Geometrical characterization of urban fill by integrating the multi‐receiver electromagnetic induction method and electrical resistivity tomography: A case study in Poitiers, France

Abstract : A geophysical survey including electromagnetic induction (EMI) and electrical resistivity tomography (ERT) methods was applied and assessed with a 40‐trench sampling grid to delineate the geometry of an urban fill layer. Classical investigation techniques, such as excavation, offer localized information and suffer from time and budget constraints for environmental assessments. Near‐surface geophysics can provide the required spatial sampling to evaluate the coverage of anthropogenic soils in a time‐effective and quasi‐continuous manner. Fast‐acquisition and high‐spatial‐coverage EMI mapping and high‐vertical‐resolution ERT were implemented to delineate a suspicious urban fill on a 5‐ha site in a suburban region, which will host the buildings of a commercial complex. The ERT data and 40 trench excavations revealed an urban fill thickness ranging from 0.4 to 3.6 m overlying a calcareous substratum. After ERT–EMI calibration, a two‐layer model was introduced into the one‐dimensional (1‐D) inversion of the EMI data for estimating the geometry of the urban fill layer across the study site. The EMI 1‐D inversion results indicated that the predicted urban fill thicknesses were consistent with 70% of the measured values (27 out of 40 excavation sites). Resistive ground, large 3‐D structures and the heterogeneity of urban fill affected the EMI and ERT measurements and increased the difficulty of estimating its spatial distribution. In this paper, we present a measurement protocol that can guide land‐use development and be reproduced to investigate brownfield sites.