Novel insights into deep groundwater exploration by geophysical estimation of hard rock permeability

Abstract. Deep groundwater exploration in hard rock is a global challenge. An accurate measurement of hydraulic parameters is essential for both effective groundwater management and the prediction of future scenarios. The permeability (k) of an aquifer is typically measured in groundwater studies. Boreholes are the traditional means of measuring k. However, conventional approaches have a lot of flaws, such as being intrusive, expensive, time-consuming, useful only for areas with relatively uniform topographies, and only providing point-scale k measurements. Moreover, traditional approaches may not be able to do deep groundwater assessments. In contrast, geophysical technologies may assess subsurface hydrogeological conditions across large areas with minimal disruption to existing structures in a shorter amount of time and at a reduced cost. Several geophysical investigations previously used empirical methods to estimate the k parameter. These studies, however, used the VES (vertical electrical sounding) method to estimate k in a homogeneous setting at shallow depths and only in 1D. It is difficult to quantify the aquifer potential in hard rock terrains using borehole or VES-based k due to the intrinsic heterogeneity of the terrain. For the first time, this work uses the CSAMT (controlled-source audio-frequency magnetotellurics) method to estimate 2D and 3D k over 1 km depth in the exceedingly diverse environments of different rocks. These findings enable the scientific planning and management of deep groundwater resources in highly varied hard rock terrains where hydrogeological data is unavailable, resulting in a more accurate hydrogeological model compared to prior studies. This, in turn, decreases the necessity for expensive pumping tests and enables a more comprehensive evaluation of aquifer potential.