Studying the Norwegian Caledonides with ambient noise tomography: fieldwork lessons and preliminary results

The Norwegian Caledonides are well mapped at the surface, but their depth extent and subsurface geometry are poorly known. Renewed interest in mineral occurrences within the Caledonian nappes comes with the need to better understand the three-dimensional geometry of mineral bearing units and their tectonic history. A recurring discussion entails whether the nappe stack in central Norway forms a synclinal or anticlinal geometry.

To address these questions, we conducted an ambient noise tomography (ANT) survey aimed at imaging the 3D subsurface structure of the Caledonian nappes in central Norway. ANT provides a comparatively inexpensive and flexible method to investigate crustal structure, allowing us to distinguish high-velocity mineral bearing units from surrounding lower-velocity metasedimentary layers as well as the topography of the underlying crystalline basement. 

The survey was carried out in October 2025 over a three-week period and involved the deployment of 300 vertical component Sercel DFU seismic accelerometer nodes. A 3D-array of ca. 15x20 km, with about 0,6-1 km inter-station spacing was supplemented with a NW-SE trending profile of 30 nodes. The fieldwork in the rugged subarctic terrain was carried out by 3-4 teams of 2 people for the installation and retrieval of the array, for 5-6 days for each operation. About a third of the array was deployed by car along gravel roads, the rest was deployed either by e-bikes along large hiking and tractor trails or on foot for small hiking trails and off-trail sites. The planning and identification of the site access modes was paramount for the efficiency of the deployment. We used the Google Earth phone app as field deployment tool to automatically build the metadata of the array based on geo-localized pictures of the nodes and their serial numbers in the field, aided by an Optical Character Recognition algorithm. 

While the 2D profile cannot resolve lateral structures, its full extension and orientation toward the north-west and thereby toward the main ambient seismic noise sources of the North Atlantic should allow us to resolve the velocity contrast between basement and nappes and the nature of their geometry. 

We present the first results from both the 3D array and the 2D profile, providing new constraints on the depth extent and internal structure of the Caledonian nappes and contributing to the ongoing discussion of their subsurface geometry.