1,720,969 research outputs found
A comparison between single- and multi-objective optimization to fit spectral induced polarization data from laboratory measurements on alluvial sediments
No full textSpectral induced polarization measurements on unconsolidated and saturated alluvial samples, sand-clay mixtures and well sorted sandy samples, are modelled with the generalized Cole–Cole phenomenological model and two simplified models: the standard Cole–Cole and the Cole–Davidson model. The goodness of fit is evaluated, as a first step, through the root mean square error, weighted on the data errors of the real and the imaginary component. At a later stage a multi-objective optimization is proposed, based on two different indicators for the resistivity amplitude and phase misfit. The analysis of the misfits variations among all the tested parameters associations is conducted to identify the Pareto set of optimal solutions. Both procedures lead to model parameter estimates comparable with literature values. However, the multi-objective approach provides information about the uncertainty of the parameter estimates and highlights the presence of more than one characteristic value for the relaxation time and the frequency exponent in many samples, thus suggesting the possible occurrence of different polarization processes in the investigated frequency range
Complex electrical resistivity measurements on alluvial sediment samples toward sedimentological and petrographic properties estimation
No full textIn the last decades, geophysical methods have been increasingly used for groundwater exploration to gain information on geometry and properties of the subsurface. However, the knowledge on petrophysical relationships achieved with the longstanding experience in hydrocarbon exploration and available for reservoirs characterization cannot be applied in a straightforward way for near-surface environments, due to typical lower temperature, pressure and consolidation degree. Moreover hydrogeological studies have less financial support and higher interactions with human activities than hydrocarbon exploration, so that different techniques are used, e.g. surface instead of well-log acquisitions. Essentially, hydrogeophysics is based on the relationships of electrical resistivity and hydraulic conductivity with the porous medium texture. In this work the resistivity-texture relationship is investigated through laboratory measurements of complex electrical resistivity on reworked alluvial sediment samples, texturally ranging from slightly sandy mud to gravelly sand, and saturated with NaCl and CaCl2 solution with electrical conductivity varying between 20 and 1400 microS/cm. Data are collected in the frequency range between 10 mHz and 100 kHz with a spectrometer connected to a sample holder, which allow to investigate a volume of about 2500 cm3. Amplitude and phase spectra are modelled through the Cole-Cole analogue electrical circuit, in order to obtain the best-fitting parameters (direct current resistivity, chargeability, relaxation time and frequency exponent) describing conduction and polarization processes within the samples. The fit of the experimental data is performed not only with a standard least-squares approach, but also with a multiobjective approach, which considers separately the amplitude and the phase spectrum. The latter approach allows to emphasise polarization phenomena primarily linked to the amount and the dispersion degree of the silty-clayey fraction, even if phase values for sediments free from metallic particles are usually smaller than a few mrad. Moreover, the multi-objective approach permits the identification of upper and lower limits for model parameters. However, preliminary results show that the superposition of comparable effects originated from different variables (e.g. both the decrease in the water electrical conductivity and the increase in silt and clay content yield an increase of polarization) prevents one-to-one correlations among the model parameters and the sediments’ litho-textural properties; a multivariate statistical analysis could be a valid support in the interpretation of experimental data
Laboratory electrical measurements on sediment samples and relation with textural parameters
No full textRelating electrical conduction of alluvial sediments to textural properties and pore-fluid conductivity
No full textElectrical conductivity of alluvial sediments depends on litho-textural properties, fluid saturation and porewater conductivity. Therefore, for hydrostratigraphic applications of direct current resistivity methods in porous sedimentary aquifers, it can be useful to characterize the prevailing mechanisms of electrical conduction (electrolytic or shale conduction) according to the litho-textural properties and to the porewater characteristics. An experimental device and a measurement protocol were developed and applied to collect data on eight samples of alluvial sediments from the Po plain (Northern Italy), characterized by different grain-size distribution, and fully saturated with porewater of variable conductivity. The bulk electrical conductivities obtained with the laboratory tests were interpreted with a classical two-component model, which requires the identification of the intrinsic conductivity of clay particles and the effective porosity for each sample, and with a three-component model. The latter is based on the two endmember mechanisms, surface and electrolytic conduction, but takes into account also the interaction between dissolved ions in the pores and the fluid-grain interface. The experimental data and their interpretation with the phenomenological models show that the volumetric ratio between coarse and fine grains is a simple but effective parameter to determine the electrical behaviour of clastic hydrofacies at the scale of the representative elementary volume
The application of hydrogeophysics to study water-based ecosystem services in alluvial plains
No full textRelating bulk electrical conduction to litho-textural properties and pore-fluid conductivity within porous alluvial aquifers
No full textEstimation of sediment texture from spectral induced polarisation data using cluster and principal component analysis
No full textSpectral induced polarisation data are usually interpreted with simple models in order to derive petrophysical relationships between electrical and sedimentological properties, such as texture, clay content, and permeability. The aim of this work is to explore the value of spectral induced polarisation in addition to conventional direct-current resistivity measurements for determining textural properties of saturated samples collected from alluvial deposits. For this, an advanced data processing approach that combines cluster and principal component analysis was developed and applied to integral parameters derived from Debye decomposition of spectral induced polarisation data. This data processing procedure allowed identifying groups of samples with a similar spectral induced polarisation response and to derive a characteristic grain-size distribution for each group of samples.The method to estimate the grain-size distribution from spectral induced polarisation data was successfully validated using independent sediment samples. The remaining uncertainty in the estimation of sediment texture from spectral induced polarisation data was attributed to the effect of pore size distribution and mineralogy, which were not considered in the present work but can be added in the future within the same conceptual workflow
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
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