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3. The 0.0167 Hz residual gravity time series after reduction for tidal constituents of gravity time series at the SLV station between July 22 and 7 August 2008. The dateline is given in UTC. ( a ) Residuals after correction using models DDW and FES2004. ( b ) Residuals after reduction using the new precision model (Table 14.1). The spike in the time series on 23 July in (a) and (b) was induced by the operator. ( c ) RSME for 5 min data bins of the residual gravity time series. Note the sharp increase from background levels associated with the VE on 29 July 2008 described in detail in Gottsmann et al. (2011).
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Gravimetric time series can provide vital clues about subsurface dynamics associated with active volcanism. Here, we report on continuous and campaign-style gravimetric observations on Montserrat between 2006 and 2009. More than 240 days of continuous gravimetric records enabled us to derive a first local joint solid Earth tides and ocean loading m...
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... Time-lapse gravity observations were successfully applied to monitor, e.g. subsurface water storage changes in general ( Naujoks et al., 2008;Pfeffer et al., 2013) and in karst regions Jacob et al., 2010;Cham- pollion et al., 2018), CO 2 storage changes ( Nooner et al., 2007;Wilkinson et al., 2017), or withdrawal or intrusions beneath volcanic edifices ( Jentzsch et al., 2004;Hautmann et al., 2014;Carbone et al., 2017). ...
... The order of gravity decrease of 1-2 µGal a −1 is small compared to the results of other studies that have investigated underground mass transfer, e.g. greater two-digit µGal range observed by Hautmann et al. (2014) in a volcanic environment. However, the significance of the results is given by the high precision level of our gravity results (SD ≈ 1 µGal) after LSA and error propagation in our local network, as shown in Sect. ...
We present results of sophisticated, high-precision time-lapse gravity monitoring that was conducted over 4 years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface.
Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Mainly in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes, occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore focus on the associated deformation processes.
In this study, we combine levelling and highly precise time-lapse gravity observations. Our investigation area is the urban area of Bad Frankenhausen in central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravimetry and 18 levelling campaigns were carried out in quarterly intervals within a local monitoring network. This network covers historical sinkhole areas but also areas that are considered to be stable. Our results reveal ongoing subsidence of up to 30.4 mm a⁻¹ locally, with distinct spatiotemporal variations. Furthermore, we observe a significant time-variable gravity decrease on the order of 8 µGal over 4 years at several measurement points.
In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the Global Land Data Assimilation System (GLDAS) Noah model to separate these effects from subrosion-induced gravity changes.
Our investigations demonstrate the feasibility of high-precision time-lapse gravity monitoring in urban areas for sinkhole investigations. Although the observed rates of gravity decrease of 1–2 µGal a⁻¹ are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks.
... They can nevertheless be used for tidal analyses (Bonvalot et al., 1998;Ducarme & Somerhausen, 1997;Meurers, 2012) but are useless for phenomena with a period longer than a few days. Metal spring gravimeters are more stable and have been used for continuous monitoring in a broad range of applications such as studies of active volcanoes (Branca et al., 2003;Carbone et al., 2006Gottsmann et al., 2011;Hautmann et al., 2014;Jousset et al., 2000). However, the long-term stability of spring gravimeters is usually not sufficient to resolve weak gravity signals, as those (typically smaller or equal to 100 nm/s 2 ) caused by hydrological systems. ...
... Combining gravity and deformation measurements permits discrimination between gas, water, and magma intrusion (Bagnardi et al., 2014;Bonvalot et al., 1998;Hautmann et al., 2014), assessing voids opening (Carbone, 2003;Furuya et al., 2003), magma density changes associated with degassing (Bagnardi et al., 2014;Poland & Carbone, 2016) or overturn of resident magma in a reservoir (Rymer et al., 1998). Terrestrial gravity measurements can also support the investigation of dike growth and migration, which can be induced by vertical intrusion (Gudmundsson, 1995(Gudmundsson, , 1998Wright et al., 2012) or lateral outflow of magma (Einarsson & Brandsdottir, 1978;Gudmundsson et al., 2016;Sigmundsson et al., 2014) ( Figure 10). ...
... Different fingerprints could be revealed a few minutes before two different vulcanian explosions in July and December 2008. The authors inferred important constraints for investigations of the effusive to explosive transition; gravity measurements were valuable to investigate the response of a shallow aquifer and fluid-saturated fault damage zones to stress changes during pressurization and depressurization of the plumbing system (Hautmann et al., 2014). Table 2 summarizes the different applications of the monitoring of time-varying gravity on volcanoes. ...
In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field.Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial-body and Earth attractions, to the mass in the direct vicinity of the instruments, and vertical displacement of the instrument itself on the ground.
In this paper, we review the geophysical questions that can be addressed byground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources and good practices. We also discuss the next challenges to be met by ground gravimetry,the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.
... the inversion of spatially dense (e.g., InSAR) data (Camacho et al., 2011), its application to often-sparse (both spatially and temporally) gravity data from volcanoes may not be appropriate. Indeed, when applied to gravity data from Montserrat, the inversion scheme by Camacho et al. (2011) did not furnish results that were statistically more significant than those obtained through the use of finite source models (Hautmann et al., 2014). ...
... Similarly, deployments that are too far from the centers of volcanic activity may not record changes related to magmatic processes. At Soufriére Hills Volcano, Montserrat, continuous gravity data collected 7 km from the eruptive vent were most useful for constructing tidal and ocean loading models, and gravity anomalies that were observed during occasional Vulcanian explosions were thought to represent second-order effects, triggered by either the explosion-induced air pressure wave or by the response of a local aquifer to pressure variations in the magmatic system (Gottsmann et al., 2011;Hautmann et al., 2014). In our experience, continuous gravity measurements are best utilized at volcanoes that are restless or actively erupting and where it is possible to install stations within hundreds of meters, or at most a few km, of the center of activity. ...
... Indeed, due to intrinsic limitations of the sensors (see Section 5.1), data from continuously recording spring gravimeters are often not reliable over time scales longer than a few days, implying the existence of a significant gap between the repetition rate of time-lapse measurements and the maximum period of changes observable through continuous spring gravimeters. Because of this limitation, some studies that have had access to both discrete and continuous gravity measurements from the same volcano (e.g., Bonvalot et al., 2008;Jousset et al., 2000b;Hautmann et al., 2014) could not perform a joint analysis of the two datasets and only gained insight into different processes, mostly occurring over different time scales. ...
... This suggests that stress relaxation in the rock mass underlying the Centre Hills may have occurred following a magmatically induced strain event and this, in turn, increased the permeability of the rock mass. Gravimeter measurements made in the western Centre Hills from 2006 to 2009 detected temporal changes in mass that may correspond to changes in the water table there (Hautmann et al. , 2014. ...
The 1995–present eruption of Soufrière Hills Volcano on Montserrat has produced over a cubic kilometre of andesitic magma, creating a series of lava domes that were successively destroyed, with much of their mass deposited in the sea. There have been five phases of lava extrusion to form these lava domes: November 1995–March 1998; November 1999–July 2003; August 2005–April 2007; July 2008–January 2009; and October 2009–February 2010. It has been one of the most intensively studied volcanoes in the world during this time, and there are long instrumental and observational datasets. From these have sprung major new insights concerning: the cyclicity of magma transport; low-frequency earthquakes associated with conduit magma flow; the dynamics of lateral blasts and Vulcanian explosions; the role that basalt–andesite magma mingling in the mid-crust has in powering the eruption; identification using seismic tomography of the uppermost magma reservoir at a depth of 5.5 > 7.5 km; and many others. Parallel to the research effort, there has been a consistent programme of quantitative risk assessment since 1997 that has both pioneered new methods and provided a solid evidential source for the civil authority to use in mitigating the risks to the people of Montserrat.
... These data have been used to calibrate a combined high-precision Earth tide and ocean loading model for Montserrat, and to investigate high-frequency gravity changes associated with eruptive activity at SHV. Details on explosion-related gravimetric and barometric signals were reported by Gottsmann et al. (2011), while the new tidal model and its derivation are described in detail in this Memoir (Hautmann et al. 2014). Gravimetric observations on Montserrat are being continued to further constrain subsurface dynamics over a wide frequency range by enlarging the current network of benchmarks and by the episodic installation of a network of continuously recording gravimeters. ...
Geodetic surveying is a core volcano monitoring technique. Measurements of how the crust deforms can give valuable insight into the mechanisms and processes that drive an eruption, and the way in which they change. Various geodetic observables, including ground deformation and gravity changes, have been recorded on Montserrat throughout the eruption. Instrumentation and surveying networks used to make such measurements have evolved significantly since 1995, providing increasingly accurate and robust observations. The detailed research that has been facilitated by these rich geodetic datasets has illuminated many aspects of the Soufriere Hills Volcano (SHV) and demonstrated eruptive mechanisms that are relevant to the study of other volcanoes. We have compiled a history of the geodetic study of the eruption on Montserrat, detailing the development of surveying techniques, network design and data processing since 1995. We then underline some of the key geodetic observations and review some of the most significant research that has contributed to our understanding of this volcanic system. Finally, we apply a series of typical deformation inversion models to deformation observations, and discuss the parameter sensitivity of such modelling approaches and how confidently they can be applied to identify the characteristics of the mechanisms feeding the eruption.
... This suggests that stress relaxation in the rock mass underlying the Centre Hills may have occurred following a magmatically induced strain event and this, in turn, increased the permeability of the rock mass. Gravimeter measurements made in the western Centre Hills from 2006 to 2009 detected temporal changes in mass that may correspond to changes in the water table there (Hautmann et al. , 2014. ...
The 1995 to present eruption of Soufrière Hills Volcano on Montserrat is one of the most important and best-studied eruptions of an explosive andesitic volcano. This volume presents scientific findings from the period between 2000 and 2010; it follows on from Memoir 21, which focused on the early years of activity between 1995 and 1999. In addition to descriptions and analysis of the growth, collapse and explosions associated with lava domes, there are papers on the deformation of the volcano caused by the deep magma, the petrology and geochemistry of the lavas and associated gases. Of particular note are: an overview of the insights into the deep structure of the volcano that resulted from a major international seismic tomography experiment; and an analysis of the quantitative risk assessment process that has run now for most of the eruption, the longest such continuous assessment in the world.
... This suggests that stress relaxation in the rock mass underlying the Centre Hills may have occurred following a magmatically induced strain event and this, in turn, increased the permeability of the rock mass. Gravimeter measurements made in the western Centre Hills from 2006 to 2009 detected temporal changes in mass that may correspond to changes in the water table there (Hautmann et al. , 2014. ...
... Using the software Leica GeoOffice, we obtained vertical accuracy of about 3 cm for most of the sites. [6] The recorded gravity data were corrected for solid Earth tides and ocean loading using a tidal model for Montserrat (MTY11) [Hautmann et al., 2013]. Standard techniques were applied in order to mitigate the influence of benchmark elevation (free-air effect;Figure 1b) and latitude [Moritz, 1980; Walsh, 1975]. ...
Although the persistently active Soufrière Hills Volcano (Montserrat, West Indies) is one of the most extensively studied active stratovolcanoes, a local Bouguer gravity map of the volcano and the island of Montserrat has yet to be constructed. We collected 157 new gravity data, which we analyzed and inverted in order to constrain the island's subsurface density distribution. Our model results reveal high-density material beneath the centers of the extinct volcanic complexes—presumably related to exposed dome cores—while the volcanic flanks and the active Soufrière Hills Volcano are underlain by low-density material. Volcaniclastic deposits and subsurface melt aggregations, respectively, may explain these negative gravity anomalies. Our results are in good agreement with previous structural observations from seismic tomography; however, a higher spatial density of the gravity survey network has allowed us to additionally capture smaller, shallow-seated anomalies in the gravity field that relate to tectonic structures and fluvial filling deposits.
Continuous gravity data collected near the summit eruptive vent at Kīlauea Volcano, Hawaiʻi, during 2011–2015 show a strong correlation with summit-area surface deformation and the level of the lava lake within the vent over periods of days to weeks, suggesting that changes in gravity reflect variations in volcanic activity. Joint analysis of gravity and lava level time series data indicates that over the entire time period studied, the average density of the lava within the upper tens to hundreds of meters of the summit eruptive vent remained low—approximately 1000–1500 kg/m3. The ratio of gravity change (adjusted for Earth tides and instrumental drift) to lava level change measured over 15 day windows rose gradually over the course of 2011–2015, probably reflecting either (1) a small increase in the density of lava within the eruptive vent or (2) an increase in the volume of lava within the vent due to gradual vent enlargement. Superimposed on the overall time series were transient spikes of mass change associated with inflation and deflation of Kīlauea's summit and coincident changes in lava level. The unexpectedly strong mass variations during these episodes suggest magma flux to and from the shallow magmatic system without commensurate deformation, perhaps indicating magma accumulation within, and withdrawal from, void space—a process that might not otherwise be apparent from lava level and deformation data alone. Continuous gravity data thus provide unique insights into magmatic processes, arguing for continued application of the method at other frequently active volcanoes.
Les dômes de lave sont associés à des éruptions volcaniques violentes et des indices d’explosivité élevés. L’observation et la surveillance de dômes actifs (e.g. St. Helens, Unzen, Montserrat) ont mis en évidence des modes de croissance caractérisés par des phases d’extrusion, d’explosion et des phénomènes d’effondrement, impliquant une structure interne souvent complexe de ces édifices volcaniques. L’étude du Puy de Dôme (Massif Central français), un dôme trachytique âgé de 11 000 ans, grâce à l’apport de l’imagerie géophysique et à la modélisation des données, ainsi qu’à une analyse morpho-structurale détaillée, a permis d’établir un modèle précis de la structure interne du dôme et a fourni de nouvelles contraintes concernant sa croissance et son évolution. L’analyse du Modèle Numérique de Terrain haute résolution (0,5 m) a permis d’identifier différentes unités sur le dôme, morphologiquement distinctes, et associées à des dynamismes éruptifs différents, ainsi que des structures volcano-tectoniques remarquables sur les édifices volcaniques voisins (Petit Puy de Dôme et Puy des Grosmanaux). Différentes méthodes géophysiques (tomographie des résistivités électriques – ERT -, gravimétrie et magnétisme) ont été mises en oeuvre afin d’étudier la structure interne du dôme, et de caractériser la nature des mécanismes à l’origine des zones de déformations identifiées dans l’environnement du Puy de Dôme. L’utilisation de plusieurs méthodes a permis d’étudier des paramètres physiques différents mais complémentaires, bien que l’interprétation globale des résultats géophysiques ait parfois été délicate dans le cas d’un édifice volcanique aussi complexe. Les modèles géophysiques 2D et 3D obtenus montrent que le Puy de Dôme repose sur des édifices volcaniques préexistants, un ensemble de volcans stromboliens dont la présence et/ou l’extension exacte étaient partiellement méconnues jusqu’alors. La structure interne de l’édifice, très hétérogène, est constituée d’une partie centrale très massive, entourée d’une ceinture de brèches d’effondrement, la zone sommitale du conduit étant affectée de nombreuses évidences d’une forte altération hydrothermale, caractéristique des dômes volcaniques. La partie supérieure du dôme est définie par une carapace de roches consolidées, de quelques dizaines de mètres d’épaisseur au maximum, alors que la base de l’édifice forme un talus constitué des dépôts d’effondrements gravitaires et d’écoulements pyroclastiques associés à la croissance du dôme. Enfin, les données gravimétriques et magnétiques ont permis la mise en évidence de la présence d’intrusions sous les édifices du Petit Puy de Dôme et du Puy des Grosmanaux. La géométrie de ces intrusions, déterminées grâce à différentes approches de modélisation, ainsi que la nature des roches qui les composent indiquent des processus de mise en place complexes.
