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Gravity measurements are increasingly used for high-precision and high-resolution Earth investigation. Recent times highlight the intention to combine both terrestrial and satellite data in order to reach higher accuracy for several purposes such as geological structures determination and geoid models construction.Here we present results of a comparison between a twenty-year (2002-2022) relative and absolute gravity data collected through the Microg LaCoste FG5#238 absolute gravimeter (AG), in the framework of repeated measurements in one station at about 1750 m above sea level and the satellite gravity data provided by CNES/GRGS RL05 Earth gravity field models, from GRACE and SLR data.The comparison allows to estimate the long-term correlation between the two dataset and a remarkably good fit was found in the long-term trend, revealing gravity changes most likely due to hydrological and volcanological effects.Our study shows how the combination of terrestrial and satellite data can be used to obtain a fuller and more accurate picture of the temporal characteristics of the studied processes. The combined use of these dataset results crucial especially in a harsh, unsteady and changing environment as well as the Etna volcano.

The gravity time series from two iGrav superconducting gravimeters and an AQG-B absolute quantum gravimeter, installed at Mt. Etna volcano, reveal a marked gravity decrease during early-June to late-July 2021, a period when more than 20 short-lasting and strongly explosive eruptions (the so-called lava fountain events) took place from one of the summit craters of the volcano. GNSS data indicate that this phase of gravity decrease was associated to deflation of the volcano edifice. We performed a joint inversion of the gravity data and deformation field to define the parameters of the common mass/pressure source. The optimal source is located beneath the summit crater area of Mt. Etna, at a depth between 2 and 3 km below the sea level. Results of the joint modeling also point to a residual mass change that is largely in excess of the corresponding volume change, for any reasonable density of the material extracted from the source. We propose that the observed gravity decrease was mostly driven by a decompression-related density change, i.e., an increase in the proportion of exsolved gas to magma in the source reservoir. This hypothesis is checked through comparing the results of the geophysical data inversion with independent estimates of the change in exsolved gas content due to pressure decrease in the source reservoir.

The excitation of long-period signals has been reported for large eruptions such as El Chichon (1982) and Mount Pinatubo (1991).We report on series of pressure and gravity signals associated with the series of violent explosions at the Hunga-Tunga eruption, on 14-15th January 2022 at different locations and set-up configurations.We present records from various stations all over the world. Those stations have different set up and local configurations.At most stations, gravity signals records reveal clearly surface waves associated to the main eruption at ~4:30, 15th January and pressure wave up to Iceland.We analyze similarity and difference in the signals in association with the location and the station configurations

In the framework of the NEWTON-g project, the field version of the Absolute Quantum Gravimeter produced by iXblue (AQG-B) was deployed in the summit crater zone of Mt. Etna volcano (Italy), in the summer of 2020. This is the first absolute atom interferometry gravimeter ever deployed on an active volcano. The device was installed in the facilities of the Pizzi Deneri volcanological observatory (PDN; 2800 m elevation, 2.5 km from the summit craters).Despite the unfavorable environmental conditions at the installation site and the occurrence of phases of high volcanic tremor, the AQG-B provided high-quality continuous data, suitable for studying volcano-related gravity changes. Indeed, it has been possible to track gravity changes with amplitudes ranging between a few tens and a few hundreds of nm/s2, occurring over a wide range of time scales.Here, we describe the main features of the AQG-B and the issues that were addressed to allow its deployment in the summit zone of an active volcano. We also present the months-long time series that were acquired in 2020 and 2021, with a special focus on the anomalies likely related to volcanic processes.

New gravimetric observations were carried out in the summit area of Mt. Etna in July 2021. Only the north-west half of the planned survey area was accessible to field work due to ongoing intense eruptive activity. The new gravimetric observation points (171 in number) were positioned using precise geodetic positioning based on GNSS technology. Due to rough conditions, unstable ground and wind, the gravity observations were collected with a precision of about 15 microGal using two relative gravimeters (CG5 and CG6). Complete Bouguer anomalies (CBA) were compiled. The computation of accurate topographic correction for CBA compilation poses a challenge because of the ever-changing topography around the summit craters due to intense eruptive activity. Precise topographic correction was computed using the Toposk software. The available high resolution (5 m) DEM released in 2016, and the reference constant topographic density of 2300 kg/m3, which resulted from our analysis as representative for the summit area, were adopted for the numerical evaluation of the topographic correction. These data will serve the 2D and 3D density modelling for determining the subsurface structural model of the summit area and the upper-most part of magma feeders of the summit craters on Etna.

The determination of Eruptive Source Parameters (ESPs) is crucial especially for very active volcanoes whose eruptive intensity can vary significantly. In this aim, new strategies are being developed to determine in near real time the total erupted mass (TEM), total grain-size distribution (TGSD) and plume height from ground sampling and remote sensing methods. Since 2011, Etna volcano has produced about 100 paroxysmal episodes characterized by the emission of fountain-fed tephra plumes whose heights reached up to 15 km (above sea level). In this work, we present multi-sensor strategies based on data acquired by the complementary set of remote sensing systems available at Etna. In fact, multi-sensor strategies may help to refine and assess the uncertainty of ESP estimates made by individual sensors, which can present various limitations such as narrow field of views (e.g., visible imagery) and/or low temporal resolution (e.g., satellite-based infrared). First, we show how the combination between tephra-fallout deposit and satellite-based estimates, along with numerical modelling, can help to refine estimates of TEM and TGSD, especially for weak explosive eruption such as the 29 August 2011 paroxysm. We use the model TEPHRA2 and compute synthetic data of ground accumulation to successfully fill significant sampling gaps in the tephra-fallout deposits. Moreover, we find that the Rosin-Rammler equation can be used to inform on missing part of the TGSD, including the tail of very fine ash also detected by satellite-based platforms. Additionally, we compare all estimates of Mass Eruption Rates, Plume height and grain-size distributions made by all available methods including Doppler radar detection, visible and infrared imagery, infrasound arrays, gravimetric signals and tephra-fallout deposit sampling. Accordingly, based on each sensor limitation and capacities, we obtain new constraints on ESP estimates acquired during several paroxysms between 2011-2013 and February 2021. We also bring new insights into the differences and complementarities that exist between the available remote sensing methods, especially in the case of future eruptive events at Mount Etna.

We present the preliminary results of combined discrete and continuous gravity measurements, carried out at Vulcano Island (Aeolian Archipelago, Sicily, southern Italy), in the period October 2021 - January 2022. The gravity observations have been aimed at investigating the dynamics of the volcanic-hydrothermal system, during an interval when significant changes in chemical properties, temperatures and emission rates of La Fossa crater fumaroles were observed.Campaigns of gravity measurements were carried out at Vulcano on an annual basis, between 1982 and 2014. The gravity network initially included 11 benchmarks and grew through time. For the period considered here, the discrete gravity measurements were repeated twice (October and November 2021) on a network consisting of 19 benchmarks. The network is linked to an external reference station, situated in Milazzo (Sicily north coast), that has been a site of absolute measurement since 1990. In order to obtain information on the time scales of the volcanic and hydrothermal processes able to induce bulk mass changes, three stations for continuous gravity measurements were installed in October 2021.Comparison between campaign data collected in 2014 and in October 2021 reveals a gravity decrease affecting the whole volcano, with a maximum amplitude of about -100 microGal in the area of La Fossa. No significant gravity changes were observed between October and November 2021. On the other hand, continuous gravity observations showed high frequency variations affecting only one of the three stations, thus indicating that they are due to fast-evolving and local processes, occurring within shallow sources. Transient signals also appear in the time series from the three stations during Very Long Period (VLP) events.Our findings indicate that the combined use of discrete and continuous gravity measurements is a promising tool for studying the volcano-hydrothermal system of Vulcano and mitigating potential hazards.

Lava fountains represent a common eruptive phenomenon at basaltic volcanoes, which consist of jets of fluid lava ejected into the atmosphere from active vents or fissures. They are driven by rapid formation and expansion of gas bubbles during magma ascent. The dynamics of lava fountains is thought to be controlled by the gas accumulation in the foam layer at the top of a shallow magmatic reservoir, which eventually collapses triggering the lava fountaining. Gravity measurements taken from a location close to summit of Mt. Etna during the 2011 lava fountain episodes showed a pre-fountaining decrease of the gravity signal. The interplay between gas accumulation in the foam layer and its subsequent exsolution in the conduit has been interpreted as the mechanism producing the gravity decrease and eventually leading to the foam collapse and onset of the lava fountaining activity. Gravity measurements have proved helpful in recording the earliest phases anticipating the lava fountain episodes and inferring the amount of gas involved. However, more accurate estimates of the accumulating and ascending gas volume and total magma mass require considering the possible effect of non-spherical magma chamber geometries and magma compressibility. Under task 4.4 of the H2020 NEWTON-g project, we are accomplishing a detailed study aimed to simulate the gravity signal produced in the stage prior to a lava fountain episode, through a magma chamber - conduit model. We use a prolate ellipsoidal chamber matching the inferred shape of the shallow chamber active at Mt. Etna during the lava fountain episodes, and calculate the surface gravity changes induced by inflow of new magma into the chamber-conduit system. We use a two-phase magma with fixed amount of gas mass fraction and account for magma compressibility. We find that a realistic chamber shape and magma compressibility play a key role and must be considered to produce realistic gravity changes simulations. We combine our physical model with empirical distributions of recurrence time and eruption size of the past lava fountains at Mt. Etna to stochastically simulate realistic time series of gravity changes. The final goal of this study is to develop a prediction model for the amount of magma and duration of lava fountains at Mt. Etna.

The NEWTON-g project [1] proposes a paradigm shift in terrain gravimetry to overcome the limitations imposed by currently available instrumentation. The project targets the development of an innovative gravity imager and the field-test of the new instrumentation through the deployment at Mount Etna volcano (Italy). The gravity imager consists in an array of MEMS-based relative gravimeters anchored on an Absolute Quantum Gravimeter [2].The Absolute Quantum Gravimeter (AQG) is an industry-grade gravimeter measuring g with laser-cooled atoms [3]. Within the NEWTON-g project, an enhanced version of the AQG (AQGB03) has been developed, which is able to produce high-quality data against strong volcanic tremor at the installation site.After reviewing the key principles of the AQG, we present the deployment of the AQGB03 at the Pizzi Deneri (PDN) Volcanological Observatory (North flank of Mt. Etna; 2800 m elevation; 2.5 km from the summit active craters), which was completed in summer 2020. We then show the demonstrated measurement performances of the AQG, in terms of sensitivity and stability. In particular, we report on a reproducible sensitivity to gravity at a level of 1 μGal, even during intense volcanic activity.We also discuss how the time series acquired by AQGB03 at PDN compares with measurements from superconducting gravimeters already installed at Mount Etna. In particular, the significant correlation with gravity data collected at sites 5 to 9 km away from PDN proves that effects due to bulk mass sources, likely related to volcanic processes, are predominant over possible local and/or instrumental artifacts.This work demonstrates the feasibility to operate a free-falling quantum gravimeter in the field, both as a transportable turn-key device and as a drift-free monitoring device, able to provide high-quality continuous measurements under harsh environmental conditions. It paves the way to a wider use of absolute gravimetry for geophysical monitoring.[1] www.newton-g.com[2] D. Carbone et al., “The NEWTON-g Gravity Imager: Toward New Paradigms for Terrain Gravimetry”, Front. Earth Sci. 8:573396 (2020)[3] V. Ménoret et al., "Gravity measurements below 10−9 g with a transportable absolute quantum gravimeter", Nature Scientific Reports, vol. 8, 12300 (2018)

Continuous gravity measurements at Mt. Etna, Sicily demonstrate spatio-temporal variations that can be related to volcanic processes. Two iGrav superconducting gravimeters (SGs) were installed in 2014 and 2016 at Serra La Nave Astrophysical Observatory (SLN; 1,730 m elevation; ~6.5 km from the summit craters) and La Montagnola hut (MNT; 2,600 m asl; ~3.5 km SE of the summit crater). Since their installation both stations have been continuously recording, resulting in high-resolution (1 Hz sampling rate) time series. The persistent activity of Etna is maintained by a regular supply of magma to the shallower levels of the plumbing system. The bulk mass redistributions induced by the newly injected material result in minor variations in the local gravity field that are recorded by the two stations. By assuming that the observed gravity changes are due exclusively to mass changes in an almost spherical-shaped source, located beneath the craters, the amplitude ratio between the two signals can be used to estimate the depth to potential mass changes beneath the surface.This study reports on the time-dependent nature of mass changes located beneath the craters of the volcano during 2019. Results highlight distinct periods of stability at different depths, as well as potential periods of transitory activity, where the predominant mass source switches between storage zones at different depth. These events are correlated to phases of strombolian and effusive activity, highlighting the potential of continuous gravimetry for the detection of eruption precursors.

Since September 2014, iGrav#016 superconducting gravimeter (SG; by GWR) has recorded continuously at the Serra La Nave Astrophysical Observatory (SLN; 1730 m elevation; ~6.5 km from the Etna’s summit craters; Italy).Here we present results of a comparison between a six-year (2015-2020) time series from iGrav#16 and absolute gravity data collected through the Microg LaCoste FG5#238 absolute gravimeter (AG), in the framework of repeated measurements that were performed at the same installation site of the SG. Both AG and SG records have been corrected for the local tides, local atmospheric pressure and for the polar motion effect.The comparison allows to estimate the long-term drift of the SG, defined as the total SG trend minus the observed trend in AG measurements, which is of the order of 9 microGal/year. Once the drift effect is removed, there is a remarkably good fit between the two data sets. The differences between absolute gravity changes and corresponding relative data in the continuous time series from the SG are within 1-2 microGal (the total error on AG measurements at this station is typically +/- 3 microGal).After being corrected for the effect of instrumental drift, the time series from the SG reveals gravity changes that are due to hydrological and volcanological effects.Our study shows how the combination of repeated AG measurements and continuous gravity observations through SGs can be used to obtain a fuller and more accurate picture of the temporal characteristics of the studied processes.

The transport of magma and magmatic fluids is a key process behind the occurrence, duration and intensity of volcanic crises. Volcano gravimetry allows for unequivocal inference of the location and mass of accumulated or removed magmatic fluids at volcanoes. This task is best accomplished through collecting gravity time series at multiple stations simultaneously. The performance of individual gravimeters and the configuration of the gravimetric array, however, determine the threshold of detectable mass change and the ability of the array to minimize the uncertainty on the inferred quantities.We utilize numerical optimization techniques to design a network including one absolute quantum gravimeter (AQG), two superconducting relative gravimeters (iGRAVs) and several microelectromechanical system (MEMS) relative gravimeters at Mount Etna. We also develop analytical solutions for simple design problems. We show that the analytical solutions are essential for validating the numerical optimization procedure. We provide practical details and caveats that should be considered in similar gravimetric network optimizations. These include 1) specifying the target zone of the network by using the history of mass transport, 2) accounting for the relative importance of different parts of the target zone, 3) accounting for logistic and instrumental constraints in the optimizations 4) calibrating the objective functions associated with various optimizations, 5) analyzing the network sensitivities to different parts of the target zone and identifying blind zones and 6) calculating the optimal number of gravimeters as a function of the sensor sensitivity and accuracies. We show that our optimal solution for Mount Etna provides an improved detection power across the target zone as compared to an equally spaced network of gravimeters with the same existing constraints, surface topography and sensor sensitivities. Furthermore, this optimal solution ensures that a certain range of mass change anywhere in the target zone can be sensed by a given minimum number of gravimeters and at the same time minimizes the impact of random observation errors on the inferred quantities.

Rock strains detection is one of the principal ways to monitor geohazards. Classic strainmeters are cumbersome, hard to install and very expensive. Opto-electronics devices based on fiber Bragg grating technology allow to realize strainmeters with high sensitivity, low-cost, small volume and high performance.We present the long term result of continuous soil strain monitoring on the Etna mount by a three-axial fiber Bragg grating sensor. The sensor has been developed in the framework of European Project MED-SUV (MEDiterranean SUpersite Volcanos). The installation site is a 8.5 meters deep borehole at a distance of about 7 km South-West from the summit craters of the Etna mount, at an elevation of about 1740 meters. This kind of sensor has a resolution better than 100 nanostrains on a daily timescale. Despite it is only a prototype, the sensor has worked for four years with a duty-cycle higher than 90% detecting both fast event, as earthquakes, and slow event, as epochal rocks strain behavior.

We study the transient effect of groundwater mass changes on the observed gravity signal from a superconducting gravimeter deployed on Mt. Etna, Italy. Gravimeters are capable of detecting minor changes in the vertical component of gravity over time scales from minutes to years. Insight on geophysical phenomena that cause mass displacements in the subsurface can be obtained through the use of gravimetry. Gravity recordings integrate multiple components that contribute to the signal with different magnitudes. The effects of earth tides, atmospheric pressure changes, hydrological processes are among the above components. They need to be precisely evaluated, in order to isolate the signal caused by the volcanic processes. Here, we study the effect of groundwater mass changes on gravity, as a result of rainfall and snow melting, the latter estimated through GNSS interferometric reflectometry. A forward charge-discharge model is used to compare gravity recordings between 2018 - 2019 with observed precipitation events. We show that the observed gravity signal cannot be explained only through changes in groundwater mass, implying that other (volcanic) processes must have been at play.

We focus on the eruption of Mt. Etna which took place on 24 December, 2018. The eruption occurred after a month of unrest and was accompanied by a seismic swarm that culminated in the M4.9 earthquake on the 26th, with epicentre on the eastern flank of the volcano. We jointly analyse ground deformation and gravity data to estimate the geometrical and kinematic parameters of the source structure, together with the density of the intruding material. The data used in this study were recorded by stations in the INGV-OE monitoring network (21 GPS stations and 2 gravity stations equipped with superconducting gravimeters), during the interval of 23 to 28 December (pre to post eruption). We assume a dike-type source for the forward calculation in the defined objective function. A pattern search algorithm (PSA) is used for the iterative minimization of the misfit error. In order to estimate the posterior probability density function (PDFs) of the model parameters, we also use a Markov Chain Monte Carlo (MCMC) approach. Indeed, the calculated PDFs provide more information about the uncertainties of the model parameters, which helps to understand overall tendencies of the solutions. We first test the constrained inversion of the gravity data, to calculate the density of eruptive magmatic body, by fixing the geometrical parameters of the dike, previously retrieved through inversion of the deformation data only. Using this approach, it is possible to suitable explain the deformation data and the gravity change observed at the station in the near field (MNT), while the gravity change at the other station (SLN) remain unexplained. We then invert jointly both deformation and gravity datasets, in order to adequately fit all the observations. The final model gives a density value of ~1.8-2.0 g/cm3. This value is significantly lower than the density of bubble-free magma and indicates either the involvement of gas in the intrusive process, or the formation of dry fissures during the emplacement of the dyke.

Gravimetry is the only method able to directly track redistributions of bulk masses. Hence, it can supply unique information on geophysical processes that involve subsurface fluids like water, hydrocarbons, and magma. Nevertheless, the high cost of currently available gravimeters and the difficulty to use them in field conditions, has limited the applicability of the gravity method, that is indeed not as widely adopted as other geophysical methods.A new system for gravity measurements is being developed in the framework of the H2020 NEWTON-g project. This system, called “gravity imager”, includes an array of MEMS gravimeters, anchored to an absolute quantum device. It will enable, for the first time, gravity measurements at high spatio-temporal resolution. After the phases of design and production of the new devices, NEWTON-g involves a 2-year phase of field tests at Mt. Etna volcano (Italy), starting in the summer of 2020.

Here we present the results of repeated Absolute Gravity and GNSS measurements, collected at Mt. Etna (Italy) between 2009 and 2018. We aim at investigating the capabilities of this integrated approach for understanding the dynamics of magmatic sources over time-scales of months to years. The absolute gravity and GNSS campaign measurements were repeated roughly once a year; in order to improve the time resolution of gravity data, in some stations we performed, besides absolute gravity measurements, also relative measurements at intervals shorter than 1 year.After being corrected for the effect of elevation changes, gravity data reveal an increase/decrease cycle, well spatio-temporal correlated with a general pattern of uplift/subsidence, during a period of intense lava fountains from the summit craters.Our results provide insight into the processes that controlled the transfer of the magma from deeper to shallower levels of the plumbing system of Mt. Etna volcano, in periods preceding/accompanying the eruptive activity during 2009–2018.Specifically, we propose that coupled changes in height-corrected gravity and elevation might be induced either by the magma storage/withdrawal below the volcanic pile, or by fluids pressurization/depressurization, or by a combination of both processes.The application of the proposed approach could led to an improved capability to identify processes heralding eruptions.

Underground pressure sources and rift zones may act jointly during phases of volcanic activity. Pressurization of magma bodies at shallow to intermediate depth, along with degradation of the mechanical properties of the host rock, can enhance tensile stress along zones of weakness, thus favoring magma intrusion. Such interactions were hypothesized at different volcanoes, including Mt. Etna, Piton de la Fournaise and Montserrat, from seismic, gravity and ground deformation data. Here we use a finite-element modeling approach to quantitatively understand possible mechanical interactions between a shallow pressure source beneath the summit caldera and the rift zones at Kīlauea Volcano (Hawai‘i). Past studies have demonstrated a strong connection between these structures, for example, with increases in seismic activity and extension across the rift, during phases of inflation of the summit. These observations suggest a coupling, which may modulate magma accumulation and transport processes along the rift.

Continuous gravity measurements at active volcanoes are mostly accomplished using spring gravimeters, that can be operated under harsh field conditions. Unfortunately, these instruments do not provide reliable continuous measurements over long time-scales, due to the instrumental drift and artifacts driven by ambient parameters.An alternative to spring devices for continuous measurements is given by superconducting gravimeters (SGs), that are free from instrumental effects and thus allow to track even small gravity changes over time-scales from minutes to years. Nevertheless, SGs cannot be deployed in close proximity to the active structures of tall volcanoes, since they need host facilities with main electricity and a large installation surface.The mini-array of three SGs that were installed on Etna between 2014 and 2016 makes the first network of SGs ever installed on an active volcano. Here we present results from these instruments and show that, even though they are installed at relatively unfavorable positions (in terms of distances from the summit active craters), SGs can detect volcano-related gravity changes that would otherwise remain hidden, thus providing unique insight into the bulk processes driving volcanic activity.

In situ values of vertical gradients of gravity (VGGs) are often needed when compiling residual spatiotemporal gravity changes that are interpreted in volcanic areas with the objective of drawing inferences about sources of volcanic unrest or pending eruptions. VGG values are seldom acquired by in situ observations. Their availability in 4D volcano-microgravimetric surveys and studies can be mediated by predicting the VGGs based on high resolution high accuracy DEMs and modelling the topographic component (constituent) of the VGG. Based on a modelling effort and in situ verification of VGG predicted on Etna in the summit craters area, on the north-east rift and on benchmarks of the monitoring network covering the volcano in a wider context, we learned that the VGG prediction can be improved by using drone-borne photogrammetry with GNSS ground control to produce a finer DEM in the closest vicinity of the VGG point (benchmark or field point) with resolution higher than the available high-resolution LiDAR-derived DEM, and using detailed modeling of gravity effect (on VGG) of anthropogenic objects such as walls and buildings adjacent to the VGG points. In this poster we present the methods used in the refined VGG prediction and the results of the verification of VGGs predicted on Etna.