1,721,005 research outputs found
Physically constrained 2D joint inversion of surface and body wave tomography
Full text linkJoint inversion of different geophysical methods is a powerful tool to overcome the limitations of individual inversions. Body wave tomography is used to obtain P-wave velocity models by inversion of P-wave travel times. Surface wave tomography is used to obtain S-wave velocity models through inversion of the dispersion curves data. Both methods have inherent limitations. We focus on the joint body and surface waves tomography inversion to reduce the limitations of each individual inversion. In our joint inversion scheme, the Poisson ratio was used as the link between P-wave and S-wave velocities, and the same geometry was imposed on the final velocity models. The joint inversion algorithm was applied to a 2D synthetic dataset and then to two 2D field datasets. We Compare the obtained velocity models from individual inversions and the joint inversion. We show that the proposed joint inversion method not only produces superior velocity models, also generates physically more meaningful and accurate Poisson ratio models
Direct MT data transform into 1-D resistivity models: a new approach based on cumulative resistance models
Full text linkMagnetotelluric (MT) data inversion seeks to recover resistivity models of the subsurface. Solving the inversion problem is a non-trivial task, as multiple plausible solutions can be recovered due to the nonlinearity of the problem. To reduce this nonlinearity, we propose a data-driven approach where a 1-D cumulative resistance model is estimated from MT data via a direct data transformation. We define the cumulative representation of layered models as the weighted sum of layer thickness divided by resistivity from surface to any depth level, which is the cumulative conductance. Its inverse, cumulative resistance, is directly related to the real part of the impedance computed from MT data. We train a neural network to transform the MT impedance into a resistance model. The corresponding 1-D resistivity model is obtained without a priori information. We validate our approach using synthetic and real data, opening the discussion for future developments of this new perspective
GPR Imaging Via Qualitative and Quantitative Approaches
No full textGround Penetrating Radar (GPR) is a non-destructive imaging system able to provide high-resolution images of the subsurface. From a theoretical point of view, it requires to solve an inverse scattering problem, where a set of parameters describing the underground scenario must be retrieved starting from samples of the measured electromagnetic field. In this chapter, an overview of different methods/algorithms for quantitative and qualitative buried scatterer reconstruction widespread in literature is provided
Non-invasive estimation of moisture content in tuff bricks by GPR
No full textMeasuring water content in buildings of historical value requires non-invasive techniques to avoid the damage that sample taking or probe insertion may cause to the investigated walls. With this aim, a stepped frequency ground penetrating radar (GPR) system was tested to assess its applicability in moisture measurements of porous masonry elements. The technique was tested on a real scale wall made with yellow Neapolitan tuff bricks, a material commonly found in historical buildings of Campania (Southern Italy). First, the antenna was calibrated to find its characteristic transfer functions. Then 64 GPR acquisitions, coupled with gravimetric measurements of the volumetric water content, were performed on the tuff wall in laboratory controlled conditions. A full inverse modelling of the GPR signal on tuff was used to retrieve dielectric permittivity and electrical conductivity of tuff at various water contents. By linking these characteristic electromagnetic parameters to the water content, the calibration relationships specific for yellow Neapolitan tuff are defined, which can be used for moisture measurements by GPR in real case studies. The experimental results lead to a robust identification of clearly defined monotonic relationships for dielectric permittivity and electrical conductivity. These are characterized by high values of the correlation coefficient, indicating that both parameters are potentially good proxies for water content of tuff. The results indicate that GPR represents a promising indirect technique for reliable measurements of water content in tuff walls and, potentially, in other porous building materials
Foreword to the Special Issue on Advances in Ground-Penetrating Radar Research and Applications
No full textThis Special Issue in IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING (IEEE JSTARS) on Advances in Ground- Penetrating Radar Research and Applications is organized in the frame of the 15th International Conference on Ground Penetrating Radar (GPR 2014), held in Brussels, Belgium, from June 30 to July 4, 2014 (http://sites.uclouvain.be/GPR2014/). This International Conference is a longstanding traditional event making the premier forum of research and applications in the field of ground-penetrating radar (GPR). GPR 2014, the 15th in the series that has been held biannually since 1986, brought together about 300 high-standard scientists, engineers, industrial delegates, and end-users working in all GPR areas, ranging from fundamental electromagnetics to the so various fields of application. In total, 42 countries worldwide were represented. GPR 2014 was organized by the Université catholique de Louvain (UCL, Belgium) and co-organized by the University of Edinburgh (U.K.), Delft University of Technology (The Netherlands), and Roma Tre University (Italy), together with EU-COST (European Cooperation in Science and Technology) via the COST Action TU1208 “Civil Engineering Applications of Ground Penetrating Radar.” The conference was sponsored by the IEEE Geoscience and Remote Sensing Society, the COST Action TU1208, the Fonds de la Recherche Scientifique(FNRS, Belgium), the European GPR Association (EuroGPR), and 14 private companies which exhibited their products and services during the conference
GPR analysis of clayed soil behaviour in unsaturated conditions for pavement engineering and geoscience applications
No full textClay content is one of the primary causes of pavement damages, such as subgrade failures, cracks, and pavement rutting, thereby playing a crucial role in road safety issues as an indirect cause of accidents. In this paper, several ground-penetrating radar methods and analysis techniques were used to nondestructively investigate the electromagnetic behaviour of sub-asphalt compacted clayey layers and subgrade soils in unsaturated conditions. Typical road materials employed for load-bearing layers construction, classified as A1, A2, and A3 by the American Association of State Highway and Transportation Officials soil classification system, were used for the laboratory tests. Clay-free and clay-rich soil samples were manufactured and adequately compacted in electrically and hydraulically isolated formworks. The samples were tested at different moisture conditions from dry to saturated. Measurements were carried out for each water content using a vector network analyser spanning the 1 GHz–3 GHz frequency range, and a pulsed radar system with ground-coupled antennas, with 500-MHz centre frequency. Different theoretically based methods were used for data processing. Promising insights are shown to single out the influence of clay in load-bearing layers and subgrade soils, and its impact on their electromagnetic response at variable moisture conditions
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
Triangulation for seismic modelling with optimization techniques
No full textThe finite-element method can easily handle complicated geometries because of the application of unstructured meshes. Unlike the Cartesian grid used in the finite-difference method, the unstructured mesh can follow the sharp interfaces that separate two layers of different properties. Therefore, the finite-element method can provide more accurate solutions for the simulation of seismic wave propagation. Meshes of good quality are required for the finite-element simulation. However, it is not trivial to set up an appropriate mesh. Firstof all, the mesh should contain elements of good shapes and sizes. In addition, the sharp interfaces should coincide with the edges of the elements instead of intersecting with them. These requirements are formulated as an optimization problem with three terms, measuring the difference between the actual and prescribed scaling field, shape quality, and the area between prescribed curves and the nearest triangle edges. The solution of the optimization problem should provide the desired mesh. The mesh generator MESH2D was applied to obtain an initial mesh. The Matlab function minFunc was used to search for the minimum of the constructed objective function. Three weights balance the three terms in the objective function. When it comes to complicated models, these weights have to be chosen carefully to produce a reasonable mesh.Applied Geophysic
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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