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

Abstract

NGU conducted an airborne geophysical survey in Stjørdal, Selbu, Malvik and Meråker municipalities, as part of NGU’s general airborne mapping program. This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 2655 line-km data, covering an area of 535 km2 flown on July 21st to 29th 2021 and August 30th to September 6th 2021. The NGU modified Geotech Ltd. Hummingbird frequency domain electromagnetic 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 100o. The average speed was 106 km/h, and the average height clearance of the bird was 45.7 m for the magnetometer and electromagnetic probe and 75.7 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 Geosoft micro-levelling algorithm. Radiometric data were processed using standard procedures as recommended by International Atomic Energy Association (IAEA). The electromagnetic data were filtered and levelled using both automated and manual levelling procedures. The apparent resistivity was calculated from in-phase and quadrature data for three coplanar frequencies (880 Hz, 6600 Hz and 34133 Hz), and for two coaxial frequencies (980 Hz and 7000 Hz). All data were gridded using cell size of 50 m x 50 m and presented as 40% transparent grids with shaded relief on top of topographic maps. The electromagnetic data are presented as 30% transparent grids on top of topographic maps.