Kennedy Doro, assistant professor at the University of Toledo, conducts his research at Texas State University, which operates a 26-acre human decomposition and forensic taphonomy facility located at the Freeman Ranch in San Marcos. Credit: Kennedy Doro, The University of Toledo
Researchers at the University of Toledo have validated an innovative method that could help law enforcement, forensic scientists and anthropologists identify unmarked mass grave sites with much less guesswork than is currently typical.
“Engaging in excavation without useful leads on the exact location of the graves means excavating large areas, which is difficult, time-consuming and destructive,” said Kennedy Doro, assistant professor at the University of Toledo and lead investigator of the study. “It could also lead to altering the target evidence.”
Instead, Doro and his team worked to validate a method based on electrical resistivity, a measure of how strongly a material opposes the flow of electric current, in combination with ground-penetrating radar.
“A central hypothesis of our study [was] that the burial and decay of human remains with time causes changes in the soil physical and biochemical properties, including porosity, electrical conductivity, temperature and redox potential,” the authors write in their study, published in Forensic Science International.
Previous studies on grave excavations have mostly assessed individual graves using pigs as proxies for humans. Doro’s team, however, designed an experiment that set itself apart from the crowd. At Texas State University’s Forensic Anthropology Research Facility. the research team dug a mass grave for six human remains, three individual graves and two empty control graves.
Before they dug the graves, however, the team used ground-penetrating radar, electrical resistivity tomography and electromagnetic imaging on the undisturbed ground, recording all information for post-burial comparison. Additionally, prior to placing the bodies in the graves, the researchers analyzed soil profiles and added high-tech sensors capable of monitoring changes in soil moisture, temperature and electrical conductivity.
According to the study results, electrical resistivity increased around all three types of graves three days after burial, but decreased when measured 1-, 2- and 6-months post-burial. Although significant decreases were noted in all graves that held human remains, the decrease was highest in the mass grave.
“The decrease is more rapid for the first two months than six months after burial. We interpret the decrease to result from…decay of the human cadavers, which would have produced a conductive leachate from liquified soft tissues,” the team explains in their paper.
Additionally, recorded electrical conductivity in the mass grave showed a sharp increase in the first 75 days post-burial. The value then decreased gradually around 190 days after burial. The researchers say this proves the potential of electromagnetic imaging for locating graves during the early post-burial stage since the conductivity contrast is high enough to isolate the graves from the surrounding undisturbed soil.
During the experiment, ground-penetrating radar (GPR) helped reveal the position of the graves, however, the technology was interrupted by an abundance of rock fragments, leading the team to conclude that the success of GPR is entirely dependent on the geology of the site. Texas soil filled with limestone bedrock was not an easy task for GPR.
Still, the results of the study validate the potential of all of the techniques, even if electrical resistivity tomography proved superior in most cases.
“Comparing the capabilities of ground-penetrating radar, electrical resistivity tomography and electromagnetic imaging to locate the graves in this study show that, given the soil condition, the electrical resistivity tomography results were better at identifying the grave locations at all post-burial measurement times,” Doro and his team concluded.
The researchers said they will continue to monitor the graves on a quarterly basis for the next 12 months.