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Near Surface 2007 - 13th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 03 Sep 2007 - 05 Sep 2007
- Location: Istanbul, Turkey
- ISBN: 978-90-73781-81-8
- Published: 03 September 2007
41 - 60 of 144 results
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CR Behaviour of Fully and Partially Brine Saturated Building Stones and FE Modelling of Microstructural Features
More LessIn order to assign the value of complex resistivity measurements to non-destructively infer salt and water damages of building stones an experiment series has been conducted on brine saturated samples varying in type, fluid concentration and water content. CR parameter are evaluated and compared to previous results by other authors. A new image based FE modelling approach has been started to discriminate chemical from microstructural implications on CR spectra.
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Geophysical Exploration of Mining Ponds – An Effective Tool for Their Geochemical Characterization
Authors E. Aracil, P. Martínez-Pagán and Á. FazIn the Murcia Province, there are eighty five mining silt ponds resulting from the treatment and exploitation of ancient metallic sulphur mines. These silt ponds are the potential focus of environmental pollution. The Technical University of Cartagena (Murcia, Spain) and Análisis y Gestión del Subsuelo Company (for geophysics) have been carrying out an integral study of these structures from the year 2003 to the present. In this work, studies from the Brunita and El Lirio mining silt ponds (La Union, Murcia) are described. These have allowed the characterization of different horizons, combining the results obtained through geochemical analysis with data derived from electrical tomography (2D and 3D) techniques. Geochemistry has demonstrated that oxidation and salinization processes are more active in the sediment surface layers than in deeper layers. Acidification has also been found to be more active in upper layers, with analyses showing pH values slightly less acidic, combined with high concentrations of Zn. The results were analyzed with the different electrical profiles, and correlated and corroborated with the geochemical sampling analysis.
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GPR Data Interpretation Using Self Organizing Maps
Authors N.S. Spanoudakis and A. VafidisA systematic workflow is proposed which includes ground penetrating radar data pre-processing and attribute calculation. The attributes are classified using a competitive artificial neural network, namely Self Organizing Maps (SOM). This workflow is applied to 3-D ground penetrating radar datasets acquired with the PulseEkko1000 system from a tank model and the archaeological site of Aptera, Chania, Crete, Greece. This proposed methodology, proved to be useful for distinguishing different pipes from the surrounding medium with high resolution and buried relics.
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GPR Characterization of Concrete Tubes and Electrical Cables Buried in the Test Site at São Paulo University – Applications to Urban Planning and Infrastructure
Authors J.L. Porsani, V.R.N. Santos, M.G. Oliveira and F.Y. HiodoThis paper presents some GPR (Ground Penetrating Radar) results on the first Brazilian test site. The test site was constructed close to Institute of Astronomy, Geophysics, and Atmospheric Science (IAG) at São Paulo University campus (USP), in São Paulo city. Different materials and object sizes were installed at several depths along seven lines. The targets buried simulate geotechnical, environmental, and archaeological studies. 200 MHz and 400 MHz GPR profiles were acquired along two lines containing empty concrete tubes, and electrical cables buried that are usually used in the great cities of whole world. Obtained results show typical hyperbolic reflections pattern characteristic for each target, and permit to estimate target dimension. These results can be used as reference pattern, and can be extrapolated to areas in which information about surface is lacking.
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Subsurface Pipe Detection with an Array of Inductive Broadband Vector Receivers
Authors S. MacIntosh, R. Birken and Q. ZhuAn electromagnetic induction survey was conducted using an array of inductive receivers (AIR) to map and verify a network of water lines in a western city in the USA. Data was collected on a 30 cm by 30 cm grid covering 2800 m2. The array utilizes 16 vector broadband electromagnetic sensors arranged in two rows of eight sensor pods. The position of the system was measured using a robotic laser tracking system which records position information with sub-centimeter accuracy. Multiple active “clamp-on” transmitters were used to induce signals into the water lines and other nearby utilities. Each transmitter operated at an individual frequency in the range of 1 kHz – 83 kHz. In addition, ambient signals, such as electric power transmission frequencies were measured. Detailed magnetic field maps are created for each particular frequency of interests and for any combination of the three magnetic field polarizations. Qualitative positions and depth of current carrying lines can be done by directly assessing the magnetic field maps. Magnetic field inversion methods were used to quantitatively determine positions and depths of current carrying lines.
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2D Inversion of Gravity Data Using Artificial Neural Networks in Order to Detect of Subsurface Cavities
Authors A. Hajian, H. Zomorrodian, C. Lucas, H. Sarooee and N. AminiWe aim to estimate the depth of subsurface cavities from gravity data by a new method of 2D inversion through a Multi Layer Perceptron(MLP) neural network.Infact, this method is an intelligent way to interpret gravity data and gain an estimation of depth and shape. The MLP neural network was trained for two main models of cavities: sphere and cylinder in a domain of radius and depth. We tested different MLP’s with different number of neurons in the hidden layer and obtained the optimum value for number of neurons in the hidden layer. Then it was tested in present of 10% noise(S/N=.1), and also tested for real data. It presented good results for depth estimation of subsurface cavities. Keywords: gravity, 2D inversion, subsurface cavities, artificial neural networks, Multi Layer Perceptron
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GPR and Magnetic Methods Applied to Characterize an Archaeological Site in Central Mexico
Authors R.E. Chavez, D.L. Argote, M.E. Camara-Moral, G. Cifuentes and P. LopezA geophysical survey was carried out in an archaeological site called Los Teteles (bunch of rocks), which covered several areas within the zone. This investigation included high resolution total magnetic field and ground penetrating radar (GPR). Previous archaeological work carried out on a northern terrace in 1974 discovered an ancient tomb. Magnetic and GPR data were collected in this site. Results depicted a very nice N-S oriented dipolar magnetic anomaly. GPR data displayed features related to the location at depth of walls and floors. Also, remains of stairways and platforms were also inferred. The magnetic study revealed interesting magnetic trends related with ancient walls and platforms. An interesting inverted magnetic dipolar anomaly found could be interpreted as the location of a tomb. GPR profiles were taken over some of those anomalies, and a correlation was performed among the interpreted profiles confirming previous interpretation. A 3D visualisation of the GPR data along with the application of an enhancement filter helped to define characteristic features related to archaeological artefacts, as the magnetic results defined. The location of the possible tomb could also be confirmed.
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Geophysical Imaging of an Early 19th Century Colonial Defensive Blockhouse
Authors D.C. Nobes and L.R. WallaceIn 1845, the French navy built three blockhouses as part of their defence of French settlers in the Akaroa area, located on Banks Peninsula, near Christchurch, New Zealand. In the 1860’s, the blockhouses were removed and the timber used for other purposes. Two of the blockhouses were situated at either end of the Akaroa township; the locations are well known and documented. The position of the third, in the neighbouring village of Takamatua, is not well known, but is thought to have been sited in what became a public reserve, first known as the Blockhouse Domain and more recently as the Takamatua Domain. To aid local archaeological studies, non-invasive, non-destructive geophysical imaging was carried out across the Takamatua Domain. We expected that little if any of the blockhouse itself remained. However, the nature of the construction was such that we expected to find the defensive trench that enclosed the blockhouse. Using horizontal loop electromagnetic (HLEM), total field magnetic, ground penetrating radar (GPR), and electrical resistivity tomography (ERT) techniques, we identified coincident linear anomalous responses. The clear and unequivocal results of the geophysical surveys suggest that we have indeed found the blockhouse and its surrounding trench or moat.
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Electrical and Seismic Imaging of Ancient Wall Foundations in Thessaloniki, North Greece
Authors P. Tsourlos, G.N. Tsokas, C.B. Papazachos, G. Vargemezis and P. BogiatzisThis work describes the application and results of an ERT and Seismic survey which was conducted at locations of the western part of the Thessaloniki ancient City Walls (N. Greece) aiming to estimate the extend of the walls’ foundations. The borehole ERT results are in agreement with the existing information but are not in full agreement with the seismic tomography results regarding the exact depth estimate of the foundations. It is possible that a joint tomographic interpretation would be able to resolve this bias and create a single interpretation model for both data sets.
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Amplitude Inversion in Seismic Traveltime Tomography – An Application in Subsurface Imaging at a Historical Site
More LessSeismic amplitude is an important parameter for it carries information about the structure of the subsurface which complements that provided by the traveltimes. However, both the need for a special pre-inversion processing of amplitudes and their complex relationship to subsurface features, has often resulted in a preference to traveltimes. Nevertheless, for relatively confined datasets and use of independent information, amplitudes can readily contribute to a better imaging and interpretation of subsurface features. The present work is concerned with the application of seismic first arrival amplitude inversion using ray theory, on a set of seismic tomographic data acquired at a historical site. Amplitude inversion results are compared with those of traveltime inversion. Results show that amplitudes and traveltimes are sensitive to different features of the subsurface. Attenuation distance is more related to the geological bedrock and deeper and broader (low wavenumber) structure, whereas velocity is more related to shallower and detailed (high wavenumber) structure. Amplitude inversion contributes thus to the improvement of the overall subsurface imaging. Seismic tomographic exploration of historical and other near surface sites of historical and engineering interest can benefit from the use of amplitude inversion.
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Monitoring In-situ Horizontal Stress in Soil Using Shear Waves
By R. GhoseWe have developed a laboratory facility where very small change in stress in soft soil can be accurately implemented. The effect of such small stress change on seismic velocity has been measured. Our observation suggests that change in shear-wave velocity can be used to monitor change in in-situ horizontal stress, even if the magnitude of stress change is small. A properly constrained stress-porosity model can successfully predict the observation in most cases.
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Imaging by Scattered Surface Waves – Preliminary Results From Seismic Field Data
Authors A. Kaslilar and G.C. Hermancontrast of the heterogeneity by using scattered surface waves. In the method three dimensional elastodynamic wave propagation and scattering in a laterally invariant embedding is considered. The scattered wavefield is expressed as a domain type integral representation. Weak contrast in density is considered and the Born approximation is used. Due to the lateral invariant medium assumption, the radial symmetry of the medium is considered and the Green’s tensor elements are calculated in an efficient way. The method is verified by numerical tests and by a scale model experiment. Preliminary results of the seismic field data are obtained. The location and the size of the tunnel are reasonably well estimated and the density contrast is qualitatively estimated. The estimation of the actual density contrast is in progress.
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Linearized Inversion of MASW Data Using Inequality Constraints
By M. CercatoInequality constraint formulation of least squares inversion is a flexible method to insert physical constraints (as well as a priori information) into the inversion process. In MASW (Multichannel Analysis of Surface Waves) inversion this has proved to be useful to stabilize convergence when applied to fundamental-mode dominated data as well as to data containing higher modes of propagation. A reliable S-wave velocity profile can be obtained from inversion of surface wave data if all available information is inserted into the inversion algorithm.
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Weight Coefficients for Representing Data Uncertainities in Refraction Microtremor (ReMi) Inversion
Authors K. Yanik and A.T. BasokurIn the ReMi (refraction microtremor) method, the phase velocities are determined at the frequencies where a slope or a peak occurs in the spectral ratio map. Once, a dispersion curve is obtained as a velocity values versus periods, then S-wave velocities of the subsurface can be derived by using some inversion methods. We use the damped least-squares method with the singular value decomposition for the matrix manipulations. An appropriate weighting scheme is suggested for the ReMi data for describing data uncertainties. First, the measured data are smoothed by a combination of simple fitting functions that simulates the behaviour of a dispersion curve. Then the weight coefficients are derived from the differences between the measured and the smoothed data sets in order to make the interpretation algorithm stable.
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Geophysics Assisting the Needs of Soil Stabilisation
By P. ValastiSoil stabilisation is an increasingly popular method to improve foundation conditions in areas of soft soils where direct construction is not possible because it would result in heavy and uneven settlements detrimental to infrastructure. In order to improve the quality of soil stabilisation, subsurface conditions of target area must be known a priori. Especially water content is the single most important factor determining the amount of binding agent to be injected. Various geophysical measurements have been carried out for determining water content independently. A method to transform measured resistivity to water content in clayrich areas where simple law of Archie will fail to function will be presented.
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A CFP Approach to Estimate the Shear-wave Velocity Distribution in the Shallow Subsoil
Authors J.W. Thorbecke and R. GhoseThe CFP approach has been used to estimate Green functions for a high- resolution shear-wave reflection field dataset. This data set contains lateral variations of the near surface and has very low velocities. To estimate a velocity model for this kind of data is not an easy task. The CFP approach gives one-way Green function times which are accurate as indicated by the flat events at t=0. These Green functions contain all the information needed to build an accurate velocity model.
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Geoelectrical Monitoring of Seasonal Moisture Content Changes in an Earth Embankment
Authors J.E. Chambers, P.B. Wilkinson, D.A. Gunn, R.D. Ogilvy, S.G. Pearson, O. Kuras, P.I. Meldrum, G. Ghataora and M. BurrowEarth structures, such as embankments, require ongoing monitoring and maintenance to identify potential failure zones and to compensate for the effects of settlement. Extreme weather events leading to prolonged periods of desiccation or saturation are becoming more frequent and threaten embankment stability. In this paper we develop electrical resistivity tomography (ERT) as a non-invasive tool for characterising and monitoring earth embankments. A study is described in which ERT was applied alongside conventional intrusive techniques to investigate and monitor a section of Victorian Great Central Railway embankment, near Nottingham, UK. A number of modes of deployment were considered including linear 2D ERT arrays both parallel and perpendicular to the long-axis of the embankment and 3D imaging arrays. The resulting ERT images, when calibrated using intrusive geotechnical testing and core samples, revealed the spatial variability of the embankment soils. Parallel ERT sections were used to identify major discontinuities between material types at locations associated with poor track geometry. Perpendicular ERT sections also revealed significant internal heterogeneity, and were used to monitor seasonal changes in the moisture content within the embankment.
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Influence of Water Content and Dry Density on Electric and Electromagnetic Properties of a Compacted Fine-grained Soil
Authors Y-L. Beck, V. Ferber, S. Palma-Lopes and P. CoteIn geotechnical engineering, there is a need for efficient techniques which could measure soil density and moisture content, volume integrating and in a non-destructive manner. Electrical resistivity and dielectric permittivity depend on both these soil parameters and it is hoped that these properties could help addressing this geotechnical issue. In order to evaluate this possibility, the influence of water content and dry density of a compacted fine-grained soil on its resistivity and its permittivity is being studied. As suggested by previously published models and experimental data, electrical resistivity decreases when dry density or gravimetric water content increases, and dielectric permittivity increases with an increase of water content or dry density. The influence of water content is the most significant on permittivity whereas both dry density and water content have a significant influence on resistivity.
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Combining 3D GPR and Subsurface Utility Engineering to Create Accurate Utility Maps in Istanbul
Authors R.A. Birken, R. Stearns, Q. Zhu and S. MacIntoshA pilot project was commissioned by the Istanbul Metropolitan Municipality to assess the combined capabilities of traditional Subsurface Utility Engineering (SUE) and Ground Penetrating imaging Radar (GPiR) to accurately map and identify the utilities underneath the historic streets of Istanbul. The array based GPR technology combined with cm accuracy land surveying and advanced imaging software allows for an efficient and complete mapping of large-areas (1000’s of square-meters) with radar traces collected on a 10 cm grid. The resulting high-resolution 3D radar images are interpreted for utility lines, which are then identified and calibrated with standard SUE techniques. The combination of SUE and GPiR is a powerful technology that produces more accurate utility maps than by each method alone. The complementary technologies were successfully applied in the Eyup district of Istanbul. Typically we achieved good radar penetration down to a depth of approximately 1.5 m, after that a clay layer attenuates the radar signal significantly. The Municipality received accurate CAD drawings with the 3D location of each pipe as determined by GPiR and SUE.
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Peat Dike Stability Monitoring with a Semi-permanent Electrical Resistivity Set-up
Authors W.A. van Leeuwen, E.C. Slob and G. de VriesOne of the results of the failure of a peat dike near Wilnis in the summer of 2003 was that peat dikes needed monitoring during dry periods to observe their sensitivity for heave. It was thought that this could possibly be done combining geotechnical and geophysical techniques. This research focuses on the possibility of electrical resistivity imaging to aid in determining the stability of peat dikes with respect to heave. To achieve this, laboratory experiments, cone penetrations tests, and electrical resistivity measurements were made. After data analysis, a relationship between volumetric weight and electrical resistivity of soil samples was found. This has resulted in a procedure that will allow the successful monitoring of peat dike stability.
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