Abstract
The geothermal characteristics of specific areas on Earth can be identified using geophysical and lithological logs based on deep boreholes, such as those more than 1000 m in depth. Based on the combined analyses of geophysical logs (temperature, caliper, electrical conductivity (EC), and natural gamma-ray logs) and lithological logs, as well as 1D steady-state heat transfer model, the deep groundwater flow and temperature were characterized in four deep boreholes (BH-1–BH-4) roughly 2000 m deep in the area of Heunghae-eup, Pohang, South Korea. The estimated thermal gradients from the temperature profiles are as follows: 22.37–30.77 °C/km for BH-1, 35.67–64.52 °C/km for BH-2, 40.85–46.44 °C/km for BH-3, and 33.33–35.71 °C/km for BH-4. According to the geophysical logs and lithology profiles, the groundwater mainly flows into and out of the boreholes through the basic dyke, rhyolite, and sandstone/mudstone. Evidently, the groundwater flows moving through the fractures and faults induce nonlinear temperature changes. The upward and downward groundwater flows passing through fractures and faults can be estimated using a 1D steady-state heat-transfer equation, by considering a fracture angle based on the lithological and geophysical profiles. To determine the direction (up/down) and rate of groundwater flow, the values for the parameter β were estimated as follows: −1.95 to 5.40 for BH-1, −13.48 to 4.87 for BH-2, −1.76 for BH-3, and −3.39 to 14.15 for BH-4.
Highlights
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
Dark greenish-gray andesitic breccia can be observed at a zone lower than that of the Figure 4 shows the stratigraphic columns of the four deep boreholes; it is evident that crystal tuff sandstone/mudstone beds in BH-4; it cannot be observed in BH-1, BH-2, and BH-3
Andesitic breccia lies below the semiconsolidated mudstone at the following depths: for BH-3 and 217–443 m generally contains dark-red volcanic blocks and is fresher than 206–428 the tuff.mMedium-grained for Crystal tuff was not observed for BH-1 and
Summary
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. 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]. Sea alkaline volcanic zone [18], occupying the central part of the Tertiary Pohang. Era and is overlain by repeated layers of Cretaceous sandstone/mudstone (Figure 1 [19])
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