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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 - 50 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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