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Understanding the behavior and the evolution of hydrothermal systems is of great interest for both scientific and commercial purposes, such as volcanic hazard assessment and geothermal energy exploitation. To this aim, a novel approach based on continuous magnetotelluric (MT) data is proposed for characterizing and monitoring hydrothermal systems. To test the effectiveness of the proposed approach, a sensitivity analysis has been performed by simulating different evolution scenarios of a hydrothermal system and calculating the MT response at different time intervals corresponding to different stages of the system dynamics. The study proved to be essential for understanding the degree of sensitivity of the MT method to changes of the hydrothermal system with reference to its possible temporal evolutions.

In the last decades, the magnetotelluric (MT) method has been proved to be a useful geophysical tool in different contexts, from geothermal reservoir characterization to crustal structures studies. Nevertheless, the MT method is very sensitive to the presence of noise. Indeed, as the method is based on the measurement of both the electric and magnetic components of the natural electromagnetic (EM) field, it fails when these components are affected by noises of different nature. In particular, in industrialized and urbanized context, the MT time series could be strongly affected by man-made noise and, as a consequence, the impedance tensor estimates, given by the ratio between the electric and magnetic components of the MT field, could be unreliable. To improve the reliability of these estimates, most of the proposed approaches rely on the robust evaluation of the impedance tensor as well as on the use of remote reference MT stations or on the combination of both approaches. However, these methods are not always effective. The robust methods fail when most of the data are affected by noise, giving as result a biased impedance tensor, while the remote reference approach is ineffective when the noise is correlated between reference and local MT station. In recent years, alternative procedures have been proposed to obtain reliable estimates of the MT impedance tensor. In the present work, a different approach based on the use of Discrete Wavelet Transform (DWT) and Self-Organizing Map (SOM) neural network analysis is proposed for improving the magnetotelluric impedance tensor estimates. The approach has been tested by changing type, level and window length of the noise affecting MT time series. Furthermore, in order to identify the most reliable apparent resistivity and phase values among the different impedance tensors clusters provided by the SOM analysis for each analyzed period, a selection criterion is provided and tested on synthetic and field MT data.

© 2018 The Author(s). Fumaroles are a common manifestation of volcanic activity that are associated with large emissions of gases into the atmosphere. These gases originate from the magma, and they can provide indirect and unique insights into magmatic processes. Therefore, they are extensively used to monitor and forecast eruptive activity. During their ascent, the magmatic gases interact with the rock and hydrothermal fluids, which modify their geochemical compositions. These interactions can complicate our understanding of the real volcanic dynamics and remain poorly considered. Here, we present the first complete imagery of a fumarolic plumbing system using three-dimensional electrical resistivity tomography and new acoustic noise localization. We delineate a gas reservoir that feeds the fumaroles through distinct channels. Based on this geometry, a thermodynamic model reveals that near-surface mixing between gas and condensed steam explains the distinct geochemical compositions of fumaroles that originate from the same source. Such modeling of fluid interactions will allow for the simulation of dynamic processes of magmatic degassing, which is crucial to the monitoring of volcanic unrest.

Magnetotellurics (MT) is a geophysical method based on the use of natural electromagnetic signals to define subsurface electrical resistivity structure through electromagnetic induction. MT waves are generated in the Earth’s atmosphere and magnetosphere by a range of physical processes, such as magnetic storms, micropulsations, lightning activity. Since the underground MT wave propagation is of diffusive type, the longer is the wavelength (i.e. the lower the wave frequency) the deeper will be the propagation depth. Considering the frequency band commonly used in MT prospecting (10-4 Hz to 104 Hz), the investigation depth ranges from few hundred meters to hundreds of kilometers. This means that magnetotellurics is inherently a multiscale method and, thus, appropriate for applications at different scale ranging from aquifer system characterization to petroleum and geothermal research. In this perspective, the application of the Wavelet transform to the MT data analysis could represent an excellent tool to emphasize characteristics of the MT signal at different scales. In this note, the potentiality of such an approach is studied. In particular, we show that the use of a Discrete Wavelet (DW) decomposition of measured MT time-series data allows to retrieve robust information about the subsoil resistivity over a wide range of spatial (depth) scales, spanning up to 5 orders of magnitude. Furthermore, the application of DWs to MT data analysis has proven to be a flexible tool for advanced data processing (e.g. non-linear filtering, denoising and clustering).

We present the results of a multi-methodological geophysical study of the Vesuvian area, essentially devoted to define the structural model of the volcanic apparatus, which is the base knowledge to understand the dynamic processes. In particular, 1993-95 we carried out: a dipolar geoelectric tomography along a N-S profile of about 15 km of length, from Pomigliano d'Arco to Trecase, in order to evaluate the resistivity distribution along the profile and down to the depth of about 3 km b.s.l.; a self-potential areal survey consisting of 1250 measurements, distributed over an area of about 170 km2; a magnetotelluric sounding in the Valle dell'Inferno area, which was added to a set of 11 soundings performed in previous surveys.

In P.Gasparini (Ed.): “Data related to eruptive activity, unrest phenomena and other observations on the Italian active volcanoes. Geophysical monitoring of the Italian active volcanoes 1993-1995”

We present the results of electrical and electromagnetic studies in the Etnean area during 1993-95, devoted both to volcanic hazard evaluation and to geologic and volcanic structural modeling. As it regards the first objective, we have performed on April 26, 1993, just a few days after the end of the last eruptive event started in December 1991, a further self-potential (SP) survey along a profile located at about 1600 m of height a.s.l. on the SP92 road and crossing the fracture system opened in 1989. The SP measurements was also repeated on November 28, 1995. These last SP profiles followed a group of 17 identical surveys, performed from October 23, 1989 to October 13, 1992, and constitute the experimental basis for the SP tomographic modeling. Moreover, we have carried out in 1994 a dipolar geoelectric (DG) tomography of about 2 km of length, along the same profile (1.32 km long), where we have performed, since 1989, the 19 SP surveys. The aim was essentially obtaining the electric structural scheme with which compare the results of the SP tomographic model. Finally, we present the interpretation of the couple of dipolar geoelectric (DG) and magnetotelluric (MT) soundings, performed at about 2500 m of height a.s.l. very close to the eruptive vent of the last effusive manifestation, and of 17 MT soundings carried out in the southern-eastern part of the Etnean area. These last MT measurements were aimed at studying the resistivity deep distribution in this sector of the volcano. Due to the proximity of the MT sites to known highly conductive structures, namely the sea water body and the Caltanissetta basin at the eastern and western sides of the surveyed area, respectively, we have also made a theoretical 2D modeling, in order to study the degree of distortion of the MT curves caused by the above mentioned lateral effects.

In ETNATECH, Contract EV5V-CT92-0191, Final Report

Fluids supplied by stored magma at depth are causal factors of volcanic unrest, as they can cause pressurization/heating of hydrothermal systems. However, evidence for links between hydrothermal pressurization, CO2 emission and volcano seismicity have remained elusive. Here, we use recent (2010−2020) observations at Campi Flegrei caldera (CFc) to show hydrothermal pressure, gas emission and seismicity at CFc share common source areas and well-matching temporal evolutions. We interpret the recent escalation in seismicity and surface gas emissions as caused by pressure-temperature increase at the top of a vertically elongated (0.3–2 km deep) gas front. Using mass (steam) balance considerations, we show hydrothermal pressurization is causing energy transfer from the fluids to the host rocks, ultimately triggering low magnitude earthquakes within a seismogenetic volume containing the hydrothermal system. This mechanism is probably common to other worldwide calderas in similar hydrothermal activity state., Journal of Volcanology and Geothermal Research, 414, ISSN:0377-0273

Fumaroles are a common manifestation of volcanic activity that are associated with large emissions of gases into the atmosphere. These gases originate from the magma, and they can provide indirect and unique insights into magmatic processes. Therefore, they are extensively used to monitor and forecast eruptive activity. During their ascent, the magmatic gases interact with the rock and hydrothermal fluids, which modify their geochemical compositions. These interactions can complicate our understanding of the real volcanic dynamics and remain poorly considered. Here, we present the first complete imagery of a fumarolic plumbing system using three-dimensional electrical resistivity tomography and new acoustic noise localization. We delineate a gas reservoir that feeds the fumaroles through distinct channels. Based on this geometry, a thermodynamic model reveals that near-surface mixing between gas and condensed steam explains the distinct geochemical compositions of fumaroles that originate from the same source. Such modeling of fluid interactions will allow for the simulation of dynamic processes of magmatic degassing, which is crucial to the monitoring of volcanic unrest., Scientific Reports, 8, ISSN:2045-2322

The recent signs of reawakening at Campi Flegrei caldera (Southern Italy) received a great deal of attention due to the issues related to the volcanic risk management in a densely populated area. This paper explores relations between ground deformations, seismicity and geochemical time series in the time span 2004&ndash, 2016. The aim is to unravel primary processes of unrest and the related indicators which may change in time. Data structure and interactions among variables were examined applying the clustering analysis, the correlations and the Granger causality test. The hierarchical agglomerative clustering detected two sub-periods which were further investigated. In both sub-period causal links were observed between variables while correlations did not appear and vice versa. Thus, well established formal approaches are required to study causal relations. Granger test results indicate that during 2004&ndash, 2011 the awakening unrest could be mainly ascribed to hydrothermal system pressure fluctuations, probably induced by deep-rooted fluids injection, and that ground deformation together with CO2/H2O appears the most suitable geo-indicators. The 2011&ndash, 2016 sub-period is characterized by enhanced dynamical connectivity. Granger test results suggest that the unrest is driven by a more localized and shallower thermohydromechanical engine. CO/CO2, He/CH4 and ground deformation velocity are mutually interacting appearing the most suitable geo-indicators.

Ground deformation in volcanic areas is linked to various, often interconnected processes such as magma intrusion, pressurized fluid migration, and thermal expansion effects. The presence of active and extended hydrothermal systems plays a key role and affects the deformation phenomenon in complex ways. In this study, we propose a generalized conceptual and mathematical model, which allows retrieving the flow rate of fluid injection in a volcanic hydrothermal system, assuming a ground deformation dataset as input. The basic assumption is that short‐term ground uplift episodes (with characteristic periods of less than 5 years) depend on the injection of volcanic fluids into the hydrothermal system. Then, assuming a deformation field shape independent of time and a linear time‐invariant relation between the amount of injected fluid and the resulting ground deformation, we define a Green's function as the product of spatial and temporal components. The case study is a 3D elastic model with permeability and porosity for the Campi Flegrei caldera (CFc), Italy. By Green's function, a 2 km‐long source at 2.4 km depth, which matches the InSAR deformation, is localized and the amount of injected volcanic fluid in the last 20 years of high‐frequency deformation episodes estimated. In conclusion, we find a good agreement between the measured and estimated temporal deformation patterns and, principally, that fluids injection rates can be retrieved from the deformation field at volcanoes.

Despite the magnetotelluric method (MT) is one of the most prominent geophysical technique for deep subsoil exploration, it is not yet completely reliable when applied in urban or industrialized areas due to the presence of anthropic electromagnetic noise. The latter, indeed, may severely affect the MT recordings and, as a consequence, the impedance tensor estimates, which allow to retrieve the apparent resistivity values describing the underground electrical behaviour. In this work, a new filtering approach for MT data denoising is proposed. The procedure is based on the clustering of the impedance tensor estimates by using the Self-Organizing Map (SOM) neural network model. The clustering is performed in the time-frequency domain by a discrete wavelet transformation of the MT signals. In addition, a criterion for selecting, in each wavelet scale, the clusters that lead to the most reliable apparent resistivity estimates has been suggested. The application of the proposed filtering approach to synthetic MT signals has shown that the SOM clustering is very sensitive to the presence of noise and that it is possible to get consistent apparent resistivity curves.

The inter-arrival times of the post 2000 seismicity at Campi Flegrei caldera are statistically distributed into different populations. The low inter-arrival times population represents swarm events, while the high inter-arrival times population marks background seismicity. Here, we show that the background seismicity is increasing at the same rate of (1) the ground uplift and (2) the concentration of the fumarolic gas specie more sensitive to temperature. The seismic temporal increase is strongly correlated with the results of recent simulations, modelling injection of magmatic fluids in the Campi Flegrei hydrothermal system. These concurrent variations point to a unique process of temperature-pressure increase of the hydrothermal system controlling geophysical and geochemical signals at the caldera. Our results thus show that the occurrence of background seismicity is an excellent parameter to monitor the current unrest of the caldera.

The development of denoising techniques of magnetotelluric (MT) data affected by cultural noise is currently one of the most important objective to make magnetotellurics reliably in urban or industrialized areas. In this work, a new denoising technique of MT data affected by temporally localized noise is proposed. It is based on the polarization analysis of the MT field in the time-frequency domain achieved through a discrete wavelet transform. This transform, thanks to the possibility to operate in both time and frequency domains, allows the automatic detection of transient components within the MT signal possibly due to disturbances of anthropic nature. Unlike the continuous wavelet transform, it permits to reconstruct the denoised signal in the time domain in order to test the effectiveness of the filter. Applications to both synthetic and field MT data have shown the ability of the implemented filter to detect and remove effectively the cultural noise.

Integration of high-resolution geophysical and hydrogeological investigations at Solfatara Volcano, Campi Flegrei, Italy, allowed us to (1) image the shallow and intermediate subsurface to the crater, (2) elucidate patterns in the shallow subsurface degassing, and (3) refine and upgrade volcano-monitoring strategies for this dynamic area. Our results show that the subsurface to the crater can be divided roughly into two zones: a dry, outcropping layer overlying a horizon saturated by hydrothermal fluids. Within this saturated zone, intersections of dominant NW-and ENE-striking structural lineaments act as preferential escape conduits for the fluids which generate high microseismic noise amplitudes in the southeastern part of the crater. Hydrogeological data suggest an uprising of the isotherms below Solfatara crater, and a marked increment of fluid degassing, over the last 40 years. Sudden variations of both seismic noise level and noise cycling are positively correlated with early stages of ground inflation during the AD 2000 uplift. We believe therefore that monitoring of seismic noise can be used for upgrading early warning strategies in this sector of the Campi Flegrei volcanic system. Copyright 2007 by the American Geophysical Union.

A geophysical model of the Mt. Etna volcanic area (Sicily, Italy) is outlined by an integrated analysis of gravity, geoelectrical, magnetotelluric and seismic data. New 3D tomography and visualization systems are applied in order to extract the maximum information and to define, connect and assemble structures and related physical properties. All geophysical methods concur to single out a dense, rigid and resistive structure in the central part of the study area, about 10 km long in E-W direction, about 4 km wide in N-S direction and extending from near surface down to about 30 km of depth. The resistivity pattern shows also two conductive zones on both sides of the E-W barrier, in the depth range 15-30 km. Resistivities of a few Qm and a few tens Qm are estimated in the southern and northern conductive zone, respectively. A non-uniform feeding system is thus assumed to exist, instead of the ellipsoid-like uniform magma reservoir previously deduced from regional seismological data in the depth range 15-25 km. Moreover, the comparison with seismic tomographies from local earthquakes allows a central high velocity nucleus to be distinguished inside the upper portion of the resistive barrier, which is thus ascribed to a system of compact dikes tending to become highly fractured along the western and eastern edges. Finally, the impedivity analysis gives no support to the existence of any permanent shallower magma chambers, contributing, instead, to argue the presence of a plumbing system hydrothermally altered at medium-to-low temperatures.

We present and discuss the results of an integrated electrical and electromagnetic survey in the active volcanic area of Mount Somma–Vesuvius (Naples, Italy). Dipolar geoelectrics (DG), self-potential (SP) and magnetotellurics (MT) were used to investigate the shallow and deep regions of the volcanic area. The DG apparent resistivity pseudosection along a N–S profile across the Vesuvius cone showed the existence of a largely extended conductive zone, closely in correspondence to the Somma caldera, including in the middle the top terminal part of the Vesuvius main plumbing system. The SP data, collected over the whole volcanic area, showed the existence of a W–E-directed wide band of weak positive anomalies, indicating again a conductive zone, not only including the whole Somma caldera but also extending towards the Tyrrhenian sea. A roughly N–S-trending narrow fracture system, cutting the lowest Mount Somma eastern slopes, was further evident from the SP data. A new SP tomographic inversion procedure allowed to detect a large positively charged nucleus in the depth range 600–2200 m b.g.l., located beneath the westernmost portion of a former caldera, related to the Avellino plinian eruption. The geophysical interpretation of this large positive anomaly was made using Onsager's theory of coupled electrokinetic and thermoelectric flows. The final interpretation was that the shallow, conductive central zone is very likely made up of an intensively altered and mineralised block of cemented volcanic breccia. Finally, the MT data, distributed along two perpendicular profiles, enabled us to obtain the first significant picture of the deep electrical structure of the volcano. The Bostick inversion revealed the existence of a conductive intracrustal layer, including a perched more conductive zone located roughly beneath the central-western sector of the Vesuvius apparatus.

he results of a magnetotelluric (MT) survey performed at Mt. Etna (Sicily, Italy) are presented and discussed. The MT interpretation is preceded by the description of the data managing strategy used for the estimate of the impedance tensor and the computation of a rotationally invariant parameter. The 1D Bostick inversion of MT soundings located in the Etnean central area highlights the existence of a wide conductive zone in the depth range 15-30 km. Resistivities of a fewW×m are estimated in the southern part of this zone, while resistivities one order of magnitude higher are estimated in the northern part. In the central sector, the MT soundings are characterized by much higher resistivity values suggesting the presence of an E-W directed resistive barrier separating the two conductive deep zones. A two-feeding system is thus hypothesized as an extension of a previous 3D model deduced from regional earthquakes and teleseisms in the depth range 15-25 km. Moreover, the comparison with previous shallow seismic tomographies from local earthquakes within the first 11 km of depth allows us to distinguish inside the upper portion of the resistive barrier a central high velocity zone. This zone can likely be ascribed to a slowly cooled dike tending to become highly fractured at its western and eastern edges. Finally, the impedivity analysis based on the comparison with previous geoelectric dipole soundings allows us to exclude the existence of a permanent magma chamber within the first 5 km of depth and to argue the existence of a shallowplumbing system consisting of a medium-to-low temperature hydrothermally altered environment.

The results of an analysis of Dipolar Geoelectrical (DG), Magnetotelluric (MT) and Self-Potential (SP) data collected over the emerged portion of the Campi Flegrei (CF) caldera (South Italy) are presented. The DG and MT data are from previous surveys, while the SP data have been recently collected during a survey consisting of 265 pickup land sites. Although the emerged part of the CF caldera appears as a highly inhomogeneous structure, a few simple features have been highlighted through an integrated analysis of subsets of consistent data. A well resolved feature is the structural pattern of the caldera depression along a roughly E-W profile, deduced from a 2D combined interpretation of the MT and DG soundings. Resistivity dispersion effects have also been observed at both ends of this profile. They have been ascribed to the presence of hydrothermally altered zones related to the main fracture systems bordering the caldera. A pressure/temperature source body at a mean depth of about 5 km bsl under the Bay of Pozzuoli has been inferred from the analysis of the 3D SP tomography imaging.

We present the results of self-potential, geoelectric and magnetotelluric studies in Italian active volcanic areas as essential contributions both to structural modeling and to hazard evaluation. On Mt. Etna and Mt. Somma-Vesuvius complexes structural modeling was emphasized due to a lack of global information involving the whole apparatuses, at least from the electrical point of view. Hazard investigation was, instead, investigated with high resolution techniques on the island of Vulcano, where intense unrest phenomena have long been recorded.