Abstract
Gravity changes associated with volcanic processes occur over a wide range of time scales, from minutes to years and with magnitudes between a few and a few hundred microGal. High-precision instruments are needed to detect such small signals and both time-lapse surveys along networks of stations, and continuous measurements at single points, are accomplished. Continuous volcano gravimetry is mostly carried out through relative gravimeters, either superconducting instruments, providing higher quality data, or the more widely used spring meters. On the other hand, time-lapse surveys can be carried out with relative (spring) gravimeters, that measure gravity differences between pairs of stations, or by absolute gravimeters, capable of measuring the absolute value of the gravitational acceleration at the observation point. Here we present the state-of-the-art of terrestrial gravity measurements to monitor and study active volcanoes and the possibilities of new gravimeters that are under development. In particular, we present data from a mini array of three iGrav superconducting gravimeters (SGs) at Mount Etna (the first network of SGs ever installed on an active volcano). A comparison between continuous gravity measurements recorded through the iGrav#016 superconducting gravimeter at Serra La Nave station (1730 m a.s.l.) and absolute gravity data collected with the Microg LaCoste FG5#238 gravimeter in the framework of repeated campaigns is also presented. Furthermore, we introduce the Horizon 2020 NEWTON-g project (New Tools for Terrain Gravimetry), funded under the FET-OPEN Research and Innovation Actions call, Work Programme 2016–2017 (Grant Agreement No 801221). In the framework of this project, we aim to develop a field-compatible gravity imager, including an array of low-costs Micro-Electro-Mechanical Systems (MEMS)-based relative gravimeters, anchored on an absolute quantum gravimeter. After the design and production phases, the gravity imager will be field-tested at Mt. Etna (Italy) during the last 2 years of the project.
You have full access to this open access chapter, Download conference paper PDF
Similar content being viewed by others
Keywords
1 Introduction
High-precision gravity measurements provide a powerful tool for volcano monitoring, since they highlight processes that induce bulk mass/density changes. This technique thus provides an important contribution to the understanding of the processes driving magma ascent during the preparatory phases of eruptive events.
In spite of its potential, volcano gravimetry is not widely adopted by research institutions and agencies in charge of monitoring active volcanoes. This depends on several factors, including: (1) the cost of instrumentation, (2) problems that are inherent with the use and deployment of instruments intended for laboratory conditions in the harsh environments that characterize the summit zones of most active volcanoes, and (3) difficulty in interpreting gravity changes that may result from volcanic processes, as well as hydrological effects (changes in the undergroung water mass), and instrumental artefacts. These difficulties are not insurmountable and efforts in a variety of volcanic settings highlight the value of time-variable gravimetry for understanding volcanic hazards, as well as revealing fundamental insight into the volcano dynamics (Carbone et al. 2017).
The Istituito Nazionale di Geofisica e Vulcanologia (INGV) has operated a relative gravity network for the monitoring of Etna volcano since 1986 (Budetta et al. 1989). The network has been developed and has evolved over the years and currently it consists of: (a) 71 benchmarks, covering an area of about 400 km2, for relative gravity campaigns (LaCoste & Romberg model D and Scintrex CG-3M/CG-5 gravimeters were used over time); (b) three continuously running gravity stations equipped with iGravs superconductive gravimeters by GWR Instruments, Inc.; (c) 14 stations for absolute gravity (AG) measurements using the Microg LaCoste FG5#238. In all stations for AG measurements and in some stations for relative measurements, the vertical gravity gradient is also measured.
The whole Etna gravity network is routinely occupied every summer (in the same season of the year to minimize seasonal variation effects). More frequent measurements are carried out through relative spring gravimeters in same elements of the network (almost monthly along the East-West profile; Fig. 1; Carbone et al. 2009; Greco et al. 2010; Del Negro et al. 2013; Bonforte et al. 2017) or during volcano activities. Since 2007, several absolute field surveys have been routinely carried out at Etna volcano to check the long term gravity variations and to confirm the gravity changes obtained by relative measurements (Pistorio et al. 2011; Greco et al. 2012, 2015). To collect the absolute gravity data, the IMGC-02 absolute gravimeter (D’Agostino et al. 2008), developed by Istituto Nazionale di Ricerca Metrologica (INRiM, Italy), was used in the period 2007–2009. Since 2009 up to date the Micro-g FG5#238 absolute gravimeter is used.
In this paper, we present data collected with SGs and absolute gravimeters at Mt. Etna. Furthermore, we introduce the field-compatible gravity imager, that will be developed under the NEWTON-g project, including an array of low-costs MEMS-based relative gravimeters, anchored on an absolute quantum gravimeter. This measuring system will provide continuous imaging of gravity changes associated with variations in subsurface fluid properties, with unparalleled spatio-temporal resolution and, consequently, with fundamental implications for risk management. We will deploy the new gravity imager at Etna volcano (Italy), where frequent gravity fluctuations, easy access to the active structures and the presence of a multiparameter monitoring system ensure an excellent natural laboratory for testing the new tools.
2 The Mini-Array of Three iGrav SGs at Mount Etna
Continuous gravity measurements have been successfully carried out at a number of volcanoes around the world using spring gravimeters. Nevertheless, these instruments do not provide reliable continuous data over intervals of weeks or more, because they are influenced by environmental factors and are subject to instrumental drift. Accordingly, most studies of continuous gravity at active volcanoes have focused on the analysis of changes over time-scales of minutes to a few days (Branca et al. 2003; Carbone et al. 2006, 2013, 2019).
An alternative to spring gravimeters for continuous measurements is given by superconducting gravimeters (SGs) that feature a much higher precision and stability than spring gravimeters. SGs are free from instrumental effects and thus allow to track even small gravity changes (1–2 μ Gal) over a wide range of time scales (minutes to months). However, even the most portable SGs (e.g., the iGrav® by GWR) are not ideal for installation in the vicinity of active volcanic structures. Indeed, they require AC power at the installation site and some kind of hut or vault to house the instrumentation (Carbone et al. 2019).
At Mt. Etna, the installation of a mini-array of three SGs (distances of 3.5–15.5 km from the active craters; Fig. 1) began in September 2014, when the iGrav#16 was installed in the facilities of the Serra La Nave (SLN) Astrophysical Observatory (1,730 m above sea level (asl); ~6.5 km from the summit craters; Fig. 1). In the summer of 2016, iGrav#20 and iGrav#25 were also installed at La Montagnola hut (MNT; 2,600 m asl; ~3.5 km SE of the summit craters; Fig. 1) and in the facilities of INGV – OE located in the village of Nicolosi (NIC; 720 m asl; ~15 km SE of the summit craters; Fig. 1), respectively.
To our knowledge, these are the first SGs ever installed on an active volcano.
Signals from these instruments have shown hydrologi-cally-induced components superimposed on gravity changes that are related to volcanic processes (Carbone et al. 2019).
Figure 2 shows the signals acquired in the period 23 July–05 September 2016 at NIC (top; iGrav#25), SLN (middle; iGrav#16) and MNT (bottom; iGrav#20) stations, after the corrections for the effect of Earth tides (Wenzel 1996), local atmospheric pressure variations (Merriam 1992), and polar motion (Hinderer et al. 2015), are accomplished.
The reduced signals show common anomalies over intervals of 10–15 days. In particular, a gravity decrease is observed in the signals from SLN and MNT stations during August 8–18. The MNT/SLN amplitude ratio during this period points to the activation of a mass source located a few kilometers below sea level.
Figure 2 also shows positive variations on time scales of the order of a few hours on the signal acquired at MNT (red arrows). Since these variations (average amplitude equal to about 2 microGal) are not present on the SLN and NIC signals and occur during heavy rainfall, they are most probably due to local hydrological effects.
3 Comparison Between the Time Series from a SG and Absolute Gravity Data
Figure 3 shows a 56-month long time series from iGrav#16 SG (at SLN station), after removing the effect of Earth Tides, atmospheric pressure and polar motion. Figure 3 also shows absolute gravity observations (red points) at the same station (the total error on AG measurements at this station is typically ± 3 microGal).
Once a linear fit, obtained through AG data, is removed, there is a good fit between the two data sets. Continuous gravity changes are within 1–2 microGal of gravity changes measured using AG at the same site. This comparison allows to estimate the long-term drift of the iGrav SG, which is of the order of 9 microGal/year (Fig. 3).
This example shows how repeated AG measurements and continuous gravity observations can be used together to get a fuller and more accurate picture of the temporal characteristics of the studied processes.
4 New Tools for Terrain Gravimetry
Gravimetry is a powerful geophysical tool that, through sensing changes in subsurface mass, can supply unique information on the dynamics of underground fluids, like water, magma, hydrocarbons, etc. This is critically important for both resource management and risk reduction. In spite of this potential, gravimetry is currently underexploited. Indeed, continuous gravity measurements have been rarely performed, owing to the fact that the available instruments are not well suited for continuous measurements under severe field conditions. In addition, the high cost of available gravimeters limits deployment to, at most, a few sensors in a given area, implying that the achievable spatial resolution is always lower than needed.
To overcome the current limits of terrain gravimetry, the NEWTON-g project, that received funding from the European Commission’s Horizon 2020 programme, under the FET-OPEN 2016–2017 call, proposes the development of a new generation of gravimeters, based on MEMS and quantum technologies (www.newton-g.eu). MEMS technology will make possible the production of a device that is two orders of magnitude cheaper and lighter than currently available gravimeters. In the framework of NEWTON-g, one goal is focused on strategies to increase the stability of ordinary MEMS accelerometers, thus making them capable of measuring, beside inertial, also gravitational acceleration, with a sensitivity of a few tens of microGal/hz1/2 (Middlemiss et al. 2016).
The quantum gravimeter that will be developed under NEWTON-g will be the ideal complement to the MEMS devices. We expect that the new quantum gravimeter would: (a) measure the absolute value of the gravity acceleration (absolute instrument), with a sensitivity and a stability at the μGal level; (b) perform continuous measurements at a high rate (2 Hz) over a long time interval (several years).
A network of low-cost MEMS gravimeters, anchored to the absolute quantum device, will form the so-called gravity imager, which will provide imaging of sub-surface mass changes with unparalleled spatio-temporal resolution (Fig. 4).
Once developed, the gravity imager will be field-tested at Etna volcano (Italy) during the last 2 years of the NEWTON-g project (summer 2020 – summer 2022). Insights from the gravity imager will also be used for volcanic hazards analysis, to demonstrate the importance of using gravity to face problems of societal relevance. A successful implementation of NEWTON-g will open new doors and represents a fundamental step that can move gravimetry into a cornerstone resource for geophysical monitoring and research.
5 Conclusions
Many geophysical phenomena are associated with mass transport and may induce gravity changes measurable at the surface over a wide range of time scales. Gravimetry can thus supply unique information on mass transport within the Earth system.
Through the experiments accomplished at Etna during the past few decades many steps forward have been made towards the regular acquisition of high-quality gravity data. The main issues with continuous gravity measurements at active volcanoes is to obtain and maintain data to a suitable standard (as for quality and continuity) against an adverse environment (high altitude, inaccessibility for several months, lack of mains electricity for power, variable temperature, pressure and humidity, seismicity, corrosive gases) and using instruments which are intended for use under laboratory conditions.
The combined use of discrete (absolute and relative) and continuous gravity measurements is a unique tool both for studying the internal dynamic of a volcano and for surveillance purposes. Such combined system has already allowed important conclusions to be drawn at Mt. Etna.
The state-of-the-art instruments are, in most cases, heavy, expensive, power-hungry and difficult to operate. This prevents the installation of dense networks of continuously running gravimeters, implying that suitable spatio-temporal resolution cannot be attained.
Less expensive and easier to deploy instruments will open new horizons for terrain gravimetry. In particular, the gravity imager that will be developed and field-tested under the NEWTON-g project will demonstrate the possibilities of a new generation of MEMS and quantum gravimeters, allowing a step that can turn gravimetry into a cornerstone resource for volcano monitoring and hazard assessment.
References
Bonforte A, Fanizza G, Greco F, Matera A, Sulpizio R (2017) Long-term dynamics across a volcanic rift: 21 years of microgravity and GPS observations on the southern flank of Mt. Etna volcano. J Volcanol Geotherm Res 344:174–184. https://doi.org/10.1016/j.jvolgeores.2017.06.005
Branca S, Carbone D, Greco F (2003) Intrusive mechanism of the 2002 NE - rift eruption at Mt. Etna (Italy) inferred through continuous microgravity data and volcanological evidences. Geophys Res Lett 30(20):L06305. https://doi.org/10.1029/2003GL018250
Budetta G, Grimaldi M, Luongo G (1989) Variazioni di gravità nell’area etnea (1986-1989). Boll GNV 5:137–146
Carbone D, Zuccarello L, Saccorotti G, Greco F (2006) Analysis of simultaneous gravity and tremor anomalies observed during the 2002 – 2003 Etna eruption. Earth Planet Sci Lett 245(3–4):616–629. https://doi.org/10.1016/j.epsl.2006.03.055
Carbone D, D’Amico S, Musumeci C, Greco F (2009) Comparison between the 1994–2006 seismic and gravity data from Mt. Etna: new insight into the long-term behavior of a complex volcano. Earth Planet Sci Lett 279:282–292. https://doi.org/10.1016/j.epsl.2009.01.007
Carbone D, Poland MP, Patrick MR, Orr TR (2013) Continuous gravity measurements reveal a low-density lava lake at Kīlauea volcano, Hawai‘i. Earth Planet Sci Lett 376:178–185. https://doi.org/10.1016/j.epsl.2013.06.024
Carbone D, Poland MP, Diament M, Greco F (2017) The added value of time - variable microgravimetry to the understanding of how volcanoes work. Earth Sci Rev 169:146–179. https://doi.org/10.1016/j.earscirev.2017.04.014
Carbone D, Cannavò F, Greco F, Reineman R, Warburton RJ (2019) The benefits of using a network of superconducting gravimeters to monitor and study active volcanoes. J Geophys Res Solid Earth 123:4035–4050. https://doi.org/10.1029/2018JB017204
D’Agostino G, Desogus S, Germak A, Origlia C, Quagliotti D, Berrino G, Corrado G, Ricciardi G (2008) The new IMGC-02 transportable absolute gravimeter: measurement apparatus and applications in geophysics and volcanology. Ann Geophys 51(1):39–49
Del Negro C, Currenti G, Solaro G, Greco F, Pepe A, Napoli R, Pepe S, Casu F, Sansosti E (2013) Capturing the fingerprint of Etna volcano activity in gravity and satellite radar data. Sci Rep 3:3089. https://doi.org/10.1038/srep03089
Greco F, Currenti G, Del Negro C, Napoli R, Budetta G, Fedi M, Boschi E (2010) Spatio-temporal gravity variations to look deep into the southern flank of Etna volcano. J Geophys Res 115(B):11411. https://doi.org/10.1029/2009JB006835
Greco F, Currenti G, D’Agostino G, Germak A, Napoli R, Pistorio A, Del Negro C (2012) Combining relative and absolute gravity measurements to enhance volcano monitoring at Mt Etna (Italy). Bull Volcanol 74:1745–1756. https://doi.org/10.1007/s00445-012-0630-0
Greco F, Biolcati E, Pistorio A, D’Agostino G, Germak A, Origlia C, Del Negro C (2015) Absolute gravity measurements at three sites characterized by different environmental conditions using two portable ballistic gravimeters. Eur Phys J Plus 130:38. https://doi.org/10.1140/epjp/i2015-15038-0
Hinderer J, Crossley D, Warburton RJ (2015) Superconducting gravimetry. In: Schubert G (ed) Treatise on geophysics, 2nd edn. Elsevier, Oxford, pp 59–115. https://doi.org/10.1016/B978-0-444-53802-4.00062-2
Merriam JB (1992) Atmospheric pressure and gravity. Geophys J Int 109(3):488–500. https://doi.org/10.1111/j.1365-246X.1992.tb00112.x
Middlemiss RP, Samarelli A, Paul DJ, Hough J, Rowan S, Hammond GD (2016) Measurement of the earth tides with a MEMS gravimeter. Nature 531:614–617
Pistorio A, Greco F, Currenti G, Napoli R, Sicali A, Del Negro C, Fortuna L (2011) High precision gravity measurements using absolute and relative gravimeters at Mount Etna (Sicily, Italy). Ann Geophys 54:5. https://doi.org/10.4401/ag-5348
Wenzel HG (1996) The nanogal software: earth tide data processing package ETERNA 3.30. Bull d'Informations des Marées Terrestres 124:9425–9439
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Rights and permissions
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Copyright information
© 2020 The Author(s)
About this paper
Cite this paper
Greco, F., Carbone, D., Cannavò, F., Messina, A.A., Siligato, G. (2020). Absolute and Relative Gravity Measurements at Volcanoes: Current State and New Developments Under the NEWTON-g Project. In: Freymueller, J.T., Sánchez, L. (eds) 5th Symposium on Terrestrial Gravimetry: Static and Mobile Measurements (TG-SMM 2019). International Association of Geodesy Symposia, vol 153. Springer, Cham. https://doi.org/10.1007/1345_2020_126
Download citation
DOI: https://doi.org/10.1007/1345_2020_126
Published:
Publisher Name: Springer, Cham
Print ISBN: 978-3-031-25901-2
Online ISBN: 978-3-031-25902-9
eBook Packages: Earth and Environmental ScienceEarth and Environmental Science (R0)