Investigating soil pollution with the aid of EMI and GPR measurements

The non-invasive detection, delineation and characterization of soil pollution is still largely experimental. Single soil sensor methods often fall short in providing sufficiently comprehensive information. Therefore, we used a combination of a multi-receiver electromagnetic induction (EMI) system, measuring the apparent electrical conductivity (ECa), and a stepped-frequency GPR operating over a frequency range of 50 to 1500 MHz with an antenna-array of 13 antenna pairs, 1 m wide. We investigated a 0.1 ha parking area close to a former petrol station. Earlier invasive investigations had indicated the presence of mineral oil in the upper few meters of the soil in the area next to the parking area. The EMI measurements indicated a zone of lower electrical conductivity and, within the same zone, the GPR reflections revealed the presence of contrasting material. Based on terrain inspections and analyzed soil samples, the observed contrast could be associated with the presence of elevated concentrations of mineral oil. We concluded that the combined interpretation of EMI and GPR signals has a strong potential to complement invasive field investigations of soil pollution.