Helicopter-borne magnetic, electro-magnetic and radiometric geophysical survey in Byglandsfjorden area, Agder County

Abstract

NGU conducted an airborne geophysical survey in Byglandsfjorden area, in Bygland, Åseral og Åmli municipalities in Agder County, as part of NGU’s general airborne mapping program. The data acquisition in Byglandsfjorden area was carried out May 20th to June 19th, 2021.This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical survey consists of 5880 line-km data, covering an area of 1176 km2, flown from the base near Byglandsfjord village.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 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 Geosoft 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 50x50 meters and presented as 40% transparent grids with shaded relief on top of topographic maps.