Six Years of Repeated Absolute Gravity Measurements at Etna Volcano (Italy)

Since 2007, absolute gravity measurements have been systematically accomplished to monitor the Etna volcano. Data are collected in 13 absolute stations distributed on the volcano edifice between 1500 and 2850 m a.s.l. with the commercial instrument FG5#238 and with the prototype instrument IMGC-02. We carried out several field surveys and confirmed that both the absolute gravimeters can still achieve a 10 µGal or better uncertainty even when they are operated in severe environmental conditions. The combined use of the transportable absolute and relative gravimeters has greatly improved the efficiency and changed the nature of the traditional gravity survey at Etna volcano. The annual changes of the gravity measured over 2007–2012 provide unequivocal evidence that during this period, several main phenomena of subsurface mass redistribution occurred in distinct sectors of the volcano, accompanying different eruptive episodes. We present here also the advantages and the disadvantages in using the theoretical and the measured free-air vertical gravity gradient to move the absolute values at different heights.