# Innovative Hydrogeophysical Approaches as Aids to Assess Hungarian Groundwater Bodies
## 1 Introduction
2D ERT measurements are often done with the aim of studying faults [13], fracture zones [14], fractures and fissures [15; 16] or cavities [17], etc., that is in the prospection of hydrogeological interest. An additional practical overview of the geophysical methods applicable in hydrological studies is given \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Challenge** \\ **No.** \\ \end{tabular} } & \multirow{2}{*}{**Hydrogeological Problem**} & \multicolumn{4}{c}{**Hydrogeophysical Method**} \\ \cline{3-6}  & & **DC** & **Geoelectric** & **Lithology** & **Porosity** & **Resistivity** \\ \hline
1 & Identification and classification of aquifers & X & X & & \\ \hline
2 &
\begin{tabular}{c} Detection of lateral and vertical extensions of \\ the groundwater bodies \\ \end{tabular} & X & X & & \\ \hline
3 &
\begin{tabular}{c} Detection of lateral and vertical changes \\ inside the groundwater bodies \\ \end{tabular} & X & X & X & X \\ \hline
4 &
\begin{tabular}{c} Detection of faults, fractures and conduits in \\ karst, volcanic and metamorphic formations \\ \end{tabular} & X & & & X \\ \hline
5 &
\begin{tabular}{c} Estimation of different hydraulic and \\ petrophysical properties \\ \end{tabular} & X & X & X \\ \hline
6 &
\begin{tabular}{c} Detection of contaminations plumes in the \\ subsurface media \\ \end{tabular} & X & & & X \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Challenges in the investigation of Hungarian groundwater bodies and their solution by the best applicable (X) geophysical methods. in [18]. Fractures proved to be detectable by ground-penetrating radar (GPR) but with only 5 m resolution [19]. The GPR investigations carried out by [20] had at the same time very good resolution. They were, however, carried out on a quarry wall because the plateau above the cliff was covered with a conductive weathered layer, which drastically reduces the penetration depth of this method. In the study shown in [21], GPR and frequency-domain electromagnetic induction methods proved to be capable of detecting discrete fracture and conduit features. In the Kadarta area, the GPR measurements, in the other study site, in Sopron seismic measurements proved to be unable to fulfill the aim of the research. The geoelectric method is one of the most often applied geophysical techniques for shallow subsurface investigations, among others, because the electric resistivity of the rocks varies in wide range enabling to distinguish different rocks. Since electrical resistivity depends strongly also on the water content of a rock, this method can be very successfully applied also in hydrogeological studies [22]. It can show the electrical resistivity distribution of the subsurface utilizing four electrodes. Between two of the electrodes (current electrodes), the current is forced to insert into the soil. The evolving electrical potential distribution can be measured on the surface using two other electrodes, the so-called potential electrodes. From the surface potential data, an inversion process enables the determination of the electric resistivity distribution of the subsurface. Traditionally two main types of geoelectric methods were applied: sounding was used to follow the vertical-, profiling the horizontal variations of the resistivity [23]. Actually, both of these types are joined by carrying out measurements by using numerous electrodes (e.g., 48, 60 or 72), which are laid out and computer controls, which ones are the current-, and which ones are the potential electrodes [24]. Such measurements are called electrical resistivity tomography (ERT), which can be two-, or three-dimensional (2D or 3D). Nowadays, the 2D measurements are the most often used, where the electrodes are laid in a line. The basis of the 2D measurements is the application of electrode arrays, and there are arrays, which are more convenient for sounding and arrays, which are better for profiling. For sounding the Wenner-Schlumberger (WS), profiling the dipole-dipole (Dp) array seems to be the best.