Investigation of hydrogeological structures in carbonate rock with ground penetrating radar

Carbonate rock aquifers are an important resource in the face of water scarcity. However, groundwater recharge processes are not fully understood in the heterogeneous matrix and fracture system. The shallow epikarst zone is important for drainage, transport and storage and needs to be investigated. Geophysical techniques are promising, particularly ground penetrating radar (GPR) due to its sensitivity to water saturation. To test the potential of GPR, hydrogeological structures were investigated in the Lower Muschelkalk of the Rüdersdorf limestone quarry near Berlin, Germany. A survey field was monitored under three different moisture conditions and the experiments included densely spaced zero offset GPR and common midpoint (CMP) profiles. The analysis focused on EM wave velocities as a proxy for water saturation, which were used for a relative comparison of the results from different methods. The more generic CMP results were significantly higher than the velocities from diffraction hyperbolas, which only represent the very local position. Structural observations from picked reflectors throughout the monitoring contributed to the interpretation. While the matrix appears to be unaffected by water variability, preferential flow paths can be identified. Diffraction hyperbolas may occur at fractured porous zones that preferentially store water and drain towards the bedding planes. Their spatial characteristics suggest that they may be precursors of potential sinkholes. The survey shows how GPR can help to understand hydrological processes in carbonate rock and locate relevant structures for further investigation. The collected dataset provides opportunities for further analysis.