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Modeling and inversion of self-potential for hydrogeological and environmental applications
Full text linkI potenziali spontanei sono potenziali elettrici quasi-statici di origine naturale
generati da sorgenti distribuite nel sottosuolo. Le componenti principali sono il
potenziale elettrochimico legato alla presenza del gradiente del potenziale redox
e il potenziale elettrocinetico prodotto dal flusso dell’acqua che circola nei mezzi
porosi.
Al fine di riprodurre tali segnali di potenziale spontaneo in mezzi saturi eterogenei
e anisotropi caratterizzati da geometrie complesse, l’attività di ricerca è
stata incentrata sull’implementazione di un algoritmo in Matlab. La soluzione del
problema diretto 2-D è stata ottenuta a partire da un modello di pressione o potenziale
redox e dalla distribuzione di resistività elettrica tramite la formulazione
numerica dell’equazione di Poisson. L’algoritmo agli elementi finiti sviluppato si è
dimostrato essere stabile e accurato mediante procedure di simulazione rispetto a
casi di studio sintetici e reali riportati in letteratura. Nel dettaglio, si è verificato
che questo approccio può essere utilizzato per produrre modelli 2-D per l’individuazione
e caratterizzazione dei flussi di infiltrazione d’acqua (seepage) attraverso
i corpi-diga, per l’interpretazione di prove di emungimento e per l’identificazione
e localizzazione di cavità isolate/forme carsiche superficiali esistenti, le quali
costituiscono naturali vie di convogliamento delle acque meteoriche incidenti e
corrivanti sulla superficie topografica nel sottosuolo.
Per approfondire lo studio del potenziale elettrico generato dalla presenza di
corpi metallici sepolti e validare l’algoritmo presentato per l’inversione dei dati
di potenziale spontaneo, è stato effettuata una prova sperimentale. Sono state
eseguite misure 2-D di potenziale e resistività elettrica su un modello fisico di laboratorio
capace di simulare un acquifero, favorire le reazioni di ossido-riduzione
e lo sviluppo progressivo dell’anomalia negativa in corrispondenza di un elemento
in ferro a seguito del processo di corrosione. Test preliminari di laboratorio hanno
messo in luce i principali problemi della campagna di misura provocati dalla
tipologia di sabbia utilizzata e dagli errori di misura sugli elettrodi. L’apparato
sperimentale, dopo opportune modifiche e la minimizzazione dei suddetti errori,
ha fornito misure stabili e ripetibili. I valori di potenziale spontaneo acquisiti
in profondità e quelli di conducibilità elettrica sono stati forniti all’algoritmo di
inversione basato sul metodo LSQR e utilizzati per ricostruire fedelmente la distribuzione
della densità di corrente esterna, nonchè per riprodurre il modello di
"geo-batteria". Inoltre, simulazioni numeriche eseguite su un sintetico di laboratorio
e su dati di campagna acquisiti nell’ambito di un’indagine condotta per
la localizzazione di sinkhole incipienti nella piana di San Vittorino (RI), hanno
fornito dei modelli finali con valori limitati di errore e una buona ricostruzione
delle anomalie sia in termini di forma che posizione.
L’intero processing geofisico (elaborazione dei dati, modellazione e inversione)
è stato incluso all’interno di un codice in Matlab interattivo, il quale consente di
lavorare sia su dati ricavati da indagini di campagna che su quelli ottenuti da
sperimentazioni in laboratorio grazie alla flessibilità e affidabilità raggiunta dalla
modellazione e dall’inversione, con l’obiettivo primario di estendere tale approccio
risolutivo alla più ampia casistica di applicazioni del metodo del potenziale spontaneo
dal punto di vista geologico, in ambito idrogeologico, in campo ambientale
e ingegneristico.The self-potential method responds to chemical potential gradients of charges
carriers creating polarization in the porous media. Of specific interest are the
streaming potential due to fluid flow and the "electro-redox" effect associated
with redox potential gradients.
To evaluate the self-potential distribution resulting from all these charge polarisation
mechanisms and enable the study of variably saturated flow problems of
complicated geometry, a two-dimensional finite element algorithm is implemented
in Matlab. This forward algorithm explicitly calculates the distribution of electric
current sources and solves for the self-potential given a model of pressure and/or
redox potential and prescribed values of electrical resistivity. The numerical procedure
is verified to be accurate and stable through a comparison with synthetic
simulations. The capacity for producing 2-D reconstruction of transient or steady
state flow models in anisotropic, heterogeneous and water-saturated porous materials
is illustrated throught the model of a leak in a dam, the case of a pumping
test in an unconfined aquifer and the problem of coupled hydroelectric problem
associated with the presence of a cavity.
A new laboratory apparatus is presented to explain the physical and chemical
mechanisms that generate self-potential anomalies in the vicinity of buried
metallic objects and to validate a linear inversion algorithm. The controlled laboratory
experiment involves a metallic body buried with vertical orientation and
centered through and in the capillary fringe within a sandbox. Preliminary tests
were performed to choose a right sand through a performance comparison among
different materials and to evaluate approaches to remove electrode effects and
provide adequate corrections. Large dipolar self-potential anomalies developed
in association with the progressive corrosion of the vertical body. The inverse
problem is solved by accounting for the electrical conductivity distribution and
the self-potential data in order to recover the source current density vector field
using a 2-D LSQR finite-element modeling approach. This experiment indicates
that passively recorded electrical signals can be used to nonintrusively monitor
corrosion processes, lending support to the geobattery model over organic contaminant
plumes. Furthermore, synthetic and real simulations, performed on
heterogeneous models with anomalies included, provide low-error inverted models
whereas anomalies are well-detected both for shape and position. In particular
the inversion algorithm was applied to a field data set collected in the San Vittorino
Plain, located in Central Italy, in order to identify the location of sinkholes
and investigate the effects of different resistivity structure assumptions on the SP
inversion results.
The whole geophysical process (data processing, forward modelling and inversion)
has been included through an interactive Matlab algorithm divided in
sections, where the operator can act to change. The algorithm is suitable for laboratory
and field investigations, because of its high flexibility and reliability now
reached by forward and inversion routines, with the primary aim of extending the
approach to engineering and environmental applications
Three-dimensional electrical and seismic tomography for assessing the state of conservation of a masonry wall
No full textGeophysical non-destructive testing has been applied worldwide during last decades for understanding the inner
geometry, the constructive materials and the degree of conservation of ancient buildings affected by the ravages of time,
human interventions or natural phenomena (e.g. Polymenakos et al. 2005; Tsokas et al., 2013).
The integrated approach presented in this work encompasses the use of Electrical Resistivity Tomography (ERT) and
seismic tomography aiming to assess the current state of conservation of a Roman masonry building, named "Casa di
Diana", located at the Ostia Antica archaeological site (Rome, Italy).
Three-dimensional ERT and seismic tomography investigations were focused on an inner wall, made in opus
caementicium, prone to both rising damp and cracking phenomena, in order to reconstruct a 3D model where anomalous
zones would be highlighted. ERT dataset were inverted with the VEMI interface (De Donno & Cardarelli, 2015), while
seismic tomography inversion was performed by means of the algorithm after Cardarelli & Cerreto (2002).
Results show that low resistivity and P-wave velocity values can be associated to the presence of the inner mortar,
while higher values of both parameters were observed for the existence of the outer brick component. Overall, with
reference to a previous work where a small-scale sample of a Roman masonry wall was analysed in the laboratory using
seismic tomography (Cardarelli & de Nardis, 1999), the brick part seems to be in good conditions (optimal VP=1400 m/s,
recovered VP=1300 m/s), while the low velocity values of the mortar (optimal VP=500 m/s, recovered VP=250 m/s) can
potentially represent an anomaly due to degradation phenomena. Therefore this approach can be employed to reconstruct
a 3D model of an archaeological wall in order to plan the recovery actions
Assessing the State of Conservation of a Masonry Building through the Combined Use of Electrical and Seismic Tomography
No full textAn integrated geophysical approach encompassing electrical resistivity and seismic tomography
investigations is presented in this work, for assessing the current state of conservation of a Roman
masonry building located in the Ostia Antica archaeological site (Rome, Italy), affected by rising damp
and cracking phenomena.
A preliminary three-dimensional electrical survey aims to detect the foundations and understand the range
of variation of the resistivity at the study site. High-Resolution electrical and seismic tomography lines
were later acquired on an inner wall, prone to both rising damp and cracking. The combined use of
electrical and seismic tomography allows us to obtain a high-resolution map of the wall made in opus
caementicium, where low resistivity and P-wave velocity values can be associated to the presence of the
inner mortar, while higher values are probably due to the outer brick component. Overall, with reference to
a previous laboratory work on a Roman masonry sample analysed using seismic tomography, we rate the
brick part in good conditions while the low velocity values of the mortar can potentially represent an
anomaly due to degradation processes. This approach can be employed to reconstruct a three-dimensional
model of the surveyed wall in order to plan recovery actions
Geophysical and geochemical techniques to assess the origin of rising damp of a Roman building (Ostia Antica archaeological site)
No full textA combined approach based on geophysical and geochemical analyses was carried out to determine the sources of rising damp in “Casa di Diana”, a Roman building located in the Ostia Antica archaeological site (Rome, Italy). The studied building is characterized by high humidity values (closed to saturation, > 90% RH) with significant efflorescence and sub-efflorescence phenomena on masonry walls.
Electrical Resistivity Tomography was performed both inside and outside the building to understand the geological setting, to estimate the freshwater-salt water interface, to locate building foundations and consequently, to evaluate the interaction between the structural elements and groundwater. On the other hand, local shallow groundwater was analysed for soluble salts and isotopic analysis (δ18O) to identify the origin of solutes, to isolate the main factors determining the isotopic content and to investigate the aquifer behaviour in terms of movement and distribution processes. The water samples were collected from one well and one tank sited inside and outside the house, respectively.
The geophysical inverted models, were able to detect a freshwater aquifer at the shallower depths (2–5 m) in correspondence to the Roman foundation walls (extended at depth up to 8 m), while below (8–10 m from surface) the salt water presence is likely. The chemical analysis confirmed that groundwater is slightly mineralized even though the salt concentration can be compatible with an interaction with sea salts, maybe due to the depth of the foundation walls.
Therefore, the proposed approach is suitable for understanding the causal relationship between the observed phenomena, assessing the current degree of conservation of the archaeological site and consequently planning the recovery actions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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