Semblance analysis to assess GPR data from a five year forensic study of simulated clandestine graves
Abstract Ground penetrating radar (GPR) surveys have proven useful for locating clandestine graves in a number of forensic searches. There has been extensive research into the geophysical monitoring of simulated clandestine graves in different burial scenarios and ground conditions. Whilst these studies have been used to suggest optimum dominant radar frequencies, the data themselves have not been quantitatively analysed to date. This study uses a common offset configuration of semblance analysis, both to characterise velocity trends from GPR diffraction hyperbolae and, since the magnitude of a semblance response is proportional to signal to noise ratio, to quantify the strength of a forensic GPR response. 2D GPR profiles were acquired over a simulated clandestine burial, with a wrapped pig cadaver monitored at three month intervals between 2008 and 2013 with GPR antennas of three different centre frequencies (110, 225 and 450 MHz) The GPR response to the cadaver was a strong diffraction hyperbola. Results show, in contrast to resistivity surveys, that semblance analysis have little sensitivity to changes attributable to decomposition, and only a subtle influence from seasonality: velocity increases (0.01 0.02 m/ns) were observed in summer, associated with a decrease (5 10% in peak semblance magnitude, S M and potentially in the reflectivity of the cadaver. The lowest frequency antennas consistently gave the highest signal to noise ratio although the grave was nonetheless detectable by all frequencies trialled. These observations suggest that forensic GPR surveys could be undertaken with little seasonal hindrance. Whilst GPR analysis cannot currently provide a quantitative diagnostic proxy for time since burial, the consistency of responses suggests that graves will remain detectable beyond the five years shown here.