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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
21 - 40 of 194 results
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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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