To determine the precise localisation of paleokarsts is a major problem where civil engineering works and limestone exploitations are concerned. This article proposes an efficient and comprehensive procedure of doing so, defines a method of prospection appropriate to the type of karsts detected and describes an experiment applied to the “Carrière du Milieu” terrain situated in the Tournaisis area (Belgium). Both radon and temperature measurements were taken and interpreted by also taking into account a preliminary study of the morphostructures of the site, an approach which enabled an accurate characterisation of morphostructural accidents to be drawn up and cryptolines to be differentiated from fault and “ghost rocks”. This procedure thus opens up the way to improving initial risk and hazard assessment.