][46][47][48][49][50][51][52][53][54].…”

Collaborative Geophysical Monitoring of Simulated Forensic 'Crime Scenes’ in the U.K.

“…Their test site has been the subject of various decomposition studies and they are replicating the conductivity of decompositional fluid experiment [3]. They also have access to a woodland environment at Lincoln University that has been used for resistivity and magnetic survey studies [20], resistivity anomalies present even when surface remains are not obvious, thought to be decompositional fluids being retained in the local soil.…”

“…These have been repeatedly geophysically surveyed by fixed-offset electrical and Electrical Resistivity Imaging (ERI) surveys as well as multi-frequency (110-900 MHz) GPR datasets at least quarterly, and every month in the first three years post-burial. Studies have been published on determining what causes relative resistivity anomalies [19], magnetic surveys [20], measurements of decompositional fluid conductivities over time [3] as well as the 0-3 years post-burial datasets [6]. Repeat surveys suggest winter surveys are optimal and resistivity surveys should be undertaken six months to two years post-burial for optimal success.…”

“…Here studies have been undertaken for initial decompositional fluid research, magnetic survey trials [20] and optimal geophysical surveys for detecting clandestine burials [21], resistivity and GPR deemed optimal. Staffordshire are also leading inorganic chemical analysis of decompositional fluid collected by the research team, Na/K major indicators and cadaverine/putrescene showing grave detection promise.…”