Radiomagnetotelluric and Direct Current Resistivity measurements for the characterization of conducting soils
Résumé
The application of geophysical techniques to hydrogeological questions has grown in importance due to the increasing concern for environmental issues. To improve the hydrogeological image of the subsurface we carried out electric and electromagnetic measurements using Radiomagnetotellurics and Direct Current Resistivity in East-Frisia, North-West Germany. The aim of our experiments was to compare the two methods and improve the hydrogeological image of the subsurface. This allows us to assess the threat groundwater exploitation poses to wetland habitats (biotopes). Pumping groundwater from deep aquifers can drain water from near surface aquifers. Hence it is vital to know ground water confining units. Prior to our measurements the hydrogeological image of those confining units was based on interpolation between boreholes. Some single Direct Current sounding measurements were also carried out and were used with the information from the boreholes to generate a hydrogeological model of the area. By combining the results from our measurement profiles with the hydrogeological model we were able to revise the hydrogeological model. By using the geophysical modeling results we could define regions where the hydrogeological model is inadequate due to errors in interpolation. We apply both Direct Current Resistivity and Radiomagnetotellurics and perform a practical comparison of the two methods. To ensure the comparability of the resulting resistivity models we examine the depth of exploration of the two methods and demonstrate the application of the depth of exploration index to Radiomagnetotelluric data. Both methods detect conducting structures, which correlate with low permeable clay structures in the hydrogeological models.