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

垃圾填埋场的地球物理表征是场地修复的一个重要步骤。目前，利用非破坏性和非侵入性地球物理技术探测废物体和成像污染羽流的情况明显增加。在德国科隆西北部的Weidenpesch废物场，在DFG项目TE 170/21-1的框架内测量了磁、ERT、TEM和CSRMT数据。由于covid-19流行期间的旅行限制以及决定改变原计划中选择的位于俄罗斯的被调查垃圾填埋场，测量推迟了几个月（c. 9个月）。这影响了在规定时间内实现计划目标。通过对磁性、ERT和TEM数据的综合解释，我们能够成功地确定Weidenpesch填埋场废物体的几何形状。此外，倾倒的磁性和非磁性材料也有明显的区别。用于确定废物场地水平边界的不同技术显示出良好的一致性。研究结果表明，与高电阻率的砾石砂层相比，倾倒材料的电阻率值较低，使废物体更容易成像。利用ERT和TEM数据在深部发现了一个有趣的导电层（小于5 Ω.m）。根据现有钻孔和地质资料，将该层解释为湿褐褐煤层。它是在20-25米的深度探测到的，使用的技术没有探测到它的底部。该层的电阻率无法求解。目前拟议的项目（工期6个月）将侧重于使用比现有环路更大的环路探测煤层底部；规划填埋场内外50m × 50m甚至80m × 80m回路的TEM站8个。此外，将对瞬变电磁法数据进行二维/三维建模，并将大回线站的瞬变电磁法数据整合到建模研究中。此外，将利用ERT、磁性和CSRMT的约束。将采用有限元算法CustTEM和著名的有限差分算法SLDMem3t。最后，结合不同使用的地球物理技术的发现将有助于改善废物体的成像，更准确地说，有助于追踪任何潜在的污染物羽流。因此，该策略对垃圾填埋场的风险评估和最终模型的构建具有重要意义。

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.