Only a limited amount of subsurface data is available is some parts of the Netherlands. Those areas were described in the report "Kader voor exploratie werkprogramma geothermie in gebieden met lage dichtheid" (in Dutch), written by EBN and TNO in 2017. For some areas, the sparse information is sufficient to determine whether an aquifer has geothermal potential or not. For instance, rocks of the Slochteren Formation in the West Netherlands Basin are thin and deeply buried. Consequently, the aquifer was drilled by few wells. Nevertheless, it can be stated with a large degree of certainty that the geothermal potential of the aquifer in this area is limited. Rocks of the same aquifer in the province of Utrecht are also relatively unknown. Here, the aquifer is locally thick (>100 m), and the maximum burial depth is unknown. The current depth (~2 km) suggests that geothermal potential of the aquifer may be good. Still, the uncertainty is large, and consequently, the Rotliegend aquifer in Utrecht is considered to constitute a 'white spot'.
A 'white spots' map was generated for all aquifers. These maps indicate where the subsurface data is sparse. These areas may be interesting for geothermal exploration, even when the currently calculated potential is low.
A scoring system was made to define a metric for certainty of the data based off of the type of data available. The following list shows what scores (denoted in the list as "classes") were assigned to which data types. The lower the class number, the more uncertain the subsurface data are.
- 3D seismic available - class 5
- 2D seismic (younger than 1980) available - class 4
- Wells having penetrated the aquifer available (influence radius 4 km) - class 4
- 2D seismic (older than 1980) available - class 2
In cases where both 3D and 2D seismic are available, class 5 prevails. Next, the seismic and/or well classes are added together. For example, an area that has 3D seismic coverage (class 5) and a relevant well nearby (class 4) receives a value of 5 + 4 = 9. A low value indicates a white spot.