Integrating GPR and EMI to three-dimensionally reconstruct a WW I training trench at Stonehenge

Given the upcoming centennial commemoration of the beginning of the Great War (WW I), the international attention for this conflict grows considerably. Questions will be raised about the nature of this heritage. In this study, we focus on a WW I training trench which was built in the Stonehenge (UK) World Heritage landscape where no fighting took place. Frequency-domain GPR and multi-receiver EMI were used to characterize this trench network in 3-D. The GPR measurements showed the trench infillings as strong reflections contrasting with the surrounding loess-chalk soil. However, converting the two-way travel times to absolute depths requires knowledge of the relative permittivity ε. Due to the impossibility to perform invasive observations in this protected landscape, we developed a procedure to integrate the GPR measurements with EMI magnetic susceptibility (κ) measurements. A fitting procedure allowed to estimate both the of the trench infilling and the surrounding soil material, and the ε of the material above and within the trench. This rendered absolute depth values to the GPR reflection data, improving the exact characterization of the trench system. Moreover, these results allowed to obtain depth-slices from EMI κ_a data. So integrating both GPR and EMI measurements enabled the 3-D reconstruction of the buried trench network.