Mapping fracture flow anisotropy using the Self Potential method: field and laboratory experiments

Recent studies have shown the potential of SP to identify and quantify groundwater flow in fractured rocks, including hydraulically active fractures, fracture connectivity, and preferential flow directions. The presented work reports combined laboratory and field SP experiments applied to a fractured gneissic aquifer system in NW Scotland characterised by the occurrence of several fracture sets and a major regional-scale fault zone. Field surveys involved both SP transects and azimuthal SP surveys and revealed clear anomalies including anisotropy that matched the dominant fracture set directions, for both local scale fracture sets and the regional fault zone, which were previously mapped from outcrop observations and electrical resistivity tomography. Collected fractured gneiss samples were analysed in the laboratory and provided values for the electrokinetic coupling coefficient and zeta potential which were further used to quantitatively interpret field SP data in terms of groundwater pressure gradients. Results showed that groundwater pressure gradients were notably higher along one of the dominant fracture set than along the other near-orthogonal one, which coincides with the fault zone. These results confirm the hydrogeological conceptual model in which the fault zone would act as a high permeability/high flow rate drain for the surrounding, less fractured rock mass.