A hydrogeophysical and hydrogeological characterization of a fractured aquifer, Saturna Island, British Columbia

Characterization of fractured aquifer systems is both critical and difficult because groundwater flow and solute transport paths are complex. In this study, 2-D electrical resistivity imaging (ERI), 3-D ERI, azimuthal apparent resistivity, square array resistance, fracture mapping and hydrogeological results were integrated to characterize a fracture system on Saturna Island, British Columbia, Canada. The geophysical results are consistent with local outcrop geology and previous hydrostratigraphic and hydrostructural conceptualizations. The ERI images show a distinction between overburden, sandstone-dominant and mudstone-dominant units, and the presence of fractured zones; however, discrete fractures were not identified. Consequently, in similar geologic conditions it is possible to use ERI to constrain the aquifer architecture for groundwater models at a regional and sub-regional scale. Azimuthal apparent resistivity and square array resistance results suggest the subsurface is electrically anisotropic. Azimuthal offset error analysis and forward resistivity modeling were used to investigate if the anisotropic response was the result of homogeneous fracturing or subsurface heterogeneities. Both offset error analyses and forward modeling indicated that the anisotropic response could be explained by relatively simple subsurface heterogeneities associated with the geology in the vicinity of a fault. If electrical anisotropy due to the fracture system orientation exists, it appears to play a secondary role in the azimuthal array response compared to geologic structure. In this geologic setting, if offset error analyses and forward modeling were not completed, the electric and hydraulic anisotropy could have been incorrectly interpreted. Therefore, anisotropy results should not be interpreted without considering variations in resistivity due to subsurface heterogeneities.