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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
141 - 160 of 164 results
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Seismic Surface-wave Analysis for Railway Platform Auscultation
Authors L. Bodet, S. Pasquet, A. Dhemaied, J. Boisson-Gaboriau, Y.J. Cui, P. Leroux, S. Nebieridze, A.M. Tang, J.M. Terpereau and Q. VitaleSummaryThe renewal of existent railways requires the characterisation of the mechanical properties of railway platforms (RP), thus raising the need to select appropriate maintenance actions. Conventional techniques (geotechnical soundings, coring) remain local, destructive, expensive and with low yields. Using nondestructive investigation techniques for local diagnosis and monitoring thus appears of great interest for enhancing RP control. Seismic surface-wave methods have been proposed to estimate in situ mechanical parameters of the superficial layers below railways. In this context, a joint geotechnical and seismic survey was carried out along the Northern Europe high-speed line (LGV) in order to precisely determine the origins of a phenomenon affecting the geometry of the track. Strong a priori knowledge of the RP structure allowed for inverting dispersion measurements for 1D VS models along the track. The results showed a contrast of VS in the loess lying below the RP, between areas where the phenomenon was observed and those it was not. This contrast was confirmed by Bender Elements measurements of VS performed on core drilling samples, and corresponded to the lateral variations observed along the track. These results encourage considering dispersion measurements as an appropriate tool of RP monitoring.
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The Limits of Narrow and Wide Angle AVA Inversions in Case of High Vp/Vs Ratios -Application to Seabed Characterization
Authors M. Aleardi and A. TognarelliSummarySince its introduction in the oil and gas industry, amplitude versus angle (AVA) inversion has become a standard tool in hydrocarbon exploration. However, with the intensification of off-shore construction activity, applications of this method have also been extended to evaluate the elastic properties of seabed sediments. These sediments are often undercompated and characterized by very low Vs and high Vp/Vs ratios. The importance of Vp/Vs ratios is usually underrated in AVA inversion which is often applied with few misgivings in case of high Vp/Vs ratios. In this work, we derive the elastic properties of the seabed interface by applying AVA inversion to a 2D well-site survey acquisition. The limited water depth, the maximum offset of the seismic acquisition and the high frequency content of the data make it possible to consider two different ranges of incidence angles: 0-30 and 0-60 degrees. The limits of AVA inversion in case of high Vp/Vs ratios are discussed together with the benefits introduced by wide angle reflections in constraining the inversion. In particular, these reflections are essential to decrease both the cross-talk between the inverted parameters and the uncertainties in the Vp and density estimations, but they do not guarantee a reliable Vs estimation.
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Characterization of Shallow Marine Sediments by Means of High Resolution Velocity Analysis and Stochastic 1D FWI
Authors M. Aleardi, A. Tognarelli and A. MazzottiSummaryWe estimate the elastic properties of marine sediments beneath the seabed by means of high-resolution velocity analysis and 1D elastic full-waveform inversion (FWI) performed on 2D seismic data of a well-site survey (WSS). A high-resolution velocity functional is employed to exploit the broad frequency band of the WSS data and to derive the compressional velocity field with a high degree of accuracy. The estimated velocity field is then used as a priori information in the 1D elastic FWI that is solved by making use of genetic algorithms as optimization method. We consider two error functions: the first uses the entire waveform to compute the misfit between modelled and observed seismograms; the second considers the envelope of the seismograms thus relaxing the requirement of an exact estimation of the wavelet phase. The very similar velocities and density depth trends derived in the two FWI tests indicate the reliability of the predicted subsurface model. Moreover, the FWI and the high-resolution velocity analysis yield comparable velocity profiles, although with different resolution.
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Geophysical Investigation for an Assessment of Sinkhole Risk in Schleswig-Holstein/Northern Germany
Authors R. Kirsch, H. Wiederhold, W. Rabbel, E. Erkul, C. Thomsen, R. Timm, S. Hintz and T. Schulze DieckhoffSummaryOn the sports ground in the village of Münsterdorf small scale (about 2 m wide) sinkholes occur in a regular time interval of about 2 years. Origins of the sinkholes are cavities formed in a Cretateaous chalk layer covered by about 20 m of unconsolidated sediments. Geophysical investigations were carried out to delineate the area of sinkhole risk.
Criteria were established to define sinkhole risk following the “dropout sinkhole” theory of Waltham and Fookes”: a) weakened chalk surface with fissures, b) sandy layers covering the chalk surface, c) cohesive layer in the sedimentary cover leading to a “soil cavity” after sandy material is washed into the fissures of the chalk.
Seismic, resitivity and GPR methods were applied in this area. In the area of sinkhole occurance reduced s-wave velocities and a diffuse reflection image of the chalk surface were found (in contrast to the clear chalk reflections outside the sinkhole area). Resistivity measurements (2D ERT and AEM) verified a sandy layer (high specific resistivity) on top of the chalk layer and a low resistivity layer (till or clay, cohesive) in the sedimentary cover. A 3D GPR survey covering the sports ground found evidence of former sinkholes in the area.
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Imaging of Karst Features to Guide Mining Activities in a Marble Quarry by Means of 3D ERT
Authors S.S. Uhlemann, J.E. Chambers, A. Tirado Alonso, A. Espín de Gea and W.E. FalckSummaryThis study shows the application of 3D Electrical Resistivity Tomography (ERT) to the characterization of a narrow and deep open-pit marble quarry, in order to develop near-surface geophysical methods as an aid to realize the concept of an invisible, low-impact quarry. The purpose of the survey was to prove the applicability of ERT to image the extent and shape of subvertical faults and cavities in a highly resistive hard rock environment, thus allowing for targeted extraction of the natural stone reserve and a reduction of mining waste. ERT faces a range of challenges when applied to a hard rock environment, namely very high contact resistances, preferential current flow through conductive surface layers, and pronounced topographic features, which have to be accounted for in the data inversion.
Despite these challenges, results show that using 3D ERT it was possible to outline the extent of a cave below the lower quarry floor and to identify the location of the fault causing the karstification.
Observations recorded during data acquisition correlated well with the features imaged in the 3D ERT surveys, indicating the reliability of the results.
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Reconstruction of Contamination Migration Based on Advanced CPTe Data
More LessSummaryThe precise environmental diagnostics requires the knowledge of main subsurface flow directions where the contamination could migrate. Using an advanced CPTe method with nuclear logs the detailed subsurface structure can be reconstructed and also the grain size distribution can be determined. Based on this information the accumulation zones and the migration routes can be identified.
The migration process takes time while the degradation of the hydrocarbon contamination is going on. Laboratory tests proved that during this process the electrical properties of the hydrocarbon change significantly. It means that from the electrical resistivity of the contaminated soil may refer to the degree of degradation and consequently to the migration time.
This study presents a field experiment focusing on the determination of migration routes of subsurface hydrocarbon contaminants based on the data of advanced CPTe data.
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Ascertaining Grain Scale Effects of Seismic or Aseismic Stimulation upon Strength of Near Surface Geological Materials
Authors B. Hassan, S.D. Butt and C.A. HurichSummaryCertain peculiarities of inelastic nonlinearity of unconsolidated near surface stressed granular media contributed at micro- scale are investigated to ascertain anomalous time dependent strength behaviour macro-effects, with geotechnical/geo-environmental implications. Comparative examination of ultrasonic P- and S-wave pulse stimulated repeatable displacement response wave-forms of confined dry and fully saturated ceramic grains analogue, endorsable by pertinent theory, hint certain persistive time and frequency restricted occurrences vouching vital insights. Examination aimed at both understanding “louder” response of “quieter” seismic and aseismic events in granular sediments, and imparting enhanced conceptual substantiation of some previously published results, show that subtle acoustic emission and/or stick-slip type micro events do occur, and, when spread over time and/or space are collectively capable of altering granular material macro strength behaviour. It is clearly deducible from resonant type spectral results that material fragmentation or force chain formation type phenomenon occurs possibly due to macro-scale friction mobilization by grain scale events. It is further speculated that invisible high frequency events may irreversibly alter grain scale surface properties and/or intergranular friction as enhanced elasticity type effect, more elusive with saturation. Temporal morphology of stick-slip type events when stimulated by P- and S-wave modes is posited to be non-identical in characteristic sense.
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GPR Images of Structure’s Foundation at Hagios Thedoros Church, Old Gumushane, Turkey
Authors A. Seren, Z. Ogretmen, A.E. Babacan, T. Sadiklar and S.B. İkizlerSummaryA restoration should be required to prevent demolition of extant other parts and walls of a church. Because any excavation cannot generally be made in historical buildings, restoration of such buildings has been decided without regarding the foundations of these structures. In these cases, some problems such as collapse and demolishing of structure may be occurred in historical buildings. Before the restoration of such historical buildings, position of foundation in the buildings must be determined by using ground penetrating radar (GPR) which is a non-invasive technique can give results with high resolution for dealing with these problems. The study was aim to determine foundation’s depths/size/shape/lateral extents of walls of the Hagios Thedoros Church by using GPR. Amplitude-slide maps with the method were created on the investigation area located at north of the Hagios Thedoros Church to be the oldest church in the Old Gumushane on NE of Turkey. These maps for different depths were interpreted. Strong reflections resulted from foundation structures remains and collapsed stones of the church were shown as dashed red lines and arrows on these maps.
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Investigating Fiber Optic Cable with GPR Methods in Infrastructure Studies
Authors Z. Ogretmen and A. SerenSummaryome elements such as cold/hot/waste/gas water pipes and telephone/electrical/fiber optic cables are situated in infrastructures of modern cities and large-scale settlements. Ground Penetrating Radar (GPR) method is used to detect these elements unknown positions from the surface. Main goal of determining the structural elements in infrastructure investigation prevents damaging of these cables and pipes in any excavation. In this paper, unknown positions of fiber optic cables in underground were aimed to determine by using GPR method with 250, 500 and 800 MHz center frequency antennas. For the purpose, numerical modelling, testing in laboratory and field studies on GPR data have been conducted in the paper. According to results from the study, the quality of radargrams obtained numerical modelling has determined geometrical and physical properties of the cables. A radargram acquired on a profile passed across on testing crate showed clearly high amplitude reflections up to 25 cm in laboratory. 2D/3D GPR images, created to identify lateral extension of fiber optic cable on campus area of Karadeniz Technical University, revealed distinctly reflections. Consequently, in order to determine a better way to do research targets, the numerical modeling and laboratory studies are of great importance before carrying out GPR studies on field.
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A Controlled Monitoring Study of Simulated Clandestine Graves Using 3D Ground Penetrating Radar
Authors M. van Schoor, W.C. Nienaber and A. Marais-WernerSummaryA controlled 3D GPR monitoring study of simulated clandestine graves was conducted in conjunction with a taphonomical study that focused on the effect of environmental variables on the decomposition process. The test site is characterised by a clay-rich loam top soil and is located in a summer rainfall region. The combination of the soil type and heavy seasonal rains had a significant impact on the GPR detectability of the burial responses. Although the burial anomalies were clearly discernible in the early stages of monitoring (0-60 days), the above environmental variables rendered these anomalies as relatively challenging to detect during subsequent scans (90-180 day). The heavy, soaking rains in the 60 to 120 day interval resulted in an increased bulk ground conductivity, higher signal attenuation, noisier data, more rapid settling of the disturbed burial zone and consequently lower observed anomaly contrasts.
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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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