Dušan Rajver

Signature of the last ice age in the present subsurface temperatures in the Czech Republic and Slovenia

Abstract Ground surface temperature (GST) history can be evaluated by analysing the present-day temperature-depth profiles measured in boreholes. Due to the diffusive character of the process, however, the resolution of the method decreases quickly for the more remote events. The reconstructed GST at a given moment in the past is a weighted average of temperature over a certain period of time. The present study shows that because the cold climate of the last (Weichselian) glacial prevailed in the period of 75–10 ka, there is a chance to obtain its mean GST, despite the large averaging intervals, from temperature profiles measured in deep boreholes. This fact is demonstrated on the GST inversions of carefully selected profiles, three from the Czech Republic and two from Slovenia, the depth of which ranges between 1.5 and 2.4 km. They yield GST histories with a minimum between 19 and 10 ka followed by a warming of 6–15 K. In order to suppress the nonclimatic noise and to extract the common signal, the joint inversions were carried out for the Czech and the Slovenian boreholes, respectively. The Czech data show the minimum at 17 ka and the warming of 8 K. The Slovenian data have the minimum at 16 ka and the warming amounts to 7 K. These results agree well with information extracted earlier from the German KTB super deep borehole, where the inversion of the temperature log indicated 10 K warming since the glacial minimum, and represent an independent estimate of the difference between glacial and interglacial conditions typical for the region of Central Europe.

The climate record inverted from borehole temperatures in Slovenia

Abstract Temperature profiles from nine boreholes in Slovenia, located mostly in the northeastern part of the country, have been studied for evidence of past climatic changes. The reconstruction of the ground surface temperature (GST) history, based on 1-D theory of heat conduction, was accomplished using the functional space inversion (FSI) method. Because of the paucity of quality thermal conductivity data and because the nature of heat conduction is such that information about GST changes decay with the passage of time, the a-priori standard deviations of the thermal conductivity model (1 W m−1 K−1) and measured temperature (0.2 K) were chosen large enough, so that the GST histories older than 200 years are considerably smoothed and essentially merge with the long-term mean. The average GST history based on consistent results from seven boreholes starts to rise at the end of the last century. The amplitude of the warming is about 0.6 to 0.7 K in the past 100 years. Two boreholes were rejected. The long-term mean GSTs lie between 9.4 and 11.1°C. The mean annual air temperatures from the Ljubljana meteorological station are in good agreement with the average GST solution. The GST history of the past 20–30 thousand years (ka) was reconstructed from the almost 2-km-deep Ljutomer temperature profile. The history evidences climate variations during the Wurm (Weichselian) ice age: it places the glacial minimum of about 3°C at 13–14 ka ago and the postglacial maximum of 10.5°C roughly at 2–3 ka ago.

Geothermal characteristics of the Krško basin, Slovenia, based on geophysical research

Abstract The Krsko basin with its thermal springs is a syncline, filled with low permeable Tertiary and some Quaternary sediments. Their thickness reaches about 1.8 km in its central eastern part. This is perceivable also in geotherms reflecting a conductive temperature field. In the syncline basement, especially in Triassic and Jurassic carbonates, but less in Cretaceous rocks, convective thermal field predominates. The syncline pattern and structure have been determined from the results of gravimetric, seismic and geoelectrical measurements and deep drilling since 1959. The most important geothermal anomaly is the Catež field with the greatest concentration of investigations. There, the highest borehole temperatures have been reached, ranging from 50 to 64 °C. Geothermal anomalies at other localities have been less investigated, and temperatures do not exceed 36 °C. Deep boreholes which were available for geothermal measurements are found mostly along the southern rim of the basin. In much wider area the only source of data for the construction of geotherms were geoelectrical soundings, applied to elaborate geothermal maps and cross-sections. This was enabled by a conversion from resistivity and borehole lithology into temperature data, using one-dimensional simplified solution of Laplace’s equation. In such a way an approximative knowledge of geothermal conditions below surface and beyond known geothermal anomalies has been extended. Circulation of meteoric water into few kilometers deep fissured and fractured hot zones is the only heating possibility for thermal water in the Catež field. Water circulation is probably the deepest there than elsewhere in the Krsko basin. Taking into account all information collected until now, we assume that geothermal reservoir could extend at least 2–2.5 km deep below the surface.

Six years of ground–air temperature tracking at Malence (Slovenia): thermal diffusivity from subsurface temperature data

To understand the processes which govern the downward propagation of the surface temperature signal and to quantify the relationship between ground and air temperatures, a borehole climate observatory was established at Malence (Slovenia). A substantial (climate?) warming has been observed; surface air temperature from 2003 to 2009 has been increasing by a rate of more than 0.5 K/year. Temperature difference between ground and air temperatures revealed a typical annual course; during the winter months the ground is warmer than the air, in the summer the ground gets slightly colder, the mean yearly offset value amounting to 1.6 K. Here we report on the thermal diffusivity distribution which was determined for a near-surface 10 m deep section using a number of six-year-long temperature–time data series. The series of daily averaged temperatures were processed by spectral analysis. Thermal diffusivity was calculated from the amplitude and phase angle of annual waves corresponding to different subsurface layers using conduction theory as well as the conduction–convection approach. Both methods lead to the generally similar results and revealed a definite heterogeneity of the subsurface medium. The results proved that the annual scale convective heat transfer did not contribute significantly to the temperature–time variations monitored in the uppermost 10 m depth interval.

The state of exploitation of geothermal energy and some interesting achievements in geothermal research and development in the world

The article presents the latest status of geothermal energy use worldwide and the comparison with the previous period, both in electricity generation as well as in the various categories of direct use. Electricity production takes place in 26 countries and has at the end of 2014 reached 73,700 GWh from geothermal power plants with nearly 12.8 GW of installed power. This is still only 0.31 % of the total electricity produced in the world and it will be interesting to monitor the future share of geothermal energy in doing so. In the last 5-year period the development was particularly rapid in countries where it was slower in the past and, however, with favorable geological (tectonic) conditions (Iceland, Kenya, New Zealand, Turkey, etc.). Direct use of geothermal energy covers a significant number of countries, today there are 82, although some of them are such where it takes place almost solely by geothermal (ground-source) heat pumps (GHP) on shallow subsurface energy (Finland). Installed capacity in the direct use is 70,885 MWt and geothermal energy used, including the GHP, is 592,638 TJ/year (end of 2014). Within the used energy the share of GHP dominates with 55.2 %, followed by the bathing and swimming pools complexes incl. balneology by 20.2 %, space heating by 15.0 % (the majority of it is district heating), heating of greenhouses and soil with 4.9 %, etc. The second part presents some interesting technological and scientific innovations in exploration and exploitation of geothermal energy. GEOLOGIJA 59/1, 99-114, Ljubljana 2016 http://dx.doi.org/10.5474/geologija.2016.007