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Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics
- Conference date: September 6-10, 2015
- Location: Turin, Italy
- Published: 06 September 2015
151 - 164 of 164 results
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Using GPR and High Frequency Seismic to Locate Under Water Buried Pipes
Authors M.A. Barsottelli-Botelho and L. MesquitaSummaryThis paper analyses the results of the application of geophysical methods of GPR (Ground Penetrating Radar) and CHIRP (high frequency seismic/acoustic) in surveys of pipes buried at the bottom of rivers and bays located at South and Southeast regions of Brazil. The GPR data acquisition uses a common-offset reflection survey array with 200 and 300 MHz antennas. The high frequency seismic surveys uses a CHIRP equipment with an emission frequency of 3.5 kHz. The GPR best results on mapping buried tubes are on rivers with sandy or gravel floor and no tidal influence and the chirp best results are on muddy floors (river or bay). Of course, the salt and brackish water also makes the GPR ineffective. The electromagnetic pulse suffers a high attenuation in wet clayey soils (10 – 100 dB/m) and under brackish water (100 – 1000 dB/m). The solution is to plan the tubes mapping campaign with an integration of GPR and CHIRP methods. The GPR has a very good portability, is easier to operate and to perform data acquisition and requires an easier data processing.
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Land Magnetic Data Smoothing Methods - Comparative Study
Authors S. Alhumidan, N. Alarifi and A. AlbassamSummaryGeophysical data processing is the use of computers for the analysis of geophysical data. A major task in geophysics is to determine as much as possible about the constitution of the interior of the earth. Where direct penetration is impractical or impossible, seismological, electromagnetic, and gravity measurements are made and the task of making inferences from these measurements is begun. Through systematic application of the laws of physics and the principles of statistics, some of these interpretation tasks can be computerized. When the number of observations is small, it may be satisfactory to match them to the adjustable parameters in known analytic solutions to the equations of classical physics. Today, however, it is common to have massive numbers of observations which contain far more information about the earth than can be modeled by analytic solutions. A typical reflection seismic marine survey ship can collect about a trillion (1012) bits of information per month (Claerbout, 1985).
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4D ERT Monitoring of Simulated Leaks from a Nuclear Storage Silo at Sellafield, UK
Authors P.B. Wilkinson, O. Kuras, P.I. Meldrum, L.S. Oxby, S.S. Uhlemann, J.E. Chambers and N. AthertonSummaryResults are presented from a 2-year full-scale field trial of Electrical Resistivity Tomography (ERT) monitoring of simulated leaks from a waste storage silo at Sellafield, UK. The trial included the monitoring of a series of injections of conductive tracer fluids to simulate leaks, as well as an extended monitoring period to assess the fate of the resulting saline plume. The trial constituted the first application of ERT subsurface monitoring at a UK nuclear licensed site. The work was undertaken to evaluate the Technology Readiness of ERT in terms of detecting, spatially characterising and monitoring potential leaks to the subsurface of silo liquor from a storage structure. Despite the weak conductivity contrast of the tracer fluids compared to the groundwater, and the loss of one of the ERT boreholes during the trial, full 4D inversion proved sufficiently sensitive to detect and image the passage of the tracers. Images of resistivity changes relative to a reference baseline have revealed likely pathways of tracer flow in the vadose zone and upper groundwater system. These pathways were found to be in close agreement with the locations of historic contamination detected in sediment cores retrieved from the ERT boreholes.
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Mapping of the Fault Zones on Basis of Geomagnetic Variations Due to Seismic Wave Propagation
Authors D.N. Loktev and A.A. SpivakSummaryThe properties of the tectonic faults differ from the properties of surround rock. The fault properties determine behavior of the fault due to different external influences. Seismic waves propagating through the fault result to local magnetic field variations. Amplitude and intensity of these variations depend on the conditions for transformation energy between mechanical oscillations and magnetic field. Registrations of the seismo-magnetic effect at the Earth’s crust surface allow determining the presence of the fault and estimating its internal properties. We carried out synchronous registration of seismic waves and magnetic variations along the profile crossing the fault zone situated at the central area of the Russian platform. Geomagnetic variations caused by seismic signals resulting from open-cast mine blasts were analyzed. As a result we determined that magnitude of seismo-magnetic effects is maximum in the central fault’s zone and decreases appreciably according to exponential law as the distance from the fault increases. It shows that the transformation of mechanical energy of vibrations into the energy of geomagnetic variations occurs more intensively in the fault. It allows determining the fault location firstly and estimating its broken state and present-day activity secondly.
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VLF Electromagnetic Data Inversion Starting from Current Density Pseudo-sections Obtained by Linear Filtering
Authors F.C.M. Andrade and T. FischerSummaryWe used current density pseudo-sections obtained by linear filtering of VLF data to create a starting model for an inversion procedure using the Polak-Ribiere variant of the conjugate gradient method. The inversion procedure looks for the minimum error between the observed data and forward modelling data, obtained by a finite-difference algorithm, of the initial subsurface model, where the model parameters are changed iteratively. The inputs for the procedure are the host medium resistivity within a range given by the user, the number of cells that should be used as an approximation of the real medium and the desired accuracy. The lateral positions of the cells, forming the initial model, are obtained automatically by looking for zeros in the second derivative in descending parts of the observed data curves. The depths of the cells, as well as their resistivities, are obtained from the maximum values of the current density pseudo sections within a given range of cells centred in the previously obtained lateral positions. Results for two simple synthetic models are shown.
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Contribution of SFM and ERI Methods to Assess an Underground Quarry Pillar
Authors Y. Fargier, L. Dore, R. Antoine, J. Lacogne, E. Pairault, S. Palma Lopes, C. Fauchard and D. MathonSummaryNatural degradation of underground quarries is a major issue from an economic and public safety point of view. Electrical Resistivity Imaging (ERI) method can assess efficiently the electrical resistivity distribution inside a medium and its evolution. However, conventional 3D ERI methods are not well suited for quarry pillar assessment. Moreover, a complete 3D ERI necessitates an accurate 3D geometry. In this paper, we propose the use of cost effective SFM techniques to generate a three-dimensional medium. A new assessment methodology based on the combination of SFM and Electrical Resistivity methods is proposed. This methodology is applied to a real limestone quarry pillar. Inversion results show the key role of the accuracy of the 3D model.
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Spectral Time Domain IP - Factors Affecting Data Information Content and Applicability to Geological Characterization
Authors A. Rezvani, T. Dahlin, P.I. Olsson, G. Fiandaca and P. AhhnfeltSummaryA DCIP survey with different timing settings, electrode arrays, cable spreads and waveforms (50% and 100% duty cycle waveform) were carried out over the Liikavaara deposit in northern Sweden. All the measured data were inverted using AarhusInv. Measured data with 100% duty cycle waveform, which is much faster than the conventional 50% duty cycle waveform, present significant improvements in the quality of IP-data. It was also attempted to increase the data quality by separating current and potential cables.
Tau and C parameters from time domain IP were compared in different time bases and spreads. These two parameters appear to be sensitive to the duration of current injections in which; decreasing the time base causes diminished anomalies in Tau and partly in C. Although measurement with separated spreads improved the data quality, it did not have any significant effect on these two parameters. Moreover correlation of sulphide content matched well with the inverted models, which confirms the applicability of the DCIP method in mineral exploration.
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Contribution of Seismic Methods to Hydrogeophysics
Authors S. Pasquet, L. Bodet, P. Bergamo, C. Camerlynck, A. Dhemaied, N. Flipo, R. Guérin, L. Longuevergne, A. Mouhri, R. Mourgues, F. Rejiba, V. Tournat and Q. VitaleSummaryThe characterisation and monitoring of aquifer systems mainly rely on piezometric and log data. Delineating spatial variations of lithology between piezometers is a delicate task, which inevitably generates errors possibly propagating into hydrogeological models. Seismic methods have been proposed to: (i) improve the low spatial resolution of borehole data, (ii) provide a characterisation of the subsurface geometry, and (iii) estimate the physical parameters of the medium influenced by the presence of water and the associated flow and transport processes. The joint study of pressure (P-) and shear (S-) wave seismic velocities (VP and VS, respectively), whose evolution is strongly decoupled in the presence of fluid, has been proposed through the estimation of the VP/VS ratio and Poisson’s ratio. A specific methodology has been developed for the combined exploitation of P- and surface waves present on single seismic records. The use of this methodology in several geological and hydrogeological contexts allowed for estimating VP/VS ratio lateral and temporal variations in good agreement with a priori geological information and existing geophysical and piezometric data. Laser-based ultrasonic techniques were also proposed to put these processing techniques in practice on perfectly controlled physical models and study elastic wave propagation in partially saturated porous media.
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Environmental Study of the Bragança City Landfill (Brazil) Applying Ground Penetrating Radar
Authors P. Chira Oliva, D. Barbalho Pires and J. Ribeiro CruzSummaryThe Brazilian municipalities, the majority of them, deposit solid residues without controlling measures, which occasionally causes air, soil, and subterranean and ground level water pollution. The case is particularly in need of attention in Bragança (Pará, Brazil), where the landfills are located close to aqueous systems of greatest importance, such as the basin of the Caeté River. In order to determine the impact caused by the location of the landfill in Bragança, this study presents the findings obtained through an application of the geophysical method Ground Penetrating Radar (GPR). These results characterize the current levels of contamination of the area studied, showing the existence of determined plumes of contamination near ground level, cause by the previously mentioned landfill.
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A Technique for Locating Leaking Points in Landfills
Authors P. Tsourlos, G. Vargemezis and I. FikosSummaryThe investigation of a newly constructed landfill in Central Greece for leak points has motivated numerical and experimental simulations in order to test modified approaches to locate leaks in geomembrane lined landfills after the protective cover (typically sand and gravel) has been laid. We adopted an approach which involves systematic survey with multiple electrode separation pole-dipole (forward and reverse) arrays and then we perform Fraser filtering and stacking to the apparent resistivity maps to produce a single filtered map which depicts the leaking points. This approach was verified with synthetic and experimental data and it produces results of increased reliability.
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Geophysical Experimental Survey on Flood Protection Dikes - The Case Study of the Loire River Basin
Authors M. Pareilh-Peyrou, P. Labazuy, K. Kelfoun and F. ChalusSummaryAn electromagnetic induction (EMI) survey has been performed along the Loire River protection dikes (France), with a 60 km long, high speed and high resolution, field acquisition campaign. The main aim of this dikes diagnostic mission was to locate and identify potential local weakness or damaged zones inside the engineering structures. We used an EMI device (EM31) and a GPS receiver on board an experimental fiberglass cart support, designed for high yield acquisition surveys. This study allowed to provide the electrical subsurface image (conductivity) of the whole structure and allowed, in addition, for precise detection of the small-scale anomalies. The outputs consist of electrical images of both the inner dike structure and the subsurface geological environment. This fast first-phase study is efficient for detecting decametric local anomalies while giving information about the dike inner structure. Once EMI anomalous zones have been detected, further detailed investigations have been performed on selected areas, including electrical resistivity tomography (ERT) profiles and geotechnical drills. Thus, the objective of our experimental study was to scale down and optimize this second phase of the prospecting campaign, which will considerably reduce the cost and the time spent on the whole survey.
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Application of the DEXP Method to the Streaming Potential Data
Authors M.A. Abbas and M. FediSummaryWe interpret the self-potential data related to groundwater flow by the depth from extreme points (DEXP) method; a multiscale method in which the data are upward-continued and scaled by a scaling law depend on the structural index. The depth to the water table is estimated from extreme points of the DEXP image without a priori estimate of the hydraulic coupling coefficient. The method is tested with a synthetic model of the water table and applied to a real self-potential dataset near a pumping well. The obtained results agree well with the known information.
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Surface-NMR in High Noise Areas
Authors J.J. Larsen and A.A. BehroozmandSummaryOne of the main noise sources in surface-NMR data is the harmonic components of the fundamental 50 Hz powerline signal. In some circumstances, the surface-NMR data are distorted by several unrelated harmonic series. We present surface-NMR data from Ristrup, Denmark, where two harmonic series are present in the data and the noise level is almost an order of magnitude higher than typical Danish surface-NMR data. Noise reduction using standard multichannel Wiener filtering is shown to be inadequate for retrieval of the NMR signal. We show that the NMR signal can be retrieved by sequential fitting and subtraction of the harmonic series using a model-based approach.
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A Waterborne GPR Survey to Estimate Fine Sediments Volume and Find Optimum Core Location in a Rockglacier Lake
Authors L. Sambuelli, N. Colombo, M. Giardino and D. GodoneSummaryRockglaciers are landforms related to ice-rich permafrost creep and represent a substantial reservoir of groundwater in the alpine region. Rockglacier lakes can occur when permafrost-ice meltwater released from rockglaciers, snowmelt, rain and groundwater find the geomorphologic conditions to be collected in an impermeable depression. This depression can be filled with fine-grained sediments (ø<1/16 mm) from at least three sources: 1) the dust contained in the snow covering the lake during winter, 2) the solid fraction of the runoff and meltwater from the rockglacier and 3) the runoff or the wind transport from the surrounding areas. Fine sediment volumes, bedding and typology offer large amount of information for interpreting the past and ongoing biological and abiotic processes in the catchment. In order to estimate the fine sediment volumes and plan a coring campaign in the Col d’Olen Rockglacier Lake (Aosta Valley, Italy) we performed waterborne GPR surveys with both 200 and 500 MHz antennas. After data processing we obtained the bathymetry of the top and the bottom of the fine sediments. Within the fine sediments unit we also found a discontinuity whose meaning will be investigated with a planned coring located in the zone with the maximum sediment thickness.
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