Z. Petrillo

Electric and electromagnetic outline of the Mount Somma-Vesuvius structural setting

Abstract 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 Onsagers 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.

Imaging 2D structures by the CSAMT method: application to the Pantano di S. Gregorio Magno faulted basin (Southern Italy)

A controlled source audiofrequency magnetotelluric (CSAMT) survey has been undertaken in the Pantano di San Gregorio Magno faulted basin, an earthquake prone area of Southern Apennines in Italy. A dataset from 11 soundings, distributed along a nearly N-S 780 m long profile, was acquired in the basins easternmost area, where the fewest data are available as to the faulting shallow features. A preliminary skew analysis allowed a prevailing 2D nature of the dataset to be ascertained. Then, using a single-site multi-frequency approach, Dantzigs simplex algorithm was introduced for the first time to estimate the CSAMT decomposition parameters. The simplex algorithm, freely available online, proved to be fast and efficient. By this approach, the TM and TE mode field diagrams were obtained and a N35°W ± 10° 2D strike mean direction was estimated along the profile, in substantial agreement with the fault traces within the basin. A 2D inversion of the apparent resistivity and phase curves at seven almost noise-free sites distributed along the central portion of the profile was finally elaborated, reinforced by a sensitivity analysis, which allowed the best resolved portion of the model to be imaged from the first few meters of depth down to a mean depth of 300 m b.g.l. From the inverted section, the following features have been outlined: (i) a cover layer with resistivity in the range 3–30 Ω m ascribed to the Quaternary lacustrine clayey deposits filling the basin, down to an average depth of about 35 m b.g.l., underlain by a structure with resistivity over 50 Ω m up to about 600 Ω m, ascribed to the Mesozoic carbonate bedrock; (ii) a system of two normal faults within the carbonate basement, extending down to the maximum best resolved depth of the order of 300 m b.g.l.; (iii) two wedge-shaped domains separating the opposite blocks of the faults with resistivity ranging between 30 Ω m and 50 Ω m and horizontal extent of the order of some tens of metres, likely filled with lacustrine sediments and embedded fine gravels.

Clues on the origin of post-2000 earthquakes at Campi Flegrei caldera (Italy)

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.

A Geophysical Study of the Mount Etna Volcanic Area

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.

Denoising Magnetotelluric Recordings Using Self-Organizing Maps

A novel approach for processing magnetotelluric data in urban areas is presented. The magnetotelluric (MT) method is a valid technique for geophysical exploration of the Earth’s interiors. It provides information about the rocks’ resistivity and in particular, in volcanology, it allows to delineate the complex structure of volcanoes possibly detecting magmatic chambers and hydrothermal systems. Indeed, geological fluids (e.g. magma) are characterized by resistivity of many orders of magnitude lower than the surrounding rocks. However, the MT method requires the presence of natural electromagnetic fields. So in urban areas, the noise strongly influences the MT recordings, especially that produced by trains. Various denoising techniques have been proposed, but it is not always easy to identify the noise-free intervals. Thus, in this work a neural method, the Self-Organizing Map (SOM), is proposed to perform the clustering of impedance tensors, computed on a Discrete Wavelet (DW) expansion of MT recordings. The use of the DW transform is motivated by the need of analyzing MT recordings both in time and frequency domain. The SOM is principally tested on synthetic dataset. Then, as a further validation of the method, it is applied on real data recorded at volcano Etna, Sicily. In both cases, the obtained results have shown the SOM capability of greatly reducing the effect of the noise on the retrieved apparent resistivity curves.

Anatomy of a fumarolic system inferred from a multiphysics approach

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.

Modelling Coulomb Stress Changes due to Fluid Injection and Withdrawal - A Step to Understand Induced Seismicity

Fluid injection and withdrawal in deep wells is a basic procedure in a series of mining and deep resources exploitation, i.e. oil and gas extraction, geothermal exploitation and EGS permeability enhancement. All these activities have the potential to indu