Porosity and Formation Factor in Coastal Sands: Micro-CT Insights into Grain-Scale Controls and Clay Influence

Abstract

An experimental and micro-computed tomography (micro-CT) study of the relationship between porosity and electrical formation factor (FF) is presented for a suite of coastal sands. Beach sands from Scarborough and Cottesloe (Western Australia) were analysed to determine bulk porosity, electrical transport behaviour, grain- and pore-scale structure, and the possible effects of clay on conductivity. Clean sands had laboratory packing porosities of 0.26–0.44. Formation factors ranged from approximately 3.96 to 8.54 in flow-cell experiments and from 4.0 to 6.9 in static-cell experiments. Micro-CT-derived porosity ranged from approximately 0.25 to 0.46, whereas finite-element-derived formation factor ranged from approximately 3.8 to 10.1, depending on cube size and microstructural heterogeneity. An Archie-type inverse relationship between formation factor and porosity was observed in the laboratory and digital datasets. Porosity and formation factor became less variable and approached laboratory-scale behaviour over cube sizes of 500³–700³ voxels, suggesting practical REV behaviour at this scale; however, the limited number of independent 700³ realisations warrants cautious interpretation. Clay-bearing samples showed higher formation factors and slower electrical stabilisation, consistent with possible clay-related interfacial effects and heterogeneous pore geometry, although the specific contribution of surface conduction was not independently demonstrated. Overall, the combined laboratory and micro-CT approach provides complementary information for understanding pore-scale controls on electrical transport in unconsolidated coastal sediments and for hydrogeophysical characterisation of coastal aquifers.