Multi-dimensional Resistivity Models of the Shallow Coal Seams at the Opencast Mine 'Garzweiler I' (Northwest of Cologne) inferred from Radiomagnetotelluric, Transient Electromagnetic and Laboratory Data
The entire Cenozoic unconsolidated fill of the Lower Rhine Embayment in Germany hosts the largest single lignite, or brown coal, deposit in Europe which covers an area of some 2,500 km2 to the northwest of Cologne. Rhineland brown coal is mined in large-scale opencast mining and accounts for around one-quarter of the public electricity supply in Germany. The present study was devoted to carrying out radiomagnetotelluric (RMT) and transient electromagnetic (TEM) investigations over the shallow coal seams at the opencast mine 'Garzweiler I.' The main objectives of the survey were to highlight the applicability and efficiency of RMT and TEM methods in an area like brown coal exploration, and to image the vertical electrical resistivity structure of these coal seams. Therefore, the vertical and lateral resolution capabilities of such methods were as necessary as the ability to cover large areas. Consequently, a total of 86 azimuthal RMT and 33 in-loop TEM soundings were carried out along six separate profiles over two opencast benches at the 'Garzweiler I' mine. The local stratigraphy at the survey areas comprises a layer-cake sequence, from top to bottom, of Garzweiler, Frimmersdorf and Morken coal seams embedded in a sand background, consisting of Surface, Neurath, Frimmersdorf and Morken Sands. A considerable amount of clay and silt intervenes the whole succession. The data were interpreted extensively and consistently in terms of one-dimensional (1D) RMT and TEM resistivity models, without using any complex multi-dimensional interpretation. However, the presence of thin, surficial clay masses (or lenses) broke down such interpretation scheme. In this case, to greatly improve the resistivity resolution for these surficial masses and the underlying coal seams, two-dimensional (2D) RMT and three-dimensional (3D) TEM interpretations have been carried out. They could be used effectively to study the local EM distortion on the measured data, where these surficial masses were found, as well as to cross-check the nearby-topography effect. Because the RMT data are usually skin-depth limited, they only provided a resolution depth between 25 and 30 m for the shallow resistivity structures. Whereas, the TEM data still have sufficiently early- to late-time information, and therefore resulted in a better resolution depth of about 100 m for the shallow to sufficiently-deep resistivity structures. The final 1D/2D RMT and 1D/3D TEM resistivity models displayed a satisfied correlation with both thicknesses derived from the stratigraphic-control boreholes and resistivities measured from direct-current (DC) and spectral induced polarization (SIP) laboratory techniques on 16 rock samples. As demonstrated, the integrated use of azimuthal RMT and in-loop TEM soundings was highly successful and effective at mapping the major stratigraphic units at the survey areas, i.e. the shallowest conductive Garzweiler and Frimmersdorf Coals within their fairly resistive sand background. They could not distinguish between Neurath Sand and the underlying sand/silt or between Frimmersdorf Coal and the underlying organic clay. The deepest Morken Coal was beyond the depth-of-investigation of the present measurements. Finally, the resistivity models revealed that both coal seams gently dip in the southwesterly direction. This should be in fairly good agreement with the regional structural makeup of the Rhineland brown coal. However, they showed that Garzweiler Coal is gradually thinned northeastwards, while Frimmersdorf Coal still has almost a regular thickness
