Estimating subsurface porosity and salt loads using airborne geophysical data

Abstract

ABSTRACTWe aim to map dissolved salts in aquifers which, with rising water tables, are a threat to fresh water resources and ecosystem health. Interpolated groundwater electrical conductivity (EC) maps from borehole measurements are combined with spatially detailed bulk electrical conductivities estimated from airborne electromagnetics to provide, through Archie’s law, a detailed prediction of subsurface porosity and total dissolved salt load of any near‐surface aquifer. This technique quantitatively integrates airborne electromagnetic and groundwater measurements of EC to provide the subsurface detail necessary for hydraulic properties. The process has been applied in Australia’s Murray River Basin, where the presence of salt is a threat to agriculture and fresh water supplies. Porosity predictions range between 1% and 50% and are spatially consistent with known geology, dissolved salt loads are estimated to be between 100 and 4000 tonnes per hectare. The predictions are yet to be compared to in‐situ measurements but if proven, will be a useful tool when in‐situ porosity is under‐sampled or is not available.