Geothermic Investigations

Abstract

The most important characteristics of the thermal field of the Earth’s crust are as follows: geothermal gradient (GTG), thermal conductivity (λ), heat flux (HF) density in the near-surface zone (q), and distribution of radiogenic heat sources (A). The geothermal gradient is derived from temperature measurements in a well after having kept it under static conditions until the thermal field becomes stationary. The heat flow (for the case of horizontal strata and stationary heat transfer) is determined as a product of GTG and thermal conductivity, q= y x λ. Consequently, the reliability and accuracy of heat flux calculations depend upon the accuracy of determination of these two quantities. The accuracy of the GTG and A measurements for the real well is now about 5% and 7–10%, respectively. The relatively low accuracy of the A evaluations is explained by the fact that this parameter is measured on cores, not in situ. When evaluating A for subsurface environments (especially at great depths), the effect of water saturation has to be accounted for. At 5–10 km depths, a considerable portion of pores and fractures becomes closed. Real values of thermal conductivity for low-porosity crystalline rocks in situ usually exceeding those measured in laboratories.