HYDROGEOLOGICAL MAPPING OF A MIOCENE AQUIFER BY TWO-DIMENSIONAL ELECTRICAL IMAGING

exploration area and for building of a detailed hydrogeological model, especially with complex geological models and 3-D structure shapes. Electrical sounding is based on a theoretical assumption of a horizontally stratified earth model, and electrical profiling maps only lateral resistivity changes at the same depth. Two-dimensional (2-D) electrical imaging (or 2-D electrical tomography), that has been introduced recently, can map the both vertical and lateral resistivity changes and provides for building three-dimensional (3-D) resistivity model of the underground. Exploration has been carried out in the area of the Seona spring to investigate the possibility for pumping more water during dry season, from the well located near the spring. The water of the Seona spring emerges from Badenian carbonate aquifer, which shows irregular 3-D shape and heterogeneous lithologic composition. A network consisting of four electrical profiles has been established to acquire a 3-D resistivity model, which can be transformed in a 3-D hydrogeological model. Inverse resistivity model sections revealed high resistivity zone which has been correlated with Badenian limestone by means of existing outcrops. Low resistivity zones point to the marly layers. Exploratory borehole located on the basis of electrical data has confirmed derived hydrogeological model from the electrical imaging data. Heterogeneous limestone aquifer has been uncovered at depth 8.2 – 55 m. The aquifer is weathered and karstificated with two main cavern intervals. Hydraulic connections between the caverns and the spring have been established during drilling, but also in a pump test by means of the airlift method. The presented case demonstrates great possibilities of the electrical imaging method in mapping of irregularly shaped aquifers and building of a precise 3-D hydrogeological model.