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Near Surface 2009 - 15th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 07 Sep 2009 - 09 Sep 2009
- Location: Dublin, Ireland
- ISBN: 978-90-73781-72-6
- Published: 07 September 2009
81 - 100 of 139 results
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4D Geoelectrical Monitoring of Natural Attenuation Processes at a Contaminated Former Gas-works Site
Authors P.B. Wilkinson, P.I. Meldrum, O. Kuras, J.E. Chambers, S.J. Holyoake and R.D. OgilvyA permanent automated geoelectrical imaging system was installed at a contaminated land site to monitor resistivity changes associated with groundwater quality after the completion of a remediation programme. The former gasworks site had been designated statutory contaminated land due to the risks of pollution of an underlying minor aquifer. The system collected data at regular scheduled intervals from a network of electrodes arranged in vertical borehole arrays on the boundaries of the site. The incoming data from the system were automatically inverted to produce 4D resistivity images. Analysis was carried out over one year, revealing resistivity fluctuations in the infilled ground beneath the tarmac that were strongly temperature dependent. But in the underlying sand and gravel aquifer there was a steady increase in resistivity that was suggestive of a reduction in groundwater contamination after the removal of the contaminant source zones. A tracer test was also undertaken to investigate the groundwater flow velocity and demonstrate rapid 4D geoelectrical monitoring of natural attenuation processes. The motion and evolution of the tracer were visualised directly in high-resolution volumetric images in near realtime. Seepage velocities measured from the images agreed closely with estimates based on the piezometric gradient and assumed material parameters.
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Evaluation of Landfill Disposal Boundary by Means of Induced Polarization and Electrical Resistivity Imaging
Authors A. Legaz, A.V. Christiansen, E. Auken and A. ViezzoliInduced polarization measurements (IP) and electrical resistivity tomography (ERT) are used to define the spatial boundaries of a former municipal landfill, namely Eskelund, Denmark. The research aimed also at investigating the contamination plume in the vicinity of the waste site deposits. A clearly chargeable and conductive anomaly was evidenced, that may represent the percolation through the landfill, in agreement with drill-hole information. A further analysis should enable to check the seepage and have a 3-D tomography of the area.
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Geoelectrical Resistivity Monitoring for Localizing Gas at Landfills
Authors T. Dahlin, V. Leroux, H. Rosqvist, M. Svensson, M. Lindsjö, C.H. Månsson and S. JohanssonIn order to assess the potential of electrical resistivity for imaging gas migration at landfills, two relatively well-known sites - one bioreactor landfill and a conventional landfill - have been monitored successively. A three-dimensional resistivity image could be constructed every two hour on both sites. Meteorological parameters were monitored at the same time, with one measurement taken every hour. Methane concentration was measured in the air at several occasions, the pore pressure was monitored at two locations and some relative estimations of moisture in the top layer were made at the second site. The results show imperfect but interesting correlations between the different parameters and give a hint of how the method could be refined. The resistivity depends on several parameters, but the areas where the resistivity is most variable seem to be clearly related to higher gas emissions.
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The Use of Satellite Remote Sensing and Helicopter Tem Data for the Identification and Characterization of Contaminated
Authors A. Viezzoli, A. Edsen, E. Auken and S. SilvestriThe present research aims at exploring an advanced methodologies to solve a serious problems, currently threatening the health of environment and populations: the illegal stocking of dangerous waste materials and the consequent contamination of soils and ground water systems. In previous studies, satellite remote sensing has been successfully used to identify numerous candidate sites that are most likely to host waste materials. This result was the basis for further monitoring activities based on the use of an helicopter transient electromagnetic (TEM) system, to be carried out at end of April 2009. Extended results of the modelling of this survey will be presented at the Near Surface conference in Dublin at the oral presentation. Based on the forward modelling carried out, and on the experience from real data acquired earlier over a disposal site in Denmark, it is anticipated that SkyTEM will be capable of identifying the contaminated sites, and of mapping the contaminant plumes. Data will be carefully processed to decouple them from man made infrastructures, and then inverted with the Spatially Constrained Inversion (SCI) to recover a quasi 3D model of the subsurface. The integration of proximal and remote sensing methods represents a useful instrument for environmental management.
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Geophysical Monitoring of Recirculation of Leachate on Landfill Sites in the Netherlands
Authors P.P. Kruiver, R.S. Westerhoff, R.P. Noorlandt, N.N. Hoekstra, H. Woelders and W.J. van VossenGeophysical investigations were carried out on two modern landfill sites in the Netherlands. The objective of the geo-electrical and EM31 measurements was to reveal the effect of recirculation of leachate on homogenising moisture content in the waste to stimulate the process of attenuation. At Vlagheide, geo-electrical and EM31 measurements were done before and after 5 months of recirculation of leachate. The patterns resistivities are consistent between the geo-electrical lines and with depth. The geo-electrical and EM31 measurements both show an average decrease in resistivity after recirculation. This is possibly linked to a less heterogeneous moisture content caused by the recirculation process. At Kragge, the recirculation system is not installed yet. The geo-electrical profiles show a marked difference between the covered and open waste due to the HDPE foil on the covered waste. From the very high resistivities measured on the covered waste, it can be concluded that there are no significant leaks in the foil at the location of the measured geo-electrical line. On the open waste, the resistivity pattern shows a higher resistivity layer of a few meters thickness on a lower resistivity zone, which probably reflects differences in moisture content.
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Near-surface Geophysical Mapping of Northern Ireland from the Air
By M.E. YoungA recent low-level high resolution airborne geophysical survey of Northern Ireland has provided detailed imagery of ground radioactivity, electrical conductivity and magnetisation. Although airborne geophysics is usually thought of as a regional mapping method, these results are being used widely for near-surface applications – for shallow mineral exploration, geological mapping, environmental management and for research in these fields. Data have been licensed to industry and have prompted significant inward investment in mineral exploration. Together with complementary geochemical datasets, these near-surface geophysical results support environmental monitoring by regulators. They provide a basis for research in other fields, including structural geology, environmental radioactivity, soil science, groundwater management and geostatistics. Some of these applications are described. The project illustrates the value of airborne geophysical data in a range of economic sectors.
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Synergical Use of Passive and Active Methods to Reconstruct the Subsoil in Urban Areas
Authors L. Piroddi, G. Ranieri, M. Manconi, C. Piga, G.P. Deidda, F. Loddo and H. BelghazalIn urban areas it is often difficult to carry out not only direct but also indirect investigations, such as geophysical surveys. In fact, because of traffic noise, power lines and pipelines it is not possible to obtain reliable and repetitive measures using seismic, magnetic, electrical and electromagnetic methods. In addition, the asphalt and the paving of the sidewalks don’t facilitate the placing of geophones and electrodes. So only a few methods can usefully be applied. The gravimetric method in particular makes it possible to reconstruct maps of vulnerability that can be a useful document for the planning of appraisal or the prevention of risk of collapse for buildings. Finally the gravimetric maps represent a basic document for municipal urban development plans. Recently a seismic passive method was also developed that uses a 3-component seismic system called Tromino, which enables us to measure over time for periods of several hours, the spatial components of acceleration and velocity produced by natural tremors and vehicular traffic. Recently seismic land streamer devices have also been produced, that allow us to make profiles of seismic refraction on unpierceable areas and also to drag the system along a profile increasing its length.
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Inversion Techniques to Improve the Resistivity Images over Karstic Structures
Authors H. Hamdan and A. VafidisThe complexity of karstic structures causes major problems in the processing and interpretation of 2D resistivity data. This paper investigates the optimum inversion method to be applied in such cases. Three inversion methods (combined, smoothness constrained and robust inversion) are applied on real data set and their results are compared. In addition, joint inversion of two electrode arrays, namely Wenner-Schlumberger and dipole-dipole, has been employed to achieve the maximum reliability of the 2D resistivity model. The resistivity data have been collected from a strongly karstic site where saline water intrusion is present.
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TEM Resistivity – Salinity Calibration in the Brine Saturated Sediments along the Dead Sea Coast of Israel
Authors M.G. Ezersky and A. LegchenkoAn importance of the TEM method as one of the surface geophysical tools used for investigating sinkhole affected areas is increasing. As the boreholes previously used for the calibration of TEM results in the Dead Sea region were located mainly several kilometers away from the shoreline a more accurate calibration of TEM results is required for interpreting TEM derived resistivity. We performed 18 TEM soundings along the DS coastal area close to observation boreholes, where groundwater salinity and electrical conductivity were measured by Geological Survey of Israel (GSI). It follows from our study that quantitative interpretation of the bulk resistivity as groundwater salinity is complicated by the lithological variations. Therefore the lithology has a strong effect on the resistivity interpretation and earlier suggested formula based on the Archie’s low for the DS region is correct for sandy materials out of the DS brine saturated sediments. Our calibration suggests that sedimentary rocks with high clay content saturated with Dead Sea brine are characterized by the resistivity values < 0.4Ohm-m , whereas sandy material saturated with Dead Sea brine has the resistivity of > 0.4 Ohm-m. It allows identification of clay saturated with the Dead Sea brine using TEM measurements.
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Application of Resistivity Tomography to the Study of the Travertine Formation of Waterfalls in Edessa, North Greece
Authors G.N. Vargemezis, P. Tsourlos, V. Naxakis and A. StampolidisIn the vicinity of the waterfalls area in Edessa (North Greece) a new tunnel has been decided to be constructed to connect the surface with an existing tunnel. For design and geotechnical reasons the formation of travertine sould be studied. Electrical resistivity tomographies have been measured in very complicated crooked lines because of the rough terrain. As a result the best location concerning topographic and geotechnical reasons revealed from geophysical data has been proposed.
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Sand and Gravel Deposit Evaluation Using Electrical Resistivity Tomography
Authors J.E. Chambers, P.B. Wilkinson, A.L. Weller, O. Kuras, P.I. Meldrum, R.D. Ogilvy, J. Aumonier, S. Penn, D.R. Wardrop, E. Bailey, P. Joel and N. GriffithsIn this study, we assess the suitability of ERT for UK sand and gravel deposit assessment. To this end, we have reviewed the characteristics of deposits in terms of geological setting, thickness and heterogeneity to inform our survey design strategy. We have collated existing data on the electrical properties of UK sand and gravel, reviewed previous examples of ERT sand and gravel surveys, and undertaken detailed geophysical studies at seven potential or active sand and gravel extraction locations in East Anglia and the East Midlands.
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Importance of Gypsum Purity in Electric Imaging
More LessGypsum deposits are the currently exploited sulphate rocks with industrial purposes. In addition to the expensive drilling projects, geophysical techniques can be considered to estimate the economical potential of these deposits. The scope of this study is to characterize the geoelectrical response of gypsum rocks to accurately define their range of resistivity and evaluate the influence of accompaining minerals in gypsum formations. Electric imaging has been successfully applied to identify the gypsum deposits interlayered in lutite units. Nevertheless, the gypsum-lutite boundaries can be shown diffuse and uncertain in the tomographyc lines. A meaningful correlation between electric resistivity and purity of gypsum has been determined. It is clear that resistivity for the investigated gypsum facies varies in a rather wide range, from 50 to 1500 Ωm; the highest values are attributed to sulphate layers with >90% of gypsum mineral. Lutite units display higher values than expected, given that lutites related to evaporite deposits are commonly marly, silty, evaporite minerals-rich and clay-poor. The results show that electric resistivity lines could be useful in prospection of gypsum deposits. However, imaging prospection should be supported by an accurate petrological study of the deposits, in order to properly interpret the resistivity profiles.
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An Analysis for Negative Apparent Resistivity Appearing in Dipole-dipole Electrical Survey
More LessIn dipole-dipole electrical resistivity survey, we have often encountered negative apparent resistivity in pseudosections of apparent resistivity. Negative apparent resistivity is defined as the apparent resistivity values with the opposite sign to the surrounding resistivity values. It has been proven that the negative apparent resistivity can be caused by some geological models. We analyze the main causes for negative apparent resistivity by plotting the electric potential and current flow distributions using 3-D electrical resistivity modeling. We could observe that the current flows and the electric potentials are severely disturbed when transmitters are located around a branch of the U-shaped conductive model. Especially, the current flows in the opposite directions to those of the homogeneous body in some regions. This is because the current flows along the U-shaped conductive body and the other branch of the U-shaped conductive body acts as a secondary source. The resulting electric potentials and current flows are expressed as the combination of electric potentials and current flows generated by both the primary and secondary sources. From these results, we can confirm that negative apparent resistivity occurs due to some geological models.
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Long-time Geoelectrical Monitoring of Groundwater Contamination – Case Studies from Romania
Authors P. Georgescu, D. Ioane, B.M. Niculescu and F. ChiteaThe paper presents two distinct case studies of long-time geoelectrical monitoring of aquifers contamination: an anthropic contamination, in fact a marine intrusion generated by the excessive exploitation of drinking water in Costineşti resort (located on the Romanian Black Sea shore), and a natural contamination due to meteoric water infiltrations along a faults system in the vicinity of Brazi oil refinery (Ploieşti region, Romania). Both the long-time monitoring of the marine intrusion from the Black Sea’s shore and the determination of meteoric water natural infiltrations dynamic along faults are confirmed by data obtained from hydrogeological wells. A possible method for the simulation of apparent resistivity anomalies within this context (media with a continuous variation of the resistivity) is represented by the application of harmonic alpha media theory.
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iSOIL – Integration of Geophysical Technologies in Measuring Platforms
Authors U. Werban, A.K. Nuesch, M. Pohle, F.M. van Egmond, T. Wunderlich, S.A. al Hagrey and P. DietrichWe present work that was done within the EU-level project iSOIL “Interactions between soil related sciences – Linking geophysics, soil science and digital soil mapping”. iSOIL focuses on improving and developing fast and reliable mapping approaches of soil properties, soil functions, and soil degradation threats. This requires the improvement and integration of advanced soil sampling approaches, geophysical and spectroscopic measurement techniques, as well as pedometric and pedophysical approaches. Integration of geophysical technologies in measuring platforms allow a rapid and economical feasible mapping of large areas with complementary parameters. Application of techniques and their combinations will improve to a great extent the significance of the resulting digital maps. Performance tests and a quantitative data evaluation, e.g. of electromagnetical data, are necessary for further integration and field application of mobile geophysical platforms. The data quality needs to be analyzed with respect to technical parameters of instruments, survey speed and possible interference between sensors and towing vehicle. An experiment was designed as first step towards a efficient mobile sensing of electromagnetical data. Results of this first experiment are presented here to illustrate challenges of iSOIL concerning measuring technologies. Furthermore we present data of different sensor types resulting in information about subsurface heterogeneities.
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Geophysical Investigation (Geo-electrical & Electromagnetical Profiling) is Still Indispensable in Environmental Issues
Authors K. Martens and K. WalraevensGeophysical investigations have been used since long time in environmental studies to delineate groundwater and soil pollution. This method is very effective in combination with the traditional method of investigation of soil and groundwater pollution. The application of both investigation methods can be continued in monitoring remediation, which is an innovative approach and is illustrated in two case studies.
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Real-time EMI Mapping and Sounding – Equipment and Software
By E.V. Balkovvisualization. Recently developed equipment NEMFIS implementing electromagnetic induction method is briefly described as well as the software that controls the instrument, collects and process the data. The software was developed for mobile operating system based Pocket PC devices. It manages the NEMFIS via wireless Bluetooth™ connection, downloads the data and performs real-time processing and visualization (maps, pseudo-sections and diagrams construction). The measured data are synchronized with the coordinates from GPS receiver, that can be wireless external or the Pocket PC integrated. Comparing the traditional approach of survey with prior layout of measuring grid and data post processing the approach presented here has wider capabilities and significantly higher productivity. The paper includes basic formulas for calculating apparent values of conductivity as well as estimation of the depth of investigation. Brief case history of the developed system is presented in the paper.
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Non-conventional Geoelectric Arrays – Results of a Research Project – Theory
More LessIn frames of a Hungarian National Scientific Research Fund project (K49604), we systematically investigated all published surface geoelectric arrays, since a large part of them are out of use, even completely forgotten. Even in case of these latter ones we were optimistic in their potential renaissance, due to the rapid advance in geophysical knowledge and technical development. This collection proved to be the basis of still on-going inter-comparisons. We revealed the original motivation of their design, as well. Then we produced parameter sensitivity maps for all possible arrays, by using a new analytical approach. Through examples we presented, how these maps can be applied. Then another characterizing parameter, the depth of investigation was studied. The so-called depth of investigation characteristics was computed for all the 30 arrays, where it exists, both in terms of Roy and Apparao (1971) and Edwards (1977). We carried out various comparisons, and revealed a complex relation among vertical resolution, depth of investigation and noise. We showed how the depth of investigation is constrained by the noise level. Therefore the maximal (theoretical) depths of investigation for 6 arrays were studied at various noise levels. Besides some further theoretical studies, our further work will concentrate on measurements.
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An Ancient Landscape Revealed – 3D Mapping of Subcrop Basement Topography in Charnwood Forest, Leicestershire, UK
Authors M.J. Harwood and A.J. MatthewsThe study occurred at Bradgate Landfill, Leicestershire, UK. Granodiorites and metasediments form the basement in this area, beneath an unconformable cover of mudstones, with Quaternary superficial deposits also present. Considerable relief exists on the basement surface, making analysis of the subcrop topography from interpolation of boreholes unreliable. The relationship between the two main units may have implications for hydrogeological modelling and this surface was the main target of the study. Resistivity tomography was employed along 14 profiles, along with FDEM ground conductivity mapping. In two open quarries adjacent to the landfill, photogrammetry was employed to survey in the basement/mudstone contact. The resistivity revealed a strong contrast between basement and cover and allowed interpretation of depth to basement along the profiles. This was combined with the results of the photogrammetry, some existing borehole logs, survey information from construction of the landfill and geological mapping to construct a xyz database of depth to basement across the study area. This was then gridded to form a model of the basement topography from which in turn were derived isopachs of cover thickness and a series of cross-sections. The techniques employed have effectively produced a 3D model of the targeted basement surface.
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Identifying Subsurface Features in the Lagan and Enler Valleys in Northern Ireland Using Airborne Tellus Geophysics Data
Authors C. Burns, U. Ofterdinger and M. YoungThe City of Belfast in the North of Ireland is underlain by the Triassic Sherwood sandstone aquifer. Increasing abstractions from the aquifer over recent years mean that quantitative assessment of the resource is becoming increasingly important. In addition to this physical pressure, the qualitative status of the groundwater body may be affected by landfilling activity in the urban fringe area surrounding Belfast City. New datasets from the Tellus survey give enough information to allow the identification of subsurface features such as individual dykes, dyke swarms, sills, or combinations of such which may be a factor influencing groundwater flow regimes. A GIS of the compiled data was developed, and using total magnetic intensity mapping and variations on this, it was possible to identify and delineate subsurface anomalies. The igneous intrusions in the study area trend almost perpendicularly to the Lagan Valley and the flow of the River Lagan, and preliminary interpretations have determined that they are subvertical. Previous investigations have shown that flow in this region is mainly from the sides of the Valley towards the course of the River Lagan. To this end, topography driven groundwater flow may be enhanced by preferential dyke-parallel flow.
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