Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey in Røros area, Trøndelag county

Abstract

NGU conducted an airborne geophysical survey in Røros, Trøndelag county as part of NGU’s general airborne mapping program in 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 2584 line km data, covering an area of 523 km2 flown between June 15th to June 22nd, 2020. 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 95 km/h, and average height clearance of the EM bird was 47 m and 77 m 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 levelled using standard levelling algorithm. Radiometric data were processed using standard procedures recommended by the International Atomic Energy Association (IAEA). EM data were filtered and levelled using both automated and manual levelling procedures. Apparent resistivity was calculated from in-phase and quadrature data for three coplanar frequencies (880 Hz, 6.6 kHz and 34 kHz), and for two coaxial frequencies (980 Hz and 7 kHz) separately using a homogeneous half space starting model of 1000 ohm-m. All data were gridded using cell size of 50x50 meters. Magnetic and radiometric data is presented as 40% transparent grids with shaded relief on top of topographic maps. EM data is presented as 30% transparent grids on top of topographic maps.