Geofysisk kortlægning af komplekse glaciogene stor-skala strukturer

• Main Author: Høyer, Anne-Sophie
• Format: Book
• Language: English
• Published: 2013
• Subjects: Hill Island; Varde; Ice; Ice Sheet; permafrost
• Online Access: https://pure.au.dk/portal/da/publications/geophysical-mapping-of-complex-glaciogenic-largescale-structures(80323ad5-8bcc-455b-ae2f-f455cb3a5cbb).html

This thesis presents the main results of a four year PhD study concerning the use of geophysical data in geological mapping. The study is related to the Geocenter project, “KOMPLEKS”, which focuses on the mapping of complex, large-scale geological structures. The study area is approximately 100 km2, and is situated near the town Ølgod on Varde hill-island in the western part of Jutland, Denmark. The area was chosen because of the complex geology encountered in existing boreholes and pits. Information from the pits formed the background for the earlier proposed glaciotectonic deformation in the area. A unique geophysical data set has been collected in this study consisting of: high-resolution seismic data, airborne transient electromagnetic data (SkyTEM), ground penetrating radar (GPR) and geoelectrical data. Furthermore, data from around 600 boreholes are available, but since the majority of these are very short, a deep investigation borehole was drilled in the framework of the study. Two fundamentally different methods, Seismic and SkyTEM, play a part in all the studies. The methods measure different parameters and therefore provide information on different properties. It can thus be beneficial to acquire both data types and co-interpret them in order to improve our geological understanding. However, in order to perform this successfully, methodological considerations are necessary. For instance, a structure indicated by a reflection in the seismic data is not always apparent in the resistivity data. In cases where contrasts in the sub-surface are measurable by both methods, a priori information can profitably be used from seismic data in the SkyTEM inversions in order to obtain geologically more realistic SkyTEM models. In the study, different inversion approaches were tested, and particularly the incorporating of a priori information resulted in significantly more realistic results. When planning geophysical mapping campaigns, the resolution capabilities of the methods are crucial for deciding which methods to use. In heterogeneous geological environments, dense data with a high degree of detail is required to understand the structures. In practice, however, also the applicability and costs of the methods are crucial. The SkyTEM method is very cost-effective in providing dense data sets, and it is therefore recommendable to use this method initially in mapping campaigns. For more detailed structural information, seismic data can profitably be acquired in certain areas of interest, preferably selected on the basis of the SkyTEM data. In areas where extremely detailed information about the near-surface is required, geoelec¬tri¬cal data (resistivity information) and ground penetrating radar data (structural information) can be collected. The geophysical data are used together with geological analyses from boreholes and pits to interpret the geological history of the hill-island. The geophysical data reveal that the glaciotectonic structures truncate at the surface. The directions of the structures were mapped from the SkyTEM data and reveal that the deformation occurred from two different directions (NNW and NNE). The deforma¬tion most likely took place during the Saalian Norwegian and/or Drenthe Advances. Subsequent to the deforma¬tion, the area has been subjected to extensive glacial erosion followed by periglacial and postglacial erosion that formed the present-day terrain. Paleoglaciological calculations based on the observed thrust structures show that the deformation was caused by a very thick ice sheet with a steep ice profile, resting on permafrost.