Experimental and Computational Approaches to Enhance the Gravimetric Monitoring of Volcanic Areas

The aim of this thesis is to improve the techniques of the gravimetric method, by enhancing the quality of discrete gravity measurements and separating the gravity signals provided by the gravimeters in continuous recording during paroxysmal events, in the gravity field component and the inertial acceleration component to assess the amount of mass redistributed during such events. The preferred scenario for this study is Mt Etna not only because it is the most intensively monitored volcano in the world but also because there was the possibility to perform directly in the field gravity measurements, both discrete and continuous, with the instruments owned by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) - Sezione di Catania - Osservatorio Etneo . In Chapter 1 is described the technical background gravimetric monitoring of active volcanoes in terms of measurements techniques with spring relative and absolute gravimeters. It also discusses on the complexity of the gravity signal and on different gravity contributions that must be quantified in volcanic areas. Finally it presents the existing gravity monitoring network of Mt Etna. In Chapter 2 are shown the results of measurements conducted using two ballistic absolute gravimeters in two different conditions: in dedicated gravity laboratories and in a place used for geophysical studies (volcano monitoring) which present unfavourable environmental conditions (low temperature, high humidity, high ground vibration, etc.). The chosen instruments represent the steady advance in ballistic gravimeter technology: the FG5#238, a commercial instrument produced by the U.S.A. Micro-g LaCoste Inc. and the IMGC-02, developed in Italy by the Istituto Nazionale di Ricerca Metrologica (INRiM) (prototype). Besides, the IMGC-02 is recognized as national standards instrument in Italy and generally the FG5 (family) is more commonly employed for the absolute gravity studies while, specifically, the FG5#238 gravimeter is normally used for different applications from volcano monitoring to the study of gas storage areas. In Chapter 3 was investigated the applicability of combined measurements of absolute and relative gravity as a hybrid method for volcano monitoring, in order to improve the quality of discrete gravity measurements and to achieve a balance between uncertainty and efficiency in gravity measurements. The use of absolute gravimeters in a field survey of the summit area of Mt Etna is unprecedented. Between 2007 and 2009, three hybrid gravity surveys were conducted at Mt Etna volcano, in June 2007, July 2008, and July 2009. Results allowed to increase the knowledge of the dynamics of the volcano through an accurate determination of the position and shape of the volcanic sources. In Chapter 4, to evaluate the coupling degree between inertial acceleration components and the gravity signal provided by spring relative gravity meters, the results of a laboratory tests on a Scintrex CG-3M and on a LaCoste & Romberg model D gravimeters using a vibrating platform to excite them along the x, y and z axes will be explained. For this purpose, to extract the parameters with which to excite the vibrating platform, the seismic signals recorded at two different stations during the 10 April 2011 lava fountain, one of the strongest paroxysmal episode in 2011 eruptive events, were analyzed. Finally, to separate the signal from the gravimeter into gravity field contribute due to the subsurface mass or density variations and the inertial acceleration components due to the ground oscillation, a mathematical approach based on the neural network was also proposed.