Development of high-resolution 3D geological subsurface models based on airborne electromagnetic data

case studies from the Cuxhaven tunnel valley and the Lutter anticline, northern Germany

authored by: Dominik Steinmetz
supervised by: Jutta Winsemann
Abstract: Airborne electromagnetics (AEM) is an efficient technique for geophysical investigations of the shallow subsurface and has successfully been applied in various geological settings to analyse the depositional architecture for groundwater and environmental purposes. However, interpretation of AEM data is often restricted to 1D inversion results imaged on 2D resistivity maps and vertical resistivity sections. The integration of geophysical data based on AEM surveys, with geological data is often missing and consequently this deficiency leads to uncertainties in the interpretation process. The aim of this thesis is to provide an improved methodology for the geological interpretation of AEM data and the construction of more realistic 3D subsurface models. This is achieved by the development of integrated workflows and 3D modelling approaches in the Paradigm® GOCAD software, based on the combination of different geophysical and geological datasets (airborne electromagnetic data (AEM), depth maps from the digital Geotectonic Atlas of Northwestern Germany and the German North Sea, 2D reflection seismic sections and well logs). The results of 1D AEM inversion were geostatistical analysed and interpolated in a 3D resistivity gridding procedure to create continuous 3D resistivity grids of the subsurface. Subsequently, geological interpretations have been performed by combining with, and validating against geological depth maps, borehole and reflection seismic data and high-quality topographic maps. For the research, two test sites with different geological settings were chosen. The primary aim of the first study area near Cuxhaven, northwest Germany, was to test the 3D resistivity gridding procedure for unconsolidated rock, where Neogene sediments are incised by a Pleistocene tunnel valley. The 3D resistivity grids clearly allow to distinguish between different lithologies and enabling the detection of major bounding surfaces and architectural elements. The Neogene succession is subdivided by four unconformities and consists of fine-grained shelf to marginal marine deposits. At the end of the Miocene, an incised valley was formed and filled with Pliocene delta deposits, probably indicating a paleo-course of the River Weser. The Middle Pleistocene (Elsterian) tunnel valley is up to 350 m deep, 0.8-2 km wide, and incised into the Neogene succession. The unconsolidated fill of the Late Miocene to Pliocene incised valley probably formed a preferred pathway for the Pleistocene meltwater flows, thus favouring the incision. Based on the 3D AEM resistivity models the tunnel valley fill could be imaged in high detail. It consists of a complex sedimentary succession with alternating fine- and coarse-grained Elsterian meltwater deposits, overlain by glaciolacustrine (Lauenburg Clay Complex) and marine Holsteinian interglacial deposits. The applied approaches and results show a reliable methodology, especially for future investigations of similar geological settings. The aim of the second case study was to test a method for the predictive mapping of rock types and fracture orientations in sedimentary rocks using a trend analysis of 3D resistivity pattern based on airborne electromagnetic high-resolution data. For areas with limited exposure, the airborne geophysical data approach is an important method for both, regional-scale geological mapping and local structural analysis. The method was tested in the area of the Lutter anticline structure in the northwestern Harz foreland, Germany. The developed 3D resistivity grid of this salt-cored anticline was used to map resistivity trends that are related to lateral and vertical changes in lithology, water content, anticline geometry, and the location of fractures. The 3D resistivity grid clearly allows to distinguish between rocks of Palaeozoic and Mesozoic rocks. Lineation patterns obtained from a curvature trend analysis based on the 3D resistivity grid reflect the orientation of the local fault and fracture systems. The comparison of the resistivity pattern and the trend of fractures and faults, derived from outcrop analyses, shows promising results, which imply that AEM data can allow the detection and visualization of near-surface, brittle, structural elements developed in sedimentary rocks. This opens the door to use AEM as an efficient tool for regional structural mapping.
Organisation(s): Institute of Geology
Type: Doctoral thesis
No. of pages: 118
Publication date: 2019
Publication status: Published
Sustainable Development Goals: SDG 14 - Life Below Water
Electronic version(s): https://doi.org/10.15488/7462 (Access: Open)