Estimation of thermal conductivity and its spatial variability in igneous rocks from in situ density logging

Characterisation of thermal conductivity of rock and its spatial variability by laboratory measurements is costly and time-consuming. There is an incentive to find more cost-effective and rapid methods. A new empirical relationship between density and thermal conductivity for igneous rocks has been found. This paper explains that the relationship is based on the rock forming processes which in turn generate typical mineral compositions. Based on this relationship, thermal conductivity can be estimated from geophysical density loggings. Experience has shown that this indirect method of determining the thermal conductivity can be used to characterise the spatial variability of thermal conductivity for certain rock types. The spatial data can be used in the modelling of thermal rock domains. Applications of the statistical relationship are presented and illustrative examples are given of how spatial variability of thermal conductivity can be estimated, based on the site investigations in a crystalline bedrock environment in Sweden. Possible deficiencies of the method are identified, such as uncertainties associated with different rock forming minerals. Methods of improvement are also discussed.