# Characteristics of Deep Groundwater Flow and Temperature in the Tertiary Pohang Area, South Korea
## 5 Discussion  
Conventional geophysical logs can qualitatively indicate the fractures and alteration zones intersecting the boreholes [58; 59]. In this study, we characterized deep groundwater flow and thermal properties by using geophysical logs and lithological logs of boreholes. The fracture zones and thermal properties were identified based on the geophysical logs, following the research of [PERSON] [60; 61] and [PERSON] [62].  
The geophysical logs and lithology profiles indicated that groundwater flows into, and out of, the boreholes in the basic dyke, rhyolite, and sandstone/mudstone that contain highly fractured zones. The caliper log identified the anomaly zones associated with highly fractured zones in the basic dyke, rhyolite, sandstone/mudstone, and andnesitic breccia. The results of the strata analysis using natural gamma-ray log and lithology log suggest that the geological strata from the upper Mesozoic sedimentary rocks to the upper part included semiconsolidated mudstone, tuffs (lapilli tuff, crystal tuff, and tufaceous sediment), rhyolite, and sandstone/mudstone; moreover, the strata possessed good horizontal connectivity, with possible fracture connections in the rock around the boreholes. The depth of the Cretaceous semiconsolidated mudstone layer increased in depth from north (BH-3 and BH-4) to south (BH-1 and BH-2). The distribution of the tufaceous sediment decreased and that of the crystal tuff increased from south to north.  
By comparing the caliper log and lithology data, anomalies appeared for BH-2 at the following depths: 424, 461, and 504 m for the rhyolite; 704, 731, and 760 m for the basic dyke; and 1087, 1125, and 1245 m for the sandstone/mudstone. This is similar to the anomalous zone of 1200-1440 m obtained based on the natural gamma-ray log. The anomalies in BH-3 appeared at a depth of 403 m for the basic dyke, 620 m for the sandstone/mudstone, and 862 and 877 m for the rhyolite. In contrast, the anomalies in BH-4 occurred at greater depths, specifically, at 1589, 1738, and 1989 m for the andesitic breccia. In BH-2, the three basic dykes (intruding into the rhyolite) were distinguished easily from the rhyolite, owing to their low natural gamma-ray intensities.  
By means of the lithological log and geophysical logs, several aquifer zones were identified, at depths of 350-410, 510-590, 600-650, 730-760, 870-890, 950-1000, and 1150-1360 m, as well as hydraulically active fractures intersecting the deep boreholes. An anomaly in the EC values can represent an inflow or outflow point of fresh and/or saline water in the borehole [45].  
Figure 9: Comparison of normalized vertical distance (\(z_{D}\)) versus normalized temperature (\(T\)\(z\)\(-\)\(T_{0}\))/(\(T_{L}\)\(-\)\(T_{0}\)) between the theoretical predictions and measured data from BH-2 and BH-4. A negative \(\beta\) value indicates upward groundwater flow with a warm origin, whereas a positive \(\beta\) value indicates downward groundwater flow with a cool origin.  
Uncertainty can be induced in the upward or downward flows of the 1D steady-state heat transfer model from the deviations in the centering of the loggers [57] or change of fracture direction or interconnection with other factor sets [27] in the boreholes, e.g., BH-2 in Figure 9. The average angle of the fracture zone, \(\omega\), representing the average direction of groundwater flow along the fracture, is not known a priori [27]. Uncertainty also exists in the groundwater flow owing to indistinct nonlinear temperature zones; these can be more clearly identified through a detailed investigation of the fractures and faults in deep boreholes.