Mark och borrhåls geofysik kombinerat för att modellera variationer av kvicklera och omkringliggande sediment vid ett jordskredsområde i södra Sverige

Today many natural disasters around the world are due to landslides. Landslides can be both life threatening and pose high extra expenses due to damages made on houses, etc. In Sweden are landslides often connected to a particular soil type called quick clay. Even though this type of soil and its connection to landslides have been investigated for a long time, more knowledge about the geological structures that control and trigger such events are still crucial. In Fråstad, an old landslide site in the southwest of Sweden, a considerable number of different investigations such as geotechnical and geophysical measurements have been made to get a better understanding about the key structures. In this report, results from direct current resistivity (DCR) and electromagnetic (EM) methods have been presented, to create resistivity sections with detailed information about the variation of the physical properties of the subsurface structures. One profile has together with borehole and reflection seismic data been analyzed and compared to results from previous investigations in the area. The analyses were used to determine spatial variations in the overburden and to evaluate the usefulness of these methods.Geoelectrical multiple electrode data, often referred to as electrical resistivity tomography (ERT) data were acquired with a small electrode distance of 2 m to increase the vertical and lateral resolution. However, the resulting resistivity model does not provide any significant extra information compared to the radio magnetotelluric (RMT) data collected along the same profile with station spacing of 20 m. The ERT method is more sensitive to high resistivity structures and the RMT to the low resistivity structures and therefore a combination can be useful to detect both types of structures. Joint inversion of ERT and RMT was also tested, but the large differences between the station spacing caused oscillatory artifacts in the final resistivity model from the inversion. Due to very low resistivity of overlying marine clays the depth of investigation was limited to about 30m and neither the ERT nor the RMT were able to resolve the resistive crystalline bedrock along the whole profile. When adding controlled source magnetotelluric (CSMT) data to the inversion the depth resolution was substantially improved. By using resistivity models alone no obvious structures could be defined as quick clay, however extensive parts of the model could be interpreted as marine clay due to its low resistivity. Based on the RMT data, a layer of higher resistivity was found and this coincides well with a coarse-grained layer resolved in the borehole measurements and the seismic section. This study shows that combining resistivity models with reflection seismic and borehole data improved the interpretation and imaging of the geological structures existing in the area.Validerat; 20140819 (global_studentproject_submitter