3-D Geologic Modelling of the Bjerkreim Lobe, Norway

In this work, we conduct 3-D geologic modelling of the Bjerkreim-Sokndal (BKS) layered intrusion's Bjerkreim lobe in southern Norway. The BKS is a folded, double-plunging syncline with an areal extent of 230km². There are five rhythmic megacyclic lithological units (MCU, I - V) that are divided into zones (a - f) based on the presence or absence of index minerals. The BKS is often used as an analogue for Martian studies due to the presence of strong magnetic remanence, 20 000 nT below background. It has also undergone significant exploration for critical minerals.

Although the BKS is well-studied, there are limitations that have hindered geophysical mapping. The layered units reside along a topographic low, limiting the lowest altitude for airborne surveys. Land use is classified for agriculture and the presence of lakes restrict ground data collection to roads and pathways. Seismic and gravity surveys have been collected over the study area; however, the gravity data is sparse, and the seismic data is restricted to a single profile. These limited studies suggest a BKS depth to base of 4 - 5 km. Drillcore has been collected, however these extend on average to a depth of 30 m. In 2021, a small-scale drone magnetic survey was collected. This survey was to complete low-altitude, high resolution magnetic sampling to complement previous ground magnetic surveys and as a segue for multiscale analysis with regional airborne surveys. Since no single geophysical dataset is sufficient for a complete geologic interpretation, joint modelling is required to better understand subsurface distribution.

A master ground sample database was compiled of over 3000 samples and 11 petrophysical properties. This database consisted of in-situ and in-lab measurements. Principal Component Analysis was conducted to reduce dimensionality and identify which properties should be incorporated into the model. K-means clustering was conducted to identify natural groupings in the data, where average values for these properties were calculated from the dominant cluster. 2-D profiles orthogonal to strike were constructed at 2 km increments along the Bjerkreim lobe with additional intermediate profiles to minimize truncation of 3-D volumes. A combination of 2-D forward and joint inversion modelling was implemented using compiled aeromagnetics and gravity as the observed values. Each MCU zone was modelled as a block with average properties from the cluster analysis and constrained at surface by mapped contacts. Depth estimation routines, including Euler deconvolution, were also executed. The modelled blocks were then wireframed into volumes to create a 3-D representation of the Bjerkreim lobe.