Broadband magnetotellurics over the Swedish Caledonides

In connection with the drilling project Collisional Orogeny in the Scandinavian Caledonides (COSC), broadband magnetotelluric (MT) data were acquired along a 60 km long transect in central Sweden. The COSC project, with drilling costs funded by the International Continental Scientific Program (ICDP) and the Swedish Research Council (VR), focuses on the study of mountain building processes through the investigation of the composition and structure of the Caledonides. The project encompasses two fully cored boreholes to ~2.5 km depth. The plan is to locate the boreholes so they penetrate structures at different levels of the tectonostratigraphy of the mountain belt. Before acquisition of the current MT data, several seismic reflection surveys were carried out to determine the location of the first borehole (COSC-1). The objectives of the MT measurements are to provide more information about the deeper structures, and complement the seismic reflection studies carried out along the same profile. In total, 86 MT soundings were recorded, three different sampling modes were used: 3000 Hz for half an hour in daytime, 1000 Hz for two hours around midnight and 20 Hz for ∼24 hours continuously. All stations recorded two horizontal components of the electric and magnetic fields, and about 40% of them, distributed along the whole profile, recorded also the vertical component of the magnetic field. The station spacing was 600 m close to possible drill sites and where there are gaps in the seismic data. Otherwise the spacing is of the order of 1 km. Data quality varies from very good to noisy nearby urban areas. Results from these data set will be combined with other geophysical methods to improve the 3D geological model of the area around the COSC-1 drill site. Additionally, it is expected that this data set will help to determine the location of the second borehole, COSC-2.