Helicopter-borne magnetic, electro-magnetic and radiometric geophysical survey in Verdal and Snåsa area, Trøndelag County

Abstract

This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical survey consists of 8500 line-km data, covering an area of 1700 km2, with 4600 km (920 km2) flown in June from Stiklestad, Verdal, and 3900 km (780 km2) flown in August from the base in Gaulstad in Ogndalen. 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 was used for data acquisition. The survey was flown with 200 meters line spacing, azimuth 100o, average speed was 116 km\/h, average height clearance of the bird was 55m and 85m 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 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.