The systems are binary karsts developed in the chalk under the cover of surface formations. The intensity of karstification varies spatially within the Paris Basin. The chalk karst observatory consists of 5 sites with high degrees of karstification from the zones of introduction to outlet zones: the first four are monitored by the M2C laboratory of the Université of Rouen Normandie and the 5th by the METIS laboratory of the Sorbonne Universités (formerly UPMC-Paris6).

• The Norville spring and borehole: this spring is supplied by a principal karst conduit into which a perennial stream disappears. Its drainage basin, which is small in size, is principally used for cattle grazing. Transfer from the surface is quite rapid (less than 24 hours)
• The Radicatel springs: these springs are highly productive (~50% of the potable water supply of the agglomeration of Le Havre) with rapid transfer through conduits. The associated drainage basin is extensive (106 km2) and very rural.
• The borehole capture of Yport: The Yport borehole is installed directly within a karst conduit that measures about 3m in diameter, located about 40 meters under the ground surface. The BAC is 230 km2
• The Bouville infiltration site: this less-than-0.5 km2 site is a test site set up for the investigation of heterogeneous infiltration via the perched aquifer of surface formations, which drains through dolines or sinkholes that are not visible on the surface topography. Hydrogeophysical investigation methods, particularly spontaneous potential (self potential) were originally developed at this site.
• The Saint Martin le Noeud underground quarry is dug into weakly karstified fractured chalk, and provides access to the water-bearing layer and to percolating waters (leaving the unsaturated zone). Transfers from the surface are slow, between several months to several decades. This site provides a manhole on slow infiltration and in this sense makes it possible to isolate this process specifically from other infiltration modalities.