Agata Siniscalchi

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.

Flank instability structure of Mt. Etna inferred by a magnetotelluric survey

This paper presents a magnetotelluric (MT) survey of the unstable eastern flank of Mt. Etna. We take thirty soundings along two profiles oriented in the N-S and NW-SE directions, and from these data recover two 2D resistivity models of the subsurface. Both models reveal three major layers in a resistive-conductive-resistive sequence, the deepest extending to 14 km bsl. The shallow layer corresponds to the volcanic cover, and the intermediate conductive layer corresponds to underlying sediments segmented by faults. These two electrical units are cut by E-W-striking faults. The third layer (basement) is interpreted as mainly pertinent to the Apennine-Maghrebian Chain associated with SW-NE-striking regional faults. The detailed shapes of the resistivity profiles clearly show that the NE Rift is shallow-rooted ( 0–1 km bsl), thus presumably fed by lateral dikes from the central volcano conduit. The NW-SE profile suggests by a series of listric faults reaching up to 3 km bsl, then becoming almost horizontal. Toward the SE, the resistive basement dramatically dips (from 3 km to 10 km bsl), in correspondence with the Timpe Fault System. Several high-conductivity zones close to the main faults suggest the presence of hydrothermal activity and fluid circulation that could enhance flank instability. Our results provide new findings about the geometry of the unstable Etna flank and its relation to faults and subsurface structures.

Sinkhole evolution in the Apulian Karst of Southern Italy: a case study, with some considerations on Sinkhole Hazards

Sinkholes are the main karst landforms characterizing the Salento Peninsula, which is the southernmost part of the Apulia region of southern Italy. They occur both as evolving recent phenomena and old or relict features testifying to ancient phases of karst processes acting in the area. Most of the sinkholes were formed by karst processes that may be reactivated, a risk to the anthropogenic structures nearby. To highlight such a subtle hazard, an area located a few kilometers from Lecce, the main town in Salento, was the subject of geological, morphological, and geophysical investigations. Historical analysis of multi-year aerial photographs, in particular, allowed identification of several phases in the recent evolution of a particular sinkhole, and demonstrated the need to carefully evaluate the likely evolution of similar features in Salento.

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.

Comment on “Deep resistivity cross section of the intraplate Atlas Mountains (NW Africa): New evidence of anomalous mantle and related Quaternary volcanism”

Citation: Jones, A. G., D. Kiyan, J. Fullea, J. Ledo, P. Queralt, A. Marcuello, A. Siniscalchi, and G. Romano (2012), Commenton “Deep resistivity cross section of the intraplate Atlas Mountains (NW Africa): New evidence of anomalous mantle and relatedQuaternary volcanism,” Tectonics, 31, TC5011, doi:10.1029/2011TC003051.

Experimental evidence of resistivity frequency-dispersion in magnetotellurics in the Newberry (Oregon), Snake River Plain (Idaho) and Campi Flegrei (Italy) volcano-geothermal areas

Abstract We show that magnetotelluric measurements carried out in volcano-geothermal areas can be significantly affected by the resistivity frequency-dispersion phenomenon. The polarization effects are recognized by comparing magnetotelluric and DC geoelectrical deep soundings performed in the same station. Indeed, the conventional separate analysis of the two data sets sometimes gives unconformable results, especially as it regards the resistivity and thickness of the conductive layers, ascribable to the geothermal fluid-filled rocks. These apparent discrepancies can be overcome by admitting the occurrence of intense induced polarization effects inside the rocks having extensive alterations due to hydrothermal paragenesis. The basic theory of the induced polarization effects over magnetotelluric measurements is at first briefly recalled. Then, three impressive field examples taken from previous experiments performed in the Newberry Volcano, Snake River Plain and Campi Flegrei geothermal areas are presented and discussed.

A method to determine the magnetotelluric static shift from DC resistivity measurements in practice

Many efforts have been made to face magnetotelluric (MT) static shift. Impedance tensor analyses give insight to the presence of this feature and allow the determination of some parameters described by the MT distortion matrix. A quantitative determination of the full distortion matrix is, however, still difficult and needs additional measurements. In addition to MT, other electric and electromagnetic methods also are effected by static shift. Using direct current resistivity techniques, e.g., we can determine the static-shift factors in a simpler way because the sources can be controlled. Generally, because the distortion matrix has four entries, four additional quantities have to be determined to describe the static shift completely. They can be achieved, e.g., through measuring two orthogonal electric field components for two orthogonal source configurations. The source electrode spacing, however, has to be sufficiently large to resemble horizontal currents and match the MT plane-wave analog. The procedure at hand extracts the static-shift factors from multielectrode measurements after this condition is met. For the sake of simplicity and demonstration purposes, only inline measurements orthogonal to the strike direction of a 2D model are considered so that the vectorial problem reduces to a scalar one. This procedure is applied to a MT field data set in a regional 2D environment that shows only two additional quantities are necessary to determine the static shift.

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.

Dynamics of internal and external origin revealed by a single-site magnetotelluric monitoring

A continuously operating magnetotelluric (MT) system was installed in Agri Valley (Southern Italy) by the Institute of Methodologies for the Environmental Analysis (IMAA) for studying the temporal stability of the electric properties of the subsoil. In a previous work, Balasco et al. (2008) analysed 1-year-long dataset evidencing possible source effects. Such effects are deeply investigated in this work considering almost three years monitoring. Two different types of dynamics are revealed: i) a seasonal fluctuation at short sounding periods, and ii) a behaviour linked to the external geomagnetic activity index at high periods.

QUANTIFYING PERSISTENT BEHAVIOR IN EARTH'S APPARENT RESISTIVITY TIME SERIES

The multiple segmenting method (MSM) has been applied to investigate the scaling behaviour in the Earths apparent resistivity time series, measured in a seismic area of southern Italy. The study of apparent resistivity represents one of the most important scientific challenges in the studies devoted to the geophysical monitoring. Our results show that apparent resistivity is characterized by a persistent scaling behaviour at all the periods considered, with the scaling exponent tending approximately to 0.5.