Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey in Okstindan and Røsvatnet area, Nordland county

Abstract

NGU conducted an airborne geophysical survey in Hemnes and Hattfjelldal municipalities, as part of NGU’s general airborne mapping program. The data acquisition in the Okstindan area was started and completed in August 2021. This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 2039 linekm data, covering an area of 394.5 km2 flown from the base at Vesterli Skistadion in Korgen, Hemnes 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 90 km/h, average height clearance of the bird was 65 m and 93 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 Geosoft microlevelling algorithm. Radiometric data were processed using standard procedures as 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