The modelled thickness of the aquifers is the gross value: the depth of the base is subtracted from that of the top. Only layers known to containing dominantly sand(stone) have been modelled. All aquifers however also contain layers with a permeability so low that they contribute little or nothing to water production. These layers often consist of clay(stone), shaly sand or cemented layers. For a correct calculation of the geothermal potential, the gross thickness of the aquifer must be corrected for the layers that do not contribute to production. This is done by multiplying the gross thickness by the percentage of the total layer thickness that does contribute to production. This percentage is called the net-to-gross ratio or N/G.
It is implicitly assumed that the average aquifer permeability is also calculated over the net reservoir thickness (an equal result is otherwise obtained if the gross thickness is used in combination with the average aquifer permeability of the gross reservoir thickness).
Net-gross ratios were first calculated per borehole and per aquifer using petrophysical evaluation availables in the TNO - GDN Rock Properties Database, based on characteristics of the gamma ray log.