# Innovative Hydrogeophysical Approaches as Aids to Assess Hungarian Groundwater Bodies
## 2 Materials and Methods
### The Applied Geophysical Methods  
#### 2.3.1. Geoelectric Surveying Methods  
Recently, a new array set has been developed. Its strategy is the opposite of that of the traditionally used arrays, e.g., the Wenner-Schlumberger (WS) and the dipole-dipole (Dp) configurations (Figure 4). It measures very small, almost zero signals above homogeneous half-space. Due to this reason, they are called quasi null arrays [28]. It was shown that their depth of detectability values is larger than those of the traditional arrays [30]. Both numerical modeling [31] and quasi field modeling [29] verified this result. According to the numerical modeling results of [28] also its horizontal and vertical resolution is better than that of the traditional arrays. On the basis of these results, it was expected that the quasi-null arrays could image the subsurface with a higher resolution. Since their application together with a traditional configuration does not significantly increase the time demand and the cost of the measurements, the joint application of a traditional and a quasi-null configuration is recommended. In this paper, several examples will be shown to illustrate the advantage of such an array set. Apparent resistivity data were acquired by a Sycal Pro Standard device. The measurement sequences of the 72-electrode acquisition system contain 1010 W-Sch, 1516 Dp-Dp, 669 St (Stummer), 1222 \(\gamma_{112}\), 964 \(\gamma_{113}\), 798 \(\gamma_{114}\), 668 \(\gamma_{115}\) and 574 \(\gamma_{116}\) data points. The Stummer (St) array has 30 electrodes and contains 669 data points. The following parameters were used (Figure 2): \(a=1\)-5, \(n=1\)-7 (W-Sch); \(a=1\)-6, \(n=1\)-4 (Dp); \(a=1\)-8 (\(\gamma_{112}\)); \(a=1\)-7 (\(\gamma_{113}\)); \(a=1\)-5 (\(\gamma_{114}\)); \(a=1\)-5 (\(\gamma_{115}\)); \(a=1\)-4 (\(\gamma_{116}\)). For the parameters of the Stummer array, see [27]. Figure 3: Overview of the study areas investigated by the proposed hydrogeophysical methods: (**a**) locations of the Hungarian sites; (**b**) Sopron area, Hungary (Section 3.1.1); (**c**) Kadárta area, Hungary (Section 3.1.2); (**d**) Khubaripur area, India (Section 3.1.3); (**e**) Baktalórántháza area, Hungary (Section 3.2). The measured data must be inverted to obtain a resistivity section that can be interpreted for hydrogeological purposes. Three inversion codes were applied in Sections 3.1.1-3.1.3. In most situations, Res2D-Hu was used [44] because the available commercial methods cannot properly handle \(\gamma_{11\text{n}}\) data. In some examples also traditional data were inverted by this code to be able to compare the data received by the different arrays. In Section 3.1.1 and Section 3.1.2, traditional data were inverted by both this code and EarthImager 2D Version 2.1.7 [45]. For the inversion of the third data set (Section 3.1.3), the RES2 DINV [46] code was applied beside of Res2D-Hu. In Res2D-Hu, forward modeling is carried out using the method of Finite Differences [47]. To avoid sudden resistivity changes, a smoothed inversion is applied. The smoothing factor automatically decreases starting from 100, through 50, 20, 10, 5, 2, 1, to 0.2. After 3 iterations by each smoothing factor, the inverted data set is used as the input model for the next inversion step, carried out with the next, smaller smoothing factor. The theoretical background of the Res2D-Hu code is discussed in detail in [28]. In forward modeling, the mesh size was taken as one-fourth of the electrode distance. In the inversion process, however, a less dense mesh was used to guarantee that the problem does not become undetermined. The vertical size of the mesh is increasing with the depth. Carrying out the inversion by the EarthImager 2D, the stop criteria were set with 3% RMS error and 5% error reduction in the resistivity inversion settings because 3% noise level was assumed. The inversion terminates on meeting one of the criteria in these settings.