Regularized solution of a nonlinear problem in electromagnetic sounding

We propose a regularization method to solve a nonlinear ill-posed problem connected to inversion of data gathered by a ground conductivity meter

Common-reflection-surface imaging of shallow and ultrashallow reflectors

We analyzed the feasibility of the common-reflection-surface (CRS) stack for near-surface surveys as an alternative to the conventional common midpoint (CMP) stacking procedure. The data-driven, less user-interactive CRS method could be more cost efficient for shallow surveys, where the high sensitivity to velocity analysis makes data processing a critical step. We compared the results for two field data sets collected to image shallow and ultrashallow reflectors: an example of shallow Pwave reflection for targets in the first few hundred meters, and an example of SH-wave reflection for targets in the first 10 m. By processing the shallow P-wave records using the CMP method, we imaged several nearly horizontal reflectors with onsets from 60 to about 250 ms. The CRS stack produced a stacked section more suited for a subsurface interpretation, without any preliminary formal and time-consuming velocity analysis, because the imaged reflectors possessed greater coherency and lateral continuity. With CMP processing of the SHwave records, we imaged a dipping bedrock interface below four horizontal reflectors in unconsolidated, very low velocity sediments. The vertical and lateral resolution was very high, despite the very shallow depth: the image showed the pinchout of two layers at less than 10 m depth. The numerous traces used by the CRS stack improved the continuity of the shallowest reflector, but the deepest overburden reflectors appear unresolved, with not well-imaged pinchouts. Using the kinematic wavefield attributes determined for each stacking operation, we retrieved velocity fields fitting the stacking velocities we had estimated in the CMP processing. The use of CRS stack could be a significant step ahead to increase the acceptance of the seismic reflection method as a routine investigation method in shallow and ultrashallow seismics

Three essays on households' financial decisions

Nel primo capitolo della tesi, ho analizzato empiricamente la relazione tra risparmio precauzionale e vincoli di liquidità in Italia usando dati panel sulle famiglie. Ho utilizzato un indicatore soggettivo della varianza delle crescita del reddito, che ha permesso di misurare il movente precauzionale al risparmio, e una varietà di indicatori dei vincoli di liquidità. Utilizzando l’approccio della switching regression endogena, ho visto come il movente precauzionale al risparmio sia più forte per i nuclei familiari che devono affrontare i vincoli vincolante o prevedano che questi vincoli siano vincolanti per il futuro. Sussiste infatti un rapporto di complementarità tra risparmio precauzionale vincoli di liquidità effettivi e attesi. Inoltre, l'introduzione di una misura soggettiva di avversione al rischio consente una migliore identificazione del coefficiente associato alla misura soggettiva della varianza della crescita del reddito. Nel secondo capitolo, basandomi su una domanda circa la quantità desiderata di ricchezza precauzionale presente nell’ Indagine sulla ricchezza delle famiglie italiane, mi propongo di analizzare le principali determinanti del movente precauzionale al risparmio. In particolare, mi propongo di fornire una valutazione empirica del legame esistente tra la composizione del portafoglio delle famiglie e la quantità di ricchezza che le famiglie desiderano avere per proteggersi da eventi inattesi. I risultati mostrano come sussista una forte correlazione negativa tra la quantità desiderata di ricchezza precauzionale e il fatto di detenere un portafoglio composto esclusivamente da attività non rischiose. Tuttavia, le famiglie non sembrano utilizzare diversificazione del portafoglio per ridurre l'esposizione al rischio totale. Infine, è stato affontato il problema della complementarità/sostituzione tra assicurazione “formale” ed “informale”. In realtà, la fiducia nel mercato dei capitali in modo da ridurre notevolmente la quantità di ricchezza delle famiglie desiderano detenere per motivi precauzionali. Tuttavia, non vi è alcuna prova a favore di un legame forte tra il negativo e il risparmio precauzionale e le assicurazioni. Nel terzo capitolo, ho analizzato il rapporto tra il livello iniziale della ricchezza delle famiglie e la probabilità di iniziare un’attività imprenditoriale in Italia e negli Stati Uniti, utilizzando i dati dell’Indagine sulla ricchezza delle famiglie italiane (SHIW) e la Panel Survey of Income Dynamics statunitense (PSID). Sono state formulate diverse previsioni teoriche, che sono state poi confrontate con i dati a disposizione. In primo luogo si è sostenuto che il livello iniziale di ricchezza iniziale debba contare di più per i potenziali imprenditori italiani, che potrebbero incontrare maggiori difficoltà rispetto a quelli statunitensi nell’ ottenere un finanziamento da una banca o da una società finanziaria per avviare un’attività imprenditoriale. Da questo punto di vista, "i mercati informali" (es. gli aiuti da amici o parenti) dovrebbero svolgere un ruolo più significativo in Italia per i potenziali imprenditori, e specialmente per coloro che hanno più probabilità di essere razionati. In secondo luogo, un mercato finanziario ben sviluppato, riducendo l'esposizione delle famiglie al rischio finanziario, dovrebbe avere un impatto positivo sulla probabilità di diventare imprenditore. Pertanto, è stata colmata una lacuna nella letteratura introducendo un indice di diversificazione del portafoglio, calcolato come l'inverso dell'indice Herfindhal, al fine di valutare il livello di sofisticazione finanziaria. Infine, è stata stimata contemporaneamente la probabilità di diventare imprenditore e le variazioni della ricchezza netta. Utilizzando un modello di switching regression endogeno è stato possibile affrontare i problemi di endogeneità dovuti al fatto che le famiglie possono effettivamente accumulare ricchezza al fine di iniziare un’attività imprenditoriale.In the first chapter of the present thesis, I empirically investigate precautionary savings under liquidity constraints in Italy using household panel data. I exploit a unique indicator of subjective variance of income growth, which allows to measure the strength of the precautionary motive for saving, and a variety of survey-based indicators of liquidity constraints. Using an endogenous switching regression approach, I eventually found the precautionary motive for savings to be stronger for those households who face binding constraints, or expect constraints to be binding in the future. Indeed, a complementarity relation exists between precautionary savings and effective and expected liquidity constraints. Moreover, the introduction of a survey-based measure of risk aversion allows a better identification of the coefficient associated with the subjective measure of variance of income growth. In the second chapter, relying on a direct question about the desired amount of precautionary wealth from the 2002 wave of the Italian Survey of Household Income and Wealth, I assess the main determinants of precautionary motive for saving. In particular, I provide an empirical assessment of the linkage existing between the composition of households' portfolio and the amount of wealth households wish to have to protect themselves against unexpected contingencies. As expected, a strong and negative correlation exists between the desired amount of precautionary wealth and the ownership of a portfolio made exclusively of safe assets. However, households do not seem to use portfolio diversification to reduce exposure to total risk. Finally, I address the issue of complementarity vs. substitution between �formal� and �informal� insurance schemes. Actually, trust in capital market would lower substantially the amount of wealth households wish to detain for precautionary reasons. However, there is no evidence in favour of a negative and strong linkage between precautionary saving and insurance. In the third chapter, I assess the relationship between initial household net wealth and the probability of switching to entrepreneurship in Italy and the United States, using household-level data from the Survey of Household Income and Wealth (SHIW) and the Panel Survey of Income Dynamics (PSID). I formulated several theoretical predictions, which are then compared with the data at hand. First of all, I argue that initial wealth should matter more for potential Italian entrepreneurs, who may encounter greater difficulties than their US counterparts in obtaining sufficient funds from a bank or financial institution to start a business. From this perspective, "informal markets" (i.e. help from friends or relatives) should play a more significant role for potential entrepreneurs in Italy, especially for those who are more likely to be constrained. Secondly, I claim that a well developed financial market, by reducing household exposure to financial risk, would positively affect transition into entrepreneurship. Therefore, I fill a gap in the literature introducing a portfolio diversification index, calculated as the inverse of the Herfindhal index, in order to assess the level of financial sophistication. Last but not least, I simultaneously estimate the probability of switching to entrepreneurship and changes in net wealth. Using a sample selection model with endogenous switching makes it possible to deal with endogeneity issues, related to the fact that households may actually accumulate assets prior to setting up a business

The Role of Financial Intermediaries in Securities Issues: A Theoretical Analysis

We consider a model of securities issues where the quality of securities is private information to the issuer, and firms of higher quality are more reluctant to issue securities than low quality firms. We show that, when the issuer directly trades with investors, market breakdown may occur. This is caused by the issuer's attempts to signal his type through the offering price. Things change if we introduce a financial intermediary which: i) underwrites the issue, ii) influences the offering price.Underwriting creates a wedge between the interests of the intermediary and those of the issuer, which allows trade with investors to be restored. A by-product of this conflict of interest is that trade is characterized by underpricing. Another implication is that the intermediary may act as a reliable screening device when she possesses private information about the firm's quality. In general, our analysis suggests that collusion between the intermediary and the issuer hinders trade, whereas collusion between the intermediary and investors may promote it.Signaling, Financial Intermediaries, Securities Issues, Underwriting.

Dynamic attentional modulation of vision across space and time after right hemisphere stroke and in ageing

This article is available open access and is shared under a Creative Commons licence (http://creativecommons.org/licenses/by/3.0/). Copyright @ 2012 Elsevier Ltd.Introduction - Attention modulates the availability of sensory information to conscious perception. In particular, there is evidence of pathological, spatial constriction of the effective field of vision in patients with right hemisphere damage when a central task exhausts available attentional capacity. In the current study we first examined whether this constriction might be modulated across both space and time in right hemisphere stroke patients without neglect. Then we tested healthy elderly people to determine whether non-pathological ageing also leads to spatiotemporal impairments of vision under conditions of high attention load. Methods - Right hemisphere stroke patients completed a task at fixation while attempting to discriminate letters appearing in the periphery. Attentional load of the central task was modulated by increasing task difficulty. Peripheral letters appeared simultaneously with the central task or at different times (stimulus onset asynchronies, SOAs) after it. In a second study healthy elderly volunteers were tested with a modified version of this paradigm. Results - Under conditions of high attention load right hemisphere stroke patients have a reduced effective visual field, over a significantly extended ‘attentional blink’, worse for items presented to their left. In the second study, older participants were unable to discriminate otherwise salient items across the visual field (left or right) when their attention capacity was loaded on the central task. This deficit extended temporally, with peripheral discrimination ability not returning to normal for up to 450 msec. Conclusions - Dynamically tying up attention resources on a task at fixation can have profound effects in patient populations and in normal ageing. These results demonstrate that items can escape conscious detection across space and time, and can thereby impact significantly on visual perception in these groups.The European Commission, Brunel University and the Wellcome Trust

Rainfall downscaling in a space-time multifractal framework

Publication History: Issue published online: 9 JUL 2010 Article first published online: 9 JUL 2010 Manuscript Accepted: 17 FEB 2000 Manuscript Received: 18 AUG 1999A space-time multifractal analysis on radar rainfall sequences selected from the Global Atmospheric Research Program Atlantic Tropical Experiment database is presented. It is shown that space-time rainfall can be considered with a good approximation to be a self-similar multifractal process, so that a multifractal analysis can be carried out assuming Taylor's hypothesis to hold for rainfall over a wide range of spatial and temporal scales. The advection velocity needed to rescale the time dimension is estimated using different tracking techniques. On each selected rainfall sequence, a very good scaling is observed for spatial scales ranging from 4 to 256 km and for timescales from 15 min to 16 hours. A recently developed scale-covariant multifractal model is then reformulated for numerical simulation of space-time rainfall fields. The two parameters of the log-Poisson distribution used as cascade generator within the model are systematically estimated from each selected rainfall sequence, and the dependence of one of these parameters on the large-scale rain rate is highlighted. The model is then applied to disaggregate large-scale rainfall, and some comparisons between synthetically downscaled and observed rainfall are discussed.1779–1794Pubblicat

a multiple threshold method for fitting the generalized pareto distribution to rainfall time series

Abstract. Previous studies indicate the generalized Pareto distribution (GPD) as a suitable distribution function to reliably describe the exceedances of daily rainfall records above a proper optimum threshold, which should be selected as small as possible to retain the largest sample while assuring an acceptable fitting. Such an optimum threshold may differ from site to site, affecting consequently not only the GPD scale parameter, but also the probability of threshold exceedance. Thus a first objective of this paper is to derive some expressions to parameterize a simple threshold-invariant three-parameter distribution function which assures a perfect overlapping with the GPD fitted on the exceedances over any threshold larger than the optimum one. Since the proposed distribution does not depend on the local thresholds adopted for fitting the GPD, it is expected to reflect the on-site climatic signature and thus appears particularly suitable for hydrological applications and regional analyses. A second objective is to develop and test the Multiple Threshold Method (MTM) to infer the parameters of interest by using exceedances over a wide range of thresholds applying again the concept of parameters threshold-invariance. We show the ability of the MTM in fitting historical daily rainfall time series recorded with different resolutions and with a significative percentage of heavily quantized data. Finally, we prove the supremacy of the MTM fit against the standard single threshold fit, often adopted for partial duration series, by evaluating and comparing the performances on Monte Carlo samples drawn by GPDs with different shape and scale parameters and different discretizations

Infectious diseases and autoimmunity

Introduction: Autoimmunity occurs when the immune system recognizes and attacks host tissue. In addition to genetic factors, environmental triggers (in particular viruses, bacteria and other infectious pathogens) are thought to play a major role in the development of autoimmune diseases. Methodology: We searched PubMed, Cochrane, and Scopus without time limits for relevant articles. Results: In this review, we (i) describe the ways in which an infectious agent can initiate or exacerbate autoimmunity; (ii) discuss the evidence linking certain infectious agents to autoimmune diseases in humans; and (iii) describe the animal models used to study the link between infection and autoimmunity. Conclusions: Besides genetic predisposition to autoimmunity, viral and bacterial infections are known to be involved in the initiation and promotion of autoimmune diseases. These studies suggest that pathogens can trigger autoimmunity through molecular mimicry and their adjuvant effects during initiation of disease, and can promote autoimmune responses through bystander activation or epitope spreading via inflammation and/or superantigens.</br

Interaction between economic and financial development

This paper presents a possible explanation of the interactive nature of the relationship between economic and financial development based on absorption of resources by the financial sector, and constant returns to physical capital accumulation in the production sector. Financial intermediaries operating in a credit market characterised by monopolistic competition emerge along with the process of economic development. This could initially have a detrimental effect on growth, so that the economy might be trapped in a low development region. If not, subsequent economic development stimulates competition among financial intermediaries which results in more efficient financial transactions, and therefore higher growth. While higher efficiency is always associated with higher growth, the laissez faire economy can still be characterised by sub-optimal levels of financial development