gradient thermique moyen

CONT: ... [A figure] shows the depths to which we would have to penetrate in order to reach various temperatures in zones of various mean temperature gradients. Of course in practice the temperature/depth relationship would never be a simple linear one ... owing to changes of thermal conductivity with depth, but in terms of average thermal gradient the figure serves to show the order or magnitude of the required penetration depths to reach various required temperatures in non-thermal and semi-thermal areas.