Helicopter-borne magnetic, electro-magnetic and radiometric geophysical survey in Totak area, Vestfold og Telemark county

Abstract

NGU conducted an airborne geophysical survey in Tokke, Vinje, Kviteseid and Seljord municipalities, as part of NGU\u2019s general airborne mapping program. The data acquisition in the Totak area was started in June 2020 and completed in September 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 4300 linekm data, covering an area of 860 km2, with 900 km (180 km2) flown in June from Gautefall, and 3400 km (680 km2) flown in September from the base in Rauland area. 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 90 degrees. Average speed was 108 km\/h, and average height clearance of the EM bird was 69m and 99m 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 (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.