The new IMGC-02 transportable absolute gravimeter: measurements and applications in Geophysics and Volcanology

The research activity carried out during the last years and aimed to develop absolute gravimeters, ended with a transportable instrument, the IMGC-02, which belongs to a new generation of riseand- falling gravimeters. It adopts the absolute ballistic method, which has been recognized at international level (Comité International des Poids et Mesures - CIPM) as primary method of measurement of the acceleration due to gravity. To assure the measurement compatibility the IMGC-02 participates regularly to the International Comparisons of Absolute Gravimeters (ICAGs) organized by the Bureau International des Poids et Mesures (BIPM). A brief history of IMGC gravimeters is given and the present features of the instrument, including the complete automation and better transportability which make the IMGC-02 a useful tools for geophysicists and metrologists, are described. Preliminary results obtained during the last ICAG05 will be showed confirming that the present apparatus has reached a point in its development where the measurement on field is reliable according to the estimated uncertainty at the microgal level. Applications in Geophysics and Volcanology of the measurement survey carried out in Italy with the IMGC will be presented. Absolute measurements on active volcanoes date back to 1986 and are carried out at the reference stations and on selected ones of the relative gravity networks covering the Neapolitan volcanoes (Vesuvius, Campi Flegrei and Ischia), Vulcano and the Aeolian Islands, the island of Pantelleria and the Colli Albani. Moreover, in the framework of an European Community Project, in 1991 two stations on Mt. Etna and Centuripe respectively were also established. The periodically surveying of them permits to check base stations and to reveal long-term variations or to confirm their stability, as well as permit periodic calibration checks and comparison for relative gravimeters and for the recording gravity stations where operating. Finally, also one station in Cosenza was settled up in 1994 to survey an active seismic area. Since the measured g is not referred to the ground, at any absolute station the local value of the vertical gravity gradient is measured to permit the comparison with the relative measurements. Most of the stations have been surveyed more than once. The obtained results and the comparison with the gravity changes revealed by relative measurements will be presented.