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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 - 20 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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