Subsurface imaging using a combination of magnetic, ERT, TEM, and CSRMT data at the Weidenpesch waste site in Cologne

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Geophysical characterization of landfills represents an important procedure in site remediation. Nowadays, the use of non-destructive and non-invasive geophysical techniques directed toward detecting the waste body and imaging contamination plumes, has been clearly increased. In the Weidenpesch waste site, northwest of Cologne/Germany, magnetic, ERT, TEM, and CSRMT data were measured in the frame of the DFG project TE 170/21-1. The measurements were delayed for several months (c. 9 months) because of the travel restrictions during the covid-19 epidemic and the decision of changing the investigated landfill chosen in the original plan located in Russia. This influenced the achievement of the planned goals within the allotted time. By the integrated interpretation of magnetic, ERT, and TEM data we were able to successfully determine the geometry of the waste body of the Weidenpesch landfill. Additionally, dumped magnetic and non-magnetic materials were obviously distinguished. The different techniques used for determining the waste site's horizontal boundaries showed a good degree of consistency. The findings demonstrate that, in contrast to the highly resistive hosting gravelly sand layer, the dumped materials have low resistivity values making it easier to image the waste body. An interesting, very conductive layer (less than 5 Ω.m) has been found at deep depths using ERT and TEM data. This layer was interpreted as a wet brown lignite layer based on the available borehole and geological information. It is detected at a depth up of 20-25 m and its base wasn’t detected by the used techniques. The resistivity of this layer could not be resolved. The current proposed project (duration 6 months) will focus on detecting the base of the coal layer using larger loops than the used ones; eight TEM stations with loops of 50m × 50m or even 80m × 80m inside and outside the landfill are planned. Moreover, 2D/3D modelling of TEM data will be carried, and the TEM data obtained from large-loop stations will be integrated in the modelling study. Furthermore, constraints from ERT, magnetic and CSRMT will be utilized. The finite element algorithm CustTEM and the well-known finite difference algorithm SLDMem3t will be applied. Finally, combining the findings from the different used geophysical techniques will aid in improving the imaging of the waste body and, more precisely, in tracing any potential contaminant plumes. Thus, this strategy will be quite useful in risk assessment and constructing the final model of the landfill.