Heat flow variations in 2 km deep borehole Litoměřice, Czechia

0570575 - GFÚ 2024 RIV GB eng J - Journal Article
Šafanda, Jan - Dědeček, Petr - Čermák, Vladimír - Uxa, Tomáš
Heat flow variations in 2 km deep borehole Litoměřice, Czechia.
Geothermics. Roč. 111, June (2023), č. článku 102708. ISSN 0375-6505. E-ISSN 1879-3576
R&D Projects: GA ČR(CZ) GA21-23196S
Institutional support: RVO:67985530
Keywords : repeated temperature logs * equilibrium temperature profile * geothermal model * climate signatures
OECD category: Volcanology
Impact factor: 3.9, year: 2022
Method of publishing: Limited access
https://www.sciencedirect.com/science/article/pii/S0375650523000627

Temperature in 2 km deep borehole Litoměřice, drilled in 2007, was repeatedly logged down to 1700 m in the period 2007 – 2020. We were able to monitor a return of the temperature to the equilibrium temperature-depth profile undisturbed by drilling. The uppermost part of the profile contains signal of the recent warming manifested by a negative temperature gradient close to the surface and a temperature minimum at a depth of about 40 m. The minimum has been migrating downward at a rate of 1.5 – 2 m per year in the period 2015 – 2020. A detailed knowledge of temperature gradient together with thermal conductivity, diffusivity and heat production measurements on the drill-core samples of mica-schist that occurs below 900 m depth enabled us to analyze the heat flow vertical variations in the lithologically homogeneous depth section 900 – 1700 m. We came to the conclusion that temperature-depth profile in this section contains a robust climate signal of the last glacial cycle. The reconstructed ground surface temperature history indicates the magnitude of the last glacial – Holocene warming 13 -15 K and existence of a minimum 15 – 20 ka. The long-term mean ground surface temperature +1 - +2 °C suggests that the borehole site was permafrost free for most of the glacial cycle. Existence of about 100 m deep permafrost is possible in the coldest part of the last glacial. The steady-state surface heat flow has been estimated at 88 mW/m2. The reconstructed ground surface temperature history used as a surface forcing function in a numerical solution of the transient heat conduction equation provided an estimate of the present-day heat flow in the well. The estimate is practically independent from the poorly constrained conductivity of the 900 m thick sedimentary cover. According to it the present-day heat flow is lower than the steady-state one by 20 - 30 mW/m2 in the first hundreds of meters below the surface and still by about 10 mW/m2 at a depth of 1 km.
Permanent Link: https://hdl.handle.net/11104/0341876