Heat flow

The temperature in the earth crust and litoshper are determined with heat flow measurements.

Information on the thermal state in the crust/lithosphere is used in many fields, e.g. from the use of geothermal energy to estimating the maturation conditions of hydrocarbons in the subsurface or characterising the quality of the natural reservoirs hosting these hydrocarbons. In addition, geothermal research has  potential applications related to Earth dynamics and tectonic deformation.

In short, heat flow determination requires knowledge on the following parameters: (1) crustal geothermal gradient (i.e. measure of the rate of temperature variation with depth) and (2) thermal conductivity of the rocks (i.e. measure of the ability of the medium to transfer heat by conduction). Heat flow is subsequently calculated using the steady-state equation for heat conduction:

q = -k dT/dZ

where q is heat flow (W/m2), T is temperature (K), Z is depth (m) and k is thermal conductivity (W/m/K).

In practice, we need to determine the crustal geothermal gradient well below the surface. Therefore, we need to drill deep wells in order to avoid thermal disturbances related to various factors like e.g. paleoclimatic changes (e.g. deglaciations) or anthropogenic effects (e.g. deforestation). Temperatures are logged in the wells using specially designed probes months after completion of the drilling. Drill-core material is systematically sampled and analysed in the lab to determine thermal conductivities of rocks.

Modelling of the thermal state at levels deep below the surface requires an additional constraint: the quantity of heat produced by radioactive decay of uranium, thorium and potassium in the rocks. Sampling and analysis of rocks as part of the Lito project contribute with quantitative data for these radioactive elements in representative rock units in Norway, which then can be used for calculating their contribution to the net heat flow.

The thermal modelling is carried out using state-of-the-art 2D and 3D numerical methods where the subsurface structure and nature is reconstructed by means of geophysical studies carried out by NGU's Geophysics team.