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

Abstract

NGU conducted an airborne geophysical survey in Harstad area in July 2019. This report describes and documents the acquisition, processing and visualization of the recorded datasets. The geophysical survey results consist of approximately 1700 line-km of data, covering an area of 340 km2. 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 an AS350-B3 helicopter was used for data acquisition. The survey was flown with 200 m line spacing, 90\u00BF direction. Average speed was 55 km\/h, and average height clearance of the EM bird was 63 m, and 93 meters 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 and gridded using proxy resistivity values. All data were gridded using cell size of 50x50 meters and presented as 40% transparent grids with shaded relief on top of topographic maps, at the scale of 1:100.000.