Helicopter-borne magnetic, electro-magnetic and radiometric geophysical survey in Øksfjord peninsula, Troms and Finnmark Counties

Abstract

NGU conducted an airborne geophysical survey in Øksfjord area, in Kvænangen, Loppa og Alta municipalities in Troms and Finnmark, as part of NGUs general airborne mapping program. The data acquisition in the Øksfjord area was carried out from 20th to 31st of August 2019. Two flights from the same area, flown September 1st, 2014, were also included in the data set for processing and presentation.This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 2155 line-km data, covering an area of 813 km2, with 245 km (49km2) flown in 2014 from Ystneset, near Øksfjord community center, and 1910 km (764 km2) flown in 2019 from Alteidet. NGU's 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 were used for data acquisition. Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and leveled using a micro-levelling 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 100x100 meters and presented as 40% transparent grids with shaded relief on top of topographic maps.