2D Reflection Seismic Imaging at a Quick-clay Landslide Site, in Southwest Sweden

Abstract: Results from a series of 2D high resolution reflection seismic profiles collected at a quick-clay landslide site are presented. This study is a part of the “Integration of geophysical, hydrogeological and geotechnical methods to aid monitoring landslide in Nordic countries” project, sponsored by the (Geoscientists Without Borders) program of Society of Exploration Geophysicists (SEG). The study area is located on the shoreline of the Göta River, about 60 km north of Göteborg. The Göta River is the largest river in Sweden which runs from Lake Vänern to Göteborg, it follows the Götaälv Zone, which is an approximately 4 km wide fault zone dipping towards the west. The site is known for its quick-clay formation and landslides. The aim of this study is to image bedrock topography and the overburden layerings above it, within the overburden layers of specific interest are a coarse-grained layer and a quick-clay layer which is responsible for “quick-clay landslides”. The area was recently studied by the Swedish Geotechnical Institutes in a nation-wide project that dealt with investigating areas along the Göta River that are prone to landslides. The study area was investigated by various geotechnical (CPTU, CPTU-R soundings, laboratory measurements) and electrical resistivity investigations. Results from this investigation are used to interpret shallow seismic reflections and better understand the near surface geology. The seismic reflection data presented in this thesis were acquired along two profiles, 484 m and 384 m long (lines 2 and 3), in September 2011. A weight-drop and sledgehammer source was used to generate the seismic signal. A receiver spacing of 4 m and source spacing of 4-8 m was used. Power spectras of various sources and raw shots (before and after vertical stack) are also discussed. Processing of high resolution data was challenging in this area where the bedrock is very shallow. Therefore, stacked sections were very sensitive to stacking velocity; a great effort was made to obtain the velocity model. Pre-stack spectral whitening and band pass filtering after stack were the key steps for successful imaging. After carefully processing the data, we were able to image very shallow reflections and bedrock topography. A coarse-grained layer interpreted at 25-50 m depth, may be playing an important role in the formation of quick-clay and hence provide the triggering mechanism for landslides. The coarse layer potential to form quick-clays and its role in landslides, however, requires further investigation using other types of hydrogeological data, geotechnical data and geophysical well logging.