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Near Surface Geoscience 2014 - 20th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 14-18, 2014
- Location: Athens, Greece
- Published: 08 September 2014
101 - 120 of 185 results
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Simultaneous Inversion of Resistivity Structure and Electrode Locations in ERT
Authors J.H. Kim, M.J. Yi, R. Supper and D. OttowitzSummaryAccurate coordinates of electrodes are essential in ERT, and electrode mislocation or inaccurate information on electrode positions inescapably results in crucial distortions of the subsurface images. This problem would be more probable in crosshole ERT due to lack of borehole deviation data or to their inaccuracy. ERT monitoring especially performed in an area where ground deformations are expected such as in landslide monitoring may also suffer from this problem owing to the electrode movements over time. To provide a fundamental way to solve these problems, a new inversion algorithm was developed so that resistivity distribution and electrode coordinates are simultaneously optimized. We performed numerical experiments to examine the performance of the developed algorithm. The numerical experiments showed that x coordinates can be recovered close to the true ones while the inverted z coordinates have higher errors. Even though the true coordinates cannot be precisely calculated, we were able to calculate the distribution of resistivity which was comparable to the inversion results based on the true electrode coordinates. Finally, the performance of the proposed algorithm is highlighted in two field applications: a surface ERT survey at a landslide-prone are and a crosshole ERT survey.
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Measuring Time Domain Spectral IP in the On-time – Decreasing Acquisition Time and Increasing Signal-to-noise Ratio
Authors P.-I. Olsson, T. Dahlin, G. Fiandaca and E. AukenSummaryResistivity and time domain IP measurements using a 100% duty cycle square waveform current injection signal, and measuring the IP responses during current transmission, was modelled numerically and tested in the field against a traditional IP current injection with a 50% duty cycle waveform and measuring during the current off-time. The results show that the approach is practically applicable and can reduce the measuring time substantially as well as increase the signal-to-noise ratio by measuring the IP-response during the current on-time.
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Negative Apparent Resistivity in 2.5D Dipoledipole Electrical Survey for a Very Simple Model
More LessSummaryNegative apparent resistivity values are often obtained in field exploration using the dipole-dipole configuration. It has been believed that negative apparent resistivity cannot exist theoretically, and most of negative readings in field data have been regarded as measurement errors or noise. In recent years, some studies and experiments presented that negative apparent resistivity can be induced by geological structures. In this study, we show that negative readings can be generated even in very simple structures, if the conductive body is located around the surface and current flows into the conductive body. By analyzing the electric potential distribution and the current flow maps, we investigate the main mechanism of generating negative apparent resistivity. Our experiments show that negative apparent resistivity is a kind of 3D effect. When we apply the line source to simulate 2D modeling, negative readings are not recorded even for the same structures, because the line source blocks the current flow. The inversion results are distorted severely by these negative readings, and deviated from the original model. To properly interpret subsurface media from field data including negative readings, we need a new interpretation algorithm that can deal with negative readings.
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Underwater ERT Survey for Site Investigation for a New Line for Stockholm Metro
Authors T. Dahlin, M.H. Loke, J. Siikanen and M. HöökSummaryAn underwater ERT survey was carried out in central Stockholm as part of pre-investigations for a new line for the Stockholm metro (T-bana). The aim was to identify variations in depth of the bottom sediments, as well as variations in rock quality and the possible presence of weak zones in the rock.
The ERT survey was performed with pole-dipole configuration and an electrode cable with 64 electrodes at 7 meter intervals placed on the sea bottom. The water depth was mapped using sonar combined with recording pressure transducers, and water resistivity as function of depth was integrated in the inversion model. The results show that the rather difficult survey conditions could be handled in a satisfactory way thanks to adequate equipment, careful planning and attention to details.
The measured data contains information that is relevant for creating coherent models of the variation in depth to rock together with data from drilling. The results also indicate that information in variation in rock quality that can be of critical importance for planning of underground construction can be derived from the data. Further comparisons against reference data are required for a full evaluation of the results.
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Airborne Gamma-ray Spectrometry Compilation and Investigation of Leveling Discrepancies
Authors A. Stampolidis, J.R. Rønning, F. Ofstad, V.C. Baranwal and B. HeinckeSummaryLarge-scale airborne geophysical data are an important part of regional-scale mineral exploration and in that context NGU conducted an airborne magnetic and radiometric survey over the Kviteseid, Notodden and Ulefoss regions, in Telemark county in summer 2013 as a part of the MINS project (Mineral resources in South Norway).
Results from a previous high resolution helicopter-borne gamma-ray spectrometry survey that was carried out in 2006 over the Fen Complex, Ulefoss region, were reevaluated in the light of the new data from the regional survey of the MINS project. We compare the thorium (eTh) ground concentrations of the two surveys, and although they have similar pattern, are showing significant scaling difference.
The differences in the level of ground concentrations between two spectrometry surveys flown over the same region were investigated. Reprocessing of the old data revealed that the discrepancies arose from inappropriate selection of sensitivity coefficients during the calculation of elemental ground concentrations from height corrected gamma-ray counts. Therefore, correct calibration of the spectrometers are essential for the quality of the processed data.
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Impact of Hydropower Plant Waters on the Destabilization of Slopes and Causing Landslides to Its Shores
By A FrasheriSummaryAlbania has numerous and biggest dams belonging to the hydroelectric power plant system. This cascade is composed by Fierza, (1978), Komani (1985) and Vau Dejës (1971) hydro power plant. Fierza Hydroelectric Power Plant at higher river flow, has an installed power of 500 MW. The volume of water in its artificial lake is 2.7 billion m3, and lake depth averagely 133 m. The dam has a length on head of 400 meters and height 167 meters. Difference between maximal and minimal levels of surface water in lake was varied over the years from some meters up to 30 or more meters, depending on the annual meteorological conditions.
The exploitation of hydrotechnical work over the last 25 has influenced the modification of their physical-mechanical properties and constructive structure, but also to the lake shore slopes and water reservoirs.
There are observed active landslides in the lakeshores. The most biggest is landslide at Porava village, about 2,5 km from the Fierza dam, etc. These landslises represent a great geological risk for hydropower plants, and Porava village. Walls of the houses are broken in Porava villages.
Results of the complex geological-geophysical-geodesic investigations in above mentioned landslides, and impact of the lake water on slope’s soil and rocks, are presented in the paper.
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Mapping the Soil-Bedrock Interface in the Alluvial Argive Basin (Greece) with TEM Soundings
Authors P.S. Soupios, H.R. Hinojosa-Prieto and K.G. HinzenSummaryThe importance of determining the shape of the soil-bedrock interface in the alluvial Argive Bain, Peloponnese, Greece, stems on the fact that it is a fundamental in a current seismic ground response analysis in areas around the archaeological sites of Midea and Tiryns, the later listed by the UNSECO as world heritage. Although newly collected Transient Electromagnetic (TEM) soundings were modeled and inverted in one-dimension (1D) and used to create two-dimensional (2D) pseudo-sections and maps of resistivity distribution at several depth intervals. This pseudo-2D and -3D imaging procedure has been successful in other alluvial basins allowing inferences of their structure. The 1D inverted models were stitched together to form thirteen 2D geoelectrical resistivity pseudo-sections, some traversing the whole basin. Additionally, the 1D models were interpolated to form maps of resistivity distribution at various depth intervals from 0 to 130 m depth. The previously uncertain soil-bedrock contact outside populated zones within the Argive Bain leading to soil thickness varying from 20 to at least 130 m thick in some areas. The bedrock is uplifted in some areas and highly fractured in others.
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Topographic Modeling for Time-domain AEM Systems Using Finite Element Method
More LessSummaryAs an effective and convenience geophysics tool, the advantage of airborne EM becomes especially obvious for explorations in ridged terrain areas. However, AEM modelling has been ignoring the topographic effect. To understand the influence of topography on AEM, we develop in this paper a time-domain AEM modelling method based on unstructured finite-element method. First, Fourier transform is applied to Maxwell equations to transform the 2.5D problem to 2D. After obtaining the governing equations from the Maxwell equations, the Galerkin method is applied and we get the final discrete equations system for the solution of EM responses in frequency-domain. Via a cosine transform, we get the time-domain Airborne EM responses. The simulation results in this paper are compared with both the published results and the semi-analytical ones and proved that the algorithm presented in this paper has a high accuracy.
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Application of PSTM on GPR Data for NAPL Contamination Detection
Authors O. Shahmoradifar, H. Hashemi and M.K. HafiziSummaryGround penetrating radar can be considered as a non – invasive geophysical method and it can be used to examine contaminated site. Multi-offset GPR measurements are used in order to locate the LNAPL plume. Because there are many similarities between GPR time sections and seismic reflection profiles, seismic reflection processing and imaging methods can be applied on multi-offset GPR data. After applying processing steps such as band pass filtering, deconvolution, gain recovery, AGC, estimation of velocity using sembalance velocity analysis and making stacking section, it can be observed, but subsurface stratigraphy and exact location of the LNAPL plume are unknown. Therefore we used seismic Kirchohoff prestack time migration on multi-offset GPR data. After applying Kirchohoff prestack time migration subsurface stratigraphy and the contaminated site has been exhibited with more clearance and the exact location and the improvement of the contaminated site is characterized by collapsing of the hyperbolas. Also another layer, which is not seen in the before migrated stacked section, is observed beneath of the water table.
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Complex Susceptibility Measurement Using Multi-frequency Slingram EMI Instrument
Authors F.-X. Simon, A. Tabbagh, J. Thiesson, J.C. Donati and A. SarrisSummaryComplex magnetic susceptibility is a well-known property both theoretically and experimentally. To achieve this measurement, different ways have been tested, like TDEM or multi-frequential measurement on soil sample. In this study we carry out the measurements by the use of a multi-frequential EMI Slingram instrument to collect data quickly and in-situ. The use of multi-frequency data is also a way to correct effects of the conductivity on the in-phase component and effects of the magnetic susceptibility on the quadrature component of the raw signal.
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Application of GPR Method for Fractured Zones Detection - Case Study from Nagorzyce site, Poland
Authors T. Golebiowski and S. Tomecka-SuchonSummaryThe paper presents selected results of GPR surveys carried out over former, shallowly located mine of sandstone. The aim of surveys was detection of fractured zones developed over the excavations which caused creation of subsidence areas and sinkhole. GPR surveys were conducted in 3D mode with profiles designed in two perpendicular directions. In interpretation influence of direction of the profiles to visualisation of spatial distribution of fractured zones was analysed. As it was shown in the paper ability of GPR detection of fractures in the rock mass depends on the direction of the profiles versus spatial distribution of fractures. Visibility od geological boundaries in radargrams for different orientations of the profiles is very similar.
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Characterization GPR and ER of Asphaltic Sandstones of the Paraná Basin, Brazil - Preliminary Results
Authors L. Bertolla, J.L. Porsani, C.F. Camargo and V.R. ElisSummaryThis paper shows the preliminary results of GPR and ER profiles on asphaltic sandstones. The study area is located in the municipality of Anhembi, situated on the eastern border of the Paraná basin, central-eastern region of the São Paulo state, Brazil. The region has been the subject of studies by Petrobras (Brazilian Oil Exploration Company) due to the presence of cumulative bitumen in sandstones. The results of ER show zones of high conductivity that match with the absences of reflections in GPR profiles. Although preliminary, the results suggest the presence of bacterial action, featuring a biodegradation problem. Despite this, detailed studies are in progress to test the biodegradation hypothesis.
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Borehole GPR Direct and Refracted Arrivals for Determination of Subsurface Void Dimensions
More LessSummaryGround-penetrating radar (GPR) borehole data were acquired in order to determine the size of shallow coal mining voids in Pittsburg Kansas, USA. A dense grid of drillholes would have been required to accurately map the lateral extent of subsurface mining voids. Borehole GPR was evaluated as a method to image the voids and potentially reduce the number of drillholes necessary to determine void dimensions. Zero-offset transmission profiles showed two distinct arrival modes in the region of the subsurface voids. Velocities estimated for each of the arrival modes indicate that the early (first) arrival corresponds to refracted energy along a carbonate bed over the voids. Slower (second) arrivals correspond to energy that propagated through water saturated void and adjacent coal pillar. The observed radar transmission arrival times were used along with the known separation of the GPR boreholes to determine the proportion of radar wave travelpath through pillar and void present between the boreholes. Using this method, void and pillar dimensions were determined between three boreholes.
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Joint Interpretation through Multiparameter Analysis of Cross-hole Resistivity and Seismic Tomography Measurements
Authors C. Orfanos, K. Leontarakis and G. ApostolopoulosSummaryCross-hole resistivity and seismic tomography methods are powerful tools for the detection of underground openings of various types. Even in cross-hole measurements, the main disadvantage of the geophysical methods still remains and is the uncertainty of the final interpretation. Joined application of both methods and integrated interpretation, can reduce to some degree this uncertainty. In this study, a novel procedure is proposed for the integrated interpretation of cross-hole resistivity and seismic tomography measurements, based on Unified Geophysical Model (U.G.M) approach and Model stacking Technique. This new processing and interpretation procedure is evaluating for its effectiveness on synthetic examples. The post clustering “full most” models of Arithmetic U.G.M. approach delineate the targets accurately, with greater resolution and with the presence of fewer artifacts. The calculation of correlation coefficient and real rms error indexes proved mathematically the improvement of the post clustering inverted models in comparison to pre clustering inverted models.
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A Cost-effective 3D Electrical Resistivity Imaging Approach for Embankment Dike Assessment
Authors C. Jodry, Y. Fargier, S. Palma Lopes, P. Côte and M. SanchezSummaryAlthough DC-electrical resistivity imaging techniques are efficient tools for embankment dike and levee investigation and monitoring, the complexity of such structures often leads to misinterpretation when conventional 2D procedures are implemented. Nevertheless, fully 3D procedures can be very slow and costly, and bring results and resolution that may exceed the needs. This paper introduces the ‘3D–inversion’ approach that allows a useful and reliable 3D vision of the investigated structure while maintaining an acceptable number of electrodes and data. To compensate the lack of dense 3D data, the approach is mainly based on i) designing data acquisition based on a reasonable number of electrodes and measurements, ii) designing a well-adapted model discretization and iii) inserting as much a priori information as possible. The building process of the numerical model requires attention, particularly on the model discretization. To exemplify this new approach, synthetic results are presented on a model that is a simplification of a real scale experimentation. Preliminary outcomes show the cost-effective potential of the 3D– approach which is to be tested on various case studies in the near future.
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Algorithmic Maintenance and Testing Sounding the Possibilities of Equipment Radial Frequency Sounding
More LessSummaryThe paper presents the results of the implementation and optimization of the algorithm for calculating the direct problem and transformation data of radial frequency sounding. Also shown are the results of its application to different horizontal models in order to obtain the parameters of the equipment produce.
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A New Structural Cooperative Inversion Approach Applied to the Tomography of DC Resistivity and Seismic Traveltime
More LessSummaryWe present a new cooperative inversion approach that allows for a combined inversion of independent physical parameters by exchanging structural information. The technique is based on an iterative cluster analysis step using a Fuzzy c-Mean technique (FCM). Cluster analysis aims at identifying groups of similar objects, and helps to discover distribution of patterns and interesting correlations in data sets. An occurring specific pattern of one parameter facilitates the development of coherent structure in the other using a reference model term in the least square solution associated to the linearized minimization of the cost function problem. In the presence of structures that can be seen by both methods, it leads to a better delineation of patterns. The technique is applied to the inversion of electrical resistivity and seismic refraction travel times. Two synthetic co-located data sets show how different structures are resolved with and without structural cooperative inversion. It is discussed how the quality of the inversion results is improved by the new approach.
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Application of Different Algorithms to an Inversion of Rayleigh Wave Dispersion Curve
More LessSummaryInversion of surface wave dispersion curve is a important process for Vs analysis. We tried three algorithms which are the genetic algorithm (GA), least squared method (LSM) and neighbourhood algorithm (NA). These algorithms were utilized for inversion of dispersion curve obtained from surface wave method. Both synthetic and measured data sets were considered. Calculated dispersion curves in the synthetic test study by using GA, NA and LSM algorithms fitted observed dispersion curve with acceptable misfits. Surface wave methods were utilized on measured data in order to determine elastic ground characteristic of Dokuz Eylul University campus in Izmir (Turkey). The variation of shear wave velocity (Vs) with depth could be determined by using dispersive behaviour of Rayleigh waves in surface wave analysis methods. The shear wave velocity cross-sections were obtained by inversion of dispersion curves. The estimated parameters which were obtained from all algorithms were compared with each other and calculated dispersion curves in the field test study by GA, NA and LSM algorithms fitted observed dispersion curve with acceptable misfits as synthetic study. Results of calculated dispersion curves which were obtained from all algorithms are compared with observed dispersion curve. Shear wave velocity cross sections were prepared for all algorithms.
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Rapid Calculation of Optimized Arrays for Long 2-D Survey Lines
Authors M.H. Loke, S. Uhlemann, P.B. Wilkinson and J.C. ChambersSummaryPrevious studies show that optimized arrays generated using the ‘Compare R’ method have significantly better resolution than conventional arrays. This method determines the optimum set of arrays by calculating the change in the model resolution for all the viable arrays (comprehensive data set) for a survey line. The number of possible arrays increases with the fourth power of the number of electrodes. The optimization method faces practical limitations for lines with more than 60 electrodes where the number of possible arrays exceeds a million. Several techniques are examined that can be used for lines with more than 100 electrodes. Recent improvements in the computer GPU reduces the calculation time by a factor of 5. The calculation time can be further reduced by half by using the fact that arrays that are symmetrical about the center of the line produce identical changes in the model resolution values. The calculation time is reduced by another hundred times by using a subset of the comprehensive data set consisting of only symmetrical arrays. Tests with synthetic models show that optimized arrays derived from this subset produce inversion models with differences of less than 10% from those derived using the full comprehensive data set.
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A New Robust Inversion Method Using Cauchy-Steiner Weights – And Its Application in Data Processing
Authors H. Szegedi, M. Dobroka and J. Somogyi MolnarSummaryThe Fourier-transform (FT) often results noisy Fourier spectrum because this operation is linear and contaminate noise in time domain dataset appear in the frequency domain also. Especially, in case of non-Gaussian nature of the noise distribution (for example outliers in the data sets) can cause appreciable distortions in the Fourier spectra. If we treat the FT as an over-determined inverse problem, the noise effect on the Fourier spectrum can be greatly reduced. The geophysical series inversion was used for the discretization. The scaled Hermite functions - note that they are eigen-functions of the FT - were chosen as basis function. During the inversion process by using the Steiner-weights we get a highly robust inversional FT method. Numerical tests show the significant noise rejection capability of the new inversion based FT algorithm.
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