G4.4

Gravimetry has many useful applications from volcanology to oil exploration; being a method able to infer density variations beneath the ground. Therefore, it can be used to provide insight into subsurface processes such as those related to the hydrothermal and magmatic systems of volcanoes. Existing gravimeters are costly and heavy, but this is changing with the utilisation of a technology most notably used in mobile phone accelerometers: MEMS – (Microelectromechanical-systems). A team at the University of Glasgow has already developed a MEMS relative gravimeter and is currently collaborating with multiple European institutions to make a gravity sensor network around Mt Etna - NEWTON-g. A second generation of the MEMS sensor is now being designed and fabricated in the form of a semi-absolute pendulum gravimeter. Gravity data for geodetic and geophysical use were provided by pendulum measurements from the 18^th to the 20^th century. However, scientists and engineers reached the limit of fabrication tolerances and readout accuracy approximately 100 years ago. With nanofabrication and modern electronics techniques, it is now possible to create a competitive pendulum gravimeter again. In this presentation the design and fabrication techniques of a new MEMS pendulum gravimeter will be outlined. The design comprises two pendula, which oscillate in anti-phase to reduce the influence of seismic noise. Nanofabrication methods have been used to create both flexure and knife-edge pivot points. An optical shadow-sensor has been developed to monitor the position of the pendula. This optical readout can provide measurements to sub-nanometre precision. Data collected from laboratory testing will be presented, demonstrating the progression being made towards a prototype field device. This data will include measurements of the influence of tilt-sensitivity and the seismic and shadow sensor noise floors.  Altitude tests of the free-air effect will be presented to demonstrate the current sensitivity of the device. If semi-absolute values of gravity can be measured, then instrumental drift concerns are reduced. Additionally, the need for calibration against commercial absolute gravimeters may not be necessary. This promotes improved accessibility of gravity measurements at an affordable cost.

How to cite: Utting, P., Walker, R., Prasad, A., Hammond, G., and Middlemiss, R.: Design and Testing of a MEMS Semi-Absolute Pendulum Gravimeter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10306, https://doi.org/10.5194/egusphere-egu22-10306, 2022.

This research provides an overview of the mathematics influencing the design of a pendulum-based semi-absolute MEMS (Micro-Electro-Mechanical Systems) gravimeter, currently being developed at the University of Glasgow. The device comprises two pendula actuated in anti-phase, allowing a differential measurement of local gravity that is isolated from seismic noise. The pendula are pivoted about a narrow flexure, and are therefore subject to changes in elastic stiffness with temperature. By adding mass this effect is diluted and the behaviour becomes asymptotically dominated by the gravitational restoring force, reducing temperature sensitivity.

The rationale behind the basic topology of the device will first be explained in terms of both the differential representation of the system and the corresponding vibratory profile modelled using finite-element software. This is followed by a brief discussion of the main factors influencing the measurement of gravity such as thermal and damping effects. The primary focus of the research will be how best to model the physics of pendulum motion, how to extract a value for gravity from this model, and the optimum sampling technique required in order to satisfy both device sensitivity and hardware limitations. A comparison between the simulated behaviour and preliminary experimental data will then be made from which the efficacy of the modelling solution can be inferred. Finally, the results of a lift-test, wherein the device is moved to various floors of a building and which is designed to estimate the performance of the device, will be discussed and contrasted with similar data from an existing relative MEMS gravimeter (the 'wee-g'), as well as any projected design modifications considered as a consequence of these results.

How to cite: Walker, R., Utting, P., Middlemiss, R., Prasad, A., and Hammond, G.: Mathematics & Analysis of a MEMS Semi-Absolute Pendulum Gravimeter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12991, https://doi.org/10.5194/egusphere-egu22-12991, 2022.

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.

How to cite: Carbone, D., Antoni-Micollier, L., Ménoret, V., Lautier-Gaud, J., Greco, F., King, T., Messina, A., Contrafatto, D., and Desruelle, B.: First results from an absolute atom interferometry gravimeter at Mt. Etna volcano, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7016, https://doi.org/10.5194/egusphere-egu22-7016, 2022.

In August 2021 Askja caldera in Iceland started to show uplift after decades of subsidence. The uplift signal is centered at the northwestern edge of lake Ӧskjuvatn and an order of magnitude larger than the subsidence in the last decade. In September 2021 a geodesy campaign was carried out at Askja, including relative microgravity measurements acquired with the use of two Scintrex CG-5 instruments. Relative microgravity campaigns at Askja are not straightforward due to the long walking distances between sites, which makes a “double loop” procedure impossible. We revisit existing Scintrex relative microgravity data sets (2015 onward) and analyse data using the same joint weighted least squares inversion routine. We define recommendations for future relative microgravity campaigns at Askja which will be important to establish the cause of the ongoing uplift. The density of subsurface magma is only identifiable with microgravity data. Knowledge of the type of magma accumulating under Askja is vital to assess possible hazard implications.

How to cite: de Zeeuw van Dalfsen, E., Sepulveda Araya, J., Hooper, A., Sigmundsson, F., Sturkell, E., Li, S., Lanzi, C., Koymans, M., and Giniaux, J.: Inflation at Askja, Iceland. New and revisited relative microgravity data., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7070, https://doi.org/10.5194/egusphere-egu22-7070, 2022.

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.

How to cite: Greco, F., Carbone, D., Contrafatto, D., Messina, A. A., and Berrino, G.: Combined discrete and continuous gravity measurements at Vulcano Island (Aeolian Archipelago, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5491, https://doi.org/10.5194/egusphere-egu22-5491, 2022.

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.

How to cite: Zahorec, P., Papčo, J., Greco, F., Messina, A., Pánisová, J., Vajda, P., and Carbone, D.: Gravimetric investigation of the structure of the Etna summit craters system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12693, https://doi.org/10.5194/egusphere-egu22-12693, 2022.

The Classical Karst region, a region shared between Italy and Slovenia, hosts one of the most archetypical karstic aquifers in the world. Here, the large limestone plateau has been continuously dissolved by meteoric waters during the past 5 million years, leading to the formation of large cavities interconnected by a well-developed network of conduits and shafts. The system is also fed by an important allogenic contribution, the Reka river, which enters the Classical Karst aquifer through the Škocjan cave system and flows underground for more than 30 km, finally outflowing in the Adriatic Sea. The Reka river experiences large flow variations during the recharge process, resulting in fast and large water accumulation in several cave systems along its underground water path. The Škocjan caves are able to store up to 3 million m³ of water during one of these flood events and represent just one example of these allogenic dominated karstic systems in the Classical Karst.In 2018 a continuously recording gravimeter (gPhone gravimeter) was installed nearby the Škocjan caves to get more insights into the water mass balance of the system during these flood events; the instrument is presently still operating. In February 2019 the gravimeter recorded one of the largest events in the past 50 years  (peak discharge > 300 m³/s) that caused flooding of the cave system with a recorded water level increase >80 m and gravity variations >400 nm/s². Modelling of both gravimetric and hydraulic responses allowed to obtain a new hydraulic model of the cave system and a refined mass flux estimate during the flood (Pivetta et al., 2021). Apart from this event, the gravimeter was able to record the response to a few smaller flood events with peak flows of less than 250 m³/s. The gravity and hydraulic response to smaller floods differs dramatically from the 2019 event both in magnitude and time difference between peak flood and peak gravity. In this contribution we aim to describe in more detail the different response of the coupled gravimetric hydrologic observations to different flood events, evidencing the complex non-linear response of this karstic system to the recharge process. By discussing this case we show the potential of terrestrial gravity observation to depict the hydro-dynamics of this system and the potential of a remote monitoring of the storage units. In future an array of MEMs gravimeters in the Classical Karst could be an excellent tool to fill the gaps of the sparse hydrologic observations, helping to obtain a full 4D model of the hydrodynamics of the system.

Pivetta, T., Braitenberg, C., Gabrovšek, F., Gabriel, G., and Meurers, B.: Gravity as a tool to improve the hydrologic mass budget in karstic areas, Hydrol. Earth Syst. Sci., 25, 6001–6021, https://doi.org/10.5194/hess-25-6001-2021, 2021. 

How to cite: Pivetta, T., Carla, B., Gabrovšek, F., Gabriel, G., and Meurers, B.: Hydrodynamics of an allogenic karstic system from coupled gravimetric and hydrologic observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11539, https://doi.org/10.5194/egusphere-egu22-11539, 2022.

Deformation source inversions have played a substantial role in our present understanding of magma plumbing systems at active volcanoes. Such inversions mostly rely on analytical models for uniformly-pressurized cavities as idealized representations of expanding magma bodies. The most common analytical cavity models used for rapid inversions are the isotropic point-source, the finite spheroidal cavity model and tensile dislocations or cracks. All these models have very specific shapes which cannot represent potentially significant deviations of magma chambers from axisymmetric geometries; thus, this aspect of volcano deformation sources has largely remained unexplored. Potential deviations from spherical and spheroidal shapes may explain the long-wavelength systematic residuals often encountered in inversions of deformation data. Even if the biases in the inferred deformation source parameters are small, they may translate into large biases in the mass change constrained through joint inversion of deformation and gravity data.
The next step to promote our understanding about volcano deformations is to explore these complexities in the source geometries and their implications. We develop a finite ellipsoidal cavity model (finite ECM) that is a solution for surface displacements and deformation-induced gravity changes caused by finite pressurized ellipsoidal cavities. The model can be used to constrain deformation source parameters and subsurface mass changes caused by magmatic intrusions and other processes pressurizing relatively shallow magma chambers. The model is in the form of a distribution of triaxial point sources with depth-dependent spacing and strengths. We systematically validate and benchmark the model by using analytical and numerical solutions. Also through these comparisons, we explore the limitations of the finite ECM. In particular, we analyze the biases in the inferred source depth, volume change and mass change due to the approximations inherent in the model. The finite ECM is computationally efficient and can be used for coupled inversions of deformation and gravity data.

How to cite: Nikkhoo, M. and Rivalta, E.: A new solution for displacements and gravity changes caused by pressurized (triaxial) ellipsoidal cavities in a half-space: source configuration and implications for joint inversions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13046, https://doi.org/10.5194/egusphere-egu22-13046, 2022.