Ester Piegari | Università degli Studi di Napoli "Federico II" (original) (raw)

Papers by Ester Piegari

Near Surface 2005 - 11th European Meeting of Environmental and Engineering Geophysics, 2005

78th EAGE Conference and Exhibition 2016, 2016

Self-Potential (SP) fields are natural fields that originate from various forcing mechanisms rela... more Self-Potential (SP) fields are natural fields that originate from various forcing mechanisms related to electrical, hydraulic, chemical and thermal gradients. Due to the complexity of the source mechanisms, inversion of SP data is not easy and motivates the development of suitable techniques depending on application field, which ranges from engineering and geotechnical investigations to geothermal and mineral explorations. In this work, quantitative interpretations of self-potential data are given when SP anomaly sources can be modelled by simple polarized bodies whose parameters have to be determined. In particular, a comparative analysis is performed for the solutions of three different methods based on high-resolution spectral analysis, tomographic approach and global optimization, respectively. The efficiency of each technique has been tested by finding depth, polarization angle and shape factor of the anomaly source on synthetic data generated by simple geometrical structures (like sphere, horizontal and vertical cylinder and inclined sheet) and on field examples. The study shows limits and potentialities of the investigated methods and suggests hybrid algorithms as suitable tools for an accurate and full characterization of the anomaly source.

Bollettino di Geofisica Teorica ed Applicata

The structure of a volcanic edifice depends on composition, volume of erupted magma, and mechanis... more The structure of a volcanic edifice depends on composition, volume of erupted magma, and mechanism of eruption. In particular, pressure variations with depth due to density variations of the volcanic rocks are crucial for magma degassing processes as well as buoyancy mechanisms. In this paper, we analyze the effects of three different pressure profiles with depth on the probability of occurrence of simulated eruptive events. We describe the dynamics of magma ascent by means of a time dependent Self-Organized-Criticality field, which controls the opening of crack networks through which discrete magma batches are allowed to rise. We find a characteristic power-law behavior for the number of eruptions with erupted volume in all considered cases. As concerns the probability of occurrence of an eruptive event with a given percentage of gas losses, we find that extreme events, i.e., events with the lowest percentage of gas losses, are more likely to occur if a step function is used to des...

The area of Naples, in southern Italy, is densely populated and seriously affect... more The area of Naples, in southern Italy, is densely populated and seriously affected by a variety of environmental hazards related to meteorological conditions. The city is characterized by a complex topography therefore the meteorological risks sensibly vary throughout its territory. In the past decade, heat waves have been increasingly frequent: the production of heat maps would help the comprehension of the differential risks in town and would allow more accurate information on the related health risks to be available to the population. At present, maps of heat wave vulnerability show the spatial distribution of single factors, whereas vulnerability is governed by several competing or additive factors, so ambiguities about the identification of the most vulnerable areas currently remain. In the attempt to overcome this limit and identify areas more vulnerable to the effects of heat waves not only confined in Naples city centre, but also in suburban areas, we will apply unsupervised machine learning algorithms to integrate information from different factors and provide detailed heat wave vulnerability maps.

Physical Review B, 2000

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseud... more Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).

Physical Review B, 2002

We study the superconducting properties of a two-dimensional superconductor in the proximity to a... more We study the superconducting properties of a two-dimensional superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T=0, the critical temperature Tc, and the impurity scattering rate are characterized by a nonmonotonic behavior, with maxima occurring close to the ETT. We derive analytical expressions for the value of such maxima both in the s-wave and in the d-wave case. Such expressions are in good qualitative agreement with the phenomenological trend recently observed for Tc^max as a function of the hopping ratio t'/t across several cuprate compounds. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness eta. Instead of vanishing at the ETT, as could be expected, thus giving rise to an increase of the fluctuation effects, in the case of momentum-independent electron-electron interaction, we find eta different from 0, as a result of an integration over the whole Fermi surface.

International Journal of Modern Physics B, 2003

We review some results concerning the superconducting properties of a 2D superconductor in the pr... more We review some results concerning the superconducting properties of a 2D superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T = 0 is characterized by a nonmonotonic behavior, with maxima occurring close to the ETT, both for s- and for d-wave pairing. Such a result is in good qualitative agreement with the phenomenological trend recently observed for T cmax as a function of the hopping ratio t'/t for several cuprate compounds. We also derive a nonmonotonic dependence of the quasiparticle inverse lifetime due to impurity scattering. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness η, for which we recover an expression analogous to the case of an isotropic dispersion relation.

We study the effects of the proximity to an electronic topological transition (ETT) on the superc... more We study the effects of the proximity to an electronic topological transition (ETT) on the superconducting properties of low-dimensional materials, such as the high-T c cuprates and the κ-phase of BEDT-TTF-based organic superconductors. An ETT takes place when some external agent, such as an increase of the impurity concentration, doping, hydrostatic pressure, or anisotropic strain, modifies the topology of the Fermi surface of an electronic system. As a function of the critical parameter z, measuring the distance of the chemical potential from the ETT, we recover a non-monotonic behavior for the superconducting gap at T = 0, the critical temperature T c, and the impurity scattering rate for quasi-2D systems, regardless of the pairing symmetry of the order parameter. This has to be contrasted with the monotonic, nearly step-like z-dependence of the same quantities in the 3D case. Such a non-monotonic behavior is in agreement with the trend observed for T c as a function of pressure and other material specific quantities in several high-T c cuprate compounds. On the other hand, higher pressures than those reported in the literature (~ 10 kbar) should be investigated, in order to find evidence for ETT effects in the BEDT-TTF-based salts. We eventually comment on the effect of an ETT on the fluctuation range of anisotropic superconductors near T c.

$Research paper thumbnail of A model of volcanic magma transport by fracturing stress mechanisms$

Geophysical Research Letters, 2008

1] Understanding the mechanisms of magma ascent preceding eruptions, and in particular the subvol... more 1] Understanding the mechanisms of magma ascent preceding eruptions, and in particular the subvolcanic system that stores and transports magma to the surface, is of crucial relevance for hazard and risk assessment. We propose here a statistical model describing the rise of magma from the reservoir through the transport region via stress induced fracturing mechanisms of the medium. The model reproduces the general statistical properties of erupted volume, P(V), and inter-eruption time, P(t), found in catalogue data for closed conduit volcanoes. We also investigate conditional distribution (e.g., the probability, P(Vjt), to have a volume, V, erupted after a waiting time, t), which can have important practical implications. Citation: Piegari, E., V. Cataudella, R. Di Maio, L. Milano, M. Nicodemi, and R. Scandone , A model of volcanic magma transport by fracturing stress mechanisms, Geophys. Res. Lett., 35, L06308,

European Physical Journal B, 2005

Polaron formation is investigated in a one-dimensional chain by taking into account both the loca... more Polaron formation is investigated in a one-dimensional chain by taking into account both the local Holstein and the non-local SSH electron-phonon interactions. The study of the adiabatic regime points out that the combined effects of the two interactions are important mainly in the weak coupling regime. Thus, using the weak-coupling perturbation theory, spectral weights, effective masses, polaronic phase-diagram, and band structures are discussed. Contrarily to what happens in the Fröhlich and Holstein models, we find that the ratio between the coherent spectral weight and the mass renormalization ratio is greater than 1. Moreover, we show that the non-local electron-phonon interaction is responsible for the largest deviations of the band structure from the cosine shape of the free energy band.

Geophysical Research Letters, 2006

Landslide inventories show that the statistical distribution of the area of recorded events is we... more Landslide inventories show that the statistical distribution of the area of recorded events is well described by a power law over a range of decades. To understand these distributions, we consider a cellular automaton to model a time and position dependent factor of safety. The model is able to reproduce the complex structure of landslide distribution, as experimentally reported. In particular, we investigate the role of the rate of change of the system dynamical variables, induced by an external drive, on landslide modeling and its implications on hazard assessment. As the rate is increased, the model has a crossover from a critical regime with power-laws to non power-law behaviors. We suggest that the detection of patterns of correlated domains in monitored regions can be crucial to identify the response of the system to perturbations, i.e., for hazard assessment.

Usually, the degree of stability of a slope is quantified by the Factor of Safety whose values de... more Usually, the degree of stability of a slope is quantified by the Factor of Safety whose values depend on physical and mechanical soil properties analyzed on samples of much reduced sizes or referring to very small soil volumes around porous probes. To overcome the limit of punctual information, we propose a semi-empirical approach based on the use of geophysical methods and the employment of a geophysical Factor of Safety recently introduced by the authors in terms of local resistivities and slope angles. In this paper, we show an application of our proposal on a test area of about 2000 m2 on Sarno Mountains (Campania Region - Southern Italy), where shallow landslides involving pyroclastic soils periodically occur triggered by critical rainfall events. Starting from two resistivity tomography surveys performed on the test area in autumn and spring, we obtained maps of the geophysical Factor of Safety at different depths for the two seasons. We also estimated the values of the Factor of Safety by using the infinite slope model in the dry and saturated scenario. A comparison between the values of the geophysical and geotechnical Factor of Safety shows advantages and disadvantages of our approach.

Journal of Applied Geophysics, 2009

In this paper, we propose a new approach for modelling the development of instabilities in 11 pyr... more In this paper, we propose a new approach for modelling the development of instabilities in 11 pyroclastic covers induced by rainfall events. The approach is based on high-resolution 2D 12 electrical resistivity tomography (ERT) and on a cellular automaton model aimed to simulate 13 landslide events. A local and time-dependent Factor of Safety of a slope, that depends on electrical 14 resistivity and the slope angle, is introduced. 15

Eruptions of volcanoes are complex natural events highly variable in size and time. Over the last... more Eruptions of volcanoes are complex natural events highly variable in size and time. Over the last couple of decades, statistical analyses of erupted volume and repose time catalogues have been performed for a large number of volcanoes. The aim of such analyses is either to predict future eruptive events or to define physical models for improving our understanding of the volcanic processes that cause eruptions. In particular, for this latter purpose we study a statistical model of eruption triggering caused by the fracturing of the crust above a magma reservoir residing in the crust. When the fracturing reaches the reservoir, magma is allowed to ascend because of its buoyancy. It will be found in batches along the transport region and it will ascend as long as fractures are developed to its tip; when a path is opened to the surface, an eruption occurs involving all batches connected to the opening. We model the vertical section of a volcanic edifice by means of a two-dimensional grid and characterize the state of each cell of the grid by assigning the values of two dynamical variables: a time dependent variable e describing the status of the local stress and a time-dependent variable n describing the presence of magma. At first step of approximation, we treat the magma presence field n as a diffusing lattice gas, and, therefore, we assume its value to be either zero or one if the corresponding cell is empty or filled by magma, respectively. We study the probability distribution, P(V), of eruptions of volume V and the probability distribution, P(t), of inter-event time t and find that the model is able to reproduce, at least in a descriptive way, the essential statistical features of the activity of volcanoes. A key component of magma is the quantity of dissolved gas as it gives magma its explosive character, because the volume of gas expands as the pressure decreases on raising towards the surface. Then, to more accurately describe the rise of magma in a volcanic structure, we have introduced in our model the presence of dissolved gas in the liquid. We relax the previous approximation made on the magma presence field n and treat it not as a step function, but as a continuous function, whose values reduce when pressure decreases as magma rises towards the surface. In particular, we focus on the modeling of Mt. Somma-Vesuvius volcano and, therefore, we consider basaltic magmas.

Journal of Applied Geophysics, 2011

ABSTRACT

In the territory of the Campania region (southern Italy), critical rainfall events periodically t... more In the territory of the Campania region (southern Italy), critical rainfall events periodically trigger dangerous fast slope movements involving ashy and pyroclastic soils originated by the explosive phases of the Mt. Somma-Vesuvius volcano and deposited along the surrounding mountain ranges. In this paper, an integration of engineering-geological and geophysical measurements is presented to characterize unsaturated pyroclastic samples collected in a test area on the Sarno Mountains (Salerno and Avellino provinces, Campania region). The laboratory analyses were aimed at defining both soil water retention and electrical resistivity curves versus water content. From the matching of the experimental data, a direct relationship between electrical resistivity and matric suction is retrieved for the investigated soil horizons typical of a ash-fall pyroclastic succession. The obtained relation turns out to be helpful in characterizing soils up to close saturation, which is a critical condition for the trigger of slope failure. In such a regime, the water content and the matric suction have small variations, while electrical resistivity variations can be appreciated in a larger range of values. For this reason, besides suction measurements on very small soil volumes through classical tensiometers, our analyses suggest the direct monitoring of in-situ electrical resistivity values as an effective tool to recognise the hydrological state of larger and more representative soil volumes and to improve early warning of dangerous slope movements.

Nonlinear Processes in Geophysics, 2009

Landslides are natural hazards occurring in response to triggers of different origins, which can ... more Landslides are natural hazards occurring in response to triggers of different origins, which can act with different intensities and durations. Despite the variety of conditions that cause a landslide, the analysis of landslide inventories has shown that landslide events associated with different triggers can be characterized by the same probability distribution. We studied a cellular automaton, able to reproduce the landslide frequency-size distributions from catalogues. From the comparison between our synthetic probability distribution and the landslide area probability distribution of three landslide inventories, we estimated the typical size of a single cell of our cellular automaton model to be from 35-100 m 2 , which is important information if we are interested in monitoring a test area. To determine the probability of occurrence of a landslide of size s, we show that it is crucial to get information about the rate at which the system is approaching instability rather than the nature of the trigger. By varying such a driving rate, we find how the probability distribution changes and, in correspondence, how the size and the lifetime of the most probable events evolve. We also introduce a landslide-event magnitude scale based on the driving rate. Large values of the proposed intensity scale are related to landslide events with a fast approach to instability in a long distance of time, while small values are related to landslide events close together in time and approaching instability slowly.