Magnetic Modelling of the Bjerkreim-Sokndal Layered Intrusion, Southern Norway

The Proterozoic Bjerkreim-Sokndal (BKS) is a 230 km2 layered intrusion in the Rogaland Anorthosite Province, Norway. BKS has been extensively explored for its critical mineral potential. The BKS has also been used as an analogue for planetary studies due to the presence of strong magnetic remanence, which has resulted in anomalies of up to 20 µT below background in airborne surveys. The BKS has five megacyclic lithological units, which are subdivided into zones based on specific mineralogy. The surficial location of these zones has been mapped based on limited outcrops due to difficult accessibility and regional airborne magnetics. However, review of recent high-resolution magnetic surveys suggests that the geologic map should be reassessed and used in conjunction with petrophysical data to better understand subsurface geometry. To address this challenge, we are developing a 3D geologic model that integrates multiscale geophysical data, from microscale lab measurements to regional airborne surveys. This includes data from a novel 2021 multicopter drone magnetic survey. A master ground sample database that includes over 3000 samples with in-situ and in-lab petrophysical measurements was compiled. The petrophysical database was analysed using Principal Component Analysis (PCA) clustering to distinguish relevant lithological units. PCA was performed using the variables: Density, Susceptibility, Natural Remanent Magnetization, Induced Magnetization, Koenigsberger Ratio, Declination, Inclination, Vx, Vy, Vz, and Residual Magnetic Field.

A combination of 2D forward and inversion modelling was implemented in Geosoft Oasis Montaj GM-SYS. The model was based on dense survey profiles orthogonal to geological strike along the eastern margin of the Bjerkreim lobe. Airborne magnetic data was used as the regional constraint and the petrophysical clusters as input data for the modelled blocks. The results were integrated into a 3D visualisation framework and used to refine the geometry of the main geological units. Visualisation of the high-resolution drone data shows complex structures beneath local lakes that were unmapped during previous geophysical surveys.