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

Abstract

NGU conducted an airborne geophysical survey in Levanger, Verdal, Stjørdal and Meråker municipalities, as part of NGU’s general airborne mapping program. The data acquisition in the Feren area started in August 2023 and was completed in September 2023. This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 2017 line-km data, covering an area of 409 km2 flown from the base at Okkenhaug in Levanger municipality. The NGU modified Geotech Ltd. Hummingbird frequency domain electromagnetic system supplemented by an optically pumped caesium magnetometer and the Radiation Solutions 1024 channels RSX-5 spectrometer mounted on a AS350-B3 helicopter were used for data acquisition. The data collected were processed at NGU using Seequent Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and levelled using Oasis Montaj micro levelling algorithm. Radiometric data were processed using standard procedures as recommended by International Atomic Energy Association (IAEA). Electromagnetic data were filtered and levelled using both automated and manual levelling procedures. Apparent resistivities were 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. All data were gridded using cell size of 50x50 meters and presented as 40% transparent grids on top of topographic maps