URBAN SOIL EXPLORATION USING ELECTROMAGNETIC INDUCTION AND GROUND PENETRATING RADAR

Abstract

Urban soils are typically made up of composite materials derived from previous uses and exogenous sources and may contain different types of soil pollutants. Therefore, they often show a strongly heterogeneous organization that is hard to reconstruct based on information from a limited number of invasive point observations. Non-invasive geophysical investigation can provide spatially comprehensive soil data, but a single geophysical method often lacks sufficient discriminating power to identify the various soil materials. Urban soils thus require a specific approach combining multiple geophysical methods. As electromagnetic induction (EMI) and ground penetrating radar (GPR) each have an established reputation for solving different types of environmental and engineering problems, we have combined a multi-receiver EMI sensor and a stepped-frequency GPR to investigate the soil of a parking area of a former garage with gas station. Besides indicating the presence of shallow metal objects and foundation remnants of a former building, the combined interpretation of the EMI and GPR measurements allowed to discern a contrasting soil layer at a depth between 0.5 and 1 m below the ground surface. Where the GPR reflections showed the strongest contrast, the EMI measurements indicated a lower electrical conductivity. Based on information from soil borings and sample analysis this anomaly could be related with a higher compaction level and slight concentrations of mineral oil in this part of the contrasting soil layer. This study indicates that the combination of EMI and GPR can provide a powerful tool to improve the reconstruction of the complex organization of urban soils complementary to invasive soil investigation. The approach allows to optimize invasive sampling for the identification of different types of soil disturbance such as mineral oil pollution.