The temperature of the subsurface increases with depth. This is called the geothermal gradient. It is determined by the local heat flux and by the thermal conductivity and radiogenic heat generation potential of the different rocks present in the subsurface. In Flanders, the geothermal gradient is approximately 3 °C per 100 m and the heat flux varies from 50 to 90 kW/m2. This variation, combined with the local geology, implies that the temperature rise with depth varies for different areas (see Figure). A simple calculation demonstrates that one must drill approximately 500 m deep to encounter water with a temperature of 25 °C. This is just about the bottom limit for direct geothermal applications. In Flanders, water-bearing layers at this depth can only be found in the Campine region and in the most southern area of West-Flanders. Direct applications for which a temperature of 40 °C or more is needed only appear possible in the Campine area. In contrast, applications that make use of heat pumps are possible everywhere.
Non-traditional geothermal applications, like EGS (Enhanced Geothermal Systems) or HDR (Hot Dry Rock), are, in principle, possible throughout Flanders. However, the economic feasibility of such projects must be evaluated on a case-by-case basis.