Monitoring the long-term variations of the gravity field at the Neapolitans volcanoes

Starting from 1981 relative gravity measurements are carried out at the Neapolitan volcanoes ( Vesuvius, Campi Flegrei and Ischia). Measurements are periodically carried out on networks covering the whole volcanoes and are referred to a common reference station located in a comparatively stable area (Naples). A program of absolute gravity measurements was started in 1986 at the reference station and on selected ones of the relative networks. The long-term periodically surveying of them permits the check of base stations and is aimed to reveal long-term variations or to confirm their stability; moreover it permits periodic calibration checks and comparison for relative gravimeters and for the recording gravity stations where operating. Several absolute field surveys have been carried out, mainly at Vesuvius and at Campi Flegrei. The last campaign was carried out on March 2009. During the survey a new absolute station was established at Campi Flegrei, in the same site where a recording gravity station will be set up in a short time. Absolute data till now collected confirm the gravity variations revealed by the relative measurements carried out using spring gravimeters. We show that the combination of absolute and relative data is a powerful way to monitor the volcanic areas. As in past, absolute data were collected with the transportable ballistic gravimeter IMGC-02. New features and metrological improvements of the IMGC-02 are also briefly described.