f 3-D HEM Modeling Studies in Northern Germany

AEM data sets are used for both geophysical and geological modeling. For generating geophysical (resistivity) models, commonly 1-D inverse modeling procedures are applied. In many cases such 1-D models are sufficient, e.g., for horizontally layered targets and groundwater applications. In case of strong 3-D targets, however, this may result in misleading interpretation due to strange 1-D models, hence multi-dimensional modeling and inversion is required. The number of 2-D and 3-D AEM modeling and especially inversion codes is limited and they are also often subject to restrictions. Limiting are mainly the requirements related to the allowed resistivity structure but also the required memory and computation time to run models of decent size. Thus, real 3-D modeling is often restricted to ‘simple’ resistivity models. Full 3-D AEM inversion codes are generally not freely available. On the other hand, geological modeling uses diverse commercially available software packages. The direct integration of AEM resistivity models in geological models, however, is still challenging due to both appropriate codes and an often non-unique relationship between resistivity and lithology. In order to get reasonable resistivity models suitable for 3-D geological modeling we try to model AEM data in 1-D to the greatest possible extent and therefore restrict 3-D modeling to areas of strong resistivity anomalies only. Those limited 3-D areas are identified either manually or automatically. The quasi 1-D data are inverted using standard 1-D inversion procedures and the anomalous data are modeled in 3-D using forward or inverse modeling procedures. Once a satisfying 3-D model is obtained it needs to be integrated into the quasi 1-D environment of the remaining data set. The challenging task is, besides 3-D inversion itself, to define those areas where 1-D inversion fails and to extract and integrate the 3-D data and models, respectively. The 3-D geophysical and geological modeling is applied to spatial data measured by geophysical surveys, particularly an HEM survey flown by BGR in Northern Germany in 2000. There, the melt-water flow carved the North-South orientated Cuxhaven buried valley into Tertiary sediments during Pleistocene glacial regression epochs of the Elsterian glaciation.