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Authors: Panepinto, Stefano
Title: Time and space domain analysis of gravimetric data
Issue Date: 30-Mar-2010
Keywords: Gravimetry, Tides, Tidal Modulation, Volcano Monitoring, Seismo-Gravity Sequential Inversion Procedure, Velocity and Density 3D models
Subject Classification04. Solid Earth::04.03. Geodesy::04.03.05. Gravity variations 
Abstract: The goal of this PhD thesis is to provide an overview on the very different aspects of modern gravimetric research. In particular, this geophysical method is applied here on the one hand as volcano monitoring tool essentially by continuous gravity observations while, on the other hand, for the construction of density-velocity 3D regional models by an integrated inversion procedure of gravimetric and seismic data. The first section concentrates on continuous gravity observation performed at different sites of both Etna and Stromboli volcanoes. The gravity studies allow investigation of mass displacements (magma) and density variations (deep structures) under volcano edifices. Results are presented from high precision gravity measurements fully corrected using tidal and drift optimization programs and having a standard error of few μgal. Tidal analyses results of the treated data sets are also shown and discussed in the first section. Moreover, the simultaneous recording of external parameters (atmospheric pressure, temperature and humidity) is essential as their effects must be removed from the gravity records. The analyses carried out with different processing techniques on several data sets led us to point out the temperature as the responsible parameter for the annual drift present in the records of spring gravimeters. During the end of 2002 one of the gravimetric signals acquired on Mt. Etna showed, in its final residuals reaching a 5 μgal precision, a strong decrease of about 400 μgal in few hours. Correlation between this gravity decrease, on the one hand, and the other geophysical and geochemical signals – in particular the seismic and ground deformation data – as well as the observed summit activities, on the other hand, enable us to qualify the recorded gravity variation as a precursor of the 2002 eruption period. By comparison with simultaneous ground deformation data it is shown that the observed gravity changes are not in general caused by elevation changes but are due to the direct gravitational effect of subsurface movements of matter. Residual gravity changes are interpretable entirely in terms of mass changes in crater conduits and in near-surface dykes lying along know fissure system. Furthermore, the summit activity is consistent with a source at greater depth. Gravity measurements may thus not only contribute to a better understanding of some important features of geodynamics in volcanoes but may also be used directly for the monitoring and the prediction of the eruptions. Section two addresses the unresolved question of the possible interference between tidal forces and volcanism. After the discussion of gravimetric tide results and the determination of tidal parameters, this section is completely devoted to “tidal modulation” of thermometric data acquired at sites very close to the summit active craters of Mt Etna. The intuition that these types of data may contain some geophysical signals related to the tidal stress-strain action, as an evidence of the tidal influence on volcanic processes, comes from the following boundary consideration: since the volcanic areas are characterised by high heat fluxes due to the presence of magma bodies near the surface, taking into account that convection is the major heat transfer mechanism, the tidal strain field within the volcanic edifice could affect this convective process. Some time variations of the efficiency of the convective process should produce corresponding temperature changes observable at shallow depth. The aim of the study is thus to investigate about the presence of a periodic variation due to the main lunar tidal component (M2, tidal period of 12.421 hours). This component is chosen in order to rule out the solar radiation effects. The data set at hand was thus processed with a stacking technique coupled with a wavelet analysis for a preliminary denoising. Through the proposed procedure an anomalous amplitude of the spectral component with a period equal to that of the M2 tidal wave was found. This evidence opens a scientific speculative argument about the interaction between tidal forces and volcanic processes highlighting the possibility, under some particular conditions, of dynamic triggering. The last section deals with a seismo-gravity integrated inversion procedure for the construction of reliable 3D models of the Sicilian area and its surrounding basins. The proposed procedure allows inverting seismic and gravimetric data with a sequential technique to avoid the problematic optimization of assigning relative weights to the different types of data. The proposed procedure underlined the necessity of the different data integration although the seismic problem seemed to be a priori well constrained. Furthermore, it allowed highlighting some velocity and density features that could play a crucial rule for the reconstruction of the geodynamic evolution of the study area.
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