An indirect electromagnetic geothermometer is used for deep temperature estimations in the Soultz‐sous‐Forêts geothermal area (France) using magnetotelluric sounding data. Validation of temperature assessment carried out by comparison of the forecast temperature profile with temperature log from the deepest borehole has resulted in the relative extrapolation accuracy of less than 2%. It is found that the resistivity’s uncertainty caused by magnetotelluric inversion errors and by possible effects of external factors very weakly affects the resulting temperature, with the latter being influenced mainly by the ratio between the borehole length and the extrapolation depth. The temperature cross‐section constructed up to the depth 5000 m manifests local temperature maxima at large depths beneath the wells GPK2 and RT1/RT3. The analysis of the temperature profile in GPK2 location beneath 5000 m indicates that its behaviour continues to be of the conductive type (as in the depth range of 3700 m–5000 m) up to the depth 6000 m, while manifesting convective type below this depth. Finally, application of the indirect electromagnetic geothermometer for the deep temperature forecasting in the Rittershoffen site enabled us to constrain the location for future drilling.

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This paper provides a review of geological, geophysical, and geochemical studies for three geo thermal zones of Hengill Volcano, Iceland: Nesjavellir, Hellisheidi, and Hveragerdi. We discuss the relation ships between global tectonics and high temperature geothermal systems in Iceland. The bulk of this review is devoted to studies of the physical, geochemical, and mineralogic parameters for the three areas. A separate discussion concerns surface phenomena, as well as the origin of thermal water. This review covers studies of the main aquifer complexes: Miocene/Lower Pliocene plateau basalts, Upper Pliocene/Pleistocene lavas and volcanoclastics involving tillite horizons, an aquifer complex of Holocene lava sheets as thick as 1 km, and an aquifer complex of Upper Pleistocene/Holocene alluvial eolian deposits and formations of bottom moraines. We consider a conceptual model of geothermal reservoirs characteristic for the Hengill geothermal fields.

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A B S T R A C TWe study the feasibility of the application of an indirect EM geothermometer, developed recently, to the temperature extrapolation in depth using magnetotelluric data collected in the seismically active northern Tien Shan faulted area (Bishkek Geodynamic Test Site, Kyrgyzstan) and Hengill geothermal zone (Iceland). The approach used is based on the artificial neural network technique, which does not imply the prior knowledge of the electrical conductivity mechanisms on the one hand and provides temperature estimates based on the analysis of the implicit conductivitytemperature relations, on the other.The samples for neuronet teaching consisted in the well temperature records and electrical conductivity values determined for the same depths from the magnetotelluric data measured in the vicinities of eight boreholes in each testing area. The testing of the taught neuronets was carried out using the temperature records not involved in the teaching process. The results indicate that the temperature extrapolation accuracy essentially depends on the ratio between the well length and the extrapolation depth. In particular, in extrapolation to a depth twice as large as the well depth the relative error is 5-6% and in case of threefold excess, the error is around 20%. This result makes it possible to increase significantly the depth of indirect temperature estimation in the Earth's interior (in particular, for geothermal exploration) based on the available temperature logs.The practical application of an indirect electromagnetic geothermometer could provide the following facilities: 1) more exact temperature estimation in the extrapolation mode; 2) remote temperature estimates in the boreholes in areas characterized by extreme conditions for conventional geothermometers.

The maps are presented of seasonal air temperature and precipitation amount anomalies averaged for the whole Volga region and adjacent territory for two time periods, 1946-1977 and 1978-2008. It is demonstrated that the considerable differences in the thermal and moistening regimes of the Volga region exist for these two periods. The relation is described between the variations of temperature and precipitation amount and the circulation types according to Vangengeim-Girs classification as well as the possibility to use these data for specifying the climatic scenarios obtained on the basis of physically complete hydrodynamic models.

Advances have been made in the subsurface temperature estimation basing on the indirect electromagnetic geothermometer. The approach used was based on the artificial neural network technique, which, contrary to other available approaches, does not imply the prior knowledge of the electrical conductivity mechanisms and rock properties. Application of the indirect EM geothermometer to the interwell space interpolation in three areas (Tien Shan, Kyrgyzstan; Soultz-sous-Forêts, France; and Hengill, Iceland) characterized by different geologic environments indicated that the temperature estimation errors are controlled by four factors: faulting, distance between the EM site and the area where the temperature is estimated, meteoric and groundwater flows, and lateral geologic inhomogeneity (although the latter factor is less restrictive if appropriate EM inversion tools are used). It was demonstrated that the extrapolation errors depend on two factors: spacing between the EM site and the borehole, and ratio between the well length and the extrapolation depth. In particular, the relative accuracy of the temperature extrapolation to the depths twice as large as the borehole depth did not exceed an average of 5%. Using the indirect EM geothermometer, it was possible to reconstruct 2D and 3D temperature models of the studied areas from EM sounding data, which, in turn, enabled us to draw important conclusions regarding the dominating heat transfer mechanisms, fluid circulation paths, and better locations for drilling new boreholes. Application of the indirect EM geothermometer during exploitation of the geothermal reservoirs may enable one to monitor the variations of subsurface temperatures basing on the ground EM monitoring data and forecast future trends.