Quantification of groundwater storage heterogeneity in weathered/fractured basement rock aquifers using electrical resistivity tomography: Sensitivity and uncertainty associated with petrophysical modelling

•Estimating groundwater storage in heterogeneous basement rocks is challenging.•Spatial porosity variations are quantified from ERT in a metamorphic rock aquifer.•Archie’s petrophysical model which neglects clays largely overestimates porosity.•Waxman & Smits model produces lower values consistent with independent datasets.•Uncertainty is reasonable and dominated by conceptual and clay data uncertainties.