Helicopter-borne magnetic, electromagnetic and radiometric geophysical surveys in the Kongsberg region: Krøderen, Sokna, Høneføss, Kongsberg and Numedalen
NGU conducted an airborne geophysical survey around Kongsberg region covering Krøderen, Sokna, Høneføss, Kongsberg, Vikersund, Hokksund and Hvittingfoss regions during 2008-2011. This report describes and documents the acquisition, processing and visualization of recorded datasets. The geophysical survey preformed herein results in total 13985 line km that covers approximately an area of 2797 km2. A Hummingbird electromagnetic frequency domain system from Geotech Ltd., modified by NGU and an optically pumped Cesium magnetometer were used for electromagnetic (EM) and Magnetic data acquisition. In the first part of the survey, a 256 channel Exploranium GR 820 gamma-ray spectrometer were used while a 1024 channels Radiation Solution’s RSX-5 were used in the last part for radiometric data acquisition. The survey was flown with 200 m line spacing, mostly E-W direction with an average speed of 108 km/h. The average terrain clearance of the helicopter was approximately 75 m. Collected data were processed at NGU using Geosoft Oasis Montaj and in-house software. Raw total magnetic field data were corrected for diurnal variation and also for IGRF. Magnetic data are presented as total field anomaly, first order vertical derivative and tilt derivative. EM data were filtered and levelled using both automated and manual levelling procedure. Apparent resistivity was calculated for each frequency from in-phase and quadrature data for three higher frequencies (6606, 7001 and 34133 Hz) however from quadrature only for two lower frequencies (880 and 980 Hz) using a homogeneous half-space model. Radiometric data were processed using standard procedures recommended by International Atomic Energy Association (IAEA) and ground concentration of potassium, equivalent uranium, equivalent thorium were calculated. All data were gridded with the cell size of 50 x 50 m and presented as contour maps at various scales.