1,720,988 research outputs found
Global inversion of electrical resistivity tomography data for cavity detection
No full textThis work aims to apply a well-known global inversion method such as the VSFA (Very Fast Simulated Annealing) for the inversion of ERT data for cavity detection. We focus our study on cavity detection, which is a problem where ERT is known to be diagnostic, due to the high resistive contrast with the background medium. Despite of that, local inversion methods are often inaccurate in the assessment of the actual position, shape and resistivity values of the target, due to the smoothness constraints applied and the loss of resolution with depth. Global inversion provides further insight into the problem with relevant information especially on the uncertainty assessment of the inverted profiles
Coupling ERT and SRT data through cross-gradient joint inversion and clustering on structured meshes incorporating topography
No full textThe joint inversion of electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) is a common practice to achieve a more accurate characterization of subsurface features, since these methods are sensitive to different properties of the subsurface. We propose an approach for integrating ERT and SRT, which involves a cross-gradient joint inversion on structured meshes even in cases with complex topography and a post-inversion procedure where a new cross-gradient index and fuzzy c-means analysis are used to assess the reliability of the joint procedure and facilitate the interpretation of the results. Our strategy does not introduce non-strictly necessary elements within the joint inversion procedure for dealing with a non-flat tomography, which can accentuate the illposedness of the inverse problem. The proposed method was applied to both synthetic and a field cases located in Central Italy, where an accurate geophysical reconstruction is needed for rehabilitation of an existing dam and boreholes are available to validate the geophysical survey. For all cases, joint inversion consistently yielded superior results compared to individual inversion, and the post-inversion tools facilitated the assessment of the impact of the joint scheme and allowed for a quantitative reconstruction (position and shape) of the main units at the site
Cross-gradient joint inversion and clustering of ERT and SRT data on structured meshes incorporating topography
Full text linkThe combination of electrical resistivity and seismic refraction tomography is a common practice for the characterization of subsurface features. Presently, the cross-gradient inversion scheme stands out as one of the most robust joint approaches, and some authors modified it to manage complex topographies on unstructured meshes even if at the expense of introducing additional parameters in the inversion process. We propose in this work a cross-gradient algorithm for jointly inverting electrical and seismic tomographic data on structured meshes in cases with non-flat topography. The proposed approach preserves the benefit of the classical cross-gradient approach without the need to impose physical length scales, as for irregular meshes. The quality of the results is evaluated in comparison with independent inversion through a new standardized cross-gradient index and a fuzzy c-means analysis that provides an assessment of the reconstruction accuracy through the membership function. The proposed method was applied to both synthetic models and field-scale examples located in Central Italy, where an accurate geophysical reconstruction is needed for the rehabilitation of existing dams. For all cases, joint inversion yielded superior results compared to independent inversion, demonstrating better agreement with available borehole data. The effectiveness of the joint approach was also demonstrated by the post-inversion tools, where the new cross-gradient index highlighted changes in structural similarity whilst fuzzy c-means clustering allowed for a quantitative reconstruction (position and shape) of the main units at the sites, facilitating the detection of site layering modifications
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
Characterization of a coastal aquifer prone to saline intrusion through integration of electrical and seismic data
No full textIn this work, we present an integration of electrical and seismic data for the characterization of a coastal aquifer prone to saline intrusion. The main goal is to achieve a near-surface reconstruction of the unconsolidated lithotypes (mainly sands) and to image lateral transition of the hydrogeological parameters as saturation or permeability. To this end we select two areas within the Circeo National Park (Central Italy), where we combine electrical resistivity tomography (ERT) and time-domain induced polarization (TDIP) and multichannel analysis of surface waves (MASW) at site A and ERT and seismic refraction tomography (SRT) at site B. Through inversion of ERT/TDIP data for Cole-Cole parameters at site A, we achieve a firstapproximation prediction of the permeability, to be used for rapid hydro-geophysical screening of the coastal areas. On the other hand, the joint inversion of ERT and SRT data at site B allows to discern a lateral transition likely related to a variation of the groundwater level
Fuzzy Mapping of Integrated Multiparameter Tomographic Data to Detect and Localize Geophysical Targets
No full textIn this work, we present an integrated and quantitative approach to detect and localize geophysical targets associated with both geological and anthropogenic complex scenarios. We complement electrical and seismic tomographic techniques and a machine-learning (ML) unsupervised algorithm [fuzzy C-means (FCM)] to get a final clustered section validated by borehole and well data, where the reliability is evaluated by the membership function. This is a good estimator of the reliability of the proposed procedure, as it ranges from 0 to 1, with one reflecting a high reliability of the clustering analysis. This method is applied to two case studies, related to the detection of leachate in a municipal solid waste landfill and the detection of the bedrock surface in a site prone to instability. For both cases, we also set up synthetic simulations by reproducing realistic models with similar layering and ranges of geophysical parameters as per the field cases. The results show the effectiveness of the method in providing a unique detection of the cluster associated with the desired targets, as highlighted by the matching with direct information. However, the accuracy of the reconstruction is reduced in areas where the resolution of geophysical methods is lower. We demonstrate that the proposed method can achieve a reliable reconstruction if it focuses on searching for one desired target only, rather than a comprehensive reconstruction of the whole layering at the site. This approach can be a valuable automatic tool for improving the benefit-to-cost ratio of projects, where new constructions or remediation interventions have to be planned
Detecting gas upwelling hazards in coastal areas through integration of active and passive electrical and seismic methods (Fiumicino, Central Italy)
Full text linkThe accurate location of gas upwelling flows is still an open problem for non-invasive imaging techniques in populated areas. Gas blowouts of deep origin may represent a serious threat to human health in urban areas and should be correctly imaged with high-resolution for assessing the related hazards. In this work, we propose an integration of active (electrical resistivity tomography and high-resolution sub-bottom profiling complemented with the multibeam bathymetry) and passive (self-potential and ambient noise recordings) geophysical methods to image gas upwelling flows in the coastal area of Fiumicino (Central Italy), where the gas presence is well-documented by previous works. We demonstrate that merging seismic sub-bottom profiling and electrical resistivity tomography has enormous diagnostic potential for gas detection, since they combine the high resolution needed to correctly image the subsurface and the interfaces between different media with the high diagnostic capability of electrical methods to detect anomalies associated with the gas emissions. Passive seismic methods complement the analysis enabling an estimation of the shear-wave velocity through array measurements. Finally, the reconstruction of the natural electrical sources, inferred from the inversion of self-potential data, confirms the location of the near-surface gas upwelling flows assessed through the resistivity model. This work demonstrates that the integration of high-resolution active and passive seismic and electrical methods can be an effective choice for the accurate location of risk-prone areas by imaging the near surface gas pathways where borehole drilling is strongly limited if not forbidden
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