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The sustainable exploitation of a geothermal reservoir is usually assessed through continuous field monitoring and structural exploration of the hydrothermal reservoir. However, accurate subsurface mass and fluid displacement as well as energy transfer model of the hydrothermal reservoir is most of the time not resolved enough both in space and time. Since 2017, both at Krafla and Theistareykir powerplants (northern Iceland), we use several multi-parameter stations each equipped with a gravity meter (superconducting or spring relative meter), a broad band seismometer, a GNSS receiver and other meteorological and hydrological sensors. With this set-up, we aim to model mass and stress transfer through the combination of absolute and micro gravity measurements, continuous signals measured at the multi-parameter stations and seismic measurements.We present results from the 2022 micro gravity and absolute gravity campaigns conducted at Theistareykir geothermal field. Through inversion and interpretation of such results, as well as the analysis of the continuous measurements, and injection and production data, we aim to assess the anthropogenic contribution in the mass and energy transfer models of the investigated area. Furthermore, we show the first continuous measurements and accurate Earth tide model for the Krafla area. The final goal of this study is to verify the conditions of sustainable exploitation of the reservoirs, and establish reservoir parameters, such as permeability, that regulate the response of the geothermal system to changes in production and injection rates.

We report the results of about 20 years of relative gravity measurements acquired on Mt. Somma-Vesuvius INGV monitoring network, together with about 9 months of continuous gravimetric recordings collected with the new generation relative gravimeter gPhoneX#116, 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 meter scale factor with a high degree of precision by comparing them with 2 superconducting gravimeters and a FG5-type absolute ballistic gravimeter. It was also possible to carry out a detailed study of 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.Since 1980s the relative gravity network of Mt- Somma-Vesuvius has evolved over time becoming progressively larger and denser. We discuss the results of the time-lapse monitoring since 2003, when the INGV network reached an almost stable configuration. The retrieved field of time gravity change shows a pattern essentially related to the ground deformation detected by the permanent GNSS network. Vesuvius is currently experiencing subsidence at a variable rate. A clear topographic effect emerges with a strong correlation with altitude, whereby higher stations subside at a greater rate, up to 7 mm/year, than those at lower altitudes. Most of the observed gravity changes can be explained by this dynamics; only a residual positive gravity is detected in the western sector of the volcano, which could be likely due to hydrological effects. A reliable tidal gravity model was derived from the analysis of the gravity records. We believe that this result should help improve the accuracy of the volcano monitoring as it will be useful for the correct reduction of tidal effects for all relative and absolute gravity measurements acquired in the area.

Most students living in disadvantaged areas do not trust that their school education will have much bearing on their future and become disaffected from school as early as in lower secondary school.We here report the results of teaching Geosciences through several practical activities carried out in suburban schools characterized by a high dropout rate. The lectures were given in the framework of the STEM project Next Land, which has the overarching goal of instilling interest towards scientific topics into young students and promoting the role of scientific education for sustainable development.We propose a sequence of four laboratory activities on the subject of natural calamities (e.g. earthquakes and tsunamis), which are based on the use of both manual and IT skills.The didactical experimentation has been conducted on 21 seventh-grade classes (age ~12) involving about 350 students from the area of Naples (Southern Italy). The final aim of the teaching is to test the potential of the Geosciences in generating attitudes and behaviours of solidarity and responsible global citizenship.All the proposed activities try to make students aware that they are part of a connected global system, therefore through these lectures we attempt to help these young students to put in perspective their local reality in a larger one.

Lanzarote is the most northeast and together with Fuerteventura is the oldest island of the Canarian Archipelago (Spain), which is located on a transitional zone, a passive margin, between oceanic and continental crust. The last volcanic eruption in Lanzarote was a 7 years voluminous eruptive cycle, occurred during the 18th century. Historical seismicity registered in the region, is customarily attributed to diffuse tectonic activity.This study is intended to contribute to understanding the surface thermal anomalies and the active tectonics on Lanzarote island, mainly in the Timanfaya volcanic area, which is located to the southwest of the island and covers the land extension generated by the last eruption..First, we describe the steps taken to implement a thermo-fluid-dynamics model to study the surface thermal anomalies detected at the Timanfaya volcanic area after the volcanic activity that took place between 1730 and 1736. The origin of these anomalies is acknowledged to be due to the intrusion of a magma body and its consequent cooling, but which still might have very high temperature. This hypothesis is based on the fact that the cooling of basaltic magma, which has an initial temperature of 1200 °C, takes about 104 ÷105 years, as indicated by some authors. Our physical model consists of a cooling magma body, with a radius of 300 m, located at a depth of 4 km and with a temperature of 800 degrees (1073,15 K).The model was developed in three steps: 1) accounting for the energy balance only, 2) both the energy and the momentum balance are accounted for, 3) mass balance is accounted too.The three thermo-fluid dynamic models are based on a finite element modelling (FEM). The novelty of our model consists in including both the steady and unsteady (transient) phase, not considered in analytical solutions under purely stationary conditions developed in past modelling by other authors.Second, we describe a detailed geodetic continuous monitoring in Timanfaya volcanic area, where, as mentioned, the most intense geothermal anomalies of Lanzarote are located.We report on the analysis of about 6 years of CGNSS data collected on a small network consisting in 9 permanent stations, spread over Timanfaya area in Lanzarote Island. The GNSS stations are operated by several owners: the Institute of Geosciences, IGEO, DiSTAR, the Geodesy Research Group of University Complutense of Madrid, the Cartographical Service of the Government of Canary Islands and the National Geographic Institute of Spain.Finally, we attempt to interpret the thermo-fluid dynamic model and the observed ground deformations in light of the tectonic framework derived from state-of-the-art geophysical studies.

[EN] We present a study of the deformation pattern in El Hierro Island through the analysis of GNSS data from surveys carried out between 2015 and 2019 as well as continuous data. The last eruption in El Hierro occurred under the sea on the south rift, lasted from October 2011 to March 2012, and it was preceded by intense seismic activity and nearly 5 cm ground inflation. After this eruptive cycle, further magmatic intrusions were detected, from June 2012 to March 2014, associated to intense seismic swarms and inflation (about 22 cm of uplift). Nevertheless, these magmatic intrusions did not culminate in any eruption. Following these post-eruptive episodes, the seismic activity became less intense. Thus, for the period of this study, about 500 earthquakes with magnitude ranging from mbLG 2 to mbLG 3.9 were recorded, the ground deformation measured is of lower magnitude, still remaining a slight uplift trend in the GNSS stations up to 2017 and followed by a slight subsidence of about 1.5 cm between 2017-2019. Our purpose is to explain the ground displacements measured and the earthquake occurrence in terms of geodynamics and seismotectonic activity along the island, for the period 2015-2019. Firstly, we retrieved the geodetic velocities from the GNSS daily solutions. Secondly, we computed the 2D infinitesimal strain rates from the velocities through a triangular segmentation approach to map the deformation pattern along the respective GNSS surveys., Projects PID2019-104726GB-I00 and CGL2015- 63799-P of the Spanish Research Agency has supported this research. We wish to acknowledge GRAFCAN (www.grafcan.es) of the Government of the Canary Islands for providing GNSS data from permanent station FRON. Likewise, to the Spanish IGN (www.ign.es) for providing the EH01 and LRES data.

We present a study of the deformation pattern along the Canary archipelago through the analysis of continuous GNSS data. We use data spanning 2011–2017 period to retrieve precise horizontal displacements and a broad calculation of the strain. Geodetic data are interpreted in light of the regional tectonics, which is proven to play a key role in the volcanic eruptions that take place in the archipelago. The common-mode component filtering technique is applied to improve the signal-to-noise ratio of the time series of the GNSS daily solutions before retrieving the geodetic velocities. Through a triangular segmentation approach, we retrieve the 2D infinitesimal strain from the velocities along the Canaries and map deformation patterns in various sectors of the volcanic archipelago. We found areas of maximum deformation west of Tenerife, Gran Canaria and Fuerteventura Islands. A sharp change in shear strain between Tenerife and Gran Canaria is recognized, delineating a sector of intense seismicity, which is mostly associated with a well-known major submarine fault that separates the two insular edifices. On this submarine tectonic structure, we have performed a tentative simulation of the horizontal deformation and strain caused by one of the strongest (mbLg 5.2) earthquakes of the region. Our strain analysis supports the possibility that the main tectonic lineaments are being influenced by the regional stress field. Furthermore, the seismic areas between islands seem mainly influenced by the regional tectonic stress, rather than by the local volcanic activity. This is in accordance with the extensional and compressional tectonic regimes, already identified by other authors, which might favour episodes of volcanism in this volcanic archipelago.

Geotagged smartphone photos can be employed to build digital terrain models using structure from motion-multiview stereo (SfM-MVS) photogrammetry. Accelerometer, magnetometer, and gyroscope sensors integrated within consumer-grade smartphones can be used to record the orientation of images, which can be combined with location information provided by inbuilt global navigation satellite system (GNSS) sensors to geo-register the SfM-MVS model. The accuracy of these sensors is, however, highly variable. In this work, we use a 200 m-wide natural rocky cliff as a test case to evaluate the impact of consumer-grade smartphone GNSS sensor accuracy on the registration of SfM-MVS models. We built a high-resolution 3D model of the cliff, using an unmanned aerial vehicle (UAV) for image acquisition and ground control points (GCPs) located using a differential GNSS survey for georeferencing. This 3D model provides the benchmark against which terrestrial SfM-MVS photogrammetry models, built using smartphone images and registered using built-in accelerometer/gyroscope and GNSS sensors, are compared. Results show that satisfactory post-processing registrations of the smartphone models can be attained, requiring: (1) wide acquisition areas (scaling with GNSS error) and (2) the progressive removal of misaligned images, via an iterative process of model building and error estimation.

The coupled effects of climate change and land sinking make deltas and coastal areas prone to inundation and flooding, meaning that reliable estimation of land subsidence is becoming crucial. Commonly, land subsidence is monitored by accurate continuous and discrete measurements collected by terrestrial and space geodetic techniques, such as Global Navigation Satellite System (GNSS), Interferometry Synthetic Aperture Radar (InSAR), and high precision leveling. In particular, GNSS, which includes the Global Positioning System (GPS), provides geospatial positioning with global coverage, then used for deriving local displacements through time. These site-positioning time series usually exhibit a linear trend plus seasonal oscillations of annual and semi-annual periods. Although the periodic components observed in the geodetic signal affect the velocity estimate, studies dealing with the prediction and prevention of risks associated with subsidence focus mainly on the permanent component. Periodic components are simply removed from the original dataset by statistical analyses not based on the underlying physical mechanisms. Here, we propose a systematic approach for detecting the physical mechanisms that better explain the permanent and periodic components of subsidence observed in the geodetic time series. It consists of three steps involving a component recognition phase, based on statistical and spectral analyses of geodetic time series, a source selection phase, based on their comparison with data of different nature (e.g., geological, hydro-meteorological, hydrogeological records), and a source validation step, where the selected sources are validated through physically-based models. The application of the proposed procedure to the Codigoro area (Po River Delta, Northern Italy), historically affected by land subsidence, allowed for an accurate estimation of the subsidence rate over the period 2009–2017. Significant differences turn out in the retrieved subsidence velocities by using or not periodic trends obtained by physically based models.

International audience; We investigate a nearly 8-year record (2010-2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). We first perform a tidal analysis with ET34-ANA v7.1 software that leads to the gravimetric amplitude and phase factors for all separable waves according to the available time duration. We test nine different ocean tide models for the main eleven tidal constituents (Ssa, Mm, Mf, Q1, O1, P1, K1, N2, M2, S2, K2). After correction for ocean tidal loading we obtain the real and imaginary parts of the residual vector. We also investigate atmospheric loading which is dominated in this equatorial location by the thermal waves S1 and S2 that are modulated in amplitude by annual and semi-annual components. After correction for ocean loading, we test different air pressure corrections on the tidal gravimetric factors for the waves Sa, Ssa, S1 and S2. We show the rather large discrepancy that exists between the classical single admittance pressure reduction and a hybrid model using global atmospheric models everywhere except in the local zone where the model pressure is replaced by the observed pressure.

Smartphones can be regarded as cameras, natively equipped with geolocation and orientation sensors, making them powerful, portable, user-friendly and inexpensive tools for terrestrial structure from motion/multiview stereo photogrammetry (SfM-MVS) surveys. Camera extrinsic parameters (i.e. camera position and orientation), required to produce fully georeferenced SfM-MVS 3D models are available for the majority of smartphone images via inbuilt magnetometer, accelerometer/gyroscope, and global navigation satellite system (GNSS) sensors. The precision of these internal sensors is not yet sufficient to directly use them as input to SfM-MVS photogrammetric reconstructions. However, when the reconstructed scene is significantly greater than the positional error, camera extrinsic parameters can be successfully used to register 3D models during post-processing. We present a survey of a 400 m wide vertical cliff to illustrate a workflow that enables the use of smartphone cameras to generate and fully georeference photogrammetric models without employing ground control points. Survey images were acquired at a distance of ~350 m to the mapped scene using a consumer-grade smartphone. This survey image dataset was subsequently used to build an unreferenced 3D model, which was registered during post-processing using orientation and position metadata tagged to each photograph.

In the framework of ERASMUS+ program Key Action “Cooperation for innovation and the exchange of good practices” with an Action Type “Strategic Partnerships for vocational education and training”, our project TRack Your ATmosphere (TRYAT) was approved in august 2017 and is co-funded by the European Union. The projects total duration is 35 months and the participants are teachers, researchers and students from three vocational schools and Research/University Institutes in France, Germany and Italy.The project is going to end next August 2020. Here we present the final report on the achieved objectives.The main research goal of TRYAT is a combination of processing and analysis of Global Navigation Satellite Systems (GNSS) data and monitoring of environmental parameters for vocational education and training (VET). Permanent high-precision GNSS stations currently operate for geodetic purposes, e.g. geodynamics, earthquake and volcano monitoring, etc. We want to capitalize and highly disseminate the fact that they also offer a reliable tool for remote sensing the atmospheric water vapour. Three professional GNSS stations and low-cost weather stations have been installed on the roof of the buildings of Lycée Saint Cricq (Pau, France), Lise-Meitner-Schule (Berlin, Germany) and Istituto Leonardo da Vinci (Naples, Italy).We show the achievements so far for each expected intellectual Output (O).O1- Learning Plattform.The Learning Web Platform is an interactive and versatile tool. It helps learners, teachers, researchers and other involved personnel to crosslink, enhance intercultural teambuilding and work on the related technological and environmental issues. The platform gives even access to online real-time and archived data, maps, evaluation and graphical visualization.O2- Starter Kit.We have been realizing a starter kit of a system to acquire and manage data from both GNSS and weather stations. The kit enables us testing not only the technology, but even the concept itself.O3- OER Learning Material ‘Physical and Technical Foundations’.It is an interactive physics course where students learn the foundations of three relevant main topics of the project: satellite technology, propagation of waves and physics of the atmosphere. The corresponding competences are elaborated for the use in different VET curricula.O4- OER Learning Material 'Informatics and Electrical Engineering'.We have developed an interactive learning unit (“learning environment”) with focus on informatics and electronics. The students are given just a problem, namely the collecting of environmental data. This problem is proposed to them in the form of an order from industry “Monitoring of renewable energy plant – measuring wind and sun strength as well as the electrical power”.O5- Educational videos.A series of 5-minute long videos have been realizing in all the official languages of the participating countries. The videos deal with the scientific subjects pertaining the project and report on the results themselves, and the way the student worked as teams to achieve them. Different subtopics will be presented in short videos, as a desirable way of dissemination.

Following the 2004 seismic unrest at Tenerife and the 2011-2012 submarine eruption at El Hierro, the number of GNSS observation sites in the Canary Islands (Spain) has increased, offering scientists a useful tool with which to infer the kinematics and present-day surface deformation of the Canary sector of the Atlantic Ocean. We take advantage of the common mode component filtering technique to improve the signal-to-noise ratio of the velocities retrieved from the daily solutions of 18 permanent GNSS stations distributed in the Canaries. The analysis of GNSS time series spanning the period 2011-2017 enabled us to characterize major regions of deformation along the archipelago through the mapping of the 2D infinitesimal strain field. By applying the triangular segmentation approach to GNSS velocities, we unveil a variable kinematic behaviour within the islands. The retrieved extension pattern shows areas of maximum deformation west of Tenerife, Gran Canaria and Fuerteventura. For the submarine main seismogenic fault between Tenerife and Gran Canaria, we simulated the horizontal deformation and strain due to one of the strongest (mbLg 5.2) earthquakes of the region. The seismic areas between islands, mainly offshore Tenerife and Gran Canaria, seem mainly influenced by the regional tectonic stress, not the local volcanic activity. In addition, the analysis of the maximum shear strain confirms that the regional stress field influences the E-W and NE-SW tectonic lineaments, which in accordance with the extensional and compressional tectonic regimes identified, might favour episodes of volcanism in the Canary Islands., Project CGL2015-63799-P

SUMMARY We analyse a nearly 8-yr record (2010–2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). After tidal analysis removing all solid Earth and ocean loading tidal contributions and correcting for the long-term instrumental drift and atmospheric loading, we obtain a gravity residual signal which is essentially a hydrological signal due to the monsoon. This signal is first compared to several global hydrology models (ERA, GLDAS and MERRA). Our superconducting gravimeter residual signal is also superimposed onto episodic absolute gravity measurements and to space gravimetry GRACE data. A further comparison is done using local hydrological data like soil moisture in the very superficial layer (0–1.2 m), water table depth and rainfall. The temporal evolution of the correlation coefficient between the gravity observation and both the soil moisture and the water table is well explained by the direct infiltration process of rain water together with the lateral transfer discharging the water table. Finally, we compute the water storage changes (WSC) using a simulation based on the physically based Parflow-CLM numerical model of the catchment, which solves the water and energy budget from the impermeable bedrock to the top of the canopy layer using the 3-D Richards equation for the water transfers in the ground, the kinematic wave equation for the surface runoff and a land surface model (CLM) for the energy budget and evapotranspiration calculation. This model forced by rain is in agreement with evapotranspiration and stream flow data and leads to simulated water storage changes that nicely fit to the observed gravity signal. This study points out the important role played by surface gravity changes in terms of a reliable proxy for water storage changes occurring in small catchments.

Defining the origin of ground deformation, which can be a very challenging task, may be approached through several investigative techniques. Ground deformation can originate in response to both natural (e.g., tectonics) and anthropic (e.g., groundwater pumping) contributions. These may either act simultaneously or be somewhat correlated in space and time. For example, the location of structurally controlled basins may be the locus of enhanced human-induced subsidence. In this paper, we investigate the natural and anthropic contributions to ground deformation in the urbanized area of the inner Sarno plain, in the Southern Apennines. We used a multidisciplinary approach based on the collection and analysis of a combination of geomorphological, stratigraphical, structural, hydrogeological, GPS, and DInSAR datasets. Geomorphological, stratigraphical, and structural data suggested the occurrence of a graben-like depocenter, the Sarno basin, bounded by faults with evidence of activity in the last 39 ka. Geodetic data indicated that the Sarno basin also experienced ground deformation (mostly subsidence) in the last 30 years, with a possible anthropogenic contribution due to groundwater pumping. Hydrogeological data suggested that a significant portion of the subsidence detected by geodetic data can be ascribed to groundwater pumping from the alluvial plain aquifer, rather than to a re-activation of faults in the last 30 years. Our interpretation suggested that a positive feedback exists between fault activity and the location of area affected by human-induced subsidence. In fact, fault activity caused the accumulation of poorly consolidated deposits within the Sarno basin, which enhanced groundwater-induced subsidence. The multidisciplinary approach used here was proven to be successful within the study area and could therefore be an effective tool for investigating ground deformation in other urbanized areas worldwide.

International audience; We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts and Rittershoffen sites. Soultz-sous-Forêts site is the first enhanced geothermal system demonstration site producing electricity in France. Here a geothermal fluid at 165 °C allows to produce around 1.5 MW thanks to one injection well and one production well 5 km deep. Rittershoffen site is dedicated to an industrial use and it is designed to produce 24 MWth heat power with 2 wells around 2.5 km deep. The most recent production episodes of Rittershoffen and Soultz-sous-Forêts geothermal plants have started respectively on May and June 2016. To study underground mass redistribution, time-lapse relative microgravity measurements have been done since 2014 on a network designed ad hoc for Soultz-sous-Forêts site and since 2015 for Rittershoffen site. After tide and drift correction, double differences are calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. Absolute gravity measurements have been also collected at one of the reference stations. Before the beginning of the production in 2016, the stability of the Soultz-sous-Forêts reference station was monitored through the repetition of absolute measurements and continuous gravity records. In 2016, regular ties between the reference stations and the Strasbourg gravity observatory STJ9 have been done. Several superconducting gravimeters operate continuously in STJ9. Thus, we approach the concept of hybrid gravity. Vertical deformation is also controlled thanks to six continuous GPS measurements: the height changes are less than 1 cm. So we consider that our gravity variations are only due to subsurface mass changes. For the Soultz-sous-Forêts network, we notice significant changes in agreement with the position of the injection and the production wells. The maximum gravity change is 31 ± 8 μGal. On the contrary, we do not detect any similar signal for the Rittershoffen network. A simplistic model using two spherical sources located at 5 and 2 km for Soultz-sous-Forêts and Rittershoffen sites respectively shows negligeable surface gravity changes when taking into account the known injection and production flow rates.

We report on a detailed geodetic continuous monitoring in Timanfaya volcanic area (TVA), where the most intense geothermal anomalies of Lanzarote Island are located. We analyze about three years of GNSS data collected on a small network of five permanent stations, one of which at TVA, deployed on the island, and nearly 20 years of tiltmeter and strainmeter records acquired at Los Camelleros site settled in the facilities of the Geodynamics Laboratory of Lanzarote within TVA. This study is intended to contribute to understanding the active tectonics on Lanzarote Island and its origin, mainly in TVA. After characterizing and filtering out the seasonal periodicities related to “non-tectonic” sources from the geodetic records, a tentative ground deformation field is reconstructed through the analysis of both tilt, strain records and the time evolution of the baselines ranging the GNSS stations. The joint interpretation of the collected geodetic data show that the area of the strongest geothermal anomaly in TVA is currently undergoing a SE trending relative displacement at a rate of about 3 mm/year. This area even experiences a significant subsidence with a maximum rate of about 6 mm/year. Moreover, we examine the possible relation between the observed deformations and atmospheric effects by modelling the response functions of temperature and rain recorded in the laboratory. Finally, from the retrieval of the deformation patterns and the joint analysis of geodetic and environmental observations, we propose a qualitative model of the interplaying role between the hydrological systems and the geothermal anomalies. Namely, we explain the detected time correlation between rainfall and ground deformation because of the enhancement of the thermal transfer from the underground heat source driven by the infiltration of meteoric water., Projects CGL2015-63799-P of Spanish Ministry of Economy and Competitiveness, and 320/2011 of National Parks Autonomous Agency have supported this research.

We propose a thermo-fluid-dynamics model to study some recent uplift episodes occurred in the period 2008–2013 at Campi Flegrei caldera (Italy). Accounting for eight overpressure sources (from 5 to 40 MPa) in the hydrothermal system, our model solves for heat and momentum balances to obtain fluid velocities responsible for the observed ground displacement. For a validation of the model we use a dataset from seven continuous GNSS stations of the Neapolitan Volcanoes Continuous GPS network (NeVoCGPS), belonging to a geodetic monitoring system covering the Neapolitan volcanic area, and operated by the Istituto Nazionale di Geofisica e Vulcanologia. We compare the observed and modelled vertical displacements to assess “threshold” values for the vertical ground accelerations below which ground displacements could be described with the classic fluid-dynamics equations applied to the hydrothermal system without invoking a direct magmatic contribution. We find out that below 280 mm/yr2 the observed ground acceleration can be explained as just due to the interaction between the deep magmatic and hydrothermal systems. On the contrary, for values exceeding the modelled “threshold”, the direct magmatic contribution can be likely invoked as source of the ground deformations. Through this study, we target to contribute to the debate on the origin of the observed ground deformation, mainly to separate the effects of hydrothermal perturbations, caused by the injection of deep magmatic fluids into the aquifer, from the direct magma intrusion.

International audience; The resonance associated with the Free Core Nutation (FCN) has been widely studied in Very Long Base Interferometry (VLBI) network measurements and in superconducting gravity records, but few experiments have been done with tiltmeters. In this study we use records collected with a pair of about 100 m long hydrostatic silica tiltmeters, orthogonally installed in an abandoned silver mine at Sainte Croix aux Mines (Alsace, in North-Eastern France). Main difficulties in retrieving FCN parameters from tidal analysis arise from the weak amplitude of PSI1 tidal wave (the closest in frequency to the FCN), as well as from the inaccuracy of the available ocean loading correction. Moreover because of the closeness in frequency of the single constituents of the diurnal tidal band, long (>1 year) records are needed for resolving K1, PSI1 and PHI1 waves. Hence we analyze a 10-year dataset of tilt records, hich has preliminarily required a critical review and a relevant editing for making records suitable for tidal analysis and subsequent inversion of the tidal parameters. A Bayesian inversion is used for a preliminary retrieval of the FCN parameters.

In this paper, we analyze the tropospheric delay observed on some ground-based CGPS stations in a dense small regional network and its time evolution during extreme weather conditions. In particular, we studied two severe weather events occurring in the Campanian Region (Italy) on October 12, 2012 and December 2, 2014, reaching 42 and 28 mm rainfall during about 1 h at Naples (MAFE) and Gragnano (GRAG) stations respectively. The main concern of this study is the retrieval of the precipitable water (PW) from co-located GPS and meteorological stations. We investigate the correlation between PW and rain amount at ground level. We analyse phase residuals for each visible GPS satellite using sky plots of the phase residuals along the GPS satellites tracks, showing that the two phenomena are shown in the phase residual plots. Moreover, we compare PW data retrieved from observed meteorological data and from models (GPT2 and ECMWF), evidencing that there is a need for co-located CGPS and weather stations to improve the assessment of water content in the troposphere.

International audience; We report on the results of a comparative analysis carried out on more than ten months of co-located records collected at the J9 gravity station in Strasbourg (France) with a new generation spring gravimeter, the Micro-g LaCoste, Inc. gPhone-054 and the GWR-C026 superconducting gravimeter (SG-C026). The gPhone is essentially a LaCoste & Romberg, model G meter, but with an improved thermal system (a double oven) for increased temperature stability, which should result in an unprecedented improvement in noise and drift of this mechanical gravimeter. We test the performances of the gPhone-054 in terms of resolution, accuracy, noise level and long-term stability (drift) with respect to the SG-C026. Our comparative analysis is performed in a wide spectral domain, ranging from the body tides to the seismic band. This study confirms that the SG has better performances over the whole analysed spectral band. The gPhone-054 instrumental drift observed during this study still remains a critical point preventing the study of long-term gravity changes. In fact, compared with the SG, the drift is large and even non-linear. We observed a drift rate evolution characterized by a decrease from 50 µGal/day to 15 µGal/day, after about 1 month of operation. This makes it hard to distinguish real-time gravity changes from the instrumental drift. We tried to improve the drift modelling by using frequent absolute gravity (AG) measurements, but unfortunately during that time no significant gravity changes have been detected which would have helped us to discriminate short-term drift excursions from real gravity changes. In terms of noise levels, the gPhone-054 turns out to be about 10 times noisier than the SG-C026 at seismic frequencies, while in the tidal band, it is twice as noisy. In between, at periods ranging from 1 h to 6 h, the gPhone-054 is about 3 times noisier than the SG-C026 but performs slightly better than the Scintrex CG5 in terms of noise level and precision.

Bathymetric and gravity surveys were carried out from 1988 to 1994, in the Gulf of Naples (Southern Italy) to offshore extend the already existing Bouguer anomaly map. In order to improve the knowledge of the structural setting beneath the active Neapolitan volcanoes (Vesuvio, Campi Flegrei and Ischia), 862 stations were surveyed within the isobath of 400 m; at the same time, and about 2000 on-land gravity values were also collected. A new Bouguer anomaly map spanning the whole volcanic region was drawn from the final data set. Gravity anomalies were referred to the new absolute gravity station in Naples and computed according to 1980 Geodetic Reference System. Finally, a density value of 2200 kg/m(3) was used in the computation of the Bouguer and terrain effects. We carried out the inversion of the gravity anomalies adopting a 2.5-D modelling along selected profiles crossing the investigated area. The interpretative models were constrained to data obtained from deep wells and other geophysical investigations. In a previous paper, we investigated in detail the area around the Vesuvio. Here we focus on the Phlegraean Volcanic District (PVD), formed by Campi Flegrei, and the islands of Procida and Ischia. Our interpretation permits to outline two separate caldera-like structures in Campi Flegrei and their rims, mainly in the offshore sector. Moreover, it highlights a sharp separation between the island of Ischia and the rest of the PVD while showing Procida as well integrated, at least from the structural point of view. Ischia appears to be a block delimited all around and stepped by a series of volcano-tectonic structures. Both the qualitative description and the quantitative interpretation of the Bouguer anomaly in the PVD area, together with the comparison with the gravity features of the rest of the Neapolitan area, would suggest that the uprising basement beneath the island of Ischia, inducing most of the gravity signal, could be the carbonate ones. The certainty and the discrimination between a carbonate or lava basement beneath Campi Flegrei is not yet deducible. In addition, for this area, the presence of a magma chamber from the Bouguer anomaly, taking into account the information provided by further geophysical and geochemical researches, was checked. The add-on body improves the model as a whole and doesn't impact on the presence of a residual magma chamber. Finally, the quantitative interpretation also permitted to define the shape and the depth of the 2600 kg/m3 basement (carbonate in places) and of the overlapping denser polygenic layer on which the inferred main structures was drawn. (C) 2008 Elsevier B.V. All rights reserved.

Since 1973, the Gravimetric Observatory of Strasbourg (France) is located in an old fort named J9 and has been the place for various gravity experiments. We present a comparison of the noise levels of various instruments that are or were continuously recording at J9, including the LaCoste&Romberg Earth-Tide Meter ET-5 (1973???1985), the GWR Superconducting Gravimeter TT-T005 (1987???1996), the Superconducting Gravimeter C026 (since 1996), the STS-2 seismometer (since 2010) and the LaCoste&Romberg ET-11 (continuously since October 2010). Besides these instruments, the J9 Observatory has hosted temporary gravity experiments with the Micro-g LaCoste Inc. gPhone-054 (May???December 2008 and May???September 2009) and the Micro-g LaCoste Inc. Graviton-EG1194 (June???October 2011). We include also in the comparison the absolute gravimeter Micro-g FG5 #206 which is regularly performing absolute gravity measurements at J9 since 1997 and a spring gravimeter Scintrex CG5 which recorded at J9 between March 2009 and February 2010. We present the performances of these various instruments in terms of noise levels using a standardized procedure based on the computation of the residual power spectral densities over a quiet time period. The different responses to atmospheric pressure changes of all the instruments are also investigated. A final part is devoted to the instrumental self-noise of the SG C026, STS-2 and L&R ET-11 using the three channel correlation analysis method applied to 1-Hz data.

A wide set of dynamics phenomena (i.e., Geodynamics, Post Glacial Rebound, seismicity and volcanic activity) can produce time gravity changes, which spectrum varies from short to long (more than 1 year) periods. The amplitude of the gravity variations is generally in the order of consequently their detection requires instruments with high sensitivity and stability: then, high quality experimental data. Spring and superconducting gravimeters are intensively used with this target and they are frequently jointed with tiltmeters recording stations in order to measure the elasto-gravitational perturbation of the Earth. The far-field effects produced by large earthquakes on records collected by spring gravimeters and tiltmeters are investigated here. Gravity and tilt records were analyzed on time windows spanning the occurrence of large worldwide earthquakes; the gravity records have been collected on two stations approximately 600 km distant. The background noise level at the stations was characterized, in each season, in order to detect a possible seasonal dependence and the presence of spectral components which could hide or mask other geophysical signals, such as, for instance, the highest mode of the Seismic Free Oscillation (SFO) of the Earth. Some spectral components (6.5’; ∼8’; ∼9’; ∼14’, ∼20’, ∼51’) have been detected in gravity and tilt records on the occasion of large earthquakes and the effect of the SFO has been hypothesized. A quite different spectral content of the EW and NS tiltmeter components has been detected and interpreted as a consequence of the radiation pattern of the disturbances due to the earthquakes. Through the analysis of the instrumental sensitivity, instrumental effects have been detected for gravity meters at very low frequency.

High-precision gravity measurements and records are used to investigate geodynamic processes. The amplitude of the gravity changes induced by these phenomena is of the order of 10-8 g to 10-9 g, where the global average of g (gravity) is 9.80 m s-2. Instrument sensitivities up to 10-9 are therefore required to detect gravity changes. As changes in instrument sensitivity can reduce the repeatability of measurements and affect the phase and amplitude of recorded gravity signals, the accurate calibration of gravimeters in high-precision gravimetry is topical. The stability of the calibration factors in two LaCoste-Romberg gravimeters has been investigated through different calibration methods. The main results show that (a) the application of multiple calibration approaches is a powerful constraint on instrumental response and identifies real or fictitious changes in instrument sensitivity; (b) a comparison between spring and superconducting gravimeters is the most suitable way of almost completely determining the transfer function of spring gravimeters in the tidal band. Moreover, a correlation was detected between stronger changes in instrument sensitivity and the occurrence of seismic events. Thus mechanical perturbation of spring gravimeters during earthquakes can affect the stability of their calibration factors.

International audience; The increasing number of hydro-gravimetry studies proves the rising interest of the hydrology community towards this monitoring method. The accuracy of Superconducting Gravimeters (SG) potentially allows the retrieval of small Water Storage Changes (WSC) down to a few millimeters of equivalent water thickness However, the importance of these corrections applied to SG data to achieve such a precision in gravity residuals should be recalled. The Djougou permanent gravity station presented in this paper and located in northern Benin, West-Africa, provides a good opportunity to review these considerations. This station is equipped since July 2010 with the superconducting gravimeter SG-060 aimed at deriving WSC at different time-scales, daily to inter-annual. In this area, WSC are 1) part of the control system for evapotranspiration (ET) process, a key variable of the West-African monsoon cycle and 2) the state variable for resource management, a critical issue in storage-poor hard rock basement contexts such as in northern Benin. The potential for deriving WSC from time-lapse gravity data partly depends on environmental features such as topography and the instrument shelter. Therefore, this issue is addressed first, with the background idea that such sensitivity analysis should be undertaken before setting up any new instrument. In Djougou, local topography is quite flat leading to a theoretical straightforward relationship between gravity changes and WSC, close to the standard Bouguer value. However, the shelter plays a significant masking role, which is the principal limitation to the retrieval of fast hydrological processes such as ET following a rain event. Several issues concerning classical gravity corrections are also addressed in the paper. These include gap-filling procedures during rain-events and drift estimates for short time series. Special attention is provided to atmospheric corrections, and different approaches are tested: a simple scalar admittance, a filtered scalar admittance, a frequency-dependent admittance and direct atmospheric loading calculations. It is shown that only the physically-based approach of direct loading calculations performs better in both residual minimization and ET retrieval. Moreover, non-local hydrological effects are investigated and account for about 20% of the gravity residuals. Finally, gravity residuals are briefly analyzed at two distinct time scales: rapid (up to a few days) and seasonal. At the rapid time-scale, it is shown that ET retrieval is hardly achievable given shelter size and state-of-the-art atmospheric corrections. Still, mean values retrieved from this study are in accordance with known values of potential ET and constant lateral flow. Direct comparison of gravity changes with hydrological data (neutron probe monitoring and water table levels) show some discrepancies, particularly for the hydrological year of 2011, for which all hydrological data show a deficit, but SG and FG5 data do not. This preliminary analysis both provides a basis and call for further hydro-gravity modeling, to comprehensively investigate the water-cycle at the Djougou station.

International audience; We investigate in this paper various approaches to correct gravity changes for the effect of atmospheric pressure changes. Two specific locations are considered: Strasbourg (France) as mid-latitude station, where regular pressure fronts occur and Djougou (Benin) as equatorial station with large thermally driven S1 and S2 waves of planetary extent. We first review the classical approaches based on a constant or frequency-dependent admittance using only local pressure and gravity data. We consider then a model of atmospheric loading and show the barometric admittance in terms of elastic, Newtonian and total load, as a function of the distance from the station. We consider both a 2D pressure model (surface loading) and a 2.5D model, where the density decreases with height (standard atmosphere). Assuming horizontal advection in the atmospheric dynamics, we convert this spatially dependent admittance into a frequency-dependent admittance. Using global pressure data from European Centre for Medium-Range Weather Forecasts (ECMWF) at about 12 km spatial resolution and 3 h sampling, we compute the model-predicted pressure admittance for Djougou and Strasbourg and we simulate the frequency dependence inferred from gravity and pressure observations below 4 cycle per day. A long gravity and pressure data set (1996-2013) from Strasbourg is used to investigate the low frequency part of the pressure admittance while a common 2.5 year data set (August 2010-February 2013) for Strasbourg and Djougou is then analyzed to investigate the high frequency part of the admittance. In both cases, our results are in close agreement with the predictions inferred from an atmospheric 2.5D loading model with a distance-time relationship due to horizontal advection. The frequency dependence of the barometric admittance is explained by the competing contributions of Newtonian attraction and elastic surface deformation according to the distance from the gravimeter. In the far field (low frequencies), the magnitude of the admittance decreases with frequency because of the combined elasticity effect and Newtonian attraction (when the atmosphere is below the horizon) while, on the contrary, in the near field (high frequencies), elasticity becomes negligible and the pressure admittance mainly decreases with increasing frequency because of the decreasing attraction effect of the atmospheric masses inside the cylindrical pressure cell centered on the sensor location of decreasing radius. In the last part, we show that there is variability in time in the pressure admittance for both stations.

International audience; The Gravimetric Observatory J9 located near Strasbourg (France), has a long history of recording solid Earth tides. We present here one of the longest available gravity records (1973-2011) from three different instruments operating at the J9 station: a LaCoste-Romberg spring gravimeter (ET005) from 1973 to 1985, a GWR (TT70-T005) superconducting gravimeter from 1987 to 1996 and a GWR compact superconducting gravimeter (C026) since 1996. We give the results of a tidal analysis performed with ETERNA on these data sets. We show the improvement in terms of noise levels from the oldest type to the newest one and check the time stability of the observed tides for every instrument. This stability is also dependent on the stability of the scale factor of the cryogenic gravimeter which is derived from a direct comparison with repeated absolute gravity measurements. The response of the three gravimeters to the atmospheric pressure changes is also checked. With all these results we found that the improvement from a mechanical spring gravimeter to a modern cryogenic instrument is obvious. And also that the improvement from the T005 SG to the C026 SG is less impressive than from L&R to the T005 SG, but still notable.

Continuous gravity measurements made with LaCoste&Romberg Graviton-EG1194 spring gravimeter are being used in several research projects to validate body and ocean tide models as well as to investigate time gravity changes in Canary Archipelago (Spain), which is an active volcanic area. Further, accurate tidal corrections are needed to obtain reliable surface gravity measurements used to investigate crustal structure models in this area. All those scientific targets require well calibrated instruments to provide accurate gravity measurements. Here we report on an experiment of intercomparison between the Graviton EG1194 with two superconducting gravimeters aimed at checking the instrumental response of the spring meter both in amplitude and phase as well as its time stability. The Graviton-EG represents a significant evolution of previous LaCoste&Romberg models G and D spring gravimeters, with most of operational functions automatically handled. Nowadays, superconducting gravimeters are the most precise instruments to study time and frequency gravity variations, and attain the maximum level of temporal stability in gravity measurements. Besides, their low instrumental drift rates allow superconducting gravimeters obtaining the most accurate tidal parameters as well as reliable time gravity changes. Therefore, co-located gravity observations of spring gravimeters with superconducting offer the best possibilities to compare the tidal parameters and thus to establish the corresponding normalization factors. In this study, we analyze about 3 months of co-located gravity records made by the Graviton-EG1194 at the Gravity Observatory J9 in Strasbourg (France) with the superconducting GWR-C026 gravimeter, and at the Gravity Observatory of Yebes-IGN (Spain) with the superconducting GWR-064. Results are presented in terms of accuracy and precision of the tidal parameters obtained in both sites.

International audience; A GWR superconducting gravimeter of the new generation (OSG-60) has been installed in July 2010 in sub-humid West Africa, at the Djougou station in Benin. This station is located in the AMMA-CATCH long term hydrological observing system. We present the first results in terms of instrumental drift as well as the calibration results using FG5 absolute gravity measurements. We show that geophysical contributions due to hydrological load can bias the initial drift estimate. The noise level is compared to the Strasbourg SG as well as to the reference New Low Noise Model (NLNM) used in seismology. We also investigate the gravity response to atmospheric pressure changes and show that, because of the presence of large thermal tides, the gravity response to mass changes in the atmosphere is more complex than in the simple case of a constant barometric admittance.

International audience; Over the past two decades, superconducting gravimeters (SGs) have been a key tool to investigate a number of geophysical processes leading to time-variable gravity changes. As SGs are relative meters, even though they are the most sensitive and stable devices currently available, they need to be accurately calibrated. Each branch of Earth sciences that benefits from high-precision gravity monitoring demands calibration of gravity sensors to accuracy of better than 0.1%. This research deals with a calibration experiment performed at the Strasbourg (France) SG site by means of two FG5 (#206 and #211) absolute gravimeters (AGs) and new-generation spring meters (Scintrex Ltd. Autograv CG-3M and CG5 and Microg-LaCoste gPhone). Our goal is to try to use the newest generation of spring mechanical gravimeters (MGs) for calibrating SGs. We discuss the results in terms of precision and accuracy of the SG calibration by means of different metrological and methodological approaches. With the FG5 #211 we derive scale factors for the SG-GWR C026 located in Strasbourg in agreement with those routinely obtained since 1997 by means of the FG5 #206. This confirms that the estimation of the scale factors is independent of the AG sensor. From a moving-window regression analysis between the synthetic body tides and both the SG and MG gravity records we detect significant fluctuations of the SG scale factors over time due to the instability of the instrumental sensitivity of the MGs. Our main results demonstrate that, owing to the time variability of their sensitivity, the used spring meters, even if well calibrated, cannot be used as a stable reference for SGs. As a result, MGs are not suitable to replace AGs for SG calibration, and we conclude that currently the method using parallel recording with absolute gravity meters is still the most feasible calibration approach for SGs.

International audience; The superconducting gravimeters (SGs) are the most sensitive and stable gravity sensors currently available. The low drift and high sensitivity of these instruments allow to investigate several geophysical phenomena inducing small and long-period gravity changes. In order to study such topics, any kind of disturbance of instrumental origin has to be identified and possibly modelled. A critical point in gravity measurement is the alignment of the gravimeter to the local vertical. In fact a tilt of the instrument will lead to an apparent gravity change and can affect the instrumental drift. To avoid these drawbacks, SGs are provided with an "Active Tilt Feedback System" (ATFS) designed to keep the meter aligned to the vertical. We analyze tilt and environmental parameters collected near Strasbourg, France, since 1997 to study the source of the tilt changes and check the capability of the ATFS to compensate them. We also present the outcomes of a calibration test applied to the ATFS output to convert the Tilt Power signals into angles. We find that most of the observed signal has a thermal origin dominated by a strong annual component of about 200μrad. Nevertheless, our analysis shows that even the tilt due to different geophysical phenomena, other than the thermal ones, can be detected. A clear tidal signal of about 0.05μrad is detectable thanks to the large data stacking (> 11 years). We conclude that (i) the ATFS device compensates the tilt having a thermal origin or coming from any sources and (ii) no significant tilt changes alter the gravity signal, except for the high-frequency (> 1 mHz) perturbations.

This research is intended to describe new strategies in the processing and analysis of continuous gravity records collected in active volcanic areas and to assess how permanent gravity stations can improve the geophysical monitoring of a volcano. The experience of 15 years in continuous gravity monitoring on Mt. Vesuvius is discussed. Several geodynamic phenomena can produce temporal gravity changes. An eruption, for instance, is associated with the ascent of magma producing changes in the density distribution at depth, and leading to ground deformation and gravity changes The amplitude of such gravity variations is often quite small, in the order of 10-102 nms-2, so their detection requires high quality data and a rigorous procedure to isolate from the records those weak gravity signals coming from different sources. Ideally we need gravity signals free of all effects which are not of volcanic origin. Therefore solid Earth tide, ocean and atmospheric loading, instrumental drift or any kind of disturbances other than due to the volcano dynamics have to be removed. The state of the art on the modelling of the solid Earth tide is reviewed. The atmospheric dynamics is one of the main sources precluding the detection of small gravity signals. The most advanced methods to reduce the atmospheric effects on gravity are presented. As the variations of the calibration factors can prevent the repeatability of high-precision measurements, new approaches to model the instrumental response of mechanical gravimeters are proposed too. Moreover, a strategy for an accurate modelling of the instrumental drift and to distinguish it from longterm gravity changes is suggested.

Air pressure reduction in local gravity measurements is usually done by means of an empirical transfer function (barometric admittance) between pressure and gravity both measured at the same site. In the local zone ( 3.0 cycles/day). The atmospheric reduction from the barometric array is also compared to global loading computations from ECMWF-4Dvar surface pressure data, having a sampling rate of 3 h and a spatial resolution of 0.5°. We also tested a hybrid method based on a spline interpolation of joint ECMWF-4Dvar surface pressure data and local observations collected on the barometric array. The findings are discussed in light of the main features (dimension, geometry and instrumental accuracy) of the barometric array.

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.

High precision relative and absolute gravity measurements are periodically carried out at Mt. Vesuvius (Southern Italy) to monitor the changes in the gravity field caused by the internal dynamics of the volcano. Moreover, a recording gravity station is also operating at the Osservatorio Vesuviano (Old Building) aimed at measuring in continuous mode the time variations of the gravity and the tidal parameters, possibly due to changes in the physical state of the volcano. The analysis carried out on an hourly data set spanning the 1994-1996 time interval and a comparison of the results with those previously obtained, exhibit a change in the tidal parameters which occurred between 1991 and 1994. A gravity decrease of about 60 µGal was observed during the same time interval in the same area. Consistent with relative and absolute gravity observations, no significant variations have been detected since 1994 in the tidal parameters and gravity residuals.