## 5 Discussion  
The interpretation of the geothermal aquifer of the Humenne Unit is derived from the large surface occurrence which was confirmed by the borehole datasets. Some essential features of the unit are: an increasing tectonic thickness towards the Piening Klippen Belt, a south-vegent duplex structure, and a Middle Triassic-Middle Cretaceous carbonate fill. The permeability of the aquifer is variable. Its joint and joint-karstic permeability is weak (T = 1.3 \(\times\) 10\({}^{-5}\) m\({}^{2}\), k = 1.3 \(\times\) 10\({}^{-7}\) m\(\cdot\)s\({}^{-1}\)). However, in tectonically fractured zones it is much better (T = 0.05 \(\times\) 10\({}^{-7}\) m\({}^{2}\), k = 0.47 \(\times\) 10\({}^{-8}\) m\(\cdot\)s\({}^{-1}\)) with a yield ranging from 0.7 to 5.3 l\(\cdot\)s\({}^{-1}\). The data shows a low interdependence between karstification, porosity, or permeability. No direct data are available on the values of the effective porosity of Triassic carbonate complexes in the Humenne Mesozoic area. The results of interpretations of logging measurements of nearby deep wells were used for this data, where the average effective porosity is 6.0 \(\pm\) 3.7% with the values in the range of 1-18%. The carbonates of the Humenne Unit (aquifer) broke their epigenic processes after the Miocene and accelerated the development of systems of fractures. Water circulation is driven by the topography gradient [38]. Based on the results from borehole GTH-1 (Figure 9B), the main hydraulic driving force [39] for geothermal water is related to the marginal faults of the buried ridge (i.e., thermal lift). The neotectonic deep faults separate, but do not isolate the carbonate aquifer from the Neogene/Paleogene volcano sediments. These sediments are resaturated by precipitation [40,41]. The relationships between tectonic elevations and permeability play a key role in the prediction of the geothermal potential of the Humenne Unit. The interpretation of the seismic cross-sections identified deep margins between each unit. From the results, it is known that the surface and the thickness of the Humenne Unit are irregular. In the northwest, the upper boundary of the Humenne Unit is at a depth of approx. 500 m (b.s.l.) and drops to approximately 1000 m (b.s.l.) in the southeast (Figure 9A). The subsurface morphology affects the heat flow density distribution. The isolines of heat flow density are significantly twisted toward this direction, which is parallel to the elevation structures. Old thermal boreholes confirm the presence of open transverse faults which are parallel to the elevations and many of them have been seismically active during the Quaternary. These relationships are especially evident in the southeastern part of the Humenne Unit [33]. The seismic cross-sections confirmed evidence of implication  
Figure 8: Geoelectric resistivity profile No. FFTU-5 (see Figure 3). Lower differential density between Miocene and Cretaceous sediments does not allow clearly divided sedimentary boundaries. With increasing depths (more than 500 m), the quality of interpretation decreases (modified after [24,37]). structures, which significantly changed the lithological composition together with erosion. The largest thicknesses, which are approximately 1700 m to 2000 m thick, are assumed to be located under the Neogene volcanics. In contrast, the lowest thicknesses are at most 1300 m thick and are connected to tectonic elevations. The average carbonate thickness of the unit is 1600-1900 m (Figure 9D). Gravimetric measurements confirm the existence of sets of density anomalies corresponding to the morphological elevations and depressions of the pre-Cenozoic basement. They may also confirm other geological structures, which has an anomalous geophysical expression. The sedimentary-volcanic complex is most evident in the gravity field of the area built by Neogene volcanic rocks. This is due to the fact that volcanic rocks are commonly heterogeneous.