1,720,972 research outputs found
Correlation between changes in water storage and deformation transients in karst systems: new insights from the European Alps
No full textWe describe and discuss a hydrological transient deformation signal detected in geodetic measurements using a blind source separation algorithm. Space geodetic methods (GPS and SAR) have demonstrated their ability in measuring ground deformation associated with precipitation and groundwater redistribution at different spatial and temporal scales. We considered the results of a variational Bayesian Independent Component Analysis applied to the ground displacements time-series obtained from the analysis of continuous GPS stations operating in the Eastern Southern Alps and northern Dinarides. This analysis highlighted both annual common mode component signals and a time-variable, non-cyclic signal characterized by a spatially variable response in the horizontal components. This latter signal has larger amplitudes in three karst areas of the study region. The GPS stations respond to this signal by moving in opposite direction, reversing the sense of movement in time, implying a succession of extensional/compressional strains, with variable amplitudes through time, oriented normal to rock fractures in karst areas. Here we focus on the Val Belluna (i.e., Piave river basin) and the pedemountain front of the Venetian Southern Alps, and tested the hypothesis that the succession of horizontal extensions and contractions recorded by the GPS stations are caused by the variation of the water storage in the hydrological basin of this area. In a hydrological basin it is possible to define the storage variation as the difference between the total precipitation and the sum of real evapotranspiration and river discharge. In this work we used different hydrological models that are able, once calibrated on existing data, to estimate the actual evapotranspiration from potential evapotranspiration data. In addition, these models can describe the behavior of the basin also in the past, when no river-flows data were available. The only input data needed are precipitation and potential evapotranspiration and the model itself calculates the discharge, the storage and the actual evapotranspiration values. We found an excellent agreement between the temporal evolution of the water storage and the ten-year-long temporal evolution of the transient geodetic deformation signal. Then, this result provides new insights on the relation between water redistribution and ground displacements. We also found that the horizontal deformation can be explained by pressure changes associated with variable water levels within vertical fractures in the vadose zones of karst systems
Sand volcano generated by a violent degassing from methane-saturated aquifers: the case study of Medolla (Modena, Italy).
No full textSand volcano is an unusual and remarkable geological feature which forms when water-saturated sand deposits are set in motion by liquefaction and are ejected onto the surface. Commonly it is generated during earthquakes, as a result of liquefaction ofwaterlogged bodies at shallow depth, often causing wide damages. The generation of a sand volcano however can also occur unrelated to a seismic event. On the 10th of October 2014, near Medolla (Italy) during a Cone Penetration Test, a large amount of natural gas (CO2 and CH4) together with a mixture of water and sandwere erupted, creating a sand volcano. The study of this event gives the possibility of understanding
the dynamics of sand volcano generation, may enable to prevent other anomalies during future CPT tests and, more importantly, underlines the role that natural gas, stored in a sand aquifer,may play in triggering a liquefaction phenomenon. Our results suggest that episodes of gas eruptions require the onset of very peculiar conditions within the reservoir that feeds the emission. The simulations suggest that a geyser discharging a mixture of gas and water, capable of building a sand volcano, requires the presence of a shallowpressurized reservoir (1.2 MPa) where water coexists with a small amount of exsolved gas (a volume fraction of 0.05).The violent degassing occurred in Medolla confirms the role that a free gas phase may have in favoring the mobilization of liquid water
and loose deposits, even in the absence of a seismic event
Ground heating and methane oxidation processes at shallow depth in Terre Calde di Medolla (Italy): Numerical modelling
No full textAbstract The area known as Terre Calde (literally “hot lands”) in the plain of the Po River (Italy) is well known
for unusual ground temperatures, and up to now, the cause o/f the heating has not been fully investigated.
These higher-than-average temperatures are commonly associated with diffuse methane seepage. A detailed
study of shallow stratigraphy, temperature profile, and associated gas concentrations and flow rates recently
suggested that the observed anomaly could be related to the exothermic oxidation of biogenic methane,
possibly rising from a shallow peat layer. In this work, a porous media flow simulator (Transport of Unsaturated
Groundwater and Heat 2) was applied to verify a conceptual model of this phenomenon. The model
describes a layered system, with a shallow unsaturated zone, where methane is continuously supplied
along the base and heat is generated as a result of its oxidation above the water table. To mimic the
oxidation process, heat sources are placed within the layer where oxidation takes place, and the heat
generation is computed as a function of methane flux entering the layer. Numerical simulations were
carried out imposing different methane flow rates along the base of the model. The simulations also
explored the efficiency of methane oxidation, considering different heat generation rates and accounting
for seasonal effects. The good match between observed and simulated temperature profiles suggests that
the main features of the process are captured by the model and that the conceptual model devised on
the base of available data is plausible from a physical point of view
Hydrological deformation signals in karst systems: new evidence from the European Alps
Full text linkThe influence of rainfall on crustal deformation has been described at local scales, using tilt and strain meters, in
several tectonic settings. However, the literature on the spatial extent of rainfall-induced deformation is still scarce. We analyzed 10 years of displacement time-series from ~150 continuous GPS stations operating across the broad zone of deformation accommodating the N-S Adria-Eurasia convergence and the E-ward escape of the Eastern Alps toward the Pannonian basin. We applied a blind-source-separation algorithm based on a variational Bayesian Independent Component Analysis method to the de-trended time-series, being able to characterize the temporal and spatial features of several deformation signals. The most important ones are a common mode annual signal, with spatially uniform response in the vertical and horizontal components and a time-variable, non-cyclic, signal characterized by a spatially variable response in the horizontal components, with stations moving (up to ~8 mm) in the opposite directions, reversing the sense of movement in time. This implies a succession of extensional/compressional strains, with variable amplitudes through time, oriented normal to rock fractures in karst areas. While seasonal displacements in the vertical component (with an average amplitude of ~4 mm over the study area) are satisfactorily reproduced by surface hydrological loading, estimated from global assimilation models, the non seasonal signal is associated withgroundwater flow in karst systems, and is mainly influencing the horizontal component. The temporal evolution of this deformation signal is correlated with cumulated precipitation values over periods of ~200-300 days. This horizontal deformation can be explained by pressure changes associated with variable water levels within vertical fractures in the vadose zones of karst systems, and the water level changes required to open or close these fractures are consistent with the fluctuations of precipitation and with the dynamics of karst systems
Vertical ground displacement at Campi Flegrei (Italy) in the fifth century: Rapid subsidence driven by pore pressure drop
No full textCampi Flegrei (Italy) caldera has experienced episodes of ground deformation throughout its geological history, alternating between uplift and subsidence phases. Although uplift periods are typically more alarming, here we focus on subsidence, looking for its driving mechanisms and its role in the caldera evolution. Historical and archaeological records constrain ground deformation over the last two millennia. Here we revise such records and combine them with published radiometric dating and with the simulation of sea level change. The resulting analysis highlights for the first time a rapid subsidence during the fifth century. We show that rate and magnitude of this subsidence are consistent with the compaction of porous material caused by a pressure drop of ~ 1 MPa within the hydrothermal system. We interpret this event as the decompression of the hydrothermal system following an unrecognized episode of unrest, during Roman times. These findings redefine the pattern of ground deformation and bear important implications for volcanic hazard assessment
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
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