We have looked at how heat generation and thermal gradient are measured or otherwise approximated within rocks. The last remaining parameter required to define steady-state heat flow is thermal conductivity. Simply put, thermal conductivity, λ, is a measure of how easily heat is transmitted through a material. It is a tensor operator that relates the heat flow vector to the thermal gradient vector within a body, and it is an inherent physical property. Many rocks are anisotropic, with conductivity dependent upon the direction of heat flow, but geothermal problems usually involve only the vertical component.

Thermal conductivity must be estimated over the same section that thermal gradient and heat generation are known. Where temperature is denned at discrete depths (e.g. Horner-corrected bottom-hole temperatures), an average thermal conductivity is required between each of those depths. Where a continuous temperature log is available, we require a continuous conductivity log. Heat flow remains undefined in any section where there is a gap in the thermal conductivity record.