Dynamics and structure of the Colli Albani volcanic district from gravity measurements

In 2005-2006 in the framework of an INGV-DPC project, in co-operation with the University of Rome “La Sapienza”, a gravity research started in the Colli Albani Volcanic District aimed: i) to realize a new gravity network to detect gravity changes due to mass redistribution in the underground inferred by the local volcanic activity; ii) to upgrade the already available Bouguer gravity map by means of new stations and to carry out new analyses and interpretations to outline a more detailed structural setting of the area. The gravity network has been designed so as to incorporate an old net of 7 stations, settled in 1981 by the same University group and periodically reoccupied until 2004. The new network is presently formed by 25 stations covering the whole volcanic area and all close to levelling benchmarks to remove the effect of the vertical ground movements. Taking into account the logistic situation of the area, three absolute gravity stations have been settled. Two of them have been located out of the volcanic area to be adopted as references. The third one has been realized inside the most active part of the volcanic district to calibrate in future, through its repetition, the gravity changes detected by relative measurements. Measurements of the vertical gravity gradient have been carried out in the absolute sites because g is not directly measured on the ground. Each absolute station is completed with an external satellite ones, then included in the relative network, where the absolute value of g has been also transported through relative measurements. Moreover, 13 selected stations of the relative network are also sites of vertical gravity gradient measurements. Those measurements helpful to reduce the effect of height changes on gravity variations and their space distribution is useful to reduce the prospecting gravity data. Two surveys of both relative and gradiometric measurements have been carried out. In order to outline a more structural setting of the investigated area, a new set of about 300 prospecting gravity points have been settled inside an old survey made by 1500 stations measured partly by the Servizio Geologico d’Italia (SGI) in 1969 and partly by the University of Rome in 1995. Up today, the new stations have been partly measured and all the available data have been reprocessed and uniformed. The preliminary results from both the dynamics and the static gravimetry will be presented and discussed.