Helicopter-borne magnetic, electro-magnetic and radiometric geophysical survey in Mjøsa

Abstract

NGU conducted an airborne geophysical survey in Mjøsa, Innlandet county as part of NGU\u2019s 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 2146 line\u0002km data, covering an area of 458 km2 flown on August 30th to September 07th, 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 135 km\/h, and average height clearance of the EM bird was 50 m and 80 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 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 (880 Hz, 6.6 kHz and 34 kHz), and for two coaxial frequencies (980 Hz and 7 kHz) separately using a homogeneous half space model of 500 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.