Uncovering the deep structure of the Koillismaa Layered Intrusion Complex, Finland using a novel 3D seismic survey

Abstract. A pioneering ~22 km² three-dimensional (3D) seismic survey was conducted in northeastern Finland within the SEEMS DEEP project to investigate the concealed Koillismaa Deep Intrusion (KDI), part of the 2.5–2.4 Ga Koillismaa–Näränkävaara Layered Intrusion Complex (KLIC). Utilizing low-cost piezoelectric nimble nodes and a single Vibroseis source, this survey represents one of the first 3D seismic investigations of a deep layered intrusion and its feeder system. The seismic data, supported by petrophysical measurements and synthetic modelling, reveal a complex internal architecture characterized by coherent reflectivity patterns indicative of modal layering. Interpretation suggests that the KDI is not a simple feeder but a more developed, chonolithic or funnel-tube-shaped intrusion. Integration of seismic data interpretation with gravity inversion and geological data enabled the construction of a Common Earth Model (CEM), which successfully reproduces the observed gravity anomaly. A major thrust fault, likely associated with the Oulujärvi Shear Zone, was imaged for the first time, offering new insights into the tectonic evolution and mineralization potential of the region. This study highlights the advances in 3D seismic methods in hardrock environment and establishes a methodological framework for future mineral exploration in similar geological settings.