Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey in Selbu, Tydal, Holtålen and Røros, Trøndelag county

NGU conducted an airborne geophysical survey in the Selbu, Tydal and Haltdalen area from May to October 2019. This report describes and documents the acquisition, processing, and visualization of the recorded datasets, presented in maps including Røros area data. The geophysical survey results consist of approximately 13000 line-km, covering an area of 2600 km2. Also included in the report is 6200 line-km of data from a survey in the Røros area, conducted in 1999 by NGU, presented in NGU-report 1999.118 by Beard and Mogaard. A NGU-modified Geotech Ltd. Hummingbird frequency domain electromagnetic system (EM) was used for data acquisition. The EM was supplemented by an optically pumped Cesium magnetometer and a Radiation Solutions 1024 channels RSX-5 spectrometer mounted on an AS350-B3 helicopter. The survey was flown with 200 m line spacing, direction 90o (Tydal and Røros areas) and 100o (Selbu and Haltdalen areas). The average speed was 80 km/h, with an average height clearance of the EM bird was 50 m, and 80 m for the spectrometer for the 2019 data. The data were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and levelled using standard micro levelling algorithm. Radiometric data were processed using standard procedures recommended by the International Atomic Energy Association (IAEA). The EM data were filtered and levelled using automated and manual levelling procedures. Apparent resistivity was calculated from in-phase and quadrature data for three coplanar frequencies (880Hz, 6.6kHz and 34kHz), and for two coaxial frequencies (980Hz and 7kHz) separately using a homogeneous half-space model. All data were gridded using a cell size of 50x50 meters and presented as 40% transparent grids with shaded relief on top of topographic maps, at the scale of 1:250000. A preliminary data interpretation was performed immediately after processing to demonstrate the potential of the data and to support ongoing fieldwork in the study area.

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