# Characteristics of Deep Groundwater Flow and Temperature in the Tertiary Pohang Area, South Korea
## 1 Introduction  
The geothermal flow of the Earth's interior is generated mainly by conduction and convection. Based on conduction and convection, the geothermal temperature increases steadily with depth. The equilibrium temperature of groundwater in a borehole with an overall linear or nonlinear thermal gradient represents the Earth's temperature at that depth. Heat transfer inside the Earth occurs predominantly through conduction. The temperature distribution in the continental crust and lithosphere is governed largely by heat generated from the radioactive decay of rocks and by heat conduction from mantle to the surface through heat-transfer media and pathways [1]. Thermal gradients have been estimated by using constant temperature gradients in states unaffected by fluid flows or based on the average slope of an entire section of boreholes from the surface and bottom temperatures (using temperature-log data) [2; 3].  
In South Korea, due to the lack of thermal energy generated from young volcanic activity [4], research on deep geothermal energy began only in the 2000s. The Pohang area in South Korea has the largest geothermal energy potential, with heat flows greater than 80 mWm\({}^{-2}\)[4; 5]. Moreover, the Pohang area, with the youngest tectonic movements in the Korean peninsula, has possible deep-stressed fracture zones [6; 7; 8]. To evaluate the geothermal energy potential, the Korea Institute of Geoscience and Mineral Resources drilled four deep boreholes (BH-1 to BH-4) from January 2003 to November 2006. BH-4 is a pumping borehole for developing deep groundwater; it has the largest diameter (200 mm at the final depth) of the four boreholes, with a depth greater than 2 km [9]. The distance between BH-1 and BH-2 is 157 m, and the distance between BH-3 and BH-4 is 123 m. As geothermal water in Korea has a higher probability of geothermal flow along secondary pores, such as faults and fractures, than along primary pores, it is important to understand how deep geothermal water is associated with faults and geological structures [10]. The construction of a geothermal binary power plant using an enhanced geothermal system with a planned generation capacity of 1.5 MW was expected to be completed in the Pohang area by 2017. However, its construction was stopped owing to the Mw 5.4 Pohang earthquake on November 15, 2017, which damaged buildings in Pohang.  
[PERSON] and [PERSON] [11] derived a simple steady-state solution for estimating vertical water fluxes from deep temperature profiles in homogeneous environments. This approach has been used in several studies, i.e., to calculate water fluxes from the curvatures of temperature profiles recorded in boreholes [12; 13]. Furthermore, the estimation of the borehole vertical flow is of specific interest, as it can be used to derive large-scale hydraulic connections [14]. In addition, the temperature profiles display a strong correlation with the vertical borehole velocity profiles [15].  
The aim of this study was to characterize deep groundwater flows and temperatures, based on the geophysical logs and lithology profiles of four deep boreholes in Heunghae-eup, Pohang, South Korea (Figure 1). The lithology profiles and geophysical logs can be used to effectively detect the fractures intersecting a borehole. Additionally, the temperature, caliper, electrical conductivity (EC), and natural gamma ray logging can be implemented for geological and stratigraphic interpretation, thermal gradient measurement, groundwater flow pattern determination, casing installation, and screen section determination [16; 17].