Combining EMI and GPR for non‐invasive soil sensing at the Stonehenge World Heritage Site: the reconstruction of a WW1 practice trench

Abstract

SummaryIncreasingly, conventional soil sampling procedures face restrictions because of their destructive character. Hence there is a growing interest in non‐invasive techniques, on which proximal soil sensors are based. There is great interest in applying proximal soil sensing to improve the characterization of the buried heritage embedded in the soil landscape at sites such as the Stonehenge World Heritage Site, UK. Because calibration and validation procedures based on invasive practices are unconventional, we turned to the investigation of a well‐documented practice trench dug during the First World War (WW1) close to the prehistoric Stonehenge monument. A methodology was tested that would simultaneously invert frequency‐domain ground‐penetrating radar (GPR) and multi‐receiver electromagnetic induction (EMI) data, with the aim of reconstructing the trench network. This trench network could not be distinguished on the EMI apparent electrical conductivity (σa) measurements, but appeared on the apparent magnetic susceptibility (κa) data. The GPR measurements showed the trench infilling as strong reflections contrasting with the surrounding soil. However, converting the two‐way travel times to absolute depths requires knowledge of the relative permittivity (εr). Because of the preference for non‐invasive observation in this protected landscape, we developed a procedure integrating the GPR measurements with κa measurements obtained with EMI. A fitting procedure, assuming a constant susceptibility and permittivity of the sub‐surface layers, allowed us to estimate both the susceptibility of the trench fill and the surrounding soil material, and the εr value of the material above and within the trench. This provided absolute depth values for the GPR reflection data, improving the lateral and vertical reconstruction of the trench system. Moreover, these results allowed depth slices to be determined from EMI κa data. So, integrating both GPR and EMI measurements enabled the detailed reconstruction of the buried trench network at Stonehenge, offering new perspectives on the investigation of features buried within the soil of protected sites.