Studio di fenomeni di subsidenza localizzata (sinkhole) con applicazione di tecniche geofisiche integrate per la programmazione in intrventi di protezione civile e di ridefinizione dell'uso del territorio

The word “sinkhole” is increasingly used to indicate localized subsidence phenomena. In spite of the increasing awareness of the risk associated with this kind of event, sinkholes receive little attention from local authorities and technicians, especially in Italy. The real threat that these natural phenomena pose to public safety and to regional development, should encourage local governments to take action through specific planned action. In fact, sinkholes cannot be thought of as occurring unexpectedly; their spatial distribution, recurrence and type of development clearly show that they occur in specific geological and environmental conditions and are triggered by several concomitant factors. Therefore the widespread belief that sinkholes are unpredictable events simply derives from a poor understanding of the phenomenon. The purpose of this work is to provide a structured and complete overview through a detailed analysis of Italian and foreign case histories, summarizing the environmental conditions in which sinkholes occur, as well as the many factors and mechanisms underlying their formation. The need for a multidisciplinary approach is emphasized, based on the comparison and integration of results achieved using different investigation methods to obtain geological, hydrogeological and geochemical information, also examining several real cases. The importance and suitability of surveys based on geophysical techniques is stressed. To illustrate this, an overview of the results achieved using a variety of geophysical methods in numerous case studies of sinkholes and sinkhole-prone areas is provided. Lastly, the experimental part of this work, developed in the frame of this PhD course, is presented. It consists of a geophysical study of the Cixerri Valley, in SW Sardinia, Italy, where several sinkholes have appeared since 1996. Among the different geophysical survey techniques used in this study, the gravity method proved especially effective for detecting the geological and structural conditions that appear to be responsible for sinkhole formation. A sinkhole hazard map of the area has been drawn up on the basis of the gravity anomalies identified. The aim of this work is not the theoretical and practical development of geophysical techniques, which goes beyond the original scope of the study. It intends instead to define a possible route to be pursued in the analysis of sinkhole prone areas, that is able to provide valuable scientific information useful for gaining a better understanding of sinkhole formation mechanisms and how they are related to the general geological setting. With this information it would be possible to identify sinkhole hazard areas and local conditions, essential information for planning prevention and monitoring, securing these areas and for their reclamation.