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We report on about 20 yr of relative gravity measurements, acquired on Mt. Somma–Vesuvius volcano in order to investigate the hydrological and volcano-tectonic processes controlling the present-day activity of the volcano. The retrieved long-term field of time gravity change (2003–2022) shows a pattern essentially related to the subsidence, which have affected the central part of the volcano, as detected by the permanent GNSS network and InSAR data. After reducing the observations for the effect of vertical deformation, no significant residuals are found, indicating no significant mass accumulation or loss within the volcanic system. In the north-western sector of the study area, at the border of the volcano edifice, however, significant residual positive gravity changes are detected which are associated to ground-water rebound after years of intense exploitation of the aquifers. On the seasonal timescale, we find that stations within the caldera rim are affected by the seasonal hydrological effects, while the gravity stations at the base of the Vesuvius show a less clear correlation. Furthermore, within the caldera rim a multiyear gravity transient is detected with an increase phase lasting about 4 yr followed by a slower decrease phase. Analysis of rain data seem to exclude a hydrological origin, hence, we hypothesize a deeper source related to the geothermal activity, which can be present even if the volcano is in a quiescent state. We infer the depth and volume of the source by inverting the spatial pattern of the gravity field at the peak of the transient. A volume of fluids of 9.5 × 107 m3 with density of 1000 kg m−3 at 2.3 km depth is capable to fit reasonably well the observations. To explain the gravity transient, simple synthetic models are produced, that simulate the ascent of fluids from a deep reservoir up to the depth of 2.3 km and a successive diffusion within the carbonate aquifer hosting the geothermal system. The whole process appears to not significantly affect the seismicity rate and the deformation of the volcano. This study demonstrates the importance of a 4-D gravity monitoring of a volcano to understand its complex gravity signals that cover different spatial and temporal scales. Discriminating the different contributions that mix up in the observed gravity changes, in particular those due to hydrologic/anthropogenic activities form those due to the geothermal dynamics, is fundamental for a complete and reliable evaluation of the volcano state. [ABSTRACT FROM AUTHOR]

We report on the results of about 9 months of gravimetric recordings acquired at Mt. Somma-Vesuvius (SV) volcano (Southern Italy) with the new generation relative gravimeter gPhoneX#116 (gPh#116), which is a gravimeter specifically designed for continuous gravity recording. We also present the outcomes of an intercomparison experiment of the gPhone#116 conducted at the J9 gravity observatory in Strasbourg (France). In this intercomparison, we were able to check the scale factor of the meter with a high degree of precision by means of an intercomparison with 2 superconducting gravimeters (SGs) and a FG5-type absolute ballistic gravimeter. Multiple calibration approaches allowed us to validate the manufacturer's original calibration constants to a level of 1% accuracy and 0.1% precision. Moreover, we carried out a comparative study of the noise level of the gPh#116 with respect to the SGs and other spring meters routinely used in both prospecting and time-lapse gravimetry. It turns out that gPh#116 exhibits lower levels at hourly time-scales than other compared spring gravimeters (Graviton, gPhone#054, Scintrex-CG5). It was also possible to carry out a detailed study of the instrumental drift, a crucial topic for reliable monitoring of the long-term gravity variations in active volcanic areas. In fact, a challenge in time-lapse gravimetry is the proper separation of the instrumental variations from real gravity changes eventually attributable to recharge or drainage processes of magma or fluids in the feeding systems of active volcanoes. A negative finding coming out from the intercomparison is that, even when applying the tilt correction, the gravimetric residuals obtained with the gPh#116 are an order of magnitude larger and quite inconsistent with those obtained with co-located superconducting gravimeters. We guess this problem could be overcome by installing the gravimeter on an auto-levelling platform. From the analysis of the gravity records, a reliable tidal gravity model was derived, which we believe will help to improve the accuracy of volcano monitoring, as it will allow appropriate correction of tidal effects for both relative and absolute gravity measurements acquired in the area. Two further interesting elements arose from our study: (1) a peculiar cavity effect of the SV underground laboratory that seems to influence the tilt change; (2) the small residual gravity signals are time correlated with the rainfall peaks and are compatible with gravity decreases induced by increases in soil moisture above the gravimeter. [ABSTRACT FROM AUTHOR]

We present the results of nearly one year of gravity recording acquired at the active caldera of Campi Flegrei (CFc). CFc is one of the three active volcanoes in the Neapolitan area (southern Italy) and is currently the most active one. In fact, the CFc is undergoing a period of unrest characterised by slow uplift of the ground, a peculiar phenomenon known worldwide as bradyseism, accompanied by seismicity and intense fumarolic emissions. Due to the increased intensity of the volcano dynamics, a permanent gravity station equipped with a gPhoneX spring gravimeter was installed to enhance the geodetic monitoring programmes. The purpose of the continuous recordings is to complement the time-lapse observations carried out periodically on networks of benchmarks, in order to continuously monitor the short-term gravity signals. We report on the various processing steps and analyses performed to obtain reliable parameters of the Earth's tides, non-tidal corrections and gravity residuals. The various methodologies employed to investigate the instrumental drift are also elucidated in depth, because it may masquerade the elusive gravity changes resulting from mass fluctuations within the volcanic and geothermal systems. Residual gravity signals, retrieved from the recordings, after reduction of body and ocean tides, polynomial drift, atmospheric, tilt and change of the Earth Orientation Parameters (EOP) effects, appear to be uncorrelated with hydrology, while they show a clear correlation with the most energetic earthquakes, that strongly characterise the current bradyseismic crisis. The residual gravity signals display peculiar trends characterized by steps or offsets (up to about 600 nm/s2) and transients in coincidence of the most energetic volcano-tectonic events (Magnitude > 2.5) and seismic swarms. The steps in the gravity residuals are likely to be of instrumental origin, while the amplitudes of the observed transients are not consistent with co-seismic or volcanological phenomena, for which there is no evidence from other monitoring techniques. Unfortunately, the lack of repeated absolute gravity measurements severely limits our ability to attribute the observed gravity variations to geological sources. From the analysis of the gravity records, reliable tidal gravity models have been derived, which will improve the accuracy of volcano monitoring by allowing a precise reduction of tidal effects for both relative and absolute gravity measurements taken in these volcanic areas. [ABSTRACT FROM AUTHOR]