Reconstructing palaeochannel morphology with a mobile multicoil electromagnetic induction sensor

Abstract Field methods to map and reconstruct the morphology of buried river systems are highly dependent on spatial interpolation. Conventional methods, such as standard borehole survey, allow a detailed vertical reconstruction of the shallow subsurface but leave lateral connections between sample locations open to interpretation. Geophysical survey techniques have recently introduced more detail. Mobile electromagnetic induction (EMI) survey combines high density sampling with full lateral coverage but fails to produce detailed information about vertical facies changes. Recently, multicoil EMI survey added vertical discrimination potential to this lateral continuity. In this study, we present an integrated approach for reconstructing the morphology of a known palaeochannel segment by modelling the depth to the sandy substrate. In addition, a calibration method based on a limited number of auger data is proposed. In a first phase, the modelling procedure was evaluated along two transects on a test site, showing palaeochannel depths ranging from 1 to > 4 m beneath the surface. In a second phase, the morphology of the entire site was reconstructed. These three resulting depth models were then compared with auger observations and electrical resistivity tomography (ERT) data. The high correlation coefficients (> 0.9) between observed and modelled depths showed that even in complex pedological environments, palaeochannel morphology could be predicted precisely using multicoil EMI data. Therefore, we concluded that a multicoil EMI survey proves to be an efficient and reliable solution for mapping and reconstructing the morphology of the shallow subsurface.