# Characteristics of Deep Groundwater Flow and Temperature in the Tertiary Pohang Area, South Korea
## 3 Methods
### Geophysical Logs  
Temperature logging is mainly used to estimate groundwater flows and the locations of aquifers and to identify different thermal gradients [32; 33; 34]. The thermal gradients can be classified into three categories: linear thermal gradients, wherein the fluid temperature in the borehole is in equilibrium with the underground temperature (Figure 3a); multiple thermal gradients, wherein the borehole consists of several strata with different thermal conductivities (Figure 3b); and nonlinear thermal gradients, wherein the fluid flows through fractures (Figure 3c,d) [35].  
A caliper log uses a mechanical caliper, acoustic caliper, and sonar caliper to continuously measure a borehole diameter. Caliper log anomalies indicate fracture depths, based on the enlargement of the borehole diameter [36; 37]. Inflows to and outflows from the borehole can be attributed to the bedding planes or fractures identified by the caliper log. The advantages of this approach are clearly demonstrated by the number of possible water-producing zones indicated by the caliper logs. In geothermal drilling activity, a caliper log is mainly used for detecting cavities in a well, measuring the wellbore volume to estimate the required cement volume, determining the placing of casing in a well, inspecting the casing for damage either during or after drilling throughout the lifetime of the well, and calibrating the well depth (as combined with the lithological and natural gamma-ray logs) [38; 39].  
EC logging detects the EC values of water in various aquifers, thereby providing a general indication of water quality and hydraulic information [40; 41]. A systematic analysis of EC logging data can predict the location of possible hydraulically active zones in a borehole [42]. In general, the EC depends on the porosity, water temperature, and salinity, as it is controlled by ionic diffusion and dispersion, as well as by the moving water. Under the equilibrium condition, the ionic content at a given depth in a water-filled borehole effectively represents a balance of the various processes at that depth [43; 44]. A depth with less saline water may represent the mixing point of formation waters, and high-salinity zones can be attributed to inflows of saline water into the borehole [45].  
Natural gamma-ray logging is useful with respect to the stratigraphic identification of basic intrusive rocks, such as rhyolite. In addition, gamma-ray logging is an effective method for identifying felsic rock among the basalt formations in a borehole, using a linear relationship between the silica content in the volcanic rock and the natural gamma-ray intensity. Moreover, a natural gamma-ray log is often used to discriminate between Mesozoic sedimentary rock and Tertiary formations. For example, the presence of sandstone in semiconsolidated mudstone exhibits a lower gamma-ray intensity [46; 47; 48].  
Figure 3: Schematic temperature–depth profiles with various underground conditions (adapted from [PERSON] [2] and [PERSON] [35]): (**a**) temperature profile with underground equilibrium; (**b**) temperature profile with different thermal conductivities (\(\kappa_{1}\) and \(\kappa_{2}\)), wherein the borehole consists of different thermal conductivities; and (**c**,**d**) temperature profile with a nonlinear thermal gradient, which is affected by the groundwater flow from both warm and cool origins.  
In this study, the geophysical logger of Robertson Geologging (UK) with the console of Micrologger II was used for temperature log with an accuracy \(\pm 0.5\)\({}^{\circ}\)C, EC log with an accuracy \(\pm 2.5\%\) at 500 \(\upmu\) S/cm, and caliper log with an accuracy \(\pm 6\) mm.