Airborne geophysical investigations for landslide risk investigation along the Ångerman River, central Sweden

Abstract

A study of landslide risks along the Ångerman River in Sweden is currently being performed by the Swedish Geotechnical Institute (SGI) with assistance from the Geological Survey of Sweden (SGU). In this work, knowledge of the distribution of different types of sediment is crucial. The area shows large variations in both sediment stratigraphy and sediment thickness (depth to bedrock) which makes it difficult to describe geotechnical properties for larger areas based on sparse geotechnical field investigations (soundings and sampling). Within the study area, the Ångerman River valley contains numerous fluvial terraces at various elevations separated from the river by steep erosional scarps. Although fluvial sand caps the terraces, their heights are often derived from the underlying silt and clay, which in turn overlies till or glaciofluvial sediment. The thickness of these general stratigraphic units is difficult to determine from existing data. Thus, in 2018, an airborne transient electromagnetic (ATEM) survey was performed along the river and the surrounding river banks with the objective of mapping and characterizing the sediments down to bedrock. The surveyed area is 2 km wide and covers 80 km along the river with a flight line distance of 75 m. From the ATEM data, the 3D resistivity distribution of the ground down to about 100 m below the surface can be determined. The silt and clay show very low resistivity compared to the overlaying sand layers. Therefore, they can be easily distinguished in the resistivity models from the ATEM data. Furthermore, the depth to the more resistive underlaying till and bedrock can be delimited in most areas. The 3D resistivity model derived from the ATEM data was integrated with other geoscientific data such as; geotechnical drillings, information from water wells and surficial deposits maps using 3D modeling software (Geoscene 3D from I-GIS). Based on the modelling results, the area was divided into six different classes based on the soil stratigraphy. The current method of analysis uses detailed geotechnical investigations performed in one location to ground truth the interpretation of geophysical data over a broad area. With the new results based on the ATEM data, the planning of new geotechnical drillings can be better focused on more poorly-understood areas. Furthermore, the number of geotechnical field investigations can be reduced.