A year-long gravity record at Astroni, Campi Flegrei (Southern Italy): some considerations on data processing for volcano monitoring and precise gravity tides

Abstract

High-precision observations of gravity plays a central role in modern approaches for active volcano monitoring. Time-lapse observations over a network of benchmarks are frequently used to detect underground mass redistribution in the plumbing system of active volcanoes. Such approach however does not allow to retrieve small mass variations occurring over short terms (i.e. few hours or days). To fill this gap, continuous gravity monitoring at a fixed station may be employed. In January 2023 the relative gravimeter gPhoneX#116 was installed at the WWF Nature Reserve of Astroni volcano, in the Campi Flegrei caldera, to further complement time-lapse observations periodically performed over a network of benchmarks. During the 1-year of recordings, the gPhone has continuously recorded the relative gravity changes, only shortly interrupted by a few technical issues. The purpose of the observations is to monitor continuously the short-term gravity signals in one of the world's highest-risk volcanoes; to pursue this objective targeted and meticulous corrections need to be applied to remove the effect of several other geophysical effects, such as tides and atmospheric effects, which may superpose on the signals of interest. Special effort was devoted to the study of instrumental drift, which can mask actual gravity changes due to mass variations occurring in the volcanic and geothermal systems. In this contribution we report the various processing steps and analysis performed to obtain reliable parameters of the Earth tides, non-tidal corrections and gravity residuals. The retrieved Earth tide model is then used to properly reduce tidal effects in high-precision relative and absolute gravity measurements.