Electrical permittivity and resistivity time lapses of multiphase DNAPLs in a lab test

Dense Non-Aqueous Phase Liquids (DNAPLs) induce variation in electromagnetic characteristics of the ground e.g. electric permittivity and resistivity. The most used indirect methods in the mapping of these physical characteristics are electrical resistivity and ground penetrating radar. To better understand the effect of DNAPL release on electrical permittivity and resistivity in a water saturated medium, we carried out a controlled laboratory experiment where the host material was simulated by glass beads and the DNAPL by HFE-7100 (hydrofluoroether). The experiment measured the electric resistivity and permittivity of each fluid, the multi-phase fluid system, and the host material, along with time-lapse electrical resistivity and GPR measurements in a controlled cell. We found that the different phases of DNAPL within a saturated medium (free, dissolved and gaseous phase) affect the physical characteristics differently. The reflection pull-up behind contaminated sediments, which is normally detected by GPR, was mainly inferred from the HFE free phase. The dissolved phase causes small variations in electric permittivity not usually readily detected by GPR measurements. Both the dissolved and free HFE phases induce variation in resistivity. The study showed that GPR and electrical resistivity differ in sensitivity to the different HFE phases, and can be complementary in the characterization of DNAPL contaminated sites

Interpretation of Tuscan gravity data

The gravity data were used to model the deep structures of geothermal tuscan fiel

Some considerations on electrical resistivity imaging for characterization of waterbed sediments

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Joint interpretation of geophysical data for archaeology: A case study

Interpret-joints within geophysical data recorded in a complex area where ruins do not outcrop and only earthenware remains within the surficial layer are present. The study area, located in central Italy, consists of Roman, medieval and modern ruins that are included in reworked sediments. The geology is formed by inhomogeneous alluvial sediments (sand and gravel) several meters thick with diamagnetic character. To reduce the ambiguity in the subsurface reconstruction, a joint interpretation of georadar, magnetic and electrical tomography data was performed. The georadar was chosen to reconstruct detailed subsurface features, the electrical tomography to distinguish resistive bodies (stones, voids, etc.) from conductive (cavities filled by clay) and, because of the diamagnetic character of in situ sediments, the magnetic method was chosen to detect the earthenware ruins. The geophysical data were controlled by excavation, which detected silos of 1 m in diameter and a concrete layer at a few centimeters from the topographic surface. Time slices in the georadar data allowed us to detect the silos and to define the lateral edge of the concrete layer. Silos were also indirectly detected by the magnetic data because of the earthenware present in the filling sediments. Electrical tomography detected the concrete layer and an ancient anthropogenic surface of few centimeter depths. The study demonstrates that, because the geophysical methods are based on different physical characteristics, they can have different resolution and therefore detect different bodies. © 2005 Springer Science+Business Media, Inc

Two different approaches for georadar data processing. A case study in archaeologocal prospecting

Advanced GPR processing is applied to reconstruct burried archaeological structures at Axum (etiopia

Pavement testing by integrated geophysical methods: Feasibility, resolution and diagnostic potential

This work is focused on the assessment of the diagnostic potential of several geophysical methods when applied to the investigation of a rigid airport pavement. The potential and limit of each technique are evaluated as well as the added value deriving from their integration. Firstly, we reconstruct a high-resolution image of the pavement by a large electromagnetic and georadar screening. An advanced processing of georadar data, implemented through the picking of the arrival times of reflections for each profile, provides a quantitative estimation of the deviation between the design and the as-built thickness of layers. Additionally, electrical tomography has been applied to unequivocally identify the anomalous zones, where higher values of resistivity would be associated to porous zones that are prone to degradation and failure. The seismic tomographic survey had the additional purpose to recover the mechanical properties of the pavement in terms of both P- and S-waves and consequently of elastic constants (Poisson's ratio), whose values were consistent with those recovered in literature. The anomalies detected by each technique are consistent in their indications and they can be correlated to failure phenomena occurring at layer interfaces within the pavement structure or to unexpected variations of the layer thicknesses. The cost-effective geophysical campaign has validated the four-layered system deduced from the original design and has been used to reconstruct a high-resolution map of the pavement in order to discriminate fractures, crack-prone areas or areas where the as-built differs from the original design

Modeling and testing of high frequency GPR data for evaluation of structural deformation

The paper describes the application, on a hollow pier, of a high frequency multi-component 2 GHz GPR antenna for the evaluation of deformation induced by mechanical stress. The study, performed in the laboratory, was made on a reinforced concrete hollow pier built to scale 1:5. The GPR survey data were obtained before and after the mechanical stress application, in reflection and transillumination modes. The data interpretation was supported by a GPR theoretical model of a pier. The interpretation of the GPR reflection data was very challenging as the iron rods used to reinforce the pier led to wave diffraction and the air inter-space inside the pier itself produced multiple reflections. No continuous fractures were revealed, probably because the stress caused only micro-fractures. This assumption was supported by analyses of the time slices from reflection data and the first direct wave arrival times from transillumination data. In the second survey we observed increased reflectivity in the time slices and decreased first arrival times of the direct wave, with respect to the first survey. The theoretical GPR data carried out on the physical model, which reproduced the actual pier, were found to be very useful tools for interpreting the actual data as they allow signal to noise separation. (C) 2009 Elsevier Ltd. All rights reserved

Imaging continental shelf shallow stratigraphy by using different high-resolution seismic sources: an example from the Calabro-Tyrrhenian margin (Mediterranean Sea)

High-resolution seismic reflection profiles of the Calabro-Tyrrhenian continental shelf were collected using different seismic sources (Sub-Bottom Profiler, Uniboom, Sparker 0.5-1-4.5 kJ). Noticeable differences and results were obtained both from a geophysical and geological-interpretative point of view. The availability of different sources permitted the definition of the most suitable seismostratigraphic characterization in terms of resolution, penetration and acoustic facies. Very high resolution stratigraphy was defined through profiles produced by different seismic systems used in parallel. This permitted the application of sequence-stratigraphy concepts with the reconstruction of a thick postglacial depositional sequence, formed by a transgressive and a high-stand systems tract. The thickness distribution of postglacial deposits reveals that the main depocenter (55-65 m) is located offshore of the Coastal Range, along a stretch of coast supplied by several small and seasonal streams ("fiumare") and characterized by the lack of a coastal plain. This suggests the greater efficiency of sediment supply and bypass in this area relatively to sectors located offshore of the main rivers. The transgressive systems tract, usually thin or nearly absent, is particularly well developed (up to 33 m) and is composed of up to three parasequences with a retrogradational stacking pattern. The high-stand systems tract, up to 30 m thick, is made up of two parasequences and has a quite regular geometry and acoustic facies.Perfis de reflexão sísmica de alta resolução da plataforma continental tirreniana de Calabro foram obtidos utilizando-se recursos sísmicos diversos (Perfilador de Sub-superfície, Uniboom, Sparker 0.5-1-4.5 kJ). Diferenças evidentes foram encontradas sob o ponto de vista geofísico e geológico-interpretativo. A disponibilidade de diferentes fontes permitiu a definição de uma caracterização sismo-estratigráfica mais acurada em termos de resolução, penetração e facies acústica. Uma estratigrafia de resolução bastante alta pôde ser obtida através de perfis produzidos por diferentes sistemas sísmicos utilizados em paralelo. Este fato possibilitou o emprego de conceitos de sequência-estratigrafia para a reconstrução de uma espessa sequência deposicional pós-glacial formada por um trato de sistemas transgressivo e de nível de base alto. A espessura dos depósitos pós-glaciais revela que o principal centro de deposição (55-65 m) está localizado ao largo da serra costeira, ao longo de uma costa distendida e alimentada por pequenos filetes sazonais ("fiumare"), sendo caracterizado pela falta de uma planície costeira. Os dados sugerem a existência de um suprimento de sedimentos bastante eficiente nessa área, em relação a setores localizados ao largo dos rios principais. O trato de sistemas transgressivo, geralmente estreito ou ausente, é aqui particularmente bem desenvolvido (acima de 33 m) e composto por até três parasequências com padrão de empilhamento retrogradacional. O trato de sistemas de nível de base alto, até 30 m de espessura, é composto por duas parasequências e apresenta geometria e facies acústica extremamente regulares

Inverse transition in the dipolar frustrated Ising ferromagnet : the role of domain walls

We present a theoretical study aimed at elucidating the origin of the inverse symmetry-breaking transition observed in ultrathin magnetic films with perpendicular anisotropy. We study the behavior of the dipolar frustrated Ising model in a mean field approximation as well as two other models with simple domain walls. By a numerical analysis we show that the internal degrees of freedom of the domain walls are decisive for the presence of the inverse symmetry-breaking transition. In particular, we show that in a sharp domain wall model the inverse transition is absent. At high temperatures the additional degrees of freedom of the extended domain walls increase the entropy of the system leading to a reduction of the free energy of the stripe phase. Upon lowering the temperature the domain walls become narrow, and the reduction of the number of degrees of freedom associated with them manifests in a reduction of entropy which eventually induces an inverse transition to the competing homogenous phase. We also show that, for a growing external field at constant temperature, the stripe width grows strongly when approaching the critical field line and diverges at the transition. These results indicate that the inverse transition is a continuous phase transition and that the domain wall profiles and the temperature have little effect on the critical behavior of the period of the domain as a function of the applied field.publishedVersionFil: Araújo Velasque, Luciana. Universidade Federal do Rio Grande do Sul. Instituto de Física. Departamento de Física; Brasil.Fil: Stariolo, Daniel Adrián. Universidade Federal do Rio Grande do Sul. Instituto de Física. Departamento de Física; Brasil.Fil: Stariolo, Daniel Adrián. National Institute of Science and Technology for Complex Systems; Brasil.Fil: Billoni, Orlando Vito. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Billoni, Orlando Vito. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Física de los Materiales Condensado