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Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics
- Conference date: September 6-10, 2015
- Location: Turin, Italy
- Published: 06 September 2015
121 - 140 of 164 results
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Detailed Mapping of a Leachate Plume from a Landfill Using Full-decay Time-domain DC-IP
Authors P.K. Maurya, A.V. Christiansen, G. Fiandaca, M. Lajaunie and E. AukenSummaryLeaching of organic and inorganic contamination from landfills are a serious environmental problem as surface waters and aquifers are being contaminated. In order to assess these risks and investigate the migration of leachate from the landfill, several time-domain DCIP profiles have been collected at a heavily polluted landfill in Grindsted, Denmark. The DCIP data were inverted using a new 2D DCIP inversion code inverting for a full cole-cole model while modelling the full system transfer response including waveforms, gates and low-pass filters. The inverted profiles describe both the variations along the groundwater flow as well as the plume extension across the flow directions. Chemical analysis of borehole data agrees well with the observations indicating a leachate plume which gradually sinks and increases in size while migrating from the landfill in the groundwater flow direction. High chargeabilities are seen on the landfill itself but there are only small IP effects in the leachate migrating from the landfill. The information obtained from the DCIP results will be combined with chemical analysis in order to compute contaminant mass discharge. Hence, the DCIP data provides invaluable spatial information on the leachate characteristics, which cannot be obtained from boreholes alone
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Improving Groundwater Models by Electromagnetic Measurements - Case of Elbe-Weser Estuaries
Authors A. Ullmann, H. Wiederhold, T. Günther, A. Steuer and B. SiemonSummaryIn coastal areas salinity of groundwater may pose problems for the sustainable exploitation of fresh groundwater. Groundwater flow modelling is the adequate tool to produce valid predictions of future development in the groundwater system. In order to set up the groundwater models it is essential to have a comprehensive understanding of the geological subsurface. How geophysical methods can contribute to the geological understanding is shown for the model area Elbe-Weser estuaries at the German North Sea coast. Techniques used are electromagnetic soundings (airborne and ground-based), magnetic resonance soundings and 2-D electrical resistivity tomography.
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CVES Resistivity Investigations for Optimizing Groundwater Protection at Highway Construction
Authors R. Wisen, E. Hammarlund and T. DahlinSummaryCVES resistivity has been used to achieve a continuous image that reflects the hydraulic properties of the ground along a road stretch. The purpose was to map the vulnerability for pollutants to penetrate into the precious groundwater magazine under the Kristianstad plains. The investigation shows that a large part of the area is covered by impermeable layers in the form of a thick clay till. Large parts of the area is covered with sandy till over clay till, and this sequence has been determined to have a permeability that gives sufficient protection of the aquifer below. In parts of the area, close to the Linderöd horst where soil cover is thin or non-existent and at the glaciofluvial deposits at and around Helgeåsen it is motivated to construct the road so that the ground water is protected. In about 10 % of the investigated area it will be necessary to perform further investigations to define the need of groundwater protection as part of the road design.
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Quantitative Interpretation of Time-lapse MALM Measurements During a Saline Tracer Injection in an Alluvial Aquifer
Authors M.T. Perri, P. De Vita, G. Cassiani, R. Masciale, I. Portoghese and G.B. ChiricoSummaryThis study presents the results of a saline tracer test conducted on an unconfined alluvial aquifer placed in the Alento River Valley (Campania region, South-Western Italy) and monitored by Mise-à-la-Masse measurements. The aim of this test is the investigation of groundwater flow field by a time-lapse analysis. The work first introduces the local hydrogeology of the investigated system and the experimental set-up. The results of the geophysical tracer test are then described and followed by the discussion of several simulations conducted on a 3D electrical model of the system. Finally, the comparison between real and simulated datasets is discussed in order to highlight advantages and limitations of Mise-à-la Masse technique when applied for hydrogeological purposes.
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Three Dimensional Time Lapse Monitoring of a Saline Tracer in an Experimental Tank
Authors S. Kirkou, P. Tsourlos, P. Soupios, N.G. Papadopoulos and K. SimiyrdanisSummaryWe present a 4D geoelectrical time lapse experiment simulating the movement of olive oil mill wastes into a sequence of impermeable (fine sand) to permeable (medium sand) formations. Prior to the full experimental set-up a smaller scale test took place in a small experimental aquarium in order to simulate the movement of the saline tracer. The actual experiment involved a large (one cub. meter) tank instrumented with electrodes deployed in six vertical and two horizontal boreholes. A saline tracer was injected in to the saturated permeable medium and its movement was monitored by collecting 21 time-lapse full 3D ERT data sets. Data were processed using a 4D inversion algorithm and the time lapse geoelectrical images captured the red dye tracer movement. The laboratory experiment confirmed that the 4D approach provided improved time-lapse monitoring of changes in a real system. The above model will be used to extract hydrogeological parameters and create a model of the saline tracer flow and evaluate the applicability of ERT monitoring in this context.
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Multi-methodological Geophysical Approach for Contamination Detection in a High Flood Risk Area (Southern Italy)
Authors R. Di Maio, E. Piegari, P. Rani and G. ForteSummaryDue to very fast industrial growth and urbanization, problems of environmental contaminations are growing rapidly. The soil and groundwater pollution is one of the most important concerns that have received attention at local, regional and global levels because of their impact on public health. The use of geophysical techniques is of great help for environmental studies because they not only give the localization of polluted areas, but also provide an estimate of the extent of contamination, through the determination of the most likely contaminated volumes. To identify possible soil and groundwater contamination, integrated analysis of resistivity, chargeability and self-potential data has been performed in a survey area located in the town of Castel San Giorgio (Salerno, Italy) in the western basin of Solofrana river valley. This area is periodically affected by flooding induced by severe or significant rainfall, which cause the river to overflow and contribute to potential soil and groundwater contamination by heavy metals discharged from the tanning plants operating in the upper part of the valley.
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Rapid Inversion of Data from 2-D and from 3-D Resistivity Surveys with Shifted Electrodes
Authors M.H. Loke, P.B. Wilkinson and J.E. ChambersSummaryGeoelectrical monitoring surveys are used to detect temporal changes in the subsurface below unstable slopes with the measurements repeated over an extended period. The positions of the electrodes are measured at the start of the campaign and possibly at regular intervals. However, ground movements sometimes occur between the times of the electrode positions measurements. For some data sets the precise positions of the electrodes are not accurately known and have to be estimated from the resistivity data. The smoothness-constrained least-squares optimization method is modified to include the electrode positions as unknown parameters to be determined. The Jacobian matrices with the sensitivity of the apparent resistivity measurements to changes in the electrode positions are required by the optimization method. A fast adjoint-equation method to calculate the required Jacobian matrices is described. It is one to two orders of magnitude faster than the perturbation method previously used. We also modify the inversion routine by using the inversion model for the initial time-lapse data set (with known electrode positions) as the starting model for the inversion of the later-time data sets. This greatly improves the accuracy of the recovered electrode positions compared to the use of a homogeneous earth starting model.
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A Flexible Interface for Tomographic Inversion of Real and Complex Resistivity Data in EIDORS
Authors G. De Donno and E. CardarelliSummaryThe aim of this work is to present a Matlab interface called VEMI (Versatile interface for Electrical data Modelling and Inversion) included in the EIDORS package. The interface is able to invert 2D and 3D electrical data acquired in the time- or in the frequency-domain, both for cylindrical and prismatic geometries. Therefore it can be applied for inversion of both real and complex-valued resistivity tomography data. Application to laboratory and field data are discussed.
The 3D laboratory example, consisting in a simulation of a shallow aquifer contaminated by a DNAPL, demonstrated the reliability of the interface to detect the DNAPL pathway within the physical model, through the analysis of the inverted phase model.
The results of the 3D inversion of electrical resistivity tomography data acquired on a masonry building were compared with those obtained a common used commercial software. Although both software were able to detect the high-resistivity zone associated with the presence of the foundation wall, the resistivity values and the shape of the anomaly shown by the VEMI inverted model are more similar to those expected for this construction element.
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Use of Resistivity to Estimate Porosity Variations in Loose Sands after Compaction by Resins Injection Treatments
Authors F. Fischanger, M. D’Attoli, G. Morelli and M. OcchiSummaryIn this paper we report the results of a study on the possibility of using Electrical Resistivity Tomography (ERT) techniques to verify the efficiency of soil treatments by injection of expanding polyurethane resins in loose saturated sands with high liquefaction susceptibility. In order to test the injection treatment and to monitor it through geophysical and geotechnical measurements, a small test site has been devised in the San Carlo — Sant’Agostino village, close to Ferrara (Northern Italy), a place affected by the effects of soil liquefaction during the earthquake occurred on May, 2012. This test is not meant to be exhaustive, also considering that the planned ERT survey has been greatly simplified and influenced by many constraints, but the study is intended to stimulate the interest on the possibility of using resistivity for evaluating, even quantitatively, the improvements in “mechanical” properties of liquefiable sandy horizons after compaction treatments. Electrical resistivity tomography techniques can be considered a good tool for the verification of compaction of liquefiable sandy horizons. The reduction of the voids that follows the consolidation is well indicated by the increase in resistivity that allows to highlight continuity and depth of treatment. Furthermore, the possibility of using petrophysical correlations for estimation of changes in porosity on the basis of the resistivity variations certainly makes the technique very powerful in evaluating the improvements in terms of “mechanical” properties.
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Finite Element Modelling of Resin Improved Swelling Soil Based on ERT
More LessSummaryThis study shows Finite Element calculations of a swelling soil improved by resin injection technique. Several buildings show cracks which are often caused by swelling clays on which the structures are founded. To overcome the problems associated with the swelling layer, it was decided to improve it by injecting stabilizing expansive resins. The in situ mechanical properties of the soil were investigated using the pressuremeter test. The current abstract summarizes how to model and predict the effect of resin injection on the soil behaviour by making use of ERT as a guide. For the numerical modelling, the Finite Element model has been selected, and the most advanced constitutive models available for geotechnical engineering have been adopted. The proposed procedure therefore is an integrated use of ERT, standard geotechnical testing and Finite Element modelling and can be extended to any other case where it is important to feed the analysis results with field data in almost real time.
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Detailed Leachate Mapping Using a 441 Electrode Full 3D Setup Using an Extendable Fishbone Layout
Authors A.V. Christiansen, P. Maurya, G. Fiandaca and E. AukenSummary3D surveying of resistivity and induced polarization has often utilized only a small number of electrodes determined by the number of addresses in the instrument. This is often in the range of 64-128 electrodes if mounted using a switching unit. Here we use a flexible and expandable 3D system that allows for arbitrary use of any set of parallel cables in a fish-bone structure. The measurements are performed at a large highly contaminated historic landfill in Denmark for a detailed mapping of the leachate plume migrating from the fill.
A total of 441 stainless steel electrodes were installed along 7 lines (63 electrodes each). Each line was 410 meters long and the inter-line spacing is 25 meters making the total area 410 by 150 m. Six switch boxes enabled data acquisition with all 441 electrodes using a resistivity meter designed for only 64 electrodes. We collected data both in-line, and crossline to build a full 3D dataset consisting of several thousand 4-electrode measurements.
The inversion identify a very conductive leachate plume migrating from the fill, with a fluid resistivity as low as 0.5 ohmm. Due to the dense 3D cover we identidfied a division of the main plume into two subplumes diverting as the distance to the landfill increses.
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Optimized Arrays for Resistivity Measurements Confined to the Perimeter of a Survey Area
Authors M.H. Loke, P.B. Wilkinson, A. Tejero-Andrade and S. KruseSummaryIn some survey problems, particularly in urban areas, the electrodes are confined to the perimeter of the survey area due to physical obstructions. This significantly limits the electrode arrangements that can be used for resistivity survey measurements. Previous methods in designing arrays for such surveys are dependent on the shape of the perimeter, such as rectangles. We use a modification of the ‘Compare R’ array optimization technique to automatically generate electrode arrays that provide the optimal resolution for perimeters of any shape. The array optimization algorithm is modified by using a weighting function to preferentially select arrays that improve the resolution of structures within the survey perimeter. Plots of the model resolution and point-spread-function help to illustrate the capabilities of the survey technique. It is shown that the vertical resolution is significantly poorer than the horizontal resolution that can be achieved with resistivity surveys. In a synthetic model test with shallow and deep high resistivity blocks in a homogeneous medium, the optimized arrays achieve better resolution of the structures compared to standard arrays designed for a square survey perimeter.
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A Special ERT-3D Array Carried Out to Investigate the Subsoil of the Pyramid El Castillo, Chichen Itza, Mexico
Authors R.E. Chavez, A. Tejero, G. Cifuentes, D.L. Argote and J.E. HernandezSummaryIt is not always possible to apply traditional 3D electrical methods to investigate the subsoil beneath ancient structures in archaeological sites. Special three dimensional electric resistivity tomography (ERT-3D) arrays have been designed to ‘illuminate’ the subsoil beneath the structure under study. The square array was designed to surround the target to acquire reliable geophysical information from the subsurface. This ERT-3D array is applied to investigate the subsoil of the pyramid of El Castillo, located in the archaeological site of Chichen Itza, in the southern lowlands of Mexico, in the Yucatan Peninsula. This building is the emblematic structure of this well-known site and elected as one of the man-made world seven wonders. Previous GPR studies provided evidences of a buried man-made tunnel excavated within the limestone rocks near this ancient structure. Now, and ERT-3D study was carried out by employing 96 flat-surface electrodes, which surrounded the edifice forming a square geometry. A total of 8,500 apparent resistivity observations were measured, with a total depth of investigation of 25 m. A low resistivity body was found beneath the pyramid, which can be associated to a sinkhole filled with sweet water.
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4D Electrical Resistivity Tomography Monitoring of Talesh Mahaleh-Rasht Coastal Aquifer Polluted by Caspian Seawater
Authors H. Sarkheil and M. Habibi RadSummarySea saltwater influx into coastal aquifers, a phenomenon brought by the flow of seawater into freshwater aquifers, has long been recognised around the world. In this study, Geophysical surveys are carried out in the coastal area of Talesh-Mahaleh (North-eastern Rasht-Gilan Province of Iran) to study the time evolution of seawater contamination of the coastal aquifer, probably increased due to human impact. The study is based on a processing and integrated analysis of hydrogeological and geophysical data. In particular, in the September to March 2012, seasonal time-lapse electrical resistivity tomography (on 15 Dipole-Dipole profiles) has been carried out. The resistivity data were acquired with electrode spacing (a) of 25 m, and expansion factor (n) of 40. The interpretation of geophysical data made it possible to reconstruct a 4D model of the electrical resistivity of the aquifer, aimed at defining the extent and the boundaries of seawater intrusions. Furthermore, the execution of a series of high-resolution time-lapse electrical tomography and a correlation analysis between geophysical measures and, geological and hydrogeological data allowed specifying the effects of the salt concentration in the groundwater and the porosity and saturation degree of the geology formations on the time variations of the measured electrical resistivity.
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Estimating a Continuous P-wave Velocity Profile with Constant Squared-slowness Gradient Models from Seismic Field Data
Authors A.V. Ponomarenko, B.M. Kashtan, V.N. Troyan and W.A. MulderSummaryWe inverted seismic field data for a continuous, laterally invariant P-wave velocity profile. Instead of the usual approach that involves horizontal layers with piecewise constant densities and velocities, we consider models of one or two layers with a constant gradient of the squared slowness above a homogeneous halfspace.
With a single layer above a halfspace, there are three parameters. With two layers, there are five. We solve the inverse problem by a direct grid search over a wide range of parameters. The results were compared to that of a piecewise-constant multi-layer inversion result. In the single-layer case, either the shallow or the deeper part of the model would match the multi-layer case, depending on which modes of the surface waves were selected. With two layers, a considerably better agreement is obtained over a larger depth range. Our method is limited to cases with a small Vs/Vp-ratio but has only 5 parameters. It could be a useful alternative to piecewise-constant multi-layer inversion, in particular if continuous P-velocity profiles are sought. These are sometimes better suited as a starting model for full waveform inversion than models with many discontinuities.
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Depth of Investigation for Multi-parameters Inversions
Authors G. Fiandaca, A.V. Christiansen and E. AukenSummaryWe have developed a new robust concept for the calculation of the depth of investigation (DOI) for inversion problems described by several intrinsic parameters, like the spectral inversion of time-domain induced polarization data or the inversion of full-decay Nuclear Magnetic Resonance soundings. A calculation of the DOI is crucial for interpreting the geophysical models, as the validity of the model varies considerably with data noise and parameter distribution. Without the DOI estimate, it is difficult to judge when the information in the model is data-driven or is strongly dependent on the constraints and/or on the starting value. The proposed method is based on an approximated covariance analysis applied to the model output from the inversion while considering the data standard deviations. Furthermore, the cross-correlations between intrinsic parameters are taken into account in the computations, which is crucial when strong cross-correlations are expected. The new DOI estimate is described in detail and compared to the full covariance analysis on the model parameters using 2D spectral inversion of time-domain induced polarization data as an example. The DOI presented here behaves as anticipated on a synthetic model and is suggested as a general approach for model-spaces with more than one physical parameter.
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Joint Inversion of Gravity Gradient Tensor at Vredefort Impact Crater
Authors V. Paoletti, M. Fedi and F. ItalianoSummaryIn recent years, Gravity Gradient Tensor (GGT) has been successfully used in applied and environmental geophysics, also in light of the development of gradiometers. In this paper, we aim at analysing the inversion, either joint or separate, of different GGT components and of the sole gravity field vertical component. We perform our analysis by inspection of the Picard Plot, a well-known Singular Value Decomposition tool, and employ both synthetic data and gradiometer measurements carried out at the Vredefort structure, South Africa. We show that the main factors controlling the quality of the inversion are algebraic ambiguity and signal-to-noise ratio. Provided that algebraic ambiguity is kept low — by different combinations of GGT components and/or only gravity field data — the choice of components involved in the inversion is non-crucial to the quality of the reconstructions. Nonetheless, the use gradiometers allows a quicker and more effective way, with respect to the sole gravity field, to improve algebraic ambiguity.
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Near Foundation Soil Stiffening Evaluation after Resins Injection by a Novel 3D Interpretation of Surface Waves Data
Authors S. Bignardi, N. Abu Zeid, M. D’Attoli, G. Morelli, M. Occhi, M. Russo and G. SantaratoSummaryThe characterization of soil settlements below foundations of buildings using surface waves methods represent an extreme challenging problem because of the presence of localized shear waves variation and shallow heterogeneity. To tackle the near-foundation shear waves distribution reconstruction challenge a method for the elaboration of surface waves recordings unbound from both rigid field geometry and from the classic layered subsurface assumption would be highly desirable. We introduce a novel elaboration strategy of surface waves data, based on the Direct Interpretation of Phase Lags (DIPL-3D)among pairs of seismic signals which aims to retrieve the three dimensional shear waves velocity subsurface distribution without using inversion. As an example application, the method is used for the evaluation of near foundation soil stiffening after an expanding resins injection intervention
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Investigating in Separation of Charak Anticline by Using Forward Modelling and 3D Gravity Inversion Method
Authors S.Y. Sayyadi and V.E. ArdestaniSummaryThe most important goal in hydrocarbon explorations is finding undersurface anomalies which have good quality for gas and oil explorations. Gravity data can be useful in fields with salt body structures. In our case study, Charak anticline has some out growing salt bodies. This anticline has arc image, it looks that an undersurface anomaly make this image. In this paper, in this paper
To clarify this anomaly we improve the precision of inversion result, with minimum inertia, in Lewi algorithm .the result shows a salt body which have at least 2kilometeres depth that is a good place for exploring gas and oil.
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Examination of a Karstic Cave with Complex Geophysical Methods in North Hungary
Authors I.T. Tóth, É.B. Bujdosó, R.C. Csabafi, T.G. Gúthy, E.H. Hegedűs, A.C.K. Kovács, I.T. Török and Z.P. PrónaySummaryThe On-Site Inspection (OSI) under the Comprehensive nuclear-Test-Ban Treaty (CTBT) shall be conducted in the least invasive manner possible, consistent with the efficient and timely accomplishment of the inspection mandate. The active seismic and geoelectrical surveys to search for and locate underground anomalies, including cavities and rubble zones caused by underground nuclear explosion belong to the approved inspection techniques. The field testing of surface geophysical methods was carried out in Felsöpetény, North Hungary between 2012 and 2014. 3D active seismic and 2D geoelectrical measurements were performed over an abandoned underground clay mine, which is connected to an oval shaped cave of karstic origin of 30m diameter situated 70m below the surface. The cave was carved in the Dachstein Limestone by hydrothermal activity during the Pleistocene close to the overlying sediments where the opening of the karstic cave could happen. A weak zone of lower velocity above the cavern was revealed by the seismic refraction method. The observed low velocity anomaly on the seismic tomography profile and the characteristic high resistivity on the geoelectrical 2D profile appear at the position of the cave.
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