G4.4

The value of the acceleration due to gravity is of interest in a wide range of fields, from geophysics, geodesy, water-floor monitoring, and hazard forecasting to oil, gas and mineral exploration. For this purpose, relative or absolute gravimeters have been developed and used for decades. While absolute gravimeters are mainly used in monitoring stations or as reference, relative gravimeters are those actually used to determine the relative variations of the local gravitational field given their smaller dimension, lighter weight, and better reading resolution, despite the high costs and the difficulty in being used under severe environmental conditions. In the last years, the advent of micro-electromechanical-systems (MEMS), in particular MEMS accelerometers, has opened up the doors to new measuring possibilities at very low-costs. As a consequence, different international research groups focused their efforts to develop relative MEMS gravimeters and showed that this technology might be really useful for monitoring the gravitational field. However, their current production is limited to a few specimens and prototypes that cannot be exploited on a large scale at the present day. For this reason, this work investigates the possibilities and the limits in the use of commercial digital MEMS accelerometers as relative gravimeters. The digital MEMS accelerometers investigated in this work are two commercial low-cost digital MEMS accelerometers (STM, model LSM6DSR, and Sequoia, model GEA). The first is composed of an accelerometer sensor, a charge amplifier, and an analog-to-digital converter and is connected by a serial cable to a separated external microcontroller (ST, model 32F769IDISCOVERY), in which other electronic components are integrated. The second is composed of the sensing element and the analog-to-digital converter. Both are connected to the computer via USB cable. The two devices are included in a thermally insulated case, in which a resistive heater and a resistance thermometer (PT1000), connected in loop, are placed in order to guarantee temperature stability during use. The system, installed on a tilting table to ensure higher accuracy in the evaluation of local g, is calibrated in static conditions by comparison to the absolute gravimeter IMGC-02 at a specific measurement location at INRIM. Calibration is repeated several times over a period of a few weeks in order to evaluate repeatability, reproducibility and stability over time. Despite the promising future prospects of this technology, at present, the levels of precisions are low compared to the ones required by most of geodynamics applications.

How to cite: Prato, A., Mazzoleni, F., Facello, A., Origlia, C., Schiavi, A., and Germak, A.: Perspectives and limits on the use of commercial low-cost digital MEMS accelerometers in gravimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10360, https://doi.org/10.5194/egusphere-egu21-10360, 2021.

The manufacture and production of a high-sensitivity cost-effective gravimeter has the potential to change the methodology and efficiency of gravity measurements. Currently, the most common method to conduct a survey is by using a single gravimeter, usually costing tens of thousands of Dollars, with measurements taken at multiple locations to obtain the required data. The availability of a cost-effective gravimeter however would allow the user to install multiple gravimeters, at the same cost of a single gravimeter, to increase the efficiency of surveys and long-term monitoring.  

Since the previous reporting on a low-drift relative MEMS gravimeter for multi-pixel imaging applications (Prasad, A. et al, EGU2020-18528), significant progress has been made in the development and assembly of the previously reported system. Field prototypes have been manufactured and undergone significant testing to investigate the stability and robustness of the system in preparation for the deployment of multiple devices as part of the gravity imager on Mount Etna. The device, known as Wee-g, has several key features which makes it an attractive prospect in the field of gravimetry. Examples of these features are that the Wee-g is small and portable with the ability to connect to the device remotely, can be powered through a mains connected power supply, or through portable batteries, weighs under 4kg, has a low power consumption during normal use of 5W, correct for tilt through manual adjustments or remotely through integrated stepper motors with a total tilt correction range of 5 degrees, the ability to read out tilt of the device through an inclinometer for either alignment or long term monitoring and numerous temperature sensors and heater servos to control the temperature of the MEMS to <1mK.

This presentation aims to report on the progress that has been achieved in the development and manufacturing of the prototype devices, various testing of the devices under various laboratory conditions (such as the measurements of the Earth tides, and a relative measurement of gravity at various floor levels), as well as additional applications that are to be explored in 2021. 

How to cite: Toland, K., Prasad, A., Noack, A., Anastasiou, K., Middlemiss, R., Paul, D., and Hammond, G.: Development and Assembly of a MEMS Based High-Sensitivity Relative Gravimeter for Multi-Pixel Imaging Applications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13167, https://doi.org/10.5194/egusphere-egu21-13167, 2021.

A vibrating beam MEMS gravimeter with an Allan deviation of 9 μGal for a 1000 s integration time, a noise floor of 10 μGal/√Hz, and measurement over the full ±1 g dynamic range (1 g = 9.81 ms⁻²) is presented. In addition to a direct digital signal output, the sensor system possesses built-in tilt compensation capabilities and a 2-stage temperature control that is stable to 500 µK.

Instances of Earth tidal tracking and ground motion records corresponding to several teleseismic events are demonstrated. The output response from tracking of the Earth tides is compared to the data obtained from the software TSoft and a statistical correlation R of 0.92 is obtained between the conditioned MEMS dataset over a period of ~4 days and the predicted Earth tides model from TSoft following correction for ocean loading effects.

The device also recorded the ground motion from several teleseismic events during the testing period, a prominent event among them is the 6.2 M_L earthquake near to Petrinja, Croatia, which occurred on December 29^th, 2020. The MEMS sensor has demonstrated excellent performance as a long-period seismometer and the response is compared to the seismograms recorded by two nearby BGS broadband seismic stations. 

Advances in microgravity sensor detection capability will be shown to match feasibility modelling for void detection. Results demonstrate that a vibrating beam MEMS accelerometer can be used for measurements requiring high levels of stability and resolution with wider implications for precision measurement. Gravimetry use to warn of imminent failures due to a range of shallow hazards include assessing damage in the built environment, transmission losses in utilities, territory breach and storage containment loss.

How to cite: Topham, A., Pandit, M., Du, Z., Sobreviela, G., Young, D., Pili, C., Baker, C., and Seshia, A.: Use of a vibrating beam MEMS accelerometer for surface microgravimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15278, https://doi.org/10.5194/egusphere-egu21-15278, 2021.

Since a few years, several laboratories, institutes or organizations through the world have acquired marketed quantum absolute gravimeters AQG developed by Muquans. Among their potentialities, these new generations of instruments are expected to complement the existing capabilities of long term monitoring of the Earth gravity field. A metrological evaluation of their performances for long-term measurements is thus a first step.

The LNE-SYRTE gravimetry laboratory in the suburb of Paris, has been designed to accommodate other gravimeters for metrological comparisons, tests and calibrations. Instruments of different classes operate in this well characterized laboratory: a laboratory-based absolute cold atom gravimeter (CAG) and a relative superconducting gravimeter iGrav. Both instruments allow for continuous measurements, Accuracy is guaranteed by the CAG and long-term stability by the iGrav.

We there have performed a more than one-year long measurement session with the initial version of the marketed quantum gravimeter AQG (AQG-A01).

An improved version of this AQG (AQG-B01) designed for outdoor measurement and recently acquired by RESIF (the French Seismologic and Geodetic Network) has been also implemented to close this session with a last month of simultaneous data recording involving all the instruments. Finally, we also performed supplementary accuracy tests, in particular to evaluate the Coriolis bias of the two AQG commercial sensors.

The talk will briefly present the different instruments to rapidly focus on the performances of the AQGs and results of the comparisons.

How to cite: Pereira Dos Santos, F., Vermeulen, P., Bonvalot, S., Gabalda, G., Le Moigne, N., Champollion, C., Antoni-Micollier, L., and Merlet, S.: One-year long common view measurements with continuous gravimeters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12405, https://doi.org/10.5194/egusphere-egu21-12405, 2021.

Recent technological advances in the field of quantum gravimetry led to the first commercially available absolute quantum gravimeters (AQG) that are designed for deployment in field surveys (AQG by Muquans, B series). Limitations of other relative or absolute gravimeters currently used for environmental applications which require highly accurate and precise data (e.g. monitoring subsurface water storage changes), are expected to be at least partly overcome with AQGs.

In this contribution, we report on the first experiences gained with the Muquans AQG-B02 during a gravimetric field survey in the vicinity of the Geodetic Observatory Wettzell (Bavarian Forest, Germany). The instrument is part of MOSES, a novel observing system of the German Helmholtz Association, comprising flexible and mobile observation modules for event-based investigation of hydrological extreme events, among other processes. To our knowledge, this is the first use of an AQG in an outdoor field campaign. During the 4-day survey, measurements with the AQG were performed on small concrete pillars at 4 field locations and partly repeated on consecutive days. In between the field measurements, reference measurements were carried out on a laboratory pillar of the Geodetic Observatory. We present the AQG field deployment with regard to transport, site design and power supply. The AQG survey is evaluated with respect to its technical and operational feasibility and the data are assessed in terms of their sensitivity, accuracy and reproducibility. Parallel recordings of environmental conditions such as wind speed and air temperature allow for assessing their potential disturbing effect on the gravity measurements. Observations with an A10 absolute gravimeter on the same sites few days before or after the AQG measurements were used for comparing the absolute gravity values.

How to cite: Güntner, A., Reich, M., Reinhold, A., Glässel, J., and Wziontek, H.: First experiences with an absolute quantum gravimeter during field campaigns, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14072, https://doi.org/10.5194/egusphere-egu21-14072, 2021.

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)

How to cite: Carbone, D., Antoni-Micollier, L., Greco, F., Lautier-Gaud, J., Contrafatto, D., Ménoret, V., and Messina, A.: Deploying and operating an Absolute Quantum Gravimeter on the summit of Mount Etna volcano, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15186, https://doi.org/10.5194/egusphere-egu21-15186, 2021.

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.

How to cite: Greco, F., Carbone, D., Messina, A. A., and Contrafatto, D.: Comparison between a six-year (2015-2020) continuous time series from an iGrav superconducting gravimeter and absolute gravity data at Mt. Etna volcano (Italy)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9119, https://doi.org/10.5194/egusphere-egu21-9119, 2021.

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.

How to cite: King, T., Carbone, D., and Greco, F.: Temporal Variations in the Depth Estimate of Mass Density Changes at Mt Etna Volcano, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8248, https://doi.org/10.5194/egusphere-egu21-8248, 2021.

The accurate deformation-induced topographic effect (DITE) should be used to account for the gravitational effect of surface deformation when analyzing residual spatiotemporal (time-lapse) gravity changes in volcano gravimetric or 4D micro-gravimetric studies, in general. Numerical realization of DITE requires the deformation field available in grid form. We compute the accurate DITE correction for gravity changes observed at the Laguna del Maule volcanic field in Chile over three nearly annual periods spanning 2013–2016 and compare it numerically with the previously used free-air effect (FAE) correction. We assess the impact of replacing the FAE by DITE on the model source parameters of analytic inversion solutions and apply a new inversion approach based on model exploration and growing source bodies. The new inversion results based on the DITE correction shift the position of the mass intrusion upwards by a few hundred meters and lower the total mass of the migrated fluids to roughly a half, compared to the inversion results based on the local-FAE correction. Our new Growth inversion results indicate that vertical dip-slip faults beneath the lake, as well as the Troncoso fault play active roles in hosting migrating liquid. We also show that for the study period, the DITE at Laguna del Maule can be accurately evaluated by the planar Bouguer approximation, which only requires the availability of elevation changes at gravity network benchmarks. We hypothesize that this finding may be generalized to all volcanic areas with flatter or less rugged terrain and may alter interpretations based on the commonly used FAE corrections.

How to cite: Vajda, P., Zahorec, P., Miller, C. A., Le Mével, H., Papčo, J., and Camacho, A. G.: Application of deformation–induced topographic effect in interpretation of 2013–2016 spatiotemporal gravity changes at Laguna del Maule (Chile), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-467, https://doi.org/10.5194/egusphere-egu21-467, 2021.