Site characterisation at the Äspö Hard Rock Laboratory through seismic refraction

Sammanfattning: The successful construction of any well-functioning structure is dependent on a good understanding of the ground on which it lies. Several methods and techniques have been developed to model subsurface conditions and properties with the purpose of gaining comprehensive knowledge of geological conditions. Seismic refraction presents a geophysical method of subsurface investigation capable of providing a model of the subsurface over a large area. Seismic refraction was performed at the Äspö Hard Rock Laboratory as part of the Geoinfra-TRUST project, which focuses on the development of tools and methods to optimize underground construction. The objective was to map the layer interface structure, determine the depth to bedrock, and locate faults along the 450m underwater profile between Äspö and Hålö. Results were presented in a continuous velocity gradient model completed through seismic tomography interpretation by Rayfract software. The outcome model showed a four-layer interface compromising of a top layer of sand and clay and a hard granite bottom. Intermediary layers consist of solidified soil and granite at varying degrees of weathering and fracture. Structurally, the Äspö-Hålö profile is complex with undulating layers and a predicted horst-graben structure on the south end of the profile. The profile is marked by disparate conditions in the model between the northern and southern side. Results support the use of seismic tomography and the use of an integrated investigation with integrated methods.