Electromagnetic survey of old landfill boundaries—A case study

Abstract This case study examines the mapping of old landfill boundaries using a short‐coil spacing frequency domain electromagnetic (EM) system in western Finland. Geophysics was used to supplement the preceding soil drillings, test pits and samples taken. In many places at the site area, there were difficult access conditions and obstacles that flexible moving geophysical surveying could bypass and avoid. Short‐coil spacing EM mapping with three frequencies was chosen as the method of investigation. The frequency‐sounding effect was analysed and shown that it is present under typical conditions met. The depth of investigation can also be calculated with the specific system parameters specified. Frequency‐traverse tabulated graphical presentation of apparent resistivities was aefficient method to present EM results for qualitative assessment. In practice, on‐site applicability was first tested with two reference lines for natural ground and landfill area characteristics. The results showed clear differences in terms of resistivities, as well as the real (Re) and imaginary (Im) component values, Re/Im ratios and signal characteristics. The main survey consisted of another 20 investigation lines at various places intersecting presumed landfill boundaries. Interpretation of the data and point‐by‐point classification were based on reference line data and inferred waste disposal material indicators. The quality of the data was assessed with calibration points and static offsets determined with the help of resistive bedrock point. Finally, EM data values were compared against test pits and drillings at close proximity, supporting the reliability of the EM survey and the differentiation between waste and contamination made. Layer modelling was tested to study the depth penetration and to assess whether additional information could be produced from calculated resistivities and magnetic material indications.