Country-scale airborne electromagnetic and gamma-ray spectrometric data for mapping sustainable resources in Norway

The Geological Survey of Norway has collected frequency-domain electromagnetic and gamma-ray spectrometric data through airborne mapping since 1972 on the mainland of Norway. These data were acquired and processed using the technologies available at the time of the campaigns. Consequently, the resolution and quality of individual surveys vary across the country.

Over the years, helicopter-borne frequency-domain electromagnetic data were acquired using various instruments with up to five different frequencies. While today, these data are inverted to determine apparent resistivity using a half-space earth model, inversion has not been consistently carried in the past. To build a homogeneous compilation, we are re-processing and inverting all existing data using modern levelling and noise-reduction tools. By limiting instrumental and environmental noise, we create a country-scale map of conductors. For each frequency, apparent resistivity data from all surveys are merged into a single, seamless compilation.

The primary objective of reprocessing and compilation is to recover the maximum amount of legacy airborne data and produce a uniform coverage map. This unique compilation serves as a crucial tool for identifying conductors in evaluating mineral resources and for general bedrock mapping. The location and continuity of conductive structures are interpreted, across survey boundaries, providing critical insights into the deeper sources of mineral systems at a regional scale.

Similarly, gamma-ray spectrometric data were collected using different instruments with varying detector volumes, leading to discrepancies in large-scale resolution. Presently, these data are corrected for live time, cosmic and aircraft background removal, radon removal, Compton stripping, and height attenuation following the International Atomic Energy Agency (IAEA) recommendations. Since 2002, the final products are the ground concentration for potassium, uranium and thorium. Prior to this, window counts of gamma rays for each respective radioelement were reported. A homogeneous compilation is obtained after a careful data re-processing including noise reduction, levelling and calibration. For surveys where original calibration parameters are missing, data from neighbouring overlapping surveys allow to derive the ground concentration. The final compiled ground concentration maps provide geochemical insight about the top half meter of the ground. Combined with electromagnetic data, links between surface lithology and deeper sources can be studied.

Airborne geophysical operations are capital-intensive. A standardized and homogeneous re-processing of frequency-domain electromagnetic and gamma-ray spectrometric data maximizes the value of Norway’s existing geophysical assets. These new regional datasets will contribute to mineral exploration, effective bedrock mapping, and societal safety by identifying natural and anthropogenic radioactivity, quick clay and rock instabilities, and other environmental hazards.