Abstract:
The goal of this paper is to describe how continuous gravity measurements can improve the geophysical monitoring of a volcano. Here the experience of 15 yr in continuous gravity on Vesuvius is presented. A wide set of dynamic phenomena (i.e. geodynamics, seismicity, volcanic activity) can produce temporal gravity changes, with a spectrum varying from short (1-10 s) to longer (more than 1 yr) periods. An impending eruption, for instance, is generally associated with the ascent of magma producing changes in the density distribution at depth, and leading to ground deformation and gravity changes observed at surface. The amplitude of such gravity variations is often quite small, on the order of 10-9-10-8 g (10-102 nm/s2; 1-10 μGal), where g is the mean value of normal gravity (9.806 199 203 m/s2), so their detection requires instruments with high sensitivity and stability, providing high quality data. Natural, man-made and instrumental sources are present on the gravity records affecting the Signal to Noise Ratio. Such effects may hide the subtle volcanic signals. The main natural noise is due to ocean-atmosphere dynamics and seismic activity. New approaches to model the instrumental response of mechanical gravity sensors (based on the inter-comparison among superconducting, mechanical and absolute gravimeters) and to investigate the temporal trends of the instrumental sensitivity are proposed. In fact, variations of the calibration factors can be considered the main cause preventing the repeatability of high-precision gravity measurements and inducing phase and amplitude perturbations in recorded gravity signals. A modelling of the background gravity noise level was performed at the Vesuvius station. Moreover, the "far field" effects produced by large earthquakes on the gravity station have been also investigated. Finally, the time dependent behaviour of the tidal gravimetric factors, the non-stationary components of the gravity field detected at Vesuvius and the results of absolute and relative gravity measurements are interpreted in the framework of its present-day dynamics, mainly characterized by the low level of seismicity, small ground deformation, gravity changes and moderate gas emission. © 2005 Elsevier B.V. All rights reserved.
