Helicopter-borne magnetic, electro-magnetic and radiometric geophysical survey in Treungen and Vegårshei area, Agder, and Vestfold og Telemark counties.
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NGU conducted an airborne geophysical survey in Fyresdal, Nissedal, Åmil, Vegårshei, Gjerstad municipalities as part of NGU\u2019s general airborne mapping program. Treungen area was completed in June 2020, and Vegårshei area completed in October 2020. This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 5500 linekm data, covering an area of 1100 km2, with 2930 km (586 km2) flown in June from Gautefall, and 2570 km (514 km2) flown in September\/October from Ubergsmoen. The NGU modified Geotech Ltd. Hummingbird frequency domain EM system supplemented by an optically pumped Cesium magnetometer and the Radiation Solutions 1024 channels RSX-5 spectrometer mounted on a AS350-B3 helicopter was used for data acquisition. The survey was flown with 200 meters line spacing, azimuth 90o. Average speed was 110 km\/h, and average height clearance of the EM bird was 57m and 87m for the spectrometer. Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and leveled using standard micro levelling algorithm. Radiometric data were processed using standard procedures recommended by International Atomic Energy Association (IAEA). EM data were filtered and leveled using both 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 cell size of 50x50 meters and presented as 40% transparent grids with shaded relief on top of topographic maps.