First results from the SEEMS DEEP seismic survey conducted over the Koillismaa Igneous Complex, Northern Finland

The Koillismaa layered igneous complex (KLIC) in northern Finland spans a large distance from the Finnish-Swedish border to the Finnish-Russian border. It has been an area of interest for several decades among geologists due to its potential to host several critical raw materials including cobalt & nickel which are key materials required for lithium-ion batteries. The KLIC intrusion comprises outcropping Koillismaa and Näränkävaara layered complex mafic-ultramafic intrusions that are interestingly connected by a zone of high gravity and magnetic anomalies. Extensive petrophysical and lab studies were conducted by the Geological Survey of Finland on a 1.7 km long deep drillhole located within the area of our interest. A pre-existing low-fold seismic study indicated the reflectivity of the ultramafic rocks. Drillhole study further established the fact that the contacts with mafic intrusion rocks having the potential to host mineralization are causing observable seismic reflections at 1.4 km depth. The ongoing SEEMS DEEP project (2022-2025), an ERA-MIN 3 sponsored project comprises an integrated approach of using seismics and electromagnetics studies to substantially improve the geomodel of the Koillismaa area which will help in better decisions in exploration drilling. In this study, we are focussing on the seismic part of the project. In August 2023, an irregular-sparse 3D seismic and two regional 2D seismic lines, roughly in the direction of E-W and NNE-SSW were acquired under the SEEMS DEEP project. The overall aim of the 2D seismic lines was to constrain large-scale information about the geological architecture of the study area, whereas the 3D survey was conducted to highlight the detailed information from the rock volumes near the deep drill hole. The acquired data, both 2D (~10 Km and ~12 Km) and 3D (~5 Km x 6 Km), are of good quality with reflections visible in the raw data. An 8-tonne Mark IV Vibroseis truck was used as a seismic source for both surveys, with sweep frequency ending at 160 Hz. Almost 3000 single-component receivers (Strydes) were deployed in varying subarctic terrains defined by swamps and forests for the 3D survey. For 2D profiles, over 700 three-component receivers (GSBs) were used next to the existing gravel roads utilized by the Vibroseis truck. For the 3D survey, receiver spacing was kept at 30 m with inline spacing of 200 m. Shot points were located mainly following the existing roads within the forest. For the 2D survey, a uniform receiver spacing of 15 m and a shot spacing of 30 m was used. Seismic data processing was applied with the overall aim of suppressing noise, boosting signal-to-noise ratio, and improving reflectivity in the data. Special emphasis was put on handling the highly heterogeneous near-surface weathering layer. Final results revealed several reflectors at various depths, which have preliminarily been interpreted to originate from mafic intrusions, diabase veins, and faults cross-cutting the intrusion.