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Near Surface Geoscience 2012 – 18th European Meeting of Environmental and Engineering Geophysics
- Conference date: 03 Sep 2012 - 05 Sep 2012
- Location: Paris, France
- ISBN: 978-90-73834-34-7
- Published: 03 September 2012
1 - 100 of 194 results
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Forensic Geophysics: how GPR Could Help Police Investigations
Authors P.M. Barone, C. Ferrara, E. Pettinelli, A.P. Annan, A. Fazzari and D. RedmanPolice regularly use GPR to uncover buried caches of drugs, money, weapons as well as locate unmarked graves. GPR's versatility and sensitivity to buried objects has lead to an ever widening use in forensics. While GPR does not deliver the fantasy results portrayed on some TV shows, GPR can provide powerful insight to forensics specialists needing to conduct non-destructively detailed subsurface site investigations.
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Comparing ERT Measuring Schemes for 3D Geoelectrical Investigation of Tumuli
Authors P. Tsourlos, M.J. Yi, J.H. Kim and N. PapadopoulosThe detection of tombs inside tumuli is an especially challenging geophysical problem. Geoelectrical methods by means of 3D inversion are increasingly popular for tumuli investigation. Typically data are obtained by establishing a regular grid and by assembling the data collected by parallel two-dimensional (2D) tomographies. In this work we extended the study into the application of radial 3D mode which is considered as the assembly of data collected by radially positioned ERT lines. We investigate the relative advantages and disadvantages of this measuring mode over the regular grid measurements and we propose optimum ways to perform 3D ERT surveys for tumuli investigations. Comparative test were performed by means of synthetic examples as well as by tests with field data. Overall all tested models verified the superiority of the radial mode in delineating bodies positioned at the central part of the tumulus while regular measuring mode proved superior in recovering bodies positioned away for the center of the tumulus. The combined use of radial and regular mode seems to produce superior results.
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The Use of Electrical Resistivity Tomography for an Archaeological Investigation in the Old Oslo Harbor, Norway
Authors A.A. Pfaffhuber, S. Bazin, K. Reiersen, H. Anschütz and M.J. LatoAn Electrical Resistivity Tomography (ERT) survey was conducted in downtown Oslo in conjunction with on-going archaeological investigations: boring, excavations and pollen analysis are presently being carried on at the site of the former medieval harbor. Seven ERT profiles were acquired to investigate subsurface conditions before the construction of a new road. The goal of this near surface geophysical survey is to detect possible archaeological artifacts in the marine clay layer down to 10 m depth without disturbing the construction site. Based on 3D visualization and integration with archaeological investigations, some resistive bodies localized by the ERT are identified as buried timbers or riverbed sediments. Other anomalies are of unknown origin, sampling is proposed in order to solve the ambiguity between recent structures and buried ships.
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Resistivity and Georadar Survey on the Nazca Lines
Authors A. Weller, M. Eidner, S. Rosas and K. HartschThe investigation of shallow structures beneath the Nazca lines aims at a better understanding of the alteration and weathering processes of the desert soils. Suitable geophysical, mineralogical, geochemical, and geotechnical methods should be identified. Our paper reports the application of a combined resistivity and georadar survey at a single test site. The resulting images of the two methods indicate similar structures. Georadar enables a fast and continuous data acquisition but is restricted in its depth of penetration in dependence of the nominal frequency. The slower resistivity imaging reveals structural features up to a depth of 3.5 m.
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Numerical Simulation of Capacitively Coupled Resistivity Imaging Measurements on Rock Samples
Authors S.S. Uhlemann, O. Kuras and A.G. GreenEstimation of electrical properties in permafrost studies can benefit from capacitive resistivity imaging (CRI), which overcomes the limitations of usual electrical resistivity imaging employing galvanic coupled electrodes. However, the response of these systems is not fully understood yet. We have used numerical finite element modelling to investigate the complex transfer impedance as measured by a quadrupole consisting of sensors with finite dimensions. Next to a parameter study over a homogeneous halfspace, also an actual experimental setup is modelled. The numerical results are compared to a quasi-static approximation based on point-poles and measured data, respectively. The parameter study shows that the real part of the complex transfer impedance can be approximated by the quasi-static approach in case the dipole separation is large compared to the sensor dimensions. It is shown that quantification of the imaginary part is difficult and depends strongly on the geometric setup. The results of the simulation of a 3D finite rock sample indicate that finite element modelling can be a practical tool for improving the understanding of the experimental data and for performing a better error analysis.
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Monitoring Rock-freezing Experiments in the Laboratory with Capacitive Resistivity Imaging
Authors O. Kuras, M. Krautblatter, J.B. Murton, E. Haslam, P.I. Meldrum, P.B. Wilkinson and S.S. UhlemannWe explore a new technology concept for the non-invasive volumetric imaging and routine temporal monitoring of permafrost-affected bedrock. Capacitive Resistivity Imaging (CRI), a technique based upon a low-frequency, capacitively-coupled measurement approach is applied to laboratory experiments simulating permafrost growth, persistence and thaw in bedrock with the aim of emulating Electrical Resistivity Tomography (ERT) methodology, but without the need for galvanic contact on frozen rocks. Experimental results using conventional ERT highlight the practical problems with maintaining adequate galvanic contact between steel electrodes and rock samples that are subjected to multiple freeze-thaw cycles. Multi-sensor CRI measurements on permafrost rock samples using newly developed CRI instrumentation demonstrate the feasibility of laboratory-scale 3D/4D imaging with capacitively coupled sensors. The results suggest that the CRI technique can usefully complement the characterisation and monitoring of permafrost rock samples. We expect that the methodology will allow us to obtain calibrated images of the temperature distribution in the sample. Controlled long-term permafrost physical modelling experiments are currently underway, monitored with CRI and ERT. Field installations of the new capacitive sensor technology are envisaged.
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Bayesian Estimation of Ice Content in a Rock Glacier from P and S Velocities
Authors P.-Y. Galibert, L. Bodet, C. Camerlynck, A. Dhemaied, S. Monnier and Q. VitaleRock glaciers are creeping ice-rock mixtures in permafrost environment which may represent significant solid water resources. In the present work the hydrologic importance of rock glaciers in mining districts of Norte Chico, Chile, is studied with seismic and GPR. Compression and shear waves velocities from travel time tomographies are used to estimate the ice content. Using a petrophysical model and Bayesian inversion, Vp Vs seismic velocities are translated into estimated porosities and volumetric fractions of ice, water and air. The average ice content of the rock glacier estimated from seismic data is 29%, in agreement with local ice measurements in a drill hole.
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Monitoring of Water Accumulation in the Tête Rousse Glacier (French Alps) using 3D Magnetic Resonance Imaging
Authors A. Legchenko, C. Vincent, J.M. Baltassat, S. Garambois, E. Thibert, A. Gilbert, M. Descloitres, J.F. Girard, O. Gagliardini and H. GuyardWe have developed and tested a 3D Surface Nuclear Magnetic Resonance imaging method (3D-SNMR), which is a new non-invasive geophysical tool for investigating 3D water distribution in the subsurface. Between 2009 and 2011 this method was successfully applied to investigation of a glacier body. In the Tête Rousse glacier (Mont Blanc area in French Alps), we have detected a large cavern containing about 50000 m3 of water, representing potential danger for local population. Basing on our results water was pumped out. After the pumping, we carried out a one-year monitoring of the glacier. Results show that after being drained the cavern was fully refilled again in one year but its volume diminished about three folds. All 3D-SNMR results were found in a good agreement with boreholes and pumping results. At our knowledge it was the first time when the 3D-SNMR was used for investigating glaciers.
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On the Eexploration of Mud Volcanoes using Electromagnetic Techniques: a Case Study from Azerbaijan
Authors B. Tezkan, H. Grossbach, J. Adrian, A. Haroon and A. Novruzov3 mud volcanoes in Perekishkul close to Baku/ Azerbaijan were chosen in order to study their conductivity structures by using shallow and deep electromagnetic techniques. Radiomagnetotelluric (RMT) and transient electromagnetic methods were applied to study the depth range up to 150 m. In order to examine depths up to 6 km, the so called long offset transient electromagnetic (LOTEM) method was used. RMT data were observed at 536 stations on 16 profiles crossing the mud volcanoes. The RMT data were then interpreted by 2D conductivity models. A 50m x 50m loop as transmitter and a 20m x 20 m receiver loop were used at 72 stations on 6 profiles for the TEM measurements. A laterally constrained inversion technique was applied on the TEM data to derive a quasi-2D conductivity model.750 m dipole was used as a galvanic source for the LOTEM method and a 33 A current was injected. Ex and dHz/dt components were measured at the receivers on a profile of the mud volcanoes. The 1D joint inversion of the LOTEM Ex and TEM data clearly resolve the conductivity structure up to a depth of 5 km indicating a relative high resistive hydrocarbon containing layer at this depth range.
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Shallow Seismic Investigations of an Abandoned Mine
Authors D. Arosio, L. Zanzi, L. Longoni and M. PapiniThe near-surface rock structure that covers an abandoned marl mine nearby the village of Montevecchia (Italy) was investigated through a combination of seismic surveys performed in two different survey campaigns. The methods selected for these investigations were Refraction Seismics and Multichannel Analysis of Surface Waves (MASW). A sort of transillumination experiment was also attempted. All the methods were successful and gave complementary information. Refraction seismics was used to characterize the shallower low velocity layers and a deeper high velocity layer of the near-surface structure. The MASW method was necessary to assess the existence of a velocity inversion revealing the presence of a low velocity layer between the faster layer identified by refraction seismics and a 4th high velocity layer covering the upper mine gallery. The transillumination experiment validated the presence of the 4th layer and gave an estimate of the average velocity that represents a lower boundary for the P-wave velocity within this layer. Both refraction and transillumination data were analysed to derive average estimates of attenuation level and rock quality factor.
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Holocene Activity of the Porto Alto Fault, Portugal
Authors J.P.G. Carvalho, R. Ghose and J. BorgesThe Lower Tagus Valley area (LTV), where Lisbon is located has been struck by several destructive earthquakes whose sources remain still to be determined. The identification of surface ruptures in the area is a challenging task that requires the use of geophysical techniques. This paper focuses on the identification, for seismic hazard purposes, of faults segments of the Porto Alto fault, one of the most important structures in the LTV. High-resolution P-wave seismic reflection data was acquired to confirm the fault activity in the Holocene but the lack of resolution to detect a fault of maximum 2m anticipated vertical throw in the 50m thick alluvium column was evident. We revisited the site to acquire high-resolution S-wave seismic and GPR data. The seismic profile shows reflectors interruptions in the stacked section, changes in amplitude/shape of the reflection hyperbolae in the shot gathers and coincident low velocity anomalies, indicating the presence of a fault segment at the anticipated location below 25m depth. The GPR profile reaches a maximum depth of about 15m and does not show the presence of any fault. Other geophysical techniques and trenching investigation are under consideration for corroborating the activity of the Porto Alto fault.
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Study of the Dynamic Response of Prone-to-fall Compartments Using Seismic Noise
Authors P. Bottelin, D. Jongmans, L. Baillet, D. Hantz, C. Levy, T. Lebourg, L. Lorier, J.D. Rouiller and J. TurpinThe dynamic response of four prone-to-fall compartments exhibiting diverse morphology, rupture mechanism, geological context and volumes has been studied using the resonance frequency technique. All sites exhibit well-defined spectral energy peaks, both in specific directions and at given frequencies. A predominant peak has been systematically measured at the lowest frequency, which has been interpreted as the first resonance frequency (f1) of the unstable compartment. The observation that the vibration direction at f1 is perpendicular to the rear main fracture at the four sites supports this interpretation and suggests that the first vibration mode is probably bending. These results show that the spectral analysis of the seismic noise might provide valuable information on unstable compartments, in various geological contexts. The first resonant frequency has also been monitored over a few months and did not show irreversible variations linked to damaging. By contrast, all sites exhibited reversible changes in fundamental frequency, clearly related to temperature variations but showing different patterns. These results suggest that the origin and control of the resonance depends on the site characteristics (volume, geology, morphology and rupture mechnism).
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Geophysical Investigations of Quick-clay Slide Prone Areas
Authors G. Sauvin, I. Lecomte, S. Bazin, J.S. L'Heureux and M. VannesteQuick clay is known to be a hazard in formally-glaciated coastal areas in, e.g., Norway, Sweden and Canada. The properties of quick clays are reviewed in order to find a suitable, integrated and multi-disciplinary approach to accurately identify the occurrence of quick clay and map their extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of methods with geotechnical data for an in-depth quick clay assessment of a given site. Such integrated approach allows moving towards a 2D or pseudo-3D site characterization for quick clays. The integrated approach is applied in practice on two Norwegian quick-clay sites. The collected data and preliminary site characterization will illustrate the high diversity of quick-clay grounds as well as the complexity related to an integrated approach.
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Correlation between 2D RMT and ERT Resistivity Models and High-resolution Reflection Data at a Quick Clay Site in Sweden
Authors C. Shan, M. Bastani, A. Malehmir and L. PerssonLandslides occur worldwide. Global damages range in billions of dollars and cost hundreds of lives each year; Sweden is not an exception. In this study, we have acquired radiomagnetotelluric (RMT), electrical resistivity tomography (ERT) and high-resolution reflection seismic along 4 profiles with the main aim of imaging the geometry of quick clays and associated unconsolidated sediments that have caused landslides in a pilot study area in southwest Sweden. Comparison between the electrical resistivity models from the 2D inversions of RMT and ERT data with the reflections observed in the seismic data suggests a good correlation between the small-scale structures and those observed in the RMT model. However, the ERT model better delineates the resistive bedrock than the RMT model. The resistivity models are also compared with the CPT-R data collected in two geotechnical boreholes and further confirm some of the layer boundaries imaged with the acquired surface geophysical data.
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Assessing Soil Moisture of a Heterogeneous Hillslope by Time Lapse Electromagnetic Mapping
Authors H.C. Paasche, D. Sauer, S. Popp-Hofmann and P. DietrichKnowledge about spatial and temporal variations of soil moisture is an important prerequisite for understanding and modelling the movement of large hillslopes. Traditionally, soil moisture is assessed by soil sample taking at a number of selected locations. Alternatively, geophysical and geoecological mapping have proven to be valuable tools when assessing soil moisture distributions. Here, we survey the Heumös hillslope in Western Austria using time-lapse electromagnetic monitoring. We can largely link relative electrical conductivity changes between different geophysical data acquisition campaigns to relative soil moisture variations as determined previously by geoecological soil moisture mapping relying on vegetation analysis.
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Geoelectric Monitoring to Investigate Landslide Dynamics
Authors R. Supper, B. Jochum, D. Ottowitz, A. Römer, I. Baron, S. Pfeiler, M. Lovisolo, G. Moser, S. Cardelini and J.H. KimAlthough the monitoring of temporal electrical resistivity changes has undergone an intensive boom within the past few years, there is still a lack of applications on landslides, allowing performing an evaluation of the methodology for monitoring and early warning. Therefore after the development of specialized equipment for geoelectric monitoring, a network of several monitoring systems was installed on several landslides in different geological environment to evaluate the applicability of the methodology. Results from two of the sites are discussed. At the site of Ampflwang geoelectrical monitoring could successfully monitor subsurface dynamics whereas at the test site of Ancona, several problems due to the low signal to noise ration were encountered. However based on these results it can be concluded that resistivity monitoring can help to map the subsurface processed, that accompany the triggering of a landslide.
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Topographic Migration of GPR Data - Case Studies from Dry Sand Dunes and Active Fault Areas
Authors J.R. Dujardin and M. BanoMost Ground Penetrating Radar (GPR) measurements are performed on nearly flat areas. If strongly dipping reflections and/or diffractions are present in the GPR data, a classical migration processing step is needed in order to determine the geometries of shallow structures. Nevertheless, standard migration routine is not suitable for GPR data collected on areas showing variable and high topographic relief. To account for the topographic variations the GPR data are, in general, corrected by applying static shifts instead of using an appropriate topographic migration which would place the reflectors at their correct locations with the right dip angle. In this paper we present an overview of Kirchhoff migration and show the importance of topographic migration in the case where the depth of the target structures is of the same order as the relief variations. Examples of synthetic and real GPR data are shown to illustrate the efficiency of the topographic migration. I am a PhD student and would like to be considered for the AGAP-Quality Award.
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Permittivity and Conductivity Reconstruction by Full Waveform Inversion of GPR Data using the L-BFGS-B Algorithm
Authors F. Lavoué, R. Brossier, S. Garambois and J. VirieuxFull waveform inversion (FWI) of ground-penetrating radar data is an emerging technique for quantitative imaging of the near surface, mainly through the estimation of the dielectric permittivity (ε) and of the electric conductivity (σ). Recent studies already succeeded to provide high resolution cross-hole images by FWI using conjugate gradient algorithms. In this study, we present a frequency-domain FWI algorithm based on the L-BFGS-B optimization which takes into account the Hessian influence in the steepest-descent direction correcting for dimensionalities between parameters. We discuss the impact of the parametrization for the simultaneous reconstruction of ε and σ, showing that a robust criterion is provided by the ratio of the gradient norms in the directions of ε and σ: the relative amplitudes of the gradients of the chosen parameters greatly impact the conditioning of the inverse problem. We show that the sensitivity of the cost function to the selected parameters needs to be taken into account in the re-parametrization. An illustration is provided using an already published benchmark. It demonstrates the great efficiency of the L-BFGS-B optimization method to deal with non-linearities of the inverse problem.
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A Second Order PML Implementation for FDTD Seismic Modelling
Authors A. Giannopoulos, S. Melling and D. ConnollyAn implementation of a second order unsplit perfectly matched layer (PML) absorbing boundary condition is presented. The formulation is given in the context of seismic modelling and simple models are used to showcase the increased performance. The formulation is presented as a correction procedure to the unchanged governing equations. This approach makes the implementation especially suited for existing FDTD modelling codes. Although, a second order PML might perform better in some cases than the classic or CFS-PML the increased computational requirements and the difficulty in optimising a greater number of PML parameters must be taken into account in deciding the most appropriate PML for a given seismic modelling problem.
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Quantitative Imagery of Shallow Structures with Multicomponent: Full Waveform Inversion and Physical Scale Modeling
Authors R. Valensi, D. Leparoux, O. Durand, F. Bretaudeau, R. Brossier and P. CôteIn near-surface quantitative seismic imaging, the mechanical properties of an heterogeneous medium are usually inferred from the measure of the normal velocity component at different locations. In this study, it is proposed to investigate the benefits of measuring also the horizontal velocity component. For that purpose, a realistic synthetic model is defined and the benefits of each component are analyzed in the framework of seismic imaging by Full Waveform inversion. The model is a shallow two-layer medium close and the synthetic data are generated using a visco-elastic finite elements code. An analysis of the information contained in the signals is carried out and the behavior of the inversion algorithm is studied for each component. The last part concerns the experimental modeling facility developed in order to experimentally validate the imaging methods. This measurement bench reproduces seismic measurement configurations at a reduced scale using an ultrasonic source and a laser interferometer. This facility has already been validated for the case of the measurement of the vertical component, and first experimental validation results of the horizontal component are presented.
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Alternative Objective Function for Inversion of Surface Waves in 2D Media
Authors C.A. Pérez Solano, D. Donno and H. ChaurisThe inversion of surface wave properties contributes to the creation of a near-surface model. In seismic exploration, the proper knowledge of the near surface can improve model building in depth. Most surface wave inversion approaches are based on 1D layered models. We propose here to estimate 2D model parameters by using a full waveform inversion approach with an alternative objective function formulated in the frequency-wavenumber domain. In the novel objective function, oscillations are reduced thanks to the exploitation of the dispersive behavior of surface waves that map into localized propagation modes in the frequency-wavenumber domain. Moreover, spatial windowing is used to allow local comparison of modelled and observed data. For the objective function minimization, a gradient-based approach will be used. We implement the adjoint-state method for an efficient gradient computation. We use simple velocity models to show the reliability of our ormulation to localize anomalies, by comparing the gradients computed with the classical full waveform inversion and the novel approach.
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Joint Inversion of Surface-wave Dispersion, P-wave Refraction and Apparent Resistivity Data.
Authors F. Garofalo, G. Sauvin, L.V. Socco and I. LecomteWe present here a joint-inversion algorithm to build a resistivity, P-wave, and S-wave velocity model from apparent resistivity, surface wave dispersion and P-wave refraction data. This algorithm can also include a-priori information available for the site, as well as any physical links among the model parameters, and the result is an internally consistent multi-parametric model. The obtained model resolves more properly the true model because the joint inversion mitigates some problems related to the individual inversion of each type of experimental data like solution non-uniqueness, illness, or lack of resolution, which might lead to interpretation ambiguities. We describe the proposed algorithm and we show the result of its application on a smoothly laterally varying synthetic model.
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Resistivity Modelling: Handling Source Singularity and Topography within a Single Step
Authors S. Penz, H. Chauris, D. Donno and C. MehlThe singularity of the potential occurring at the source location is a key point of electrical resistivity forward modelling because it might lead to large numerical errors. To tackle this problem a classical method consists of splitting the total potential into a primary part containing the singularity and a secondary part. The primary potential is defined analytically for flat topography but requires numerical computation in the presence of topography. In that case, an accurate solution happens to be computationally expensive. For any geometry we propose to keep for the primary potential the analytic solution defined for homogeneous models and flat topography, and to modify accordingly the free surface boundary conditions for the secondary potential. The primary potential still contains the singularity and new free surface conditions ensure that the total potential still satisfies the Poisson equation. The modified singularity removal technique thus remains fully efficient even in the presence of topography, without additional numerical computation. The modified secondary potential in a homogeneous model is not null in the case of topography as it would be in the classical approach. We implement the approach with a Finite Difference method. We present potential distributions computed with this technique to illustrate its versatility.
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4D Inversion of Resistivity Monitoring Data through Lp Norm Minimizations
Authors J.H. Kim, R. Supper, P. Tsourlos and M.J. YiA new four-dimensional (4D) inversion algorithm is developed so that any of data misfits and model roughness in the space and time domains can be selectively minimized either in terms of L1 norm or L2 norm. This study is motivated by the realization that a particular criterion of either L1 or L2 norm cannot be universally optimal for accurately reconstructing the subsurface condition. To overcome difficulties of jointly choosing two optimal regularization parameters for the inverse model constraints in the space and time domains, we devise automatic determination methods of two different kinds of the Lagrangian multipliers. We conducted inversion experiments using synthetic and field monitoring data to test the proposed algorithms. Both of the synthetic and field data experiments proved that the automatic determination method developed in this study is effective for calculating the ground changes that are closer to the ground truth. Inversion experiments showed that L1 norm minimization of the time-domain roughness could cure the problem of unnecessary smooth model changes when the subsurface changes are locally confined, but at the same time, the L2 norm approach would be more reasonable when the changes are expected widespread.
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Surface-wave Dispersion Stacking on a Granite-micaschists Contact at Ploemeur Hydrological Observatory, France
Authors S. Pasquet, L. Bodet, L. Longuevergne, A. Dhemaied, F. Rejiba, C. Camerlynck and R. GuérinIn the context of a geophysical survey at the Ploemeur hydrological observatory (France), we performed surface-wave profiling for the characterisation of shallow subsurface Shear-wave velocities. Since we anticipated lateral variations but needed great investigation depth, we deployed multifold acquisition geometries and used roll-along dispersion stacking to enable efficient measurements of multi-modal dispersion data. Several offset moving windows have been tested. Represented as pseudo-sections, the phase velocities extracted using a 12-trace window clearly showed three areas coherent with field observation and interestingly consistent with electrical conductivities and P-wave first arrival times. This cross-quality control has been of great help in the choice of the moving window size and revealed itself to be a rewarding step prior to the inversion process.
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The Near Surface Profiling Based on the Rayleigh Wave Dispersive Properties for Vector Displacement
Authors A.I. Konkov, A.V. Lebedev and S.A. ManakovIn the work presented the joint analysis of the frequency dependence of the Rayleigh wave velocity (dispersion characteristics) and the frequency dependence of the ratio between horizontal and vertical projections of displacement amplitudes was conducted. The inversion of the shear wave velocity profile together with the Poisson’s ratio was performed by satisfying the minimum root-mean-square deviation between the measured and calculated values. The Rayleigh wave velocity and the ratio between vertical and horizontal projections of displacement were measured. The density was assumed either constant or corresponding to the Gardner’s equation. Seismic waves launching was provided by different types of seismic sources: vertical vibrator (vertical force) mounted on the free surface, impulsive force source (blow of the hammer on an anvil) and pseudorandom source (two pedestrians walking along the line connecting the receiving geophones). The signals were recorded by two digital multichannel engineering seismic stations «Lakkolit X-M2» (produced in «Geotech», Moscow). The technique proposed in this paper enables eliminating the ambiguity of the inverse problem solution and reveals new possibilities for monitoring and diagnostics natural media in-situ.
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Precise Near-surface Interval Velocity Section Derived from High-resolution Shear Wave Seismic Data
Authors M.R. Sule, A.A. Valencia, A. Hendriyana, U. Polom and C.M. KrawczykPartial Common Reflection Surface (CRS) Stack and Normal-Incident-Point-wave (NIP-wave) tomographic methods are applied to high resolution shallow reflection seismic data in order to obtain enhanced image of subsurface with the target up to 1.2 s (shallow target) and more precise interval velocity section. A ca. 600 m long high-resolution multichannel SH-wave reflection seismic land profile in the Trondheim harbor area was accordingly reprocessed. Partial CRS Stack method is the generalization of ZO CRS-Stack which has an ability to provide more detail information about subsurface, which consist of emergence angle and the two radii of wavefront curvatures RNIP and RN. Those CRS attributes extracted from prestack seismic data by using optimization scheme and coherence analysis in order to obtain the best stacking surface in every ZO sample. Since the final product of Partial CRS-Stack is CRS supergathers, which are regularized and have better signal-to-noise ratio compared to original CMP gathers, one could implement better and easier velocity analysis after applying this method. Moreover, the CRS attributes could be used as input for NIP-wave tomography in order to determine macro velocity model in depth. This velocity model could then be used as input for Prestack Depth Migration.
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Physical Modelling of Seismic-wave Propagation over a Two Dimensional Granular Medium
Authors P. Bergamo, L. Bodet, L.V. Socco, R. Mourgues and V. TournatLaboratory small-scale physical models and non-contacting ultrasonic techniques are used to tackle theoretical or methodological issues of seismic wave propagation and seismic methods. Literature shows a wide range of experiments, both in terms of materials used for the production of the physical models but also regarding the models geometry and the recording techniques, which are chosen according to the issue that is being addressed. With a proper choice of granulometries and deposition processes, we managed here to create a two-layer granular physical model with a relatively complex geometry and characterized by 2D structures, property contrast and velocity gradients within layers. We performed several small scale seismic acquisitions using a mechanical source and a laser vibrometer. The acquired seismograms were interpreted by applying the surface-wave method and by extracting P-wave refraction data. We managed to correctly reconstruct the geometry of the model and estimated the parameters controlling the velocity gradients of P and S waves for both layers. The results we got are coherent with the different compaction degree we obtained for the two layers and with previous studies conducted over similar media.
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Gradient-based Seismic Inversion using a Finite Frequency Assumption for Imaging Velocity and Attenuation
Authors G. Grandjean, J. Gance, K. Samyn and A. BitriVarious studies operate seismic methods for imaging landslide structures. All those methods are more or less based on strong approximations and require complex data preprocessing. The issue of recovering the structural image of a landslide from the seismic velocity field estimated with an accurate, but not too unstable, method is thus posed. To solve it, we choose to revisit first arrival tomography approach which is a good compromise between the strong assumptions featuring simple refraction methods and the complexity of Full-Wave Inversion approach when used in very heterogeneous soils. The proposed method is based on a Quasi-Newton formulation to ensure an optimum convergence of the velocity model. We only use here the first arrivals of the seismic signal due to direct or refracted waves. Nevertheless, we show that some regularization strategies (Fresnel volumes approach, increasing finite frequencies) allow detecting sharp velocity variations tending to reach FWI performances.
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The Use of Seismic Methods for Imaging Complex Mineral Bodies in Hard Rock Environments
Authors S. Yavuz, K. Tertyshinikov, E. Strobach and M. UrosevicA seismic survey was designed and conducted at the Hillside mineral deposit, York Peninsula, South Australia as part of an experimental program within the Deep Exploration Technologies Corporative Research Centre (DETCRC). By using a low-power seismic source, low number of channels, specific survey geometries and small seismic crew we acquired high resolution 2D and a mini 3D seismic survey in a short time frame. The experiment was aimed as a proof of concept for using cost-effective, integrated high resolution surface seismic methods to map regolith and deeper structures in a complex hard rock environment. The forward outlook of such an experiment is the deployment of dynamic systems for the acquisition of seismic data such as land streamers or draggable geophone arrays. 2D seimic lines were subject to MASW, refraction and reflection studies. Produced P and S-wave velocity profiles provided a detailed map of elastic properties within the regolith, in good agreement with near surface geology inferred from high-density drillhole information. Reflection profiles and a mini 3D cube produced very good indications of sub-vertical mineralised bodies. Results of this comprehensive test survey suggest that cost effective seismic investigations could be a valuable tool for mineral exploration across the Yorke Peninsula.
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Rock Mass Classification Systems and Seismic Velocities
More LessAccording to their case study the authors recommend to use seismic velocities especially shear wave velocity to characterize the rock mass instead of classical rock mass classification systems. Their experimental work includes a case study comparing the RQD, RMR and Q-value with seismic velocities in a tunnel excavated in granitic rock. The best correlation of shear-wave velocity with RQD was about 90%. It means that using seismic transmission the rock properties are predictable by a nondestructive method. By sophisticated multivariant analyzis the connection between parameters were investigated in details.
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Assessment of Ground Compaction using Multi-channel Analysis of Surface Wave Data and Cone Penetration Tests
Authors K. Samyn, A. Bitri, S. Brûlé and E. JavelaudThe construction of a new industrial and commercial area has involved heavy dynamic compaction work. The objective was to improve the ground mechanical properties of 7 to 15 m of well-graded gravel backfills lying on the geotechnical bedrock. To assess the quality and depth of ground compaction, cone penetration tests are often performed before and after the compaction. The method is intrusive, covers only one location at a time and requires a substantial amount of time to evaluate a large area. The evaluation quality is very dependent on the operational technique and soil type. In this paper, the quality and extent of ground compaction were evaluated using the results from in situ Multi-Channel Analysis of Surface Waves (MASW) seismic method and cone penetration tests (CPTs). The MASW tests were used to determine the shear wave velocity (Vs) profiles before and after the compaction, and the CPTs were utilised to determine the correlation between the Vs and the measured penetration resistance (qc) improvement along the profiles. The results of this study demonstrate both the effectiveness of using surface waves for the evaluation of compaction performance and the potential application of this technique to engineering and environmental problems.
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Combined Analysis of Geophysical and Geotechnical Data for Investigating the Alluvial Plain of Beyrouth
Authors D. Jongmans, S. Nancy, D. Youssef Abdel Massih, F. Hage Chehade, C. Voisin and A. MariscalGeophysical methods are now increasingly used for sub-surface recognition. Geophysical and geotechnical tests were carried out on a site in -Beirut (Lebanon). The survey included 2 electrical tomography profiles and ambient vibration measurements using 2 passive seismic arrays with different apertures. The computed Rayleigh wave dispersion curves were inverted to derive vertical Vs profiles. Geological and geotechnical information was obtained through 14 boreholes with SPT tests. The combined interpretation of the different sets of data highlighted the strong lateral and vertical heterogeneity of the site, along with the relations between the geology, the geophysical parameters and the geotechnical properties (SPT). In particular, the combination of methods has allowed detecting a few m thick Low Velocity Zone (LVZ) of varying depth, corresponding to a soft clay layer embedded in coarser layers. Without prior information given by the other investigation techniques, the dispersion curve inversion would have led to erroneous interpretation in the Vs profiles.
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Estimating the Geotechnical Parameters from CSEM Monitoring Data at the City of 15th May, Egypt
Authors M.A. Atya, A. Olga Hachay, Y. Oleg Hachay and A. El Said RagabThe present work provides a technical procedure to estimate the geomechanical and geoelectrical parameters as key information of the dynamic state and stability of the massive structure. It involves the application of Control Source Electromagnetic "CSEM" approach developed by IGF UB RAS to image the ranked deformation levels in the massive structure. The Wide Profile System of Observation had been used to monitor the three component electromagnetic field along predefined measuring lines at Quarter 27 in the City of 15th of May, Helwan, Egypt. Four cycles of observation had been carried out in 2008, 2010, 2011, and 2012. The acquired data set had been subjected to analytical processing procedure to estimate the changes in the geotechnical parameters over time of observation. Interpreting the measured data concluded that the general dynamic state at the area is getting worse over the time, this is reflected in the crack's densities and positions, also on the changes in the lateral distribution of the geoelectric heterogeneity as indicator for water saturation. This is clearly evinced at profiles 2 and 5. Profile 7 represents a deformation case of instability close to destruction level, so that, it is recommended to annually monitor the massive structure beneath it.
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Correlation between HVSR Predominant Frequencies of Microtremor Records and Theoretical H/V Elliptical Frequencies
Authors K. Tokeshi and C. LeoIn the last decade, the H/V Spectral Ratio (HVSR) inversion has been explored as a low cost technique for estimating the Vs ground profile (shear wave velocity – thickness) of a site, since the HVSR curve can be obtained easily from 3 component microtremor records carried out at the ground surface. Recently, some researchers suggested that more detailed estimation of near surface layers might be inverted when the HVSR curve contains information on high predominant frequencies too. In this paper, the correlation between the experimental HVSR predominant frequencies and the minimum fundamental and/or higher mode theoretical Rayleigh H/V elliptical frequencies is discussed using available Vs ground profiles and microtremor records gathered at 19 Akita Kik-net stations in Japan. The fair correlation obtained in this case study suggests us that the assessment of theoretical Rayleigh H/V elliptical frequencies of fundamental and higher modes should be carried out to select the potential ground models before the inversion process.
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Joint Global Inversion of Crosshole P-wave and GPR Traveltimes - A Field Example
Authors M. Rumpf, J. Tronicke, U. Böniger and H. PaascheJoint inversion of different data sets is an increasingly popular strategy because it helps to reduce uncertainties and ambiguities in data analysis and interpretation. In this study, we use a particle swarm optimization (PSO) based inversion strategy to jointly invert crosshole P-wave and GPR traveltime data sets for the detailed characterization of near-surface sediments at a well-constrained test site in Horstwalde, Germany. At this site, the subsurface is characterized by sand- and gravel-dominated glaciofluvial sediments and exhibits a well-defined layering at the scale of our crosshole data, which allows for using a flexible layer-based model parametrization. Our inversion strategy includes the generation and analysis of a representative ensemble of acceptable models, which allows us to appraise uncertainties and non-uniqueness of the inverted velocity models. Analysing the final ensemble and comparing the resulting velocity models with borehole and direct-push (DP) data, demonstrates that the layered structure, which was found by PSO-based inverson, is in reasonable agreement with major variations in the borehole and DP-parameters. I am a PhD student and I would like to be considered for the AGAP-Quality Award.
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Cross-hole Seismic Travel Time Tomography Applied before Tunnel Excavation by using TBM
More LessA tunnel with a dual function was built in the city of Kuala Lumpur - Malaysia in order to solve flood and traffic congestion problems. Before tunnel was built by using Tunnel Bore Machine (TBM), several surface geophysical surveys were conducted in order to predict subsurface condition, especially the possibility of cavities inside limestone/marble of Kuala Lumpur Limestone formation to be occurred. Cross-hole seismic travel time tomography was conducted before the TBM excavated the subsurface below Istana Island area in Kuala Lumpur, Malaysia. This area is known as very difficult area, in which it is situated on a highway that is connected Kuala Lumpur and other cities in southern part. There is no possibility to conduct geophysical surveys, except cross-hole seismic tomography. The reprocessing of tomographic data is aimed in order to obtain better velocity model. The obtained result quality is improved, since the used inversion scheme is more stable in handling the data. Checker-board resolution test is also conducted in order to determine the tomogram areas that are reliable to be interpreted. As conclusion, it is hoped that this method could be accepted in the tunnel business if the TBM path must excavate though complicated surface situation.
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ERT Interpretation - Now you See it, now you Don't, now What?
More LessERT is an established technique for imaging 2D targets. ERT studies in areas of complex geology need careful planning so that local lateral variations and anisotropy do not distort or even mask the response of the targets to be located. ERT orientation must be chosen and, if possible, different orientations used to investigate the ground properly. However field conditions, available space, buildings, infrastructure and access to land use, can restrict the orientation and the length of the ERT profiles. The expansion of urban areas to old mining areas needs the location of abandoned shafts and underground works. Often there are no records of old mining activities and ERT can assist in the location of those works. Herein it is demonstrated that local geological conditions and space restrictions limiting the geophysical survey can distort and mask ERT response of old mining structures and, as shown, it is even possible to miss the targets completely. It is also proposed to use the square root of the ratio of model resistivities, obtained from the different ERT orientations, as a further tool for the diagnosis of 2D targets in anisotropic media.
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Excavation Damage Zone Mapping Using EDZ GPR Method
Authors P. Kantia, E.J. Heikkinen, T. Lehtimäki and M. SilvastPosiva and SKB are preparing for disposal of spent nuclear fuel deep in the crystalline bedrock. Fuel assemblies are planned to be encapsulated in copper canisters and placed in wells into tunnel floor. Disposal tunnels will be constructed using Drill and Blast method which unavoidably causes EDZ, being one concern in terms of long term safety as it may constitute a hydraulic flow path. As a non destructive and effective method the GPR technique was introduced for EDZ characterization. The GPR EDZ method was tested in several field campaigns in Äspö and in Olkiluoto. High frequency GPR antenna was used in the work on lines parallel to the tunnel. On high frequencies the resistivity is highly dispersive. Detection of the EDZ is based on computing of dispersivity index in a moving window. The EDZ data can be displayed as profiles, maps or volume visualisation. The developed GPR EDZ technique proved to be a quick and effective in locating areas suffering EDZ. The EDZ was indicated to be discontinuous in character. The next stage of method development will use rock sample data and other geophysical methods in verification. Hydraulic conductivity of EDZ volume is also necessary to define.
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Characterization of Textural and Hydric Heterogeneities in Clay-rock with Induced Polarization Method (Aveyron, France)
Authors P. Cosenza, G. Okay, A. Ghorbani, C. Camerlynck, J. Cabrera, N. Florsch and A. RevilThe extension of the excavation damaged zone (EDZ) has been investigated thought electrical resistivity and induced polarization methods in time and spectral domain from three galleries belonging to the French Institute of Radioprotection and Nuclear Safety (IRSN)’s experimental underground research laboratory of Tournemire (Aveyron, France). Combined electrical resistivity and Induced Polarization methods show the possibility to delineate textural changes associated to desaturation of the clay-rock induced by the ventilation of galleries. The impact of the desaturation is particularly observed on the gallery’s walls. In addition, Time domain induced polarization indicates the presence of mineralization (e.g., especially pyrite) located in the structural discontinuities such as tectonic fractures (mm-cm), tectonic fault (m) and calcareous nodules (cm). We cannot see these nodules on the resistivity tomogram because the contrast in resistivity between these nodules and the matrix in which they are embedded is too low. Also, the resistivity tomograms do not allow distinguishing between the calcite-filled tectonic fractures and the new fractures which are associated with the mechanical damage (both of them show high resistivity values).New fractures and hydric fractures associated with hydric changes of the formation exhibit low values of chargeability.
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High Resolution Electrical Resistivity Tomography in Superficial Limestones at Tournemire Site, France
Authors C. Gélis, M. Noble, J. Cabrera, H. Chauris, S. Penz and P. DesveauxDeep argillaceous formations are considered in many countries as potential host media for high-level long-life radioactive waste due their confining properties. The precise sedimentary, structural and hydrogeological characterization of such potential host sites is a key point in determining their appropriateness for the long-term deep underground disposal of radioactive waste in geological formations. The presence of faults in clay–rock formations should be carefully assessed, since these features could modify the confining properties. This study focuses on testing the potential of the electrical resistivity method to detect fault or fractured zones in the near subsurface layers above an argillaceous formation. We present in this paper results from a high-resolution electrical resistivity survey carried out at the IRSN Tournemire Experimental Platform (TEP). The electrical resistivity profile was located transversely to the fault and fractured zones location, inferred from geological data, that affect the Jurrassic formations at the TEP. Electrical resistivity data were successively acquired with 8m, 4m and 2m-electrode spacing. This multi-resolution acquisition allows to investigate the near subsurface limestones and dolomites to a depth of 100 metres. In particular, two sub vertical conductive corridors reaching the surface through higher resistive layers are correlated with fractured zones.
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Robust Acoustic Waveform Inversion from Underground Galleries to Image Strike-slip Faults in Clay-rock Formations
Authors F. Bretaudeau, C. Gelis, J. Cabrera, D. Leparoux and P. CôteDeep argillaceous formations are potential host media for high-level long-life radioactive waste due to their confining properties. In order to improve its ability to expertise possible projects of radioactive waste disposal in a geological clay-rock formation, the French Institute for Radioprotection and Nuclear Safety led various experiments in the undeground Tournemire Experimental Platform (TEP). The presence of strike-slip faults with small vertical offsets in clay-rock formations, as they are observed in the TEP, must be well assessed since they could modify the confining properties of the host rock. The TEP is composed of several galleries excavated in a 200 m thick Toacian clay-rock layer. Various studies have shown this type of fault is difficult to detected with seismic reflection from the surface or from galleries. Thus we tried to image a 10 m thick strike-slip fault with a transmission survey involving several galleries. We present here how the use of a robust adaptation of weighted acoustic full waveform inversion can provide a high resolution P-wave velocity model and revealed a much more complex structure than expected.
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CO2 Field Laboratory at Svelvik Ridge (Norway). Electrical Resistivity and Sonic Monitoring during CO2 Shallow Injection
Authors P.A. Pezard, N. Denchik, J. Paris, M. Geeraert, G. Henry, O. Jaafar, L. Baqué, S. Barry and D. NeyensThe CO2FieldLab shallow injection experiment is a mutual effort from several research groups and organizations (SINTEF, NGI, BRGM, BGS, CNRS, imaGeau and Schlumberger). The objective is to create a downhole leakage of CO2 in order to determine sensitivity of CO2 monitoring systems with respect to CO2 distribution and leakage detection. The CO2 displacement in the subsurface and at the surface was monitored with an exhaustive set of techniques. For this, the field Laboratory for monitoring CO2 migration and leakage was established in glacial deposit that forms Svelvik ridge, 50 km south of Oslo. The shallow injection experiment was conducted in fall 2011 with a monitoring plan designed to spatially and temporally monitor the expected plume development. The monitoring equipment was distributed around the 20m deep injection point of an inclined well. An integrated set of surface and downhole strategies was deployed across a 64m2 square monitoring area. The data recorded by CNRS are presented here. The CNRS techniques included: permanent downhole electrical resistivity observatory, time-lapse downhole induction and sonic logging. The CO2 plume was successfully detected and characterized by these methods even if the trajectory of migrating CO2 deviated from the monitoring predictions.
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TAU-Transformation of Time-Domain IP Data Measured Above a Hungarian Refuse Dump
Authors E. Turai, M. Dobróka, Á. Gyulai, N. Paripás and J. Somogyi Molnáre values of the time constant spectra. This method was tested over several contaminated sites. In this extended abstract we show a new interpretation result measured over a refuse dump in Hungary near Felsőtelekes. The analysis of the IP data shows that there are some very high contaminated parts in waste-rock bodies. The types of the polarization were mainly redox and metallic.
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Electrical Resistivity Monitoring on an Experimental Landfill Clayey Cover
Authors F. Genelle, C. Sirieix, J. Riss, S. Renié and P. BégassatIn France, a cover is set on the top of waste at the end of the exploitation of the landfill. The monitoring of landfill cover after closure of the site is a local problem, since its tightness must be ensured over time. Leaks in the cover allow water to infiltrate the stored waste. In order to define the ability of electrical resistivity tomography, an experimental clayey cover has been built in which defects have been intentionally made. Repeated measurements taken on this site showed that the detection of defects need to take into account the weather conditions preceding the measurements, as they affect the water content in the cover material. A statistical analysis carried out on the electrical resistivity values for all surveys at different dates has defined four clusters. Moreover, analysis performed on several cover material samples corresponding with the different clusters showed that the material was heterogeneous, because of a difference in particle size (fines content). This study has outlined, in addition to the detection of cracks, that electrical resistivity varies with the water content measured on site exponentially for each cluster.
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Electrical Resistivity Tomography and Transient Electromagnetic Survey in Luikonlahti Tailings Impoundment Area
Authors T.K.T. Huotari-Halkosaari and J.M. LerssiElectrical resistivity tomography (ERT) and transient electromagnetic (TEM) surveys were done in Luikonlahti tailings impoundment area, Eastern Finland. Purpose of the measurements were to find the structure of the tailings impoundment area and possible seepage areas. Totally 5 ERT profiles and 4 TEM profiles were measured in the tailings area. ERT and TEM measurements revealed clearly the structure of tailings impoundment. Results of the both methods agreed and coincide with the drilling results.
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Estimation of Gas Frequency Characteristics to Identify Microfault at Terra Drilling Site
Authors W.W. Parnadi, A.M. Nabeilah, A. Muis, R.P. Hutapea and T. MeidianaWe investigated the frequency characteristics of processsed GPR signals influenced by gas at a drilling site in ARTA block, Pelalawan,Riau, Indonesia. In the drilling site we found some areas with gas bubbles migrating from subsurface. This investigation was done to examine the appropriateness of this area for drilling activities. By using GPR methode we obtain radargram for 6 measurement lines. Furthermore, we calculated the frequency characteristics to find migration mechanism of the gas bubble emerging in the surface. From 9 frequency characteristics we find major differences between gas and non-gas area in subsurface enabling us to estimate gas migration mechanism. We got information of medium characteristics from two parameters, spectral bandwidth(fb) and variance σ. From aanalyzing these two frequency characteristics , we conclude that the presence of gas bubble is most likely caused by the migration of gas to the surface as the result of the existence of micro-fault. Therefore this area is classified as potential geohazard area.
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Quantifying the Volume of Potential Landslides: a Case Study
Authors F. Bahloul, T. Haenssler, D. Orlowsky, R. Elsen and S. ThorDestabilized hillsides and potential landslides can be the cause for large scale safeguarding measures at construction sites. The volume of destabilized material plays a major role in determining the amount and cost of safeguarding measures. In this case study we present the process of estimation of potential landslide volumes using geophysical methods in a geologically complex environment. The field work for this case study was carried out at potentially hazardous locations on the construction site of the railroad and highway leading from the city of Sochi, Russia to the venues of the Olympic winter games 2014.
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The Accident at Sayano-Shushenskaya Hydroelectric Power Plant (Seismologist View)
Authors V.S. Seleznev, V.M. Soloviev and A.V. LiseikinOn 17 August 2009 Sayano-Shushenskaya Hydroelectric Power Station suffered a catastrophic accident. In the “Report of technical investigation of the accident cause” as the main cause of the accident it was called “repeated appearance of additional loads of variable nature on the hydroturbine unit”. But what kind of loads, what is their source? The research presents the method, which allows to control by seismic records not only work of various sources, but also to determine parameters of their dynamic forces to the structure.
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The Influence of Electromagnetic Soil Properties on EMI and GPR Sensors in Landmine and UXO Detection
Authors J. Igel, H. Preetz, K. Takahashi and S. AltfelderElectromagnetic induction technique (EMI) has been used for landmine detection for a long time. Recently, ground-penetrating radar (GPR) is used more and more often as a stand-alone detector or in combination with EMI sensors. Both techniques are strongly influenced by electromagnetic soil properties, albeit in a different manner. There is a lack of information on electromagnetic soil properties of different soils under various conditions and on how these properties influence different detectors. We present measuring techniques and results of lab and field measurements of physical soil properties and their spatial variability on different scales. A classification system is deduced for magnetic susceptibility and its frequency dependence that can be used to predict the soil influence of tropical soils on EMI sensors and a performance map is designed for the country of Angola. Further, several dual sensors (EMI + GPR) were tested on different soils. The soil influence as predicted by geophysical field investigations and analysis of the electromagnetic soil properties and their spatial variability is in good accordance to the detector performance of the dual sensor test.
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2D-Electrical Resistivity Imaging for Sike Survey: Impact of the a Priori Information Management
Authors Y. Fargier, S. Palma Lopes, C. Fauchard, D. François and P. CôteIn France, EDF (Électricité De France) is in charge of managing the large hydraulic structure stock dedicated to producing hydroelectricity. This responsability is associated with high economical and safety issues. Electrical Resistivity Imaging (ERI) is being widely used on earth embankment dikes, either for extensive survey of large sections (a few km) or for higher resolution surveys on shorter sections (a few 100m). However, the processing of extensive longitudinal survey with conventional 2.5D software is likely to yield image artefacts. This study aims to show that a bad management of the dike complexity in the inversion process is the origin of these artifacts. To overcome this limitation, a new inversion algorithm specially created to take into account the complex behaviour of the dike is presented. This development aims at filling the gap between more classical 2D and purely 3D approaches, by optimising a compromise between acquisition cost and model reliability. Applied to real data sets this tool supplies a new understanding of the interpretation capability of the inversion results.
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Volumetric Remote Monitoring of an Earth Embankment: Part I - Geophysical – Geotechnical Property Translations
Authors C.J. Munro, J.E. Chambers, D.A. Gunn, P.B. Wilkinson, P.I. Meldrum, M. Kirkham, J. Wragg, S. Holyoake and O. KurasIn this work we describe a study where automated time-lapse electrical resistivity tomography (ALERT) monitoring technology has been installed on a section of Victorian embankment on the Great Central Railway (Nottingham, United Kingdom). Raw datasets collected by the ALERT system have been processed/filtered, and inverted to yield a 3D resistivity distribution which is temperature corrected and converted to gravimetric moisture content using a relationship established by laboratory testing. Electrical resistivity tomography monitoring has been used to characterize the internal structure of the embankment, and image moisture content changes and wetting front development at a high spatial resolution. Monitoring has been undertaken at the test site to determine seasonal temperature changes in the subsurface; these data have been used to correct for temperature effects. We fitted the resistivity data as a function of gravimetric moisture content by modifying the Waxman-Smits model. Using results from laboratory testing, a best fit is computed and used to establish a resistivity, gravimetric moisture content relationship, used to facilitate property translation from temperature corrected resistivity to gravimetric moisture content. These results indicate that ERT has potential to identify structures and processes related to instability at an early stage in their development.
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Fissure Detection in Flood Embankments using Electrical Resistivity Tomography
Authors G. Jones, P. Sentenac and M. ZielinskiFissures forming due to desiccation in clayey soils have been shown as a potential source of embankment failure during overtopping. Current methods of visually identifying fissures in flood embankments are inadequate particularly when faced with dense vegetation. The extent of fissuring cannot be discovered without the use of trenching, which is costly and destructive to the embankment structure. Electrical Resistivity Tomography (ERT) offers a non-invasive method for appraising the embankment structure. The presented study compares the use of a survey using ERT on the Thorngumbald embankment, near Kingston Upon Hull, UK, with previous laboratory studies on scale models using miniature resistivity arrays. Large scale scans across the slopes of the embankment showed areas of confirmed fissuring, while miniature arrays were used to fully map the fissures in the subsurface. The positions of surface fissures matched with those resolved from the resistivity data and the results compare favorably with those achieved in the laboratory. Additionally scans were taken across the embankment revealing serious fissuring on the landward face, the side most affected by heavy vegetation.
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Detection of Ground Movement using the Shape of Brillouin Spectrum
Authors E Buchoud, S. Blairon, G. D'urso, J.M. Henault, A. Girard, J. Mars and V. VrabieDistributed Optical Fiber Sensing systems (DOFSS) are composed by optical fibers wrapped in strain sensing cables, coupled with Brillouin interrogators. DOFSS are increasingly used for Structural Health Monitoring (SHM) as they can provide continuous strain profiles along the optical fiber localized in the structure. Raw Brillouin measurements consist in gain – frequency curves with a Lorentzian shape. Strain is generally assessed thanks to the abscissa of the maximum of the gain curve. Two new factors are introduced. They are sensitive to asymmetry and broadening of the Brillouin gain curve which can highlight strain gradient within the spatial resolution of the interrogator. These parameters could be used to detect more efficiently local events and improve instrument algorithm.
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The Robustness and General Applicability of Optimal Resistivity Surveys Designed by Maximising Model Resolution
Authors P.B. Wilkinson, J.E. Chambers, P.I. Meldrum, O. Kuras and C.J. MunroMost optimal survey design algorithms for resistivity imaging have not incorporated prior knowledge of the resistivity of the subsurface. The resulting surveys are optimal for a homogeneous earth, but little investigation has yet been carried out to test whether they are robust, i.e. that they remain optimal when applied to imaging heterogeneous subsurface resistivity distributions. This paper compares a generic survey, which is designed to maximise the estimated model resolution evenly across a homogeneous earth, with specific surveys similarly designed for a number of heterogeneous resistivity distributions. In terms of both the average estimated model resolution and the correlations between the inverted and true resistivity models, the generic and heterogeneous survey designs give near-identical results. This suggests that surveys designed using homogeneous earth approximations are robust in the presence of resistivity heterogeneities and are therefore generally applicable. Traditional dipole-dipole surveys with the same number of measurements do not give such good inverted images, and their degree of optimality (measured either by average resolution or image correlation) is less robust in the presence of heterogeneity.
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Simultaneous Inversion of Time-domain SIP Data
More LessWe developed a new inversion algorithm of time-domain SIP data in which the entire multi-channel data sets are simultaneously inverted. In the proposed algorithm, the channel numbers are incorporated as a new dimension. We define the multi-channel data sets of chargeability data as a single data vector in the space and channel domains, and similarly many chargeability distributions as a single changeability model which spans in the space-channel domain. These definitions allow us to introduce inverse model regularizations not only in the space domain but also in the channel domain. We demonstrated the performance of the proposed algorithm through numerical experiments as well as real data application; the new inversion approach can generate a reasonable chargeabilty model in the space-channel domain in which the curves of the inverted chargeability are smoothly and monotonously decaying. This performance of the proposed algorithm enables us to fully utilize the own capabilities and merits of SIP method.
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A Fast Least-squares Method for Inverse Modeling of Gravity Anomaly Profiles due Simple Geometric-shaped Structures
By K.S. EssaAn inversion technique using a fast least-squares method is developed to estimate, successively, the shape factor (q-parameter), the depth (z-parameter) and the amplitude coefficient (A-parameter) of a buried structure using normalized residual anomalies obtained from gravity data. By defining the anomaly value at the origin and the anomaly value at different points on the profile (N-value), the problem of shape factor estimation is transformed into a problem of finding a solution of a non-linear equation of the form f(q)=0. Knowing the shape factor and applying the least-squares method, the depth is estimated by solving a nonlinear equation of the form ψ(z) = 0. Finally, knowing the shape factor and the depth, the amplitude coefficient is determined in a least-squares way using a simple linear equation. This technique is applicable for a class of geometrically simple anomalous bodies, including the semi-infinite vertical cylinder, the infinitely long horizontal cylinder, and the sphere. The technique is tested and verified on a theoretical model with and without random errors. It is also successfully applied to real data from mineral exploration in India, and the interpreted shape and depth parameters are in good agreement with the known actual values.
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Modeling and Inversion of Self-potential Anomalies due to Sheet-like Bodies under the Presence of Arbitrary 2-D Resistivity Distribution
Authors P. Tsourlos, I. Giannakis, C. Papazachos and G. VargemezisVarious methods have been proposed to interpret the self-potential anomalies due to sheet-like bodies. A common point of all existing interpretation methods is the assumption that earth is geoelectrically homogeneous. We propose a non-linear inversion approach for SP anomalies which finds the optimized sheet-like body for a given arbitrarily resistivity distribution. Tests of the new technique with synthetic data suggest that it can produce more accurate and reliable results. Further, test with field data demonstrate the applicability of the scheme in real problems.
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Layered and Laterally Constrained 2D Inversion of Time Domain Induced Polarization Data
Authors G. Fiandaca, J. Ramm, E. Auken, A. Binley and A.V. ChristiansenIn a sedimentary environment, quasi-layered models often represent the actual geology more accurately than smooth minimum-structure models. We have developed a new layered and laterally constrained inversion algorithm for time domain induced polarization data. The algorithm is based on the time transform of a complex resistivity forward response and the inversion extracts the spectral information of the time domain measures in terms of the Cole-Cole parameters. The developed forward code and inversion algorithm use the full time decay of the induced polarization response, together with an accurate description of the transmitter waveform and of the receiver transfer function allowing for a quantitative interpretation of the parameters. The code has been optimized for parallel computation and the inversion time is comparable to codes inverting just for direct current resistivity. The new inversion algorithm retrieves consistent values for both the Cole-Cole parameters and the layer thicknesses and is a promising tool for identifying formation boundaries, e.g. in for discriminating sand and clay layers or pollution fans, due to the chargeability of these layers.
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Results of Geophysical Surveys to Locate Zones of Increased Fracturing of Rocks,Zones of Filtering Groundwater and to Identify Placements of Wells
Authors A.F. Volynin and A.P. PodsevalovComplex geophysical studies were conducted in the area of the settlement “Selchostechnika” for the selection of sites for placement of water wells for the monitoring observations to determine the cause of flooding of the territory and the basements of houses. The complex of methods included the method of spectral seismic profiling (SSP) (mapping areas of fractured limestone) and the method of vertical electrical sounding (VEZ) on sites allocated SSP. As a result of the work, it was found area of extensively damaged fractured limestone, within which identified two linear tectonic zones of heavy water-saturated rocks. It was specified placements of three bushes of wells (depth of wells of up to 70 m). The nine wells entirely confirmed the recommendation. It was carried out monitoring of the changing of the hydro geological regime and composition of groundwater. The applied method can be used to solve analogical problems on the areas of fractured limestone
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Evaluation of the GPR Effectiveness from the Results of Measurement of Dielectric Properties of Soils
Authors Z.B. Khakiev, K.Y. Kislitca, Z.B. Khakiev and V.A. YavnaThe work considers the depth evaluation of Ground Penetrating Radar (GPR) surveys using the attenuation factor of electromagnetic radiation in a medium. A method of determining the attenuation factor of low-conductive non-magnetic soils is developed based on the results of direct measurements of permittivity and conductivity of soils in the range of frequencies used in the practice of GPR. The method relies on measuring the shift and width of the resonance line after a soil sample is placed into a tunable cavity resonator. The advantage of this method is the preservation of soil structure during the measurement.
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GPR Method for Determining the Electrical Properties of Soils
Authors Z.B. Khakiev, A.V. Morozov, V.L. Shapovalov and V.A. YavnaThe paper studies the possibility of quantitative processing the GPR data obtained in a laboratory, and determining the dielectric permittivity and conductivity of a medium. Experimental studies are made using radargrams of soil samples with specified values of moisture and conductivity. The methods of quantitative processing of radargrams are proposed to determine dielectric permittivity and conductivity of soils. The results of theoretical studies and experimental measurements are in good agreement and presented in the paper.
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Geophysical Investigations of Early Middle-ages Turkic Fortresses
Authors S.A. Erokhin, I. Modin, A. Pavlova and M. KatsThe investigation of early middle-ages Turkic sites Is a very complicated task because of the shortage of written sources, huge amount of archaeological excavations needed and the low contrast between the physical properties of buildings and virgin layer. The paper presents the capabilities of integrated geophysical researches for the investigation of early middle-ages Turkic sites. The paper describes the investigations of mud brick-made Djankent site (Kyzyl-Orda region, Kazahstan) and Semikarakorsk site (Rostov region, Russia). The results of integrated geophysical surveys enable to determine the planning of the site, the construction of buildings made of filled soil and mud brick, and to achieve valuable information for historical and paleogeographic reconstructions.
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Use of Geophysics Methods to Characterize the Hydrology of Assateague Island National Seashore, Worcester County, Maryland
Authors C. Johnson, J. Masterson, E. White, E. Voytek, P. Joeston, B. Fleming and J. LaneA suite of borehole and surface geophysical methods was used to characterize the lithology and salinity of the subsurface of Assateague Island, a barrier island on the Atlantic Coast of Maryland and Virginia. The U.S. Geological Survey, as part of its Climate and Land Use Change Research and Development Program, is conducting a multi-year investigation to assess potential impacts to the natural resources of Assateague Island National Seashore that may result from changes in the hydrologic system in response to projected sea-level rise. The geophysical data collected as part of the characterization phase of this investigation will serve as the foundation for development of numerical groundwater-flow models to assess the potential effects of climate change on the coastal groundwater system of Assateague Island. Geophysical data were collected along five transects across the island. Methods used included borehole electromagnetic induction and gamma, and surface time-domain electromagnetics, frequency-domain electromagnetics, ground-penetrating radar, and coupled-capacitance resistivity. Two of the transects were in areas subjected to frequent and periodic overland wash and two of the transects were in the oldest, most stable part of the island where there is no overland wash. Based on an integrated interpretation of the geophysical data, it was concluded that the geophysical methods used are effective for determining the following: approximate location of the freshwater/salt-water interface, the distribution of fresh and brackish water in the shallow aquifer, the geometry of the fresh groundwater lens beneath the island, and the distribution and thickness of fine-grained lagoonal and peat deposits in the subsurface throughout the Island.
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ALIS - GPR 3-D Imaging for Humanitarian Demining
Authors M. Sato, K. Takahashi and Y. YokotaMore than 10 years has passed since the Ottawa Treaty or the Anti-personnel Mine Ban Treaty became effective in 1997, humanitarian demining is still important and unsolved problems in may mine affected courtiers. Antipersonnel mines have been left buried in many of regions where conflicts have occurred, and after many years, the land has to be returned to the local people.
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Gravity-measured Water Storage Change and Subsurface Hydraulic Properties at a Managed Recharge Facility in Tuscon, AZ, USA
Authors J. Kennedy, B. Creutzfeldt, T. Ferre and A. GuntnerThe storage of subsurface water is central to Arizona’s future sustainable water supplies. Presently large amounts of water delivered from the Colorado River by Central Arizona Project canals (over 400,000 acre-feet/year permitted capacity) are stored underground at artificial recharge facilities to augment future water supplies. As the demand for Colorado River water increases and the energy required to transport, store, and recover this water becomes more costly, the importance of proper management and siting of recharge facilities increases. In this study, we combine different gravimeters (superconducting, absolute, and spring gravimeters) and advanced analysis methods to improve monitoring of subsurface storage and the characterization of subsurface hydraulic properties at Tucson Water’s Southern Avra Valley Storage and Recovery Project (SAVSARP) infiltration basins. Waterstorage changes will be monitored continuously with two iGrav™ superconducting gravimeters. These observations will be supported by time-lapse monitoring of spatial gravity variations with an A-10 absolute gravimeter and spring gravimeters. We will test novel approaches to gravity monitoring network design that make intelligent use of different gravimeter types for hydrological applications. These data will be analyzed in a coupled hydrogeophysical inverse framework to estimate groundwater-storage changes and to constrain effective hydraulic properties at the field scale. Here we will present the study concept, preliminary measurements, and modelling results.
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A Novel Surface NMR Pulse Sequence for Improved Estimation of Longitudinal T1 Relaxation Times
Authors E. Grunewald and D. WalshObtaining estimates of permeability from NMR methods requires the ability to measure relaxation time parameters that are most sensitive to pore geometry. While surface-NMR (SNMR) measurements can reliably quantify the effective transverse relaxation time T2*, transverse relaxation times are sometimes dominated by the influence of magnetic geology. The longitudinal relaxation time T1 is known to provide a more robust link to pore geometry, but previous attempts to measure T1 by SNMR, have shown mixed success due to fundamental limitations the commonly applied “pseudo-saturation recovery” (PSR)experiment. Recognizing a need for improved measurements, we have developed a new paradigm of experiments for quantifying T1, which we refer to as the “crush recovery” (CR)sequence. The CR sequence utilizes two pulses: a first pulse of fixed amplitude and a second smaller pulse with an amplitude that is varied between measurements. The first pulse of the CR sequence is not intended to generate saturation with 90 degree tip angles, as in the PSR sequence; rather, the initial pulse acts to crush or decoherently scatter the magnetization orientation, leaving an effectively saturated condition of zero net longitudinal magnetization over a range of shallow to intermediate depths. A second pulse, applied after a short delay, is then used to probe the magnetization that recovers by T1 between the two pulses. The amplitude of the second pulse is varied to provide depth resolution of T1 recovery within the “crushed” zone. Fixing the amplitude of the initial pulse provides constant initial conditions, such that the data measured after the second pulse can be inverted using the standard SNMR imaging kernel. Inversion of a CR dataset for a single delay time yields depth-separated free induction decay signals where the amplitude of the signal components are linearly weighted by the observed T1 recovery at each depth. Given a complete set of CR data acquired with multiple delay times, the inversion provides estimates of both T1 and T2* as a function of depth. Further, the covariance of the observed signal with T2* and T1, provides the opportunity to estimate a two-dimensional distributions of T2* versus T1 at each depth. We illustrate the success and advantages of this new approach though a combination of synthetic and real field data examples.
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Mapping Seismically Masked Seabed Structures with a New DC Resistivity Streamer
More LessWe developed an underwater electromagnetic-based streamer to image the near-shore marine substratum. The system was tested to assess its feasibility to map the electrical resistivity of the near-surface geological seabed. The results are very encouraging to characterise heterogeneities in areas with seismic masks. We present results in two regions, one with submerged karstic structures and the other with shallow free gas
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A Bayesian-based Methodology to Detect Suspicious Electrodes for Long-term Permanent ERT Monitoring Experiments
Authors J. Deceuster, O. Kaufmann and M. Van CampDuring long-term ERT monitoring experiments, the data quality may reveal time-dependent mainly due to changes in galvanic contact resistance between buried electrodes and soil. Identifying suspicious electrodes in a permanent spread is of major importance as a faulty electrode may affect the quality of tens to hundreds of measurements on each time-slice. An automated methodology was developed to detect suspicious electrodes based on a Bayesian approach. This methodology allows pointing out faulty electrodes based on the analyses of temporal sets of measurements, each one containing multiple electrode arrays. Standard and studentized estimators of the influence of each electrode in the global data quality are computed for each time-slice based on the measurement quality factor Q given as a coefficient of variation of repeated measures. The automated detection of faulty electrodes is obtained by comparing the computed studentized estimators to values expected when every electrode can be considered as good for the given data set. These expected values are computed by Monte Carlo simulations using a distribution of Q factor values of quadripoles selected as good based on reciprocal errors. The efficiency of the proposed methodology is assessed on a field experiment.
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A Method to Detect Displacements of Borehole Electrodes through Electrical Resistivity Tomography
Authors P. Tsourlos, I. Giannakis, N. Papadopoulos and A. GiannopoulosIn this work a new geophysical methodology is presented which is able to detect any displacements of buried electrodes that are permanently installed in boreholes. These displacements can seriously affect the quality of the measurements and the subsequent resistivity inversion image. The exact knowledge of the electrode displacements will either help in correcting the electrode coordinates and the geometric factors of the measurements associated with these electrodes or just to reject all corresponding erroneous measurements. During this method pole-pole type measurements are conducted, where the current is injected in every buried electrode in the borehole and the potential is measured on a number of electrodes that are spread out along a line on the ground surface. A finite-difference least-squares non-linear inversion algorithm with damping constraints has been developed in an effort to recover the accurate coordinates of the borehole electrodes. Although a priori resistivity model resulted by a surface ERT or by any other geological and geophysical information can be used to constrain the optimization algorithm it is also shown that that the lack of this knowledge does not affect the resulting electrode positioning. The validity and effectiveness of the approach was tested through synthetic modeling and real data.
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Optimizing Surface-to-tunnel ERT Measurements
Authors K. Simyrdanis, P. Tsourlos, P. Soupios and J.H. KimThe applicability of tunnel-to-surface electrical resistance tomography (ERT) measurements for imaging subsurface targets is studied in this work. Various issues of this special arrangement are discussed and explored by means of synthetic, simulation tank and real data examples. In particular different electrode arrays including experimentally chooses optimum ones are examined and compared for various targets. Further, the significance of the tunnel effect as well as the accuracy of the electrode positioning in relation to the measured arrays is examined. Ways for taking tunnel effect as well as electrode positioning errors into account while selecting optimum electrode arrays are proposed.
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Data Quality Quantification for Time Domain IP Data Acquired at a Former Waste Deposit in Lund
Authors T. Dahlin and T. SandströmTests were done measuring resistivity and time domain induced polarisation using standard multi-core cable spreads and a special layout with separate cable spreads for transmitting current and measuring potentials. For both types of cables spreads both normal and reciprocal measurements were done in order to estimate the measurement errors. The tests were done on a former landfill in Lund in Sweden. The electrode contact was favourable with resistances of some hundred ohm. The results gave low average error levels for both types of cable spreads, but the single cable spread showed a larger variation with more scatter in the IP data. Data for both types of spreads gave very consistent models that delineate the lateral distribution of waste as well as the contaminant leachate flow path. The results agree well with observations of contaminant outflow and test pit results. It is concluded that the standard approach with a single cable spread is adequate for a site with favourable characteristics as the one studied here.
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Determining Background Variations to Identify Significant Changes in Resistivity and IP for Time-lapse Experiments
Authors A. Etienne, J. Deceuster and O. KaufmannElectrical Resistivity Tomography conducted in time-lapse mode is frequently used for the monitoring of time-varying processes. Nevertheless, it is tricky to identify significant changes in resistivity and chargeability when monitored processes vary slowly in time. Moreover, the errors on acquired measurement might not be negligible as well as the uncertainties on the blocks of the reconstructed models. The uncertainty distribution depends especially on selected arrays, resistivity and chargeability distributions and model blocks sensitivities. An estimate of the background variations for every block of the models is thus required prior to the mapping of resistivity and IP changes during the monitoring experiment. A new methodology is proposed to estimate these background variations and to point out significant changes in resistivity and chargeability. This methodology is based on a two steps approach. The first step consists in determining a sensitivity cut-off value to estimate the depth of investigation. The second step aims to estimate resistivity and chargeability confidence intervals of each model block based on Monte-Carlo simulations. This methodology was applied to a field monitoring experiment conducted on a site contaminated with chlorinated solvents where biodegradation remediation is performed to assess its efficiency.
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Simultaneous Mapping of the Soil Electrical Conductivity and Dielectric Permittivity using a Medium Frequency EM Device
Authors P. Kessouri, S. Buvat and A. TabbaghElectrical conductivity and dielectric permittivity are two useful tools to characterize the soil state parameters such as its water or clay content. A new EM device working in the medium frequency range has been developed to simultaneously measure these parameters. The prototype, working between 1 and 5 MHz with an inter-coil spacing of 1.2m has been tested on a clay loam plot at INRA Estrée-Mons (ORE-ACBB) during the spring dry period. As only half of the plot was covered with growing wheat, significant water content variations were expected. The prototype detected these variations and electrical conductivity and dielectric permittivity were mapped. These first results are in good agreement with the spatial distribution observed by d.c. electrical profiling and by mass water content direct measurements.
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Application Results of Compact EM Tool at the Geoelectric Test Site of Institute of Petroleum Geology and Geophysics
By E.V. BalkovA geoelectric prospecting test site was constructed within the area of Kluchi geophysical station (Novosibirsk, IPGG SB RAS). The aim of the test site is to study resolution of the near surface geophysical equipment and to improve methodical base of field works. The ten squares of 100 m2 were used test site design, in which 33 targets simulating various real objects were organized. The following objects were laid down: plastic and metal pipes with different length and diameter, aluminum flasks, plastic jerry cans, etc. In the course of our work the distinctive features of every measured component of the signal were identified. Various targets are identified either in real or imaginary components of the signal. The imaginary component describes the position of the test object more clearly. The phase of signal appears to be an efficient way to display the field data. Targets with different shapes and depths can appear as form of one to five images. It can lead to difficulties connected with the interpretation of this material without prior accumulated methodical information.
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Pseudo-3D Characterization of a Geothermal Field through Magneto Telluric
Authors C. Comina, L.V. Socco and A. ManzellaThe use of Magneto Telluric (MT) data for geothermal resource assessment is widely documented in scientific literature. Full 3D inversions are however seldom performed since they are time consuming and provide, in most situations, smoothed models which are not suitable for accurate prospection of local anomalies. To overcome these limitations, an algorithm for the pseudo-3D interpretation of MT data based on a spatially constrained inversion is applied. An application to a real dataset is presented to show the potentialities of the selected approach. Data from an extensive MT survey carried out in the geothermal system of Travale (Tuscany, Italy) are presented. Geological constrains from a 3D seismic reflection survey, from geological data and from available stratigraphic logs were used as a-priori information for the inversion and for a correct tuning of the spatial constraints.
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Radiomagnetotelluric Measurements on the Lake Mälaren along a Planned New Motorway in Stockholm, Sweden
Authors L. Persson, M. Bastani, A. Vass and C. MellqvistIn this study we have tested a new technique with RMT measurements on water which we call Marine Tensor RMT. The measurements were performed on the Lake Mälaren close to the city of Stockholm along a planned new motorway “E4 The Stockholm bypass”. The RMT system was slightly modified to be adapted for marine applications and part of the equipment was placed on a float that was dragged behind a small motorboat. The first results from the two test profiles on the Lake Mälaren are very promising. It was possible to make the measurements in a continuous way with an approximate speed of 2 km/hour. This makes it ten times faster compared to measurements on land. With this technique large areas can be covered with parallel profiles within a short period of time. The results from the 2D inversions show a good correlation with the results from the refraction seismic investigations in the same area. Moreover the resolution is fairly good even at greater depth and the resistivity models can give additional quantitative information about the bedrock quality at depth down to 60 m which is the depth for the planned road tunnel.
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Experience of Application of the Radiomagnetotelluric Sounding Method for the Solution of Environmental Tasks
Authors A. Saraev, A. Simakov, A. Shlykov and B. TezkanProspects of application of the radiomagnetotelluric (RMT) sounding method at the solution of environmental tasks are considered. Features of the foot, mobile and controlled source modifications of the RMT method are described. Results of surveys using the RMT method for the investigations of landfills of industrial and domestic wastes and subsurface structure at a gas station are analyzed.
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The TEM Method in Studies of Near-surface Magnetically Viscous Rocks
Authors N.O. Kozhevnikov and E.Y. AntonovThe paper discusses the effect of magnetically viscous near-surface rocks on the TEM response. In geological materials magnetic viscosity is caused predominantly by relaxation of magnetization of superparamagnetic mineral grains. Magnetic viscosity is shown to have a pronounced effect on the TEM data. More often than not it is regarded as a noise hampering the interpretation of TEM data in terms of conductivity. By contrast, we show, through forward calculation of TEM responses and inversion of the field TEM data in terms of time-dependant magnetic susceptibility, how magnetic viscosity effects can be used as a source of important information on the near-surface geology.
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Magnetic Properties of the French Soil Monitoring Network: First Results
Authors J. Thiesson, L. Boulonne, S. Buvat, C. Jolivet, B. Ortolland and N. SabyThe magnetic properties of the French soil Monitoring Network have been measured. This presentation show the first results obtained and especially maps of the magnetic properties at the country scale for France. Relationships between geology and the magnetic properties are clearly shown by the maps. Advanced statistics and geostatistics have applied to evaluate the possibility for magnetic properties to be used in pedotransfer function for assessing some soil chemical or physical properties.
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3D Geological Modelling Improvement with Airborne Time Domain Electromagnetism
Authors P.A. Reninger, G. Martelet, J. Perrin, J. Deparis and Y. ChenStandard geological modelling based on boreholes and geological maps can be strengthened using geophysical data. Constrains such as gravity, magnetic and seismic data have already been used. We propose a novel method combining boreholes and the resistivity model resulting from inversion of airborne time domain electromagnetic data. First, the “geophysical” top of the chalk has been identified in the resistivity model after a detailed cross-analysis of resistivities versus boreholes. Then, we jointly interpolated slopes extracted from this geophysical surface together with the top of the chalk in boreholes. Comparison of uncertainties between this model together with pure geological and geophysical models shows that the joint modelling yields the most accurate top of the chalk. A cross-section, intersecting five boreholes (two used as control boreholes) and displaying each of the three surfaces, highlights the usefulness to take into account “geophysical slopes” when modelling. The proposed joint modelling improves what is commonly obtained with geological or geophysical data. This makes the method very attractive for detailed 3D geological modelling.
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MRS Parameter Estimation – Improvement by Joint and Laterally Constrained Inversion of MRS and TEM Data
Authors A. Behroozmand, E. Auken, G. Fiandaca and A.V. ChristiansenWe present a new scheme for joint and laterally constrained inversion (LCI) of magnetic resonance sounding (MRS) data and transient electromagnetic (TEM) data, which greatly improves the estimation of the MRS model parameters. MRS is a non-invasive geophysical technique which directly quantifies the water content distribution from surface measurements. The resistivity information of the subsurface is obtained from a complementary geophysical method such as TEM or DC resistivity methods. The conventional inversion of MRS data assumes the resulting resistivity structure to be true and considers a constant MRS kernel through the inversion. We show that this assumption may introduce an error to the forward modeling and consequently result in erroneous parameter estimations. We discuss the advantage of TEM for the joint inversion compared to DC resistivity. A fast and numerically efficient MRS forward routine makes it possible to invert the MRS and TEM data sets simultaneously along profiles. As results, a more reliable and robust estimation of all parameters is achieved. We examine the approach through a field example in Denmark where good agreement with borehole data is demonstrated with clear correlation between the relaxation time T_2^* and the grain size distribution of a sandy aquifer.
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Model-based Noise Cancelling of Multichannel Magnetic Resonance Sounding Measurements
Authors J.J. Larsen, E. Dalgaard and E. AukenThe signals recorded in a multichannel magnetic resonance sounding measurement are often heavily dominated by noise, in particular spikes and powerline harmonics. The efficiency of the standard techniques for noise cancelling; multichannel adaptive and Wiener filtering are limited by the fact that they fail to properly account for the presence of multiple noise sources. In this paper, we address this problem through a model-based noise cancelling approach. A model of the powerline harmonic interference is constructed using a weighted least squares method to estimate the fundamental powerline frequency and the amplitude and phase of all relevant harmonics. The model is subsequently subtracted from the signal records. The proposed method efficiently removes the large contribution from powerline harmonics in the MRS signals. Simulations with synthetic signals embedded in real noise records show that the synthetic signals can be recovered with a fidelity similar to or exceeding the standard noise cancelling methods.
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ERT Monitoring of Tide-induced Water Flow in a Shallow Beach Aquifer
Authors M.C. Chouteau, M. Labiau, C. Taviau, R. Chapuis and C.T.K. ChouA joint geophysical-hydrogeological experiment was performed in 2011 to monitor groundwater level changes in a shallow aquifer caused by tidal loading. Electrical resistance tomographies (ERT) were recorded in time-lapse mode for two days over a 5 hour period and consisted of a 1-m spacing profile of 41 electrodes. Eleven piezometers were installed with an average separation of 10m; pressure and temperature data was recorded at 2 min interval. Other hydrogeological data included water electrical conductivity, and porosity estimations from laboratory and in-situ measurements. The ERT monitoring experiment has shown that it was possible to monitor cyclic seawater intrusion into the shallow coastal aquifer driven by tide loading, and the subsequent hydrodynamic regime of the aquifer. Salinity changes with times caused by the telescoping salt water and freshwater flows and tide loading generate lateral subsurface resistivity changes that can then be monitored. The hydrogeology study has shown that water head data displayed minute changes up to 30 m inland; however, ERT is not sensitive enough to monitor those small vertical changes. The combination of spatially coarsely sampled piezometer data combined with geophysical monitoring allows yielding a more complete picture of the hydrodynamic system occurring in this shallow coastal aquifer.
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Coastal Aquifer Characterization Based on Seismic and Resistivity Imaging Methods: a Case Study from Costa Brava, Spain
Authors B. Benjumea, A. Gabàs, A. Macau, S. Figueras, J. Piña and I. TubauCoastal aquifer formations located in Northeast of Spain affected by saline intrusion has been the focus for a multi-technique geophysical study. Two aquifers are present: a shallow aquifer in the quaternary sediments and a deeper karstic aquifer. The objectives were: (1) estimate quaternary sediment thickness and characterize weathered/fractured rock (2) qualitative assessment of water degradation changes within the study area. The methodology consisted of combining and integrating seismic and resistivity imaging techniques: seismic refraction tomography (SRT), electrical resistivity tomography (ERT) and H/V microtremor technique. Three ERT and SRT coincident profiles have been acquired from sea to inland. Comparison of velocity and electrical models has allowed obtaining quaternary and weathered/fractured bedrock thickness. Electrical resistivity differences from one to another profile help mapping changes in saline intrusion. H/V technique has been suitable to obtain bedrock depth in the profile close to sea. However, strong attenuation of the microtremor signal has precluded its application in the rest of the study area. This attenuation could be related to the dissipation of seismic energy in moving fluids through a thick zone of fractured/weathered bedrock. All this information is critical in order to establish preferential water flows and groundwater degradation of the aquifers.
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High-resolution 2D Landstreamer Vibroseis Applied in Groundwater Mapping of Miocene Deposits
Authors T. Vangkilde-Pedersen, E.S. Rasmussen and M. KristensenThe Miocene succession in central and southern Jylland was formed during four phases of shoreline progradation into the basin that covers present-day Denmark. The deposits comprise several aquifers with potential drinking water resources and have been investigated by drilling and acquisition of seismic data integrated with sedimentology and biostratigraphy. The multi-disciplinary approach and an exclusively large dataset of 1200 km high-resolution 2D onshore seismic data and 150 boreholes, 100-400 m deep, has formed the basis for an extensive mapping of the Miocene succession. One of the cornerstones has been the generally very high data quality of the seismic data acquired in the past 10 years using the landstreamer vibroseis method. A detailed interpretation of the seismic data has been performed, focussing on mapping the extent of sand-rich bodies, by identifying top and base of separate delta lobes and fluvial deposits. The result of the investigations is a dynamic 3D geological model that reflects the basin development and the depositional processes as well as the palaeogeographical development during the Miocene. The model is intended to serve as a geological database of lithological and stratigraphical information and provides a better understanding of the geological architecture of the aquifers than traditional lithofacies models.
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Observing Sedimentary Deposition in an Artificial Lake using 3D Ground-penetrating Radar
Authors G. Hamann, J. Tronicke and M. WeberIn February and 2010 and February 2012, we have collected 3D ground-pentrating radar (GPR) data across the lake of the artificial water catchment Hühnerwasser (“chicken creek”) situated in the post-mining landscape of the opencast mine Welzow-Süd (Germany). This study shows, that the GPR technique can provide information relevant for analyzing fluvial erosion processes within the artificial catchment. We employed a conventional GPR surveying approach based on cart-mounted100 MHz antennae and a combination of the GPR instrument with a self-tracking total station on the frozen lake. Clear reflection from the lake bottom in the entire processed data cube are visible. Furthermore, deeper reflections can be recognized. These are interpreted to represent sedimentary structures associated with fluvial sedimentary input into the lake. In addition to detailed 3D images, our GPR data allow to quantify the amount of sedimented material. I am a PhD student and I would like to be considered for the AGAP-Quality Award.
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Applicability of MRS Soundings for the Characterisation of the Unsaturated Zone of Karst Systems
Authors N. Mazzilli, K. Chalikakis, M. Boucher, A. Legchenko, H. Jourde, H. Guyard, A. Chevallier and S. CarriereThis study aims to assess Magnetic Resonance Soundings (MRS) potential for the characterisation of the unsaturated zone of karst systems and thereby groundwater vulnerability assessment. The issues addressed are: (i) does water storage within the unsaturated zone of karst yield quantifiable MRS measurements ? if so, (ii) do different hydrogeological contexts yield different MRS signals ?, and (iii) what are the key factors influencing MRS signal ? The proposed methodology is based on a careful characterisation of the experimental sites, involving both hydrogeological and geomorphological analyses. MRS measurements have been performed on a total of 19 sites with varied hydrogeological and geomorphological setting. Analysis of the measured MRS signals shows that : (i) water storage within the unsaturated zone of karst does yield quantifiable MRS measurements, (ii) the first-order determinant of the MRS signals is the nature of the geological formation. These results confirm MRS applicability for the characterization of the unsaturated zone of karst aquifers and thereby vulnerability assessment. Future work will focus on the variability of the MRS response within a given geological formation.
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Identification of Past and Present Groundwater Recharge Pattern in the Lake Chad Basin with Magnetic Resonance Soundings
Authors M. Boucher, G. Favreau, A.M. Moussa, M. Ibrahim, M. Descloitres, M. Le Coz and A. LegchenkoLarge changes in the Lake Chad surface area occurred during the Holocene and had an impact on aquifer recharge, yet little is known on past and present pattern of the aquifer hydrodynamics. Two complementary MRS field surveys (23 soundings) were conducted in 2008 and 2010 for accessing of the free water content and hydraulic properties of aquifers in the semiarid northwestern part of Lake Chad Basin. High MRS water contents (10-35 %) and permeability values (10-3 to 10-2 m/s) were shown below present-day Lake Chad. At distance, below the former clayey deposits of the Megalake Chad, lower MRS water content (8-13 %) and permeability (10-4 to 10-3 m/s) values were estimated for the unconfined aquifer. These low porosity and permeability values suggest limited aquifer fluxes, in accordance with the occurrence of isotopically determined palaeo-groundwaters below the clayey plain. Intermediate and homogenous hydraulic properties (MRS water content and permeability values ranging from 16 to 25% and 1.10-3 to 3.10-3 m/s, respectively) were found in the valley of the Komadougou River which currently represents the main source of recharge for the aquifer. This study proved the decisive contribution of MRS for quantifying past and present surface-water/groundwater exchanges interactions in flat sedimentary basins.
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An up-scaling Approach to Groundwater and Aquifer Characterisation in the Australian Outback
Authors T.J. Munday, C. Sorensen and K. CahillAustralia's sustained economic growth and development over the past decade has been accompanied by an increased demand for water, particularly groundwater. Drivers for that demand include new industries, new mines, the need to secure urban and regional water supply. With reference to an example from the Murchison region of Western Australia we examine an approach that could use AEM to aid the scaling-up from a local to catchment-scale hydrogeological framework for groundwater assessment. It draws on local scale AEM surveys and hydrogeological studies, along with a regional scale model of the landscape geomorphology to determine an optimal approach to AEM data acquisition at the catchment scale, as an aid to groundwater resource assessment. Local scale studies using TDHEM data, linked to both water resource investigations and mineral exploration have demonstrated application in defining the extent and quality of groundwater resource in the sedimentary palaeovalley aquifers of the study area. This knowledge was used in combination with terrain indices to define an optimal AEM survey resolution given a limited, fixed budget.
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Magnetotelluric Method in the Exploration of Deeper Aquifers
By F. SumanovacElectrical methods are commonly used in groundwater investigations but in case of deeper aquifers the electrical tomography cannot achieve deeper targets and the electrical sounding cannot ensure a complete and dense space coverage. In such a case the use of the magnetotelluric method can provide for great depths of research, as well as a good space sampling. The capabilities of the MT-method are shown in two examples of deeper groundwater investigations carried out in the north-western Croatia. In the Gotalovec area the aquifers are fractured carbonates (Triassic and Miocene) underlying thick series of Neogenic clays and marls. On the basis of all results a conclusion can be drawn that the MT-method should be applied first in similar model in order to define potential aquifers and indicate the most favourable micro-locations where reflection seismics can be applied as the latter allows for a more precise mapping of aquifer depths and faults. The second geological model, investigated in the Apatovac area, is very complex consisting of clastic and magmatite bodies. The MT-method provided for a complete and clear solution to the structural relationships in the area and gave a precise definition of the deep narrow diabase body which is mineral water aquifer.
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Evolution of Electrical Resistivity Measurements during Process of Waste Biodegradation
Authors M. Sylvain, M. Bergeron and R. ClementElectrical Resistivity Tomography (ERT) is applied in bioreactor landfill for many years to monitor leachate variation in waste body (Moreau et al., 2007, Rosqvist et al., 2006), mainly during recirculation events when moisture content variations induced by liquid diffusion are the most important. Influence of moisture content and temperature is recognised to be significant on waste electrical resistivity (Clément et al., 2011). Nevertheless, impact of waste biodegradation should not be forgotten (Moreau et al., 2011): the assumption of a medium, electrically neutral, could not be considered in waste studies. Influence of the different steps of waste mass biodegradation on electrical resistivity measurements was studied with laboratory tests. Resistivity of well known waste composition was compared to biogas production to achieve this goal. Resistivity device was evaluated using simulation software developed by Clément (F3D-Lab, 2011) and the interpreted resistivity was calculated using BERT software package (Guenther, 2011. The resistivity variation observed on one cell can be linked to biogas production, which can not be confirmed by the tests on the other cells for the moment. Field capacity seems limiting to induce a fast biodegradation of waste mass which can be problematic for landfill generally operated with that hydraulic condition.
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Application of Electrical Resistivity Tomography Technique for Investigating Subsurface Flow Constructed Wetlands
More LessConstructed wetlands are widely used for removing pollutants from wastewater in small communities because their simplicity and low operation costs. Nevertheless, with time the cleaning process can result in gradual clogging of the porous layer by suspended solids, bacterial film, chemical precipitates and compactation. The clogging development causes decrease of hydraulic conductivity, reduced oxygen supply and further leads to a rapid decrease of the treatment performance. As the investment involved in reversing clogging can represent a substantial fraction of the cost of a new system it is essential to assess in advance the evolution of clogging process and detect potential failures in the system. Since there is a lack of experiences for monitoring the functionality of constructed wedlands a combination of non-destructive geophysical methods have been tested in this study. With this purpose electrical resistivity tomography and ground probing radar have been conducted at different horizontal subsurface flow municipal wastewater treatment wetlands of Catalonia (Spain). The obtained results have shown that the applied geophysical techniques may delineate the clogging expansion and help take the preventive measures for enlarge the lifetime of the treatment system.
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Innovative Project for In-situ Geophysical Instrumentation of Bioreactor Landfill: Short and Long Term Monitoring
Authors J. Grossin-Debattista, R. Clément, S. Loisel, A. Poulain, J.L. Böhm and S. MoreauOptimizing landfill operated in bioreactor mode is to date important challenge. Moisture content and temperature are two key parameters of the anaerobic biodegradation process. In order to improve the waste degradation and the recovery of biogas (renewable energie), more specific bioreactor studies are requiered. Indeed, waste is a very heterogeneous porous media. The water recirculation and temperature repartition is poorly known at the scale of the bioreactor site. Started on January 2012, as part of a multi-stakeholder project, experiments are being prepared: at the laboratory to optimize recirculation strategies and at the scale of the cell with a multi-instrumentation methods. For this purpose, a combination of innovative methods is being installed to allow has both a short-term follow-up during episodes including recirculation and a long-term monitoring of biodegradation process waste. Innovative methods implemented across the cell include ERT to study the distribution of moisture within the solid waste, optical sensor to measure temperature all accross the cell and moisture sensor. The objectives of this project are multiple: optimization of methods implemented, acquire a better understanding of the biodegradation process and leachate reinjection influence, achieving a long-term monitoring with a multi methods and allowing an optimization of bioreactor operating methods
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Pilot Study of using Resistivity-IP, Magnetometry and Slingram for Separation of Industrial Waste
Authors T. Dahlin and K. HellmanA controlled experiment was carried out by doing geophysical measurements on a purpose-built heap consisting of two different industrial waste fractions, called NF ( non-ferrous metal) and SLF (shredder light fraction), from the recycling plant of Stena Metall in Halmstad, Sweden. The purpose of the experiment was to see if it was possible to distinguish the different types of material using geophysics. Resistivity-IP, magnetics and slingram were measured over the test heap. The results gave distinctly different signatures for the different materials. In conclusion the test shows a large potential for the tested methods for material characterisation of buried waste in connection with landfill mining.
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Complementarity of Electrical Resistivity Tomography and Ground Penetrating Radar to Study Karst Unsaturated Zone
Authors S. Carriere, K. Chalikakis, C. Danquigny, G. Sénéchal and A. ChapeletThe structure of karst hydrosystems is complex and the related hydrodynamic functioning mechanism can be highly heterogeneous in both unsaturated and saturated zones. The Low-Noise Underground Laboratory of Rustrel (LSBB) is located in unsaturated zone (UZ) of Fontaine de Vaucluse (FdV) karst hydrosystem. In this way, the LSBB is a unique place to apply surface-based geophysical surveys due to numerous and various geological and hydrogeological knowledge under the investigated zone. We exhibit in this paper complementarity and efficiency of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR). Both GPR and ERT are well suited for the first phase of our study, in order to characterize the general structure of the investigated chosen zone. GPR results highlight the tectonic features which are also, but less accurately, seen by ERT apparent resistivity maps. ERT 2D inverted results have shown the presence of moderate resistivity zones within the limestone. The provided information allowed carrying out following step of our study to enhance understanding of local karst UZ e.i. apply Magnetic Resonance Sounding (MRS), Gravimetry and Low Frequency Electromagnetics (LF-EM).
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Detection of a Fracture Network Near a Cluster of Sinkholes using 3D Electrical Resistivity Tomography
Authors N. Dupont, M. Attali, J. Deceuster and O. KaufmannOver the last two decades, 2D electrical resistivity tomography (ERT) surveys have been conducted to detect and map karstic features as the expected electrical resistivity contrast is high between sound limestone and weathering residue as well as between sound limestone and cavities filled with clayey or sandy sediments. However, karstic features often show complex 3D geometries which are not well reconstructed through 2D surveys. In this paper, the efficiency of 3D ERT in the identification of a fracture network located at the top of limestone bedrock is assessed through a field experiment. The results of the 3D ERT interpretations were confirmed by visual observations of an outcrop located 100 meters away. Moreover, former sinkholes which occurred near the investigated area are aligned following the directions pointed out with the 3D ERT survey. This indicates that local karstification probably affect fractures located at the surface of the bedrock leading to the formation of a cryptolapiaz.
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Electrical and Electromagnetic Resistivity Imaging Environmental Applications and Engineering Study
Authors M. Djeddi and F. KhaldaouiA frequency-domain electromagnetic (FDEM) and electrical resistivity imaging (ERI) surveys can be used for delineating underground cavities and characterizing pseudo-karst dissolution of limestone sandstone on the northwest of Algiers. This site has complex geology still requires a detailed survey prior to the execution of civil works. The FDEM survey was performed using both a very low frequency resistivity (VLFR) and a controlled source EM system (Geonis EM31). The electrical resistivity imaging has been conducted using dipole-dipole array. The results of this study have identified a number of karst cavities in the calcareous sandstone associated with carbonate dissolution by groundwater from a depth of about 4 m, some of these cavities and fracture zones were identified by boreholes at the site. The networks of cavities are organized along a N110 ° and N60 °.
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The Use of In-river Resistivity to Classify Recharge Rates in Losing Disconnected Streams in the Murray Basin, Austr in
Authors A.C. Davis, T.J. Munday, S. Lamontagne and A.J. TaylorMany of the waterways in the lowlands of the Murray-Darling Basin are rivers and creeks that are typified by having long and meandering channels that are underlain by a clogging clay layer of varying quality and thickness. It is the presence and thickness of these clay layers, along with depth to water table, which are the primary parameters in the determination of the rate loss from the watercourse to the underlying aquifer. Using an in-river resistivity survey, corroborated by along-shore VES and hand auger profiles, we have produced a geo-referenced recharge/loss map for a 2 km section of Billabong Creek near Jerilderie, NSW. We have shown that in-river resistivity surveys are a possible geophysical technique that can be used to estimate recharge rates and clay thickness along the river bed at a river reach scale. Maps such as the one produced here are useful for groundwater flow modelling, and may assist in the determination of water allocations in the future.
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Geoelectrical Measurements for Agricultural Canal Seepage Detection
Authors C. Comina and L. SambuelliThe protection of water resources is a current problem at a global scale, especially in regions where this resource is poorly available. In this respect there is an increased pressure on water managers to improve water use efficiency. The use of geophysical tests as a tool for managing the efficiency of irrigation networks is well documented in literature since they have an high potential for canal seepage identification and quantification. In this respect, the present study is focused on the use of electrical resistivity methods on some sample canals of the Piedmont Region network. The results obtained have underlined the usefulness of geophysical methods in detecting local anomalies in the lining, potentially related to seepage losses, by means of waterborne CVES and in imaging the passage of saline tracer tests plumes, to be used for a quantification of the seepage losses, using cross-flow high-speed ERT.
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