Effect of thermal conductivity anisotropy of rocks on the subsurface temperature field

Abstract

SUMMARY The effect of thermal conductivity anisotropy on the heat flow is evaluated by solving numerically the heat conduction equation in 2-D models of anisotropic structures. The most common case of sedimentary rock anisotropy with two principal conductivities, parallel and normal to the bedding, is considered. For the effect due purely to anisotropy to be studied, both diagonal terms of the conductivity matrix of the anisotropic prism are put equal to the conductivity of the surrounding isotropic half-space. The results are presented in the form of figures showing isolines of horizontal and vertical heat flow densities (HFD). The greatest vertical HFD variations occur along the vertical sides of the prism and attain *7 per cent of the basal heat flow for the realistic conductivity model adopted. In addition, the vertical HFD within the prism was compared with the value calculated as if the data were measured in a vertical borehole. In this case, only the vertical temperature gradient is known. The difference between the two quantities represents the contribution of the horizontal temperature gradient to the vertical HFD. It attains 1-3 per cent of the HFD in models with an aspect (length-height) ratio of the prism of less than 2. An anisotropic body acting also as a heat flow refractor was investigated with a model of horizontally deposited sedimentary rock, with a factor of anisotropy of 1.5, surrounding a prism of the same rock with inclined layers. Vertical HFD changes concentrate along the sides of the prism and vary from -10 to +15 per cent of the HFD .