Integrated Characterization of Fracture Orientations and Hydraulic Properties in Crystalline Bedrock

Understanding hydraulic properties in fractured crystalline bedrock is essential for predicting groundwater flow and heat transport in deep geological settings. This study focuses on the Kopparnäs Test Site in southern Finland, where the bedrock mainly consists of granites, granodiorites, and, commonly migmatitic mica gneiss.

Fracture orientation and aperture were characterized using acoustic (ABI) and optical borehole imaging (OBI), combined with hydraulic conductivity measurements obtained from zone-based slug tests in a borehole drilled to a depth of 233 m. Seven test zones at depths ranging from 18 m to 132 m were selected for integrated hydraulic conductivity and fracture analysis. The borehole intersects a nearly vertical east–west striking fault zone at approximately 100 m depth, where three core zones were targeted for hydraulic conductivity measurements. Each zone was analysed in terms of fracture frequency, orientation, and infilling to achieve an integrated understanding of hydraulic behaviour.

Structural analysis indicates that most fractures are steeply dipping, with dominant NNE-SSW orientations and dips towards the SSE. Sub-horizontal fractures mainly occur at shallow depths within the upper 100 m, which is typical of Finnish crystalline bedrock due to glacial unloading after the latest glaciation. Between 100-130 m, the borehole intersects a sub-vertical fault zone that significantly increases fracture frequency. Below this zone fracture frequency decreases markedly with only sporadic fractures observed.

Hydraulic conductivity remains within the same order of magnitude (10^-9 m/s) but varies between zones reflecting differences in fracture characteristics rather than fracture density alone. Higher hydraulic conductivity is observed at shallow depths where fractures are predominantly sub-horizontal and partially open. In contrast, deeper sections are dominated by steeply dipping, mineral-filled fractures associated with reduced conductivity. Intermediate conductivities reflect mixed orientations and aperture conditions. Overall, fracture orientation and infilling exert a stronger control on hydraulic conductivity than fracture frequency, the role of fracture connectivity, aperture and mineral filling in governing fluid flow.

Core analysis revealed porosity values of about 30% at 140 m depth within heavily altered zones. A similar pattern is observed at the Kivetty site in central Finland, where increased alteration intensity correlates with higher total porosity, improved pore connectivity, and enhanced permeability. Future work will extend hydraulic testing to intervals with high porosity and include fracture aperture and spacing measurements to assess their combined influence. This integrated approach provides a robust framework for distinguishing hydraulically significant fractures from inactive ones, improving site characterization for groundwater resource management, geothermal energy exploration and deep geological repository safety assessments.