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Near Surface 2011 - 17th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 12 Sep 2011 - 14 Sep 2011
- Location: Leicester, UK
- ISBN: 978-90-73834-15-6
- Published: 12 September 2011
1 - 100 of 127 results
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Geological Geophysics - An Example Using AEM Data
By D. BeamishGeophysical information, when spatially extensive, can be used to examine the relationships that may exist between geological classification schemes and their geophysical responses and/or properties. Geological geophysics essentially refers to any geophysical map that is based on, or classified according to, existing geological knowledge. Here the geological classification of subsurface values of conductivity, derived from airborne EM survey data, is assessed using survey data obtained across a small (36 x 22 km) area in the south of England. The characteristics of the highest frequency (closest to outcrop) conductivity distributions are examined in relation to two existing sedimentary bedrock classification schemes. Geological classifications of the data are found to explain over 75% of the variability in observed values. It is then demonstrated how the central moments and dispersion statistics of the distributions obtained can be used to predict the continuous near-surface conductivity distribution across a major area of southern England containing, as it does, a high population density and extensive infrastructure.
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Geological Mapping Using Airborne TEM at Mayotte
Authors N. Foged, E. Auken, P. Nehlig, J. Deparis and J. PerrinAirborne electromagnetic methods are practical tools for large scale geological and hydrogeophysical mapping. In this abstract we show results from the SkyTEM survey on the volcanic islands of Mayotte. The SkyTEM survey is an essential part of a project aiming on establishing a basic geological model of Mayotte. Mayotte is a small group of islands between Africa and Madagascar. The results will for instance be used for future urban planning with respect to the risk of landslides and ground stability. Also ground water resources are of high interest and hereby the need for detailed knowledge about the hydrological system and especially the risk of saltwater intrusion to the aquifers. The results from the SkyTEM survey are very promising. The results reveal geological structures both at large scale as well as at the small scale. Though, there is still a large and challenging task at hand in the geological interpretation of the geophysical result and linking resistivity to lithology. Project partners: BRGM, France, Conseil Général de Mayotte, France, and Aarhus University, Denmark.
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Hybrid Inversion of Airborne TEM Data - Mixed Precision Responses for Tunable Performance and Accuracy
Authors C. Kirkegaard, A.V. Christiansen and E. AukenModern airborne TEM surveys often produce extremely large datasets that can be very time consuming to invert using a full system forward model description. Often, approximate forward models are used providing very fast results at the expense of accuracy. We show how forward models of different precision can be combined in a versatile hybrid inversion, providing tunable accuracy and performance. Our study includes inversions of actual field data using different degrees of approximation. We show how this influences the resulting earth resistivity model, discuss the implications and provide performance metrics.
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Singular Value Decomposition Denoising of Airborne Time Domain ElectroMagnetic Data in Anthropized Area
Authors P.A. Reninger, G. Martelet, J. Deparis, J. Perrin and Y. ChenIn anthropized areas, airborne Time-Domain ElectroMagnetic (TDEM) data may be affected by spikes, oscillations, shifts and/or bumps. Since EM noise spectrum is complex, thresholding and stacking standards techniques are hardly efficient in such environment. Time-consuming and subjective manual cleaning is therefore required. We propose an alternative fast and efficient user-assisted filtering approach. We adapted the Singular Value Decomposition (SVD) to denoise TDEM data. The SVD uses principal component analysis to separate noise and geological components of the signal; the latters are used to reconstruct the EM decays with the geological signal only. The SVD procedure was applied on an anthropized area, in the center of France. The comparison between each reconstructed decay and its corresponding measured decay allowed rejecting efficiently mainly spikes and oscillations. In a second phase, an ad hoc analysis of the map of weights of the “noisy components” showed high correlation with man-made installations and provided a tool to reject mainly most likely soundings biased by shifts. Bumped decays were also localized based on the analysis of specific SVD components. The established denoising procedure provides accurate denoising tools and makes, at least, the manual cleaning much less time consuming and less subjective.
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Improvements in Seismoelectric Acquisitions at Field and Lab Scales and Records Interpretation
Authors J. Holzhauer and U. YaramanciTackling the subject of seismoelectric investigations first at field then at lab scale, we have repeatedly been confronted with the pervasive and challenging issue of noise reduction. In our case, noise can take different forms, being either environmental or internal, last being not least. Thus, we developed our laboratory setup introducing five levels of shielding at the seismic source and choosing differential over single-ended transmission. On this basis, we were eventually capable to reduce the internal noise due to the operating seismic source by 4 orders of magnitudes. This enabled a clear record of the coseismic electric field as well as the observation of flat events distinct from the source pulse not yet totally removed, which amplitudes and arrival times proved consistent with an interfacial converted response. In the field, after having forced us to trigger manually, triggering noise was finally remedied by using optical fiber as triggering cable, thus reducing the pre-processing time tremendously. Being ridden of plaguing noise sources, we are now free to conduct systematic studies in both field and lab as well as to address the issue of environmental noise reduction using numerical filtering.
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Automatic Classification of Metallic, Plastic and Concrete Targets Buried at IAG/USP Geophysical Test Site Using ANN and GPR Methodologies - First Results
Authors V. Dos Santos, J. Porsani and N. HirataA methodology for classifying automatically metallic, plastic and concrete targets using pattern recognition techniques on GPR data under controlled field conditions was developed. The method consists to develop an Artificial Neural Network (ANN) classifier, using the multilayer perceptron (MLP), with features extracted from GPR profiles over targets in subsoil, and then using it to classify diffraction hyperbolas indicating their position and depth. The classification allows a high resolution reconstruction of the subsurface with reduced computing time. The system was developed in MATLAB and applied to data obtained from the IAG-USP test site, located in the city of São Paulo, Brazil, containing metallic and plastic drums and pipes and concrete tubes under controlled field conditions. The results using real data indicate that the automatic classification of the targets in the subsoil is efficient, contributing for ambiguities reduction in the near surface geophysical data interpretation, besides having application on mapping of targets in subsoil.
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First in Situ Test of a New Electrostatic Resistivity Meter
Authors A. Tabbagh, S. Flageul, M. abas, J. Thiesson and F. RejibaIf field applications of the electrostatic method are limited to roughly the first ten meters due to the necessity of staying in the low induction number domain, the possibilities it opens in non-destructive testing, dry hole resistivity logging and laboratory studies of the complex resistivity justify the design of a new multi-frequency resistivity meter presenting a very low input capacitance and a high phase sensitivity. After a first series of sample measurements in laboratory, the new resistivity meter was tested in two different field contexts: the mapping of building remains in a Gallo-roman archaeological site under a flat meadow, the assessment of the anthropogenic layers thickness in a town. The first test allowed a comparison with galvanic resistivity previous measurements and proved a very good agreement between both magnitude and spatial distribution of the resistivity. The second test established its reliable measuring abilities in a very disturbed environment.
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Modeling Seismic Inversion in Mature Oil Field of West Area, Coastal Plain Pekanbaru, Central Sumatera Basin, Indonesia
More LessThis paper presents a practical approach for the application modeling seismic inversion of real seismic amplitude data. The estimation of recorded seismic amplitudes from reflection seismic gathers is performed with the aid of post-stack time migration, which enhances continuity and reflection strength. This approach is applied to new acquisition 3D seismic of West Area Coastal Plain Pekanbaru,Indonesia. The application example represents overlaying structure map with reservoir properties and facies. This will help us to develop and manage reservoir better in mature oil field within the 3D seismic area.
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Wave Equation Datum Continuation Based on Simulated Annealing Optimized FFD Operator
More LessIn this article, we present a new technique to do wave equation datum continuation. The method is based on a global optimization FFD operator whose parameters are optimized by simulated annealing algorithm. We extend the number of the FFD operator’s parameters from 2 to 4 to improve the precision of wave-field after continuation on the premise of no increasing calculation. Example on the theoretical model data shows that the method could remove the influence of shallow subsurface and make the structures of underlying strata recovered.
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The Normalized Integration Method - An Alternative to Full Waveform Inversion?
Authors J. Liu, H. Chauris and H. CalandraClassical full waveform inversion is a powerful tool to retrieve the Earth properties from seismic measurements. However, the associated objective function that measures the misfit between observed and computed data, suffers from many local minima. We propose an alternative method referred as the Normalized Integration Method, where the objective function measures the misfit between the integral of the absolute value, or of the square, or of the envelope of the signal. Because we only compare functions increasing with time, the objective function has a more convex shape. We first present the different formulations and show how to efficiently compute the gradient of the misfit functions. We then compare the new approach to the classical full waveform inversion and to the Laplace transform through a simple application on a 2D synthetic data set. This example shows that the new approach can be useful for the determination of the long wavelengths of the velocity model.
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Depth Estimation of Cavities from Microgravity Data Through Multi Adaptive Neuro Fuzzy Interference System
Authors A. Hajian, P. Styles and H. ZomorrodianWe aim to estimate the depth of subsurface cavities from gravity data by a new method through a Multiple Adaptive Neuro Fuzzy Interference System (MANFIS); this method is an intelligent way to interpret microgravity data and gain an estimation of depth and shape of the most probable cavities. The MANFIS model was trained for two main models of cavities: sphere and cylinder in the related domains of radius and depth. We tested different MANFIS’s with different number of rules and obtained the optimum value for number of in the hidden layer. Then it was tested in the presence of 20% Gaussian noise and showed good robusnesst to noise. The method was also tested for real microgravity data from Bahamas Free Port. The results are in good agreement with ground-thruthed drilled values for the depth of subsurface cavities.
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Strike-slip Faults Imaging by Seismic Methods from Underground Galleries in the Station of Tournemire, France
Authors F. Bretaudeau, C. Gélis, D. Leparoux, J. Cabrera and P. CôteDeep argillaceous formations are considered in many countries as potential host media for high-level long-lived radioactive waste due to their physical properties. In the experimental station of Tournemire, strike-slip faults with small vertical offsets were intercepted from underground galleries in the clay layer. Our aim is to assess the capacities and limits of high resolution seismic methods to detect and characterise the fault zones from underground works. Elastic Full Waveform Inversion (FWI) and Reverse Time Migration (RTM) are tested using original measurement configurations involving one, two or three perpendicular galleries. In the frame of numerical simulations, we show how RTM can detect only a part of the fault, and how FWI can provide a full or partial quantitative image of the fault zone depending on the measurement configuration.
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Combined Gravity and Seismic Measurements for Mapping a Buried Tectonic Valley in Western Sweden
Authors N. Juhojuntti, S. Aaro, J. Jönberger and O. LarssonCombined seismic and gravity measurements have been used to map an old buried tectonic valley close to the river Klarälven in western Sweden. A gravity low of around 4 mGal is observed along the valley. Interpretation of the gravity data and seismic refraction/wide-angle reflection data indicates that the depth to bedrock exceeds 150 m in the valley. The river Klarälven previously followed the tectonic valley throughout its length, however, at the end of the latest glacial period the river was diverted to a much wider valley. The observation of such large depths to bedrock should have implications for the interpretation of the Quaternary history of the area. The seismic measurements do not indicate that permeable sediments such as coarse-grained sand or gravel occur near the surface in the valley. However, the possibility that such sediments are present at larger depths should justify further investigations for purposes of groundwater extraction and/or for geothermal applications. To conclude, we argue that the combination of gravity and seismic measurements has been a cost-effective method for mapping the buried valley.
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Effect of a Discontinuity’s Aperture on the GPR Signal
Authors M. Markovaara-Koivisto and T. HokkanenThe aim of this paper was to study effect of an opening to GPR signal, measured with 800 MHz and 1600 MHz transmitter/antenna. Tests were carried out in laboratory scale using two sawn rock blocks. The opening between them varied from 0 to 10 cm. First GPR signal’s velocity was defined in the blocks by calculating two way travel time, as the thickness of the blocks was known. Next the top and bottom of the opening were interpreted from the GPR profile by utilising theoretical change of the reflected signal’s phase in the opening. Openings apertures were calculated from the interpretations and compared to the real ones. Good results were achieved for the greatest apertures, but apertures smaller than 1/6 of the signals wavelength could not be interpreted due to signal summing. Summing could be less of a problem when using other filling materials than air in the opening. These are the next research subjects for the authors.
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Geophysical Assessment of Unsaturated Hydraulic Conductivities – Examples from the CLIWAT Project Area Föhr
Authors R. Kirsch, M. Kordanska, S. Lessing, N. Blindow and W. ScheerA geophysical approach for an assessment of hydraulic conductivities based on geophysical measurements is presented. It is based on the van Genuchten (1980) relation between the relative hydraulic conductivity and the saturation degree of the unsaturated zone. For the determination of the porosity we make use of Gassmanns equation linking the saturated and the unsaturated porosity. If we use seismic p-wave velocities instead of bulk moduli, the influence of the unknown shear wave velocity is small and can be neglected. For the determination of water content of the unsaturated zone GPR measurements are used. It is shown that the irreversible pore water of the sediment can lead to an overestimation of the water content, especially if the sediment is poorly sorted.
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Optimal Spread Design for Passive MASW
Authors B. Leitner, R.D. Miller and J. IvanovThe passive surface wave method has the potential to reach investigation depths significantly greater than active sources and can be effectively used in very noisy settings. In this study the optimal spread layout and orientation is investigated and determined that would allow surface wave penetration deep enough into the bedrock to permit studies of stress and strain conditions. An array of 336 receivers was deployed in a 2-D spread. The 4.5 Hz vertical geophones were orientated at parallel and orthogonal alignments to nearby railroad tracks. The dominant energy source were trains passing both along the north and west sides of the site. By selecting the appropriate recording times it was possible to uniquely capture trains in a wide range of azimuthal orientations. The relatively large overall spread made it possible to compare various sub-arrays by processing only selected receivers. Arrays orientated parallel to the direction of incident low frequency waves, resolve frequencies as low as 4 Hz even using small number of receivers. These lower frequencies allow a marked increase in the maximum depth of investigation, thereby, helping to solve engineering problems at greater depths than possible with active source investigations.
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Measured and Modeled Gravity Anomalies above the Tunnel in Clays – Implication for Errors in Gravity Interpretation
By V. BlechaWe measured gravity anomaly above the tunnel excavated in stiff clays. Technical parameters of the tunnel were known. In order to obtain material parameters of clays the core hole was drilled in the study area. For calculation of gravity anomaly of the tunnel we used wet bulk densities of clays derived from laboratory measurements of drill core samples. But the fit between observed and calculated gravity data were poor in this case. If we use for interpretation densities calculated from laboratory measurements we interpret the roof of the tunnel in the depth of 18 m instead of correct 12 m. The reason is that the borehole clay samples can expand after removal from the depth and they do not retain natural moisture probably. Much better fit between observed and calculated gravity values we received when we used for modeling densities derived from gamma-gamma log. We also modeled gravity effect of the deformation zone which forms around the tunnel. Gravity effect of the deformation zone was negligible.
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On the Geoelectrical Characterization of an Old Landfill Cover
Authors V. Naudet, F. Genelle, M. Dabas, C. Sirieix, J. Riss, S. Rénié, B. Dubéarnes and P. BégassatTwo geoelectrical methods were used to characterize the state of an old French landfill cover. The objectives were to locate different materials used for the covering such as clay material or geomembrane and to identify heterogeneities that could be linked to possible defects in the cover due to fractures or cracks. These damages can induce preferential water pathways and unusual increase of leachate within the waste mass. The geoelectrical methods used were the electrical resistivity cartography with an Automatic Resistivity Profiling (ARP©) and the Self Potential method (SP). Results have put in evidence three distinct zones with different geoelectrical signatures that are correlated with three different phases of landfill covering. ARP also seems to have detected the presence of geomembrane installed all around the old landfill to improve its stability and geodrains over two closed alveoli. Local differences in apparent resistivity and self-potential signals have also been identified and could result in default in the clay cover due to thickness variations but also in different lithology, compaction and water content.
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Geophysical Investigation of a Dutch Levee and Canal Using Various Seismic and GPR Techniques
Authors R.P. Noorlandt, M.P.E. de Kleine, P.P. Kruiver, M.A.J. Bakker, C.S. Mesdag and R.M. HoogendoornA geophysical survey has been carried out on the Juliana Canal and levee in the south of the Netherlands. For a stretch of about 35 km along this canal various construction works are planned, to deepen and widen the canal. The canal is in a geological setting such that leakage from the canal to the surrounding landscape during the construction works is a significant risk. To be able to adapt the construction works to the specific circumstances on hand, a number of geophysical methods were selected to determine the required characterisation of the subsurface. From these techniques the sub bottom profiles and side scan sonar on the canal, the S-wave reflection and GPR measurement on the levee provided most useful information.
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Urban Geoelectrical Survey and Spatially Constrained Interpretation for a New Metro in Copenhagen
Authors K. Martinez and J.A. MendozaGeophysical methods are prone to fail in an urban environment as all sources of noise are present in cities. The Fælledparken Park, a large open space area in Copenhagen, Denmark is not an exception. However, an extension of the metro along sections of high variability on the surface of a limestone surface motivated the use of geoelectrics as complement to seismic methods. The site conditions have implications for hydrogeological understanding and groundwater modelling validation needed for dewatering design during construction. The geo-electrical survey aim was the mapping of sediment layers in open areas where the recharge through Quaternary layers to the limestone aquifer is important for reliable groundwater modelling of dewatering scenarios. The main objectives of the surveys were to; a) Produce information that can be used to refine or update the site specific hydrogeological model for Cityringen tunnel and station locations. b) Map the sediment layers in the area, including an assessment of their areal distribution. c) Map the top of the limestone. The main outcome was that the area had heterogeneous conditions, where the west part was comprised of both sands and tills and a buried valley structure, and in the east a continuous sequence of sands.
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Geophysical Techniques Improving Geological Mapping in Urban Areas at Two Different Scales
Authors B. Benjumea, A. Gabàs, F. Bellmunt, A. Macau, S. Figueras and M. VilàWe present a geophysical study where different geophysical techniques are combined to support an urban geological mapping project in NE of Spain. The study has been carried out at two different scales. Seismic and electrical resistivity tomography methods have provided a subsoil image up to 30/40 m depth supported by surface-wave and H/V microtremor analysis. The Quaternary/Neogene contact is related to a sharp velocity contrast in one of the profiles producing a significant secondary peak in the H/V microtremor ratio. Therefore, the identification of this second peak could be used for Quaternary thickness estimation in urban areas. Furthermore, large scale study has been carried out combining CSAMT/MT and H/V microtremor methods. The resistivity model obtained from electromagnetic data allows imaging Neogene/Paleogene contact and Paleozoic bedrock. Combination of this model and bedrock depth estimation obtained from H/V technique has led to identify the geological contact linked to the soil fundamental frequency. Therefore, H/V measurements extended to the whole urban areas can be used to assess bedrock depth. The methodology introduced in this study will improve geological mapping at 1:5000 scale in other urban areas of Catalonia region.
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3D Electrical Resistivity Tomography to Locate DNAPL Contamination in an Urban Environment
Authors V. Naudet, J.C. Gourry, F. Mathieu, J.F. Girard, A. Blondel and A. SaadaThis study presents results from electrical resistivity campaigns performed around a housing estate located downstream to an old coke tar site in France. This coke tar has been previously studied with geochemical analyses and geophysical prospections. The previous results have shown a possible migration of the contaminant plume toward the housing estate. As geophysical measurements are difficult to perform in such an urban environment, the electrical array has been deployed all around the housing estate with an innovative arrangement of surface electrodes in C-shape and acquisition geometries in order to achieve a real 3D imaging of the subsoil and locate the contaminant plume above the housing estate. The electrical resistivity data were inverted with the ERTLabTM 3D inversion software developed by Multi-Phase Technologies and Geostudi Astier. Results show, at the depth of the aquifer, a very conductive plume emanating from old tar ponds and a slag heap and spreading through the housing estate.
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Suppression of Traffic Noise on Vibroseismic Data
By E. NørmarkIn Denmark, vibroseismic data acquisition is commonly used for mapping groundwater reservoirs. A significant part of the reflection seismic profiles are acquired on asphalt roads, which makes the data very exposed to traffic noise. Normally, several sweeps are carried out at each source location. This gives an option for suppressing traffic noise on uncorrelated data. The method assumes that the vibroseismic source signal shows high repeatability in terms of both amplitude and phase. In this context, noise problems from cars and pedestrians will be addressed, but the present procedure is capable of suppressing any kind of noise that is non-repeating and reasonably localized in space and time. The method is simple and robust. It improves signal-to-noise ratio and reduces the need for manual trace editing when processing the data. Thus, by addressing the problem before correlation, significant improvements in the noise suppression can be achieved.
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Geophysical Monitoring of Simulated Clandestine Graves Using Electrical and GPR Methods - 0-3 Years after Burial
Authors J.K. Pringle, J.R. Jervis, J.D. Hansen, N.J. Cassidy, G.M. Jones and G.T. TuckwellThis study provides forensic search teams with systematic geophysical monitoring data over simulated clandestine graves for comparison to active cases. Simulated ‘wrapped’ and ‘naked’ burials were created. Multi-geophysical surveys were collected over a three-year monitoring period. Bulk ground resistivity, Electrical Resistivity Imaging, multi-frequency Ground Penetrating Radar and grave ‘soil water’ conductivity data were collected. Resistivity surveys revealed the naked burial had consistently low-resistivity anomalies, whereas the wrapped burial which had small, varying high-resistivity anomalies. GPR 110-900 MHz frequency surveys showed the wrapped burial could be detected throughout, with the ‘naked’ burial difficult to resolve after 18 months. 225 MHz frequency data was optimal. ‘Soil water’ analyses showed rapidly increasing (year one), slowly increasing (year two) and decreasing (year three) conductivity values. Results suggest resistivity and GPR surveys should be collected if target ‘wrapping’ is unknown, with winter to spring surveys optimal. Resistivity surveys should be collected in clay-rich soils.
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MASW Characterization of Ground Subsidence Induced by an Outburst of Water From a Sump Gallery at St-Vaast, Belgium
Authors J. Deceuster, P. Lizin, T. Martin and O. KaufmannIn February 2009, several damages were recorded on 9 houses and a road at St-Vaast (Belgium) a few days after a sudden outburst of water coming from a former coal mining sump gallery. To identify the origin of these damages and understand the mechanism of the ground subsidence, geological, hydrogeological geotechnical and geophysical investigations were conducted. Five MASW (Multichannel Analysis of Surface Waves) profiles were carried out inside and around the impacted zone to delimit its extent. S-velocity anomalies were pointed out near the surface and at depth just beneath the centre of the impacted area.
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Reliability of Surface Wave Inversion
Authors J. Hausmann, H. Steinnel, S. Popp-Hoffmann, U. Werban and P. DietrichSurface wave seismic can be used to evaluate near surface geological information out. In order to get a reliable bedding model of the near surface loose soil layers the quality of the data and the inversion constraints are the most important influencing factors. This study discusses the influence of the thickness model as a boundary constraint doing data inversion according to MASW processed data (Park et al. 1999, Park et al. 2007). It can be shown that there is a relevant influence of the used thickness model of the uses SurfSeis sofware on the inversion results. A fixed depth model leads to unwanted stretching effects. The assessment of a free depth model using an equal or a variable thickness model gives good and comparable results. But there are high differences between the results if the profile was shot in opposite direction. The geological interpretation may vary according to the direction of the profiling. Forward and reverse measured profiles show significant differences in the inversion results, even there is only one geological setting. Therefore it is recommended to have a critical view of one dimensioned profile section.
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GPR for the Inspection of Industrial Railway Tracks
Authors J. Hugenschmidt, C. Kasa and H. KatoIndustrial railway tracks are important for the loading and unloading of goods and therefore for the railway system in general. Often, industrial tracks are embedded in concrete and/or asphalt to enable truck and other traffic across rails. Thus, most of the construction is hidden from visual inspection. If repair work is planned or if damage occurs, details of the construction have to be known for the planning of repair work or for the evaluation of damages. This paper describes the non-destructive testing of industrial tracks using GPR. Typical testing problems are described and the application of GPR for these problems is demonstrated using data from field measurements. Data from different types of equipment are compared and benefits and limits of the method are discussed. It is shown that GPR is a powerful tool for the inspection of industrial railway tracks.
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Noise Cancellation for Surface NMR - Application of Time and Frequency Domain Approaches
Authors M. Mueller-Petke and U. YaramanciEven though surface NMR is the only geophysical technique that provides hydrogeophysical rock properties on the base of direct sensitivity to subsurface water, in many cases surface NMR measurements suffers bad signal-to-noise ratio, and measurements can be carried out only far from sources of electromagnetic noise. To overcome these restriction some approaches using a reference loop based system were developed during the last years (Radic, 2006; Walsh, 2008; Müller-Petke and Yaramanci, 2010; Neyer, 2010). All approaches have demonstrated useful capability to improve S/N. But a comparison that allows for determining properties, i.e., pro and contra of each approach is missing. Thus, to compare time domain with the frequency approach, we developed an own frequency domain code beside the existing time domain code (Müller-Petke and Yaramanci, 2011). The algorithms were tested using synthetic examples and a field examples. It shows that with increasing complexity of the noise the frequency domain approach provides better results compared to the time domain for the synthetic examples. Concerning the field data both approaches are equivalent. We found that using the multi-channel reference the signal enhancement is improved.
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Noise Cancellation of Multichannel Magnetic Resonance Sounding Measurements with Wiener and Adaptive Filters
Authors E. Dalgaard, E. Auken and J. J. LarsenMagnetic resonance sounding (MRS) is a unique and promising hydrogeophysical technique. Due to its ability of providing a direct estimate of the water content and estimates of the porosity in the subsurface, MRS has the potential of being a great team player in the field of hydrological prospecting. However, the MRS measurements suffer from a low signal to noise ratio due to the signal being inherently weak and the susceptibility to electromagnetic interference. The low signal to noise ratio currently impedes the applicability of the technique, particularly in urban areas. In recent years the development of multichannel MRS has opened up new possibilities for advanced noise cancellation methods. Before multichannel MRS can realize its full potential robust and reliable methods for noise cancellation must be developed. In this work we compare two noise cancellation methods: The multichannel Wiener filter and a multichannel adaptive noise cancellation filter. The comparison is performed on noise records from a multichannel MRS instrument with or without synthetic signal added. Our results show that adaptive noise cancellation performs better than the Wiener filter for both pure noise removal and for recovery of the parameters of the synthetic signal.
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Full FID Inversion of MRS Data
Authors A. Behroozmand, E. Auken, G. Fiandaca, E. Dalgaard and J. LarsenThe Magnetic Resonance Sounding (MRS) method, also known as surface NMR, is a non-invasive geophysical method which directly studies groundwater reservoirs from surface measurements. It is capable of direct estimating of water content distribution (from initial amplitudes) and indirect information on pore sizes (from relaxation time). The purpose of this paper is to introduce a new inversion scheme for the full MRS data set. The multi-exponential behaviour of the NMR signal is taken into account and estimated by the Stretched-Exponential model (Hilfer, 2002), in which the time constant and stretching exponent determine the centre and the width of the distribution. The model space is then described with much less parameters, compared with the multi-exponential description. The FID time series are logarithmically gated and the discretization in the z-direction in the forward algorithm is decoupled from discretization of the inversion model. The inversion routine supports smooth and block inversion of the data and a full linear model parameter sensitivity analysis can be calculated (Tarantola and Valette, 1982), an essential criteria for evaluation of the inversion results.
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Block Joint Inversion (BJI) of MRS and DC Resistivity Soundings for Aquifer Imaging at the North Sea Island Borkum
Authors T. Gunther, J. Liebau, I. Akca and M. Müller-PetkeWe present a joint block inversion of DC resistivity and spectral inversion of magnetic resonance sounding (MRS) data. Coupling is achieved by using identical layer thicknesses. We apply the method to data from the north sea island Borkum and demonstrate that the joint inversion is superior to single inversions. Spectral inversion is necessary and finally allows for distinguishing lithology and salinity. Both deterministic, Gauss-Newton (GN), and stochastic, Genetic Algorithm (GA), approaches yield identical results within the resolution limits.
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3D Monte Carlo Inversion of Surface Magnetic Resonance Measurements
Authors AC Chevalier, A. Legchenko, M. Descloitres, H. Guyard, C. Vincent and S. GaramboisMagnetic Resonance Tomography (MRT) is the only geophysical method where the measured signal is directly related to water distribution in the ground. This property allows three-dimensional imagery of water content by signal inversion routines. Because of its non linearity, the inverse problem has a quasi-infinite number of solutions implying as many possible spatial distributions of water content. A good answer to this problem, relevant for ice cavity detection and karstic structures mapping, is to provide a set of solutions consistent with the measured data. Markov Chain Monte Carlo (MCMC) algorithm applied to the MRT inverse problem provides a random exploration of the solutions giving the ability to compute probabilistic answer to a particular data set. For saturated structure detection, first results on synthetic cases demonstrate the routine ability to show an important anisotropy in the MRT resolution. Finally, a real case study, where the MCMC and linear inversion are compared, also shows the benefit of the method for a French Alp glacier cavity detection.
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Defining Formation Boundaries in Multi-layer Models Through Statistical Analysis
More LessEstimating formation boundaries from 1D inversion of electric and electromagnetic data with multi-layer models can be challenging because of the inevitable regularization of this type of inversion resulting in more or less smeared transition zones between formations. Conventional wisdom has it that inversion with few-layer models will solve the problem by providing models with well defined layer boundaries. However, in modern profile-oriented, laterally correlated inversion, the number of layers is the same for all models along the profile. This may cause lateral formation boundaries to be poorly indicated, and sometimes a specific formation will "change layers" along the profile meaning that layer boundaries are no longer formation boundaries. I suggest a new approach to the definition of formation boundaries. It is based on multi-layer inversion models and finds formation boundaries through a statistical analysis of the set of equivalent models obtained in a stochastic process with a correlation function defined by the posterior covariance matrix of the inversion. The method surmounts several of the difficulties mentioned above. A field example will show a successful application of the method.
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How to Incorporate Prior Information in Geophysical Inverse Problems - Deterministic and Geostatistical Approaches
Authors T. Hermans, D. Caterina, R. Martin, A. Kemna, T. Robert and F. NguyenMany geophysical inverse problems are ill-posed leading to non-uniqueness of the solution. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by regularizing the inverse problem and incorporating all available prior information in the inversion process. We compare three different ways to go beyond standard Occam’s inversion for electrical resistivity tomography (ERT) using electromagnetic logging data in the context of salt water infiltration: a simple reference model, a structural constraint and a geostatistical constraint based on a vertical correlation length. Results with the traditional smoothness constraint yield small contrasts of resistivity, far from the reality revealed by borehole measurements. Incorporating prior information from boreholes clearly improves the misfit with logging data. If a good reference model can always be used, it can lead to misinterpretation if its weight is too strong. When the computation of the correlation length is possible, the geostatistical inversion gives satisfactory results everywhere in the section. In this specific case, the geostatistical approach seems to be a more robust way to incorporate prior information. The structural constraint seems to be more indicated when integrating information from other geophysical methods such as GPR or seismic.
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A Complicate Response of Compact EMI Sensors over Shallow Local Conductive Targets
By E.V. BalkovThe paper presents a modeling of different type of compact EMI sensors response over a local conductive shallow depth targets. The target as well as Tx and Rx coils are approximated by the vertical magnetic dipoles. All the calculations are made under the model of uniform conductive space. Three types of coil configuration are considered. First is semi two coil device with fixed transmitter that carries out the sounding by the changing both the intercoil separation and frequency. The second type is three coil multi frequency sensor with fixed geometry. All the coils in previous configuration are arranged at the same plane. The last new type is multi frequency sensor with the receiver coils that placed at the line inclined to the horizon. A field example is considered as well. The theoretical and practical investigations show that shallow local conductive anomalies produce sophisticated response for compact EMI sensors with spaced coils. It yields from one to four anomalies over one target. The new proposed coil configuration produces the single anomaly
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3D Quantitative Interpretation of Archaeo-magnetic Datasets
Authors S. Cheyney, I. Hill, N. Linford, S. Fishwick and C. LeechRecent advances in magnetic surveying have meant high-resolution data over archaeological sites can now be quickly obtained. However, post-survey processing still generally comprises a sequence of data correction and filtering prior to a 2D visual interpretation based on pattern recognition. Developments in the processing and modelling of aero-magnetic datasets have led to techniques that can identify the location and shape of anomalous sources, including providing depth information. This paper explores the possibility that several of these techniques can be adapted for use on archao-magnetic datasets. 3D models of subsurface magnetic susceptibility can be generated using inverse methods. Prior to inversion it is important that processing and filtering techniques do not alter the character of the measured signal. Inverse models suffer from the “inverse problem” where many models fit the data equally well. In order to help overcome this Euler deconvolution has be used to constrain the inversion routine, and provide confidence in the final result. Results from a case study over a Romano-Celtic temple, Silchester, UK are shown, and in order to test the robustness of the technique, the model has been compared to a GPR survey collected concurrently. Both models show equivalent features for each depth slice.
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Some Geophysical Survey Methodologies for Archaeological Research in the Brú na Bóinne World Heritage Site, County Meath, Ireland
More LessThe presented paper is a project update of integrated research to evaluate the potential for developing engineered geothermal systems (EGS) in Alberta, Canada. This process balances both locating the highest concentrations of the resource and assessing where there is a demand for those resources. In the context of the geothermal exploration being undertaken by Helmholtz Alberta Initiative (HAI), this requires that heat sources for heavy oil and bitumen production are found close to the deposits. In the passed year, 25 2D seismic lines in different locations of Northern Alberta were reprocessed and interpreted. Reference and Horizon maps were produced in Athabasca area and it would be developed by adding and processing more data sets. This work is innovative as we are developing the first geological model of the area that integrates seismic data, all data prior to this has been developed on the basis of well logs.
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Geophysical Investigations in UK Graveyards - Re-use of Existing Burial Grounds
Authors J.D. Hansen and J.K. PringleThis series of linked studies tests the capabilities of ground penetrating radar (GPR) and fixed-offset bulk resistivity surveys in the search for unmarked burials in UK graveyards. 225 MHz GPR antennae was deemed the optimum dominant frequency due to success in delineating the positions of known burials and also the relatively rapid data acquisition rate compared to higher frequencies. Resistivity data were collected using two mobile probe spacings (0.5m and 1m) simultaneously, 1-m probe spaced data was judged to be less affected by near-surface heterogeneities. Comparisons of both resistivity datasets were deemed worthwhile to detect unmarked burials. Soil type has a major influence on the effectiveness of each geophysical technique. It is recommended that both GPR and resistivity surveys be conducted to optimise the detection of unmarked burials.
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Bedrock Detection and Mineral Thickness Assessment Using 3D Electrical Resistivity Tomography (ERT)
Authors J.E. Chambers, P.B. Wilkinson, A. Hameed, I.A. Hill, C.A. Jeffrey, D. Wardrop, P.I. Meldrum, O. Kuras, R.D. Ogilvy, D. Gunn, M. Cave and J. AumonierIn this study, we demonstrate the use of 3D ERT for detecting depth to bedrock below river terrace sand and gravel, and quantify its performance against borehole control data. The approach to ERT bedrock detection considered here assumes that the interface is located at the maximum slope of the resistivity-depth curve, and is therefore referred to as the ‘steepest gradient method’ (SGM). The study site was located within a geological setting that has proven to be impossible to adequately characterise using conventional discrete sampling approaches (i.e. boreholes and trial pits) due to the considerable heterogeneity of the deposit. Comparisons between borehole and the SGM derived bedrock surface elevations indicated a reasonable agreement between the two, thereby establishing a basis, in this case, on which to estimate mineral volumes using 3D ERT and the SGM. The bedrock surface calculated from the resistivity data provided a means of estimating mineral volumes at a site for which a meaningful reserve calculation could not be made using conventional approaches to site investigation.
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Geophysical Quality Assessment of Sand and Gravel Deposits
Authors I. Hill, C.A. Jeffrey and A. HameedQuality assessment of onshoresand and gravel deposits must be low-cost and efficient to be economically attractive. This paper reports initial results of a thorough comparison of assessment methods for such deposits, by comparing results from geophysical surveys and exploration drilling with a variety of drilling methods, with ground truth obtained by direct sampling of the deposits as the working face of the quarry moved through the areas of study. Here, the results of the geophysical assessment of the deposits uses resistivity and EM methods. While immedaite results are practically useful to the industry, further development of 3D inversion, and petrophysical models are needed to fully exploit the data quality from current survey instruments.
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Industrial Raw Material Screening - Development of Cost-beneficial Screening Approach to Validate Clay Extraction Zones
Authors K. Martinez, J. Wibroe, J. Henssel and G. HydeThe Capitol Region of Denmark is in the process of mapping the industrial raw materials potential within northeastern Denmark. Raw industrial materials in the region consist primarily of sands and gravels and meltwater clays used for brick production. There are several areas that have been registered by landowners as having rights to meltwater clay deposits, however, there is not ground truth or surveys to verify these zones contain meltwater clays of value for industrial extraction. Due to the many extraction right zones existing, an approach that would verify the presence of industrial use meltwater clays in a cost-beneficial manner was desired. The project had the following three phases: Phase 1: Collection, review and evaluation of existing data and conceptualisation of the geological conditions. Based on the evaluation the screening approach with geophysical methods was determined; Phase 2: Geophysical screening; Phase 3: Validation of clay deposits and volume estimation. The results from one case study are presented. The results indicated that geophysical screening with GEM2 and multi-electrode profiling with follow up drilling was able to delineate zones where meltwater clay was present. The cost-benefit model determined from this project will be adopted for screening of further raw material sites.
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Geoelectrical Properties of Calcium Sulphate Rocks
More LessSulphates are one of the principal groups of evaporitic rocks, Gypsum tends to become into anhydrite when buried because of dehydration and the opposite process also takes place when anhydrite is affected by weathering and superficial waters. One of the most important problems found while quarrying gypsum rock is the presence of anhydrite; the drilling machines can be damaged because of this hardness and when an anhydrite body appears, the exploitation must be stopped at the moment. A geoelectrical classification of calcium sulphate rocks has been elaborated comparing the resistivity values obtained from theoretical models, laboratory tests, and field examples. A Gypsum-Anhydrite-Lutite system has been elaborated using Hashin-Shtrikman bounds. The lower bound fits with the data obtained experimentally for a matrix presence below 60% and the upper bound for the rest. With this ternary system it is possible to interpret the composition of calcium sulphate rocks from ERT profiles. The electrical value of pure gypsum and anhydrite rocks has been defined (1000 and 5000 ohm.m respectively). The most important component in determining the electrical resistivity of the bulk rock is the quantity of lutitic matrix while the significance of gypsum and anhydrite presence only is important in the purest rocks.
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Spatio-temporal Variations in MRS Signal Near a Temporary Pond in the Sahel
Authors M. Boucher, G. Favreau, A. Legchenko, J. Pfeffer, Y. Nazoumou, J. Hinderer and B. CappelaereThe deep percolation below temporary ponds in the Sahel (semi-arid West Africa) is a major source of aquifer recharge. The aim of this study is to assess the ability of the magnetic resonance sounding (MRS) method to help quantify temporal changes in groundwater storage near ponds in a sedimentary porous aquifer. In the well-documented Wankama site, a magnetic resonance tomography (MRT) with 8 coincident loops was carried out during the dry season and the MRS signal was monitored as close as possible from the pond (6 measurements from 2008 to 2010). The MRT shows 1) ranges of water content in agreement with previous observations (absolute gravimetry and pumping tests) and 2) smooth lateral heterogeneities in the water content distribution. These heterogeneities were taken into account for modelling the response of MRS to water table fluctuations observed in a series of piezometers. It was shown that the detection by MRS of changes in piezometric levels is limited by the accuracy of measurements. A significant increase in MRS amplitude was observed for extremely wet conditions, partially due to the presence of the pond at the edge of the MRS loop.
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Hydrogeophysical Monitoring of Groundwater Level Changes Induced by Tides in a Shallow Beach Aquifer
Authors J. Giraud, M. Chouteau, C. Taveau and R.P. ChapuisA joint geophysical-hydrogeological experiment was performed to monitor groundwater level changes in a shallow aquifer caused by tidal loading. Electrical resistance tomographies (ERT) were recorded every 45 minutes for two days and consisted of a profile of 41 electrodes, using a 1-m spacing dipole-dipole array protocol. Eleven piezometers were installed with a separation of 10m; pressure data was recorded at 2 min interval. Other hydrogeological data included tide levels, temperature, water electrical conductivity, and porosity estimations. Time-lapse resistivity imaging shows subtle changes in resistivity close to the sea water front and in a region of large lateral gradient of resistivity thought to be the limit between waters of contrasting salinity. Reasons for the small water-induced vertical resistivity variations were investigated using numerical modelling. It is shown that ERT may not resolve changes smaller than 25 cm over a depth of 3m using the survey acquisition parameters. Coupling hydrogeological data with groundwater ERT allowed displaying a likely contact between saline and fresh groundwater. Measured hydraulic conductivities were used to model the effect of tidal loading on the piezometric level and the computed damping and time-lag of the tide induced effects with distance inland fit well with the observed data.
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Geoelectrical Monitoring Experiment of In-situ Bioremediation of a Chlorinated Hydrocarbon Plume
Authors A. Etienne, J. Deceuster and O. KaufmannRisk management of contaminated sites requires accurate tools to monitor plume evolution especially when dealing with chlorinated or aromatic hydrocarbon. Geoelectrical methods could be valuable tool to reduce uncertainties linked with usual monitoring techniques based on punctual analyses. In this paper, the first results of on-going field experiments conducted on a site where bioremediation of a TCE plume is induced by the injection of a reducing solution will be presented. An increase in time-domain chargeability seems to emerge from surveys carried out before, during and after the solution injection which would be linked to biodegration process.
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Monitoring Water Migration Processes in Cracking Clay Soil with Depth Profiles of Square Array Resistivity Measurements
By A.K. GreveDepth profiles of 0.05 m spaced coplanar horizontal square arrays are installed in a weighing lysimeter filled with cracking clay soil. During two water applications with different application methods and intensities, time lapse series of electrical resistivity measurements with the α, β, and γ square arrays are collected and the anisotropy index (AI) and mean apparent resistivity (ρam) are calculated for each measurement depth. Differences in the pre irrigation soil moisture content and cracking intensity were identified by the initial AI and ρam values and were consistent with the initial weight of the lysimeter and the observed surface cracks intensity. Differences in the progression of the AI and ρam values during the two irrigation events highlighted different water migration processes in the soil profile, which was supported by differences in the drainage regime out of the lysimeter. The results show that time series of AI and ρam depth profiles can be used to distinguish between soil moisture and cracking stages as well as water migration processes within a soil profile, which will provide valuable insight during the investigation of the complex water migration processes in cracking clay soils.
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Volumetric Monitoring of Dynamic Moisture Distribution in an Aged Railway Embankment
Authors D.A. Gunn, J.E. Chambers, P.I. Meldrum, R.D. Ogilvy, P.B. Wilkinson, E. Haslam, S. Holyoake and J. WraggThe condition of aged embankments relates to the engineering geological properties of the source materials, the internal heterogeneity produced during construction and how these factors have affected long term processes leading to deterioration in integrity and performance. Remotely operated, automated monitoring systems providing non-invasive geophysical measurements provide insight into the processes driving long term deterioration compromising stability, such as dynamic moisture movement throughout embankments. Automated time Lapse Electrical Resistivity Tomography (ALERT) technology provides high resolution information relating to the internal structure of an embankment. The dynamic moisture distribution throughout the embankment can be interpreted from a series of time lapse, differential resistivity images based upon robust resistivity-moisture content relationships. A 3D ALERT system was installed to monitor moisture movement within a 22 m section of embankment operated by the Great Central Railway (Nottingham) Ltd. This included: 12 cables of 31 m length laid from toe to toe, across the transect of the embankment, spaced at 2 m, each comprising 32 electrodes with a 1 m spacing. This paper presents baseline 2D and 3D images of the resistivity distribution within a section of the embankment that will be used to assess the impact of vegetation and drainage characteristics upon moisture movement.
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Automatic Resistivity Profiling and Electrical Resistivity Tomography for Landfill Cover Surveying
Authors F. Genelle, C. Sirieix, J. Riss, V. Naudet, M. Dabas, S. Renie, P. Begassat, B. Dubearnes and F. NaessensOn landfills, an impermeable cover (compacted clay and a Geosynthetic Clay Liner - GCL) is set on the top of waste. However, this protection cover may be damaged: creation of defects’ could provoke preferential water pathways and therefore an unusual increase of leachate within the waste. That’s why three geophysical methods have been carried out on a French landfill in order to determine their ability to detect damages in the cover. Two of these methods will be presented in this paper: the Automatic Resistivity Profiling (ARP©) which enables cartography on a large area and the Electrical Resistivity Tomography (ERT). These measurements, supplemented by manual augers, have put in evidence several heterogeneities linked with different lithology used in cover materials. The ERT has supplied additionnal infomation about the variability in the cover thickness and the discontinuity of the GCL.
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Characterisation and Clearance of Ordnance on Former Military Ranges
Authors A.C. Butcher and B. HodkinsonNumerous coastal areas of the UK have historically been used as military ranges, with a wide variety of munitions fired for training and proofing purposes. Although years of clearance have been carried out at many locations, a significant amount of buried ordnance is still present on some sites. As a result these sites require regular monitoring, as the dynamic nature of the coastal environment can result in the regular exposure of buried ordnance through variations in sand elevations and erosion of sand dunes. The Ministry of Defence operates several towed magnetometer arrays with the objective of identifying shallow buried ordnance of sufficient mass to cause significant harm. Two case sites are discussed; the first comprising of a former air weapon range approximately 570Ha in size, while the second is a five-mile stretch of beach used as a military ordnance trials range. Both were used extensively during and immediately after World War II. In order to increase the recovery of unexploded ordnance, the characteristics of the magnetic anomaly generated by recovered ordnance is regularly reviewed. The results of eighteen 25lb projectiles and twenty-eight 500lb airdropped bombs are presented, with both sets of ordnance displaying items with significantly lower amplitudes than anticipated.
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Near Surface Variations and its Impact on Seismic Repeatability at Naylor Field, Australia
Authors Y.Y.M. Al Jabri, C. Dupuis and M. UrosevicThe micro-array investigation of the near-surface layers at Naylor site indicated that the velocities and Q-factor have same directional variations at this site. The first 0.5m layer of the agricultural soil (elasto-plastic zone) has a low velocity and low Q-factor, hence this zone significantly attenuated seismic energy. Micro-VSP measurements show a consistence increase in velocity with depth and no seismic anisotropy. The change in the water saturation in near surface can cause the changes in velocity and attenuation. Around 30% of RMS amplitude difference can be measured at the reservoir level from the simulated models and that is by simulating the measured parameters of the near surface properties of wet and dry conditions of the near surface with no change in the reservoir properties.
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The Potential of GPR AVO Analysis for Rock Fracture Characetrization
More LessSuccessful characterisation of fractures requires quantification of the reflectivity (reflection coefficient (R)) of individual fractures. The AVO method involves measuring the reflection amplitude variation with offset (incidence angle)l. Measuring the full AVO response using CMP (Common Mid Point) surveys enables estimation of these fracture properties which has proved difficult to achieve with normal-incidence data such as that typically collected in Common Offset (CO) profiling. We carried out AVO analysis of CMP data acquired over a bedding plane fracture in Carboniferous Limestone in Yorkshire, United Kingdom. Raw amplitudes were corrected for spreading and antenna radiation/receiver patterns; offsets (0.29m-2.9m) were converted to incidence angles (8°-55°) to obtain a diagnostic AVO curve. The antenna patterns were measured through numerical modelling using the GprMax 3D modelling code. Being a low loss formation, conductive attenuation was considered insignificant over the ray-path length. Because of the difficulty in characterizing the source pulse strength, both theoretical and measured amplitudes were normalized to the minimum offset amplitude. Fracture properties are then obtained by comparing the normalised field derived AVO curve with normalized theoretical reflection coefficient curves for thin fractures with various apertures and fills.
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A Comparison Between GPR and Ultrasonic Techniques for Void Detection Beneath Reinforced Concrete Sections
Authors N.J. Cassidy, R. Eddies and S. DodsThis study provides practical users with a systematic comparative study GPR and Ultrasonics as applied to the detection and location of voids beneath steel-reinforced concrete sections. Complementary surveys were collected across a 0.3m-thick, steel-reinforced concrete test ‘slab’ that covered a sub-metre scaled target void buried in the underlying clayey soils. To make the study as realistic as possible, and ensure data compatibility, the surveys were collected ‘blind’ (i.e., without a detailed a priori knowledge of the target depths and/or sub-surface conditions) and across the same survey grid. The 2D and 3D results (sections and time-slices) show that despite the relative popularity of GPR, ultrasonic methods can perform equally well for this type of specific investigation scenario and have shown the potential for overcoming some of the inherent limitations of GPR when faced with dense, complex rebar meshes.
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Towards Joint Inversion/Interpretation for Landslide-prone Areas in Norway - Integrating Geophysics and Geotechnique
Authors G. Sauvin, S. Bazin, M. Vanneste, I. Lecomte and A.A. PfaffhuberQuick clay may be described as highly sensitive marine clay that changes from a relatively stiff condition to a liquid mass when disturbed. Extended quick clay layers account for a lot of geo-hazards in Scandinavia and North-America and hence their occurrence and extent need to be mapped. Geophysical methods have been tested for small scale quick-clay mapping at a research site (Vålen) close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) and integrating them with geotechnical borehole data with the help of a resistivity logging tool (RCPTu), we confirm the value for such integrated studies in for quick-clay hazard zonation. Geophysical investigations allow indeed interpolation in between limited borehole results and thus provide a more cost-efficient and extended result than with boreholes alone.
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Investigation of Landslide by Using 2D/3D Resistivity Imaging
Authors M.E. Candansayar, N.Y. Gundogdu and H. AktarakciIn this study we investigated landslide area by using DCR method. The landslade happened close to Ilgaz, nortwestern Turkey on March 31,2011, and cut the highway connect to two cities. The main aim of this study was to delinate landslide body and its surface depth and other weak zone to take measure before reconstruct the highway. We collected DCR data for DD and PD arrays and interpreted by using 2D and 3D inversion algorithms incorporating surface topography.
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Mapping of Landfills Using Time-domain Spectral Induced Polarization Data - The Eskelund Case Study
Authors G. Fiandaca, E. Auken, A. Gazoty, J. Pedersen and A.V. ChristiansenThis study uses time domain IP for the delineation and the characterization of the former Eskelund landfill, Denmark. With optimized acquisition parameters combined with a new inversion algorithm it is now possible to use the full content of the decay curve and retrieve spectral information from the time domain IP data. Thirteen IP/DC profiles were collected, supplemented by en el-log drilling for an accurate correlation between the geophysics and the lithology. The data were inverted using a laterally constrained 1D inversion considering the full decay curves to retrieve the four Cole-Cole parameters. For all profiles, the results reveal a highly chargeable unit with an impressive agreement to 15 boreholes. The thickness and the depths have been validated by el-log measurements giving in-situ values for the Cole-Cole parameters. The 3-D shape of the waste body has been delineated with a very high accuracy, and the south boundary of the landfill has been redefined based on these results. The inversion of the IP data also allows reconstruction of the waste history combining aerial maps with the results. Thus, this case study shows the relevance and the full benefits of using the TDIP data for the recognition and the characterization of landfill areas.
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Application of Geophysical Techniques for the Diagnosis of Underground Conditions Promoting Distress - Case Study
Authors F.A.I. Hamdi and G. MorelliIntegration of Geophysical techniques with Geotechnical techniques (Drilling of Calibration boreholes) lead to that definitive answers regarding subsurface conditions were provided to the end users: • Defining the scope of the site problem pointing to adverse strata and hazardous ground conditions. • Guidance to diagnosis of Ground Conditions Promoting Distress • Recommendation for the safety design and construction of engineering structures with or without the mitigation procedures for ground improvement. In this study the recent geophysical techniques used in civil engineering investigations and its integration into the overall geotechnical information in relation to detailed design, Road & Building construction, are discussed and illustrated with three case histories.
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Evaluating the Effectiveness of Rolling Impact Compaction at a Brownfield Site with High and Low Frequency Seismic Surface
More LessAn effective procedure has been developed for guiding environmental sampling and verification of the effectiveness of impact rolling compaction for ground improvement at a brownfield site. Extensive surface wave testing within an uncontrolled fill area have demonstrated good correlations between S-wave velocities obtained from high frequency surface wave measurements using a modified MASW system, CPT and in-situ densities This generally increases with increasing number of machine passes but by very little after a certain number of passes. On average, post-compaction in-situ densities increased by 6% to 10% and S-wave velocities increased by up to 40%, clearly demonstrating the sensitivity of S-wave velocities to soil and fill modulus. The S-wave velocity sections also illustrated the consistency achieved in the landform created by the impact roller. A statistical approach was used to develop a linear relationship between S-wave velocity and Dry Density Ratio (DDR). This formed the basis for the compaction specification for the entire brownfield site. Electromagnetic and improved surface wave geophysics provide alternatives to costly excavation, removal and re-engineering of soils at brownfield sites and represent an advance over current practice.
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EngGeo1 Experience of Borehole Seismic Surveys by Method of Reversed Vertical Seismic Profiling
Authors N.A. Ragozin, I.G. Mindel and B.A. TrifonovAt designing of especially responsible constructions, such as NPP, high-rise buildings etc., reception of detailed data about velocities of longitudinal and shear waves in layers on depth up to 100-120 m is required. Ground seismic methods in such cases cannot be reliable and sufficient detail of researches on such depths. As the basic method here it is necessary to apply method VSP (Vertical seismic profiling, 1984). However a number of condition is necessary for reception of the qualitative information: casing borehole with a pipe of one diameter, cementation of space behind a pipe, careful washing of a hole, use of probes by a special design to provide a good clip to walls of a pipe. Often for the objective reasons it is not possible to reach necessary results. In these cases combine classical VSP and torpedoing of boreholes are useful.
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EngGeo2 GPR Study of Depressed Areas in the Upper Cross-section
Authors A.M. Romanova, A.V. Starovoytov and A.Y. KalashnikovOne of the major problems in the process of GPR investigation is searching for so-called weakened zones in subsurface potentially dangerous for engineering constructions in connection with possibility of development in them any deformations. This method allows to allocate these areas without infringement of sediments' solidity. But the problem is to describe indications of such weakened zones on radargrams. We consider two groups of questions: possibility of detection of emptiness under rigid coverings (asphalt, concrete etc.) and allocation of the weakened zones in subsurface.
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Improvement in Time-domain IP Data Quality with Multi-electrode Systems by Separating Current and Potential Cables
More LessGood quality time-domain IP data can be obtained using multi-electrode resistivity-IP equipment and standard electrode cable layouts at sites with favourable electrode contact. High contact resistances often result in low signal levels and capacitive coupling problems making the IP signals drown in noise. We tested an approach for measuring with separated cable spreads for current transmission and potential measurement with an instrument with transmitter, receiver and relay switch housed in the same box. Stainless electrodes and standard non-shielded multi-electrode cables with were used throughout. Data quality assessment was done via pseudosections, IP decay curves and full waveform plots. The results show that it is possible to improve the IP data quality at a site with unfavourable electrode grounding conditions. The results suggest that most of the coupling problems arise in the multi-core electrode cables, and that the problems can be reduced dramatically by separating the cable spreads for transmitting current and measuring potentials. The procedure used is relatively simple and applicable for current field measurements. Inversion of the recorded data results in low residuals and produces models that fit well with the geology at the site, although more detailed information would be needed to fully explain the observed phenomena.
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Importance of Transmitter Waveform and Receiver Transfer Function Modelling in Time Domain Induced Polarization
Authors G. Fiandaca, E. Auken, A.V. Christiansen and A. GazotyIn the computation of the forward response for Time Domain Induced Polarization the incomplete description of the transmitter waveform causes dramatic errors in the estimation of the magnitude and time characteristic of the IP phenomenon. In fact both the duration of the current pulse and the sequence of pulses used for the stacking procedure have a strong effect in the magnitude and shape of the IP decays. Furthermore, it is important to model, if present, the low-pass filters of the receivers, in order to use all the information contained in the acquired data. For these reasons, a new 1D forward and inversion algorithms have been developed using the full time decay of the IP response and the receiver transfer function to reconstruct the distribution of the four Cole-Cole parameters of the earth. The waveform implementation in the forward response for TDIP is a significant improvement that allows moving from a qualitative interpretation of TDIP data for recognition of anomaly patterns towards a quantitative analysis, able to discriminate soil lithotypes and, if present, some contamination patterns.
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2D Time Domain Inversion of Induced Polarization Data
Authors J. Ramm, G. Fiandaca, A. Binley, E.A. Auken, A. Gazoty and A.V.C. ChristiansenWe have developed a new, robust 2D inversion approach for modelling time domain induced potential data. Instead of considering the integral chargeability, the full IP waveform is modelled allowing inversion for spectral content in terms of the Cole-Cole parameters. The method is based on a finite element complex resistivity forward response, which is transformed to the time domain using a Fast Hankel Transform. The forward response is implemented in a general inversion code where constraints can be applied on all model parameters to allow migration of information along the profile. A priori information from e.g. boreholes may also be included to constrain the inversion.
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Minimisation of Electrode Polarisation Errors when Using Optimised or Generic Resistivity Imaging Arrays
Authors P.B. Wilkinson, P.I. Medlrum, M.H. Loke, J.E. Chambers, O. Kuras, D.A. Gunn and R.D. OgilvyPolarisation potentials are caused when metallic electrodes are used to transmit current in resistivity imaging surveys. If these electrodes are subsequently used to measure potential differences, the decaying polarisation potentials can be a source of significant error. In this paper we describe a general method to minimise the impact of polarisation errors by rearranging the resistivity measurements to maximise the time between any electrode injecting current and later measuring potential. This method does not rely on the existence of a natural ordering of the measurements and can therefore be used with arbitrary resistivity imaging arrays, specifically including those generated by automated optimisation schemes. The method uses a global minimisation algorithm ("simulated annealing") to attempt to avoid local minima without performing an exhaustive search of the configuration space. We determine the control parameters and permutation types for the method from the results of a series of numerical experiments on a randomly generated measurement sequence. We then demonstrate the efficacy of the method using real data measured with a permanently installed resistivity monitoring system on an active landslide. The results show that polarisation errors can be effectively eliminated when using optimised resistivity imaging arrays.
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Finite Difference Resistivity Modeling on Unstructured Grids with Large Conductivity Contrasts
Authors S. Penz, H. Chauris and D. DonnoThe 3-D geo-electrical forward problem solved with a finite difference approach faces several difficulties. Besides the singularity at the source location, major issues are caused by the definition of the computational domain to match a particular topography, and by high conductivity contrasts. To address these issues, we combine here two methods. First, we implement a specific finite difference method that takes into account specified interfaces in elliptic problems. Here, the contrasts are defined along grid lines. Second, we extend the method to unstructured meshes by integrating it to the generalized finite difference technique. In practice, once the conductivity model is defined, the approach does not need to explicitly specify where the large contrasts are located. Several numerical tests are carried out for various Poisson problems and show a high degree of accuracy.
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Constrained Time-lapse Inversion of 3-D Resistivity Surveys Data
More LessThree-dimensional surveys and inversion models are required to accurately resolve structures in areas with very complex geology where 2-D models might suffer from artefacts. 3-D data sets collected at different times are inverted simultaneously using a least-squares methodology that uses roughness filters in both the space and time domains. The spatial roughness filter constrains the model resistivity to vary smoothly in the x, y and z directions. A temporal roughness filter is also applied that minimizes changes in the resistivity between successive temporal inversion models. This method can accurately resolve temporal changes in the resistivity even in the presence of noise. The use of the L1 and L2 norm constraints for the temporal roughness filter are examined using a synthetic model. The L1 norm temporal constraint produces significantly more accurate results when the resistivity changes abruptly with time. A test with field data from a landfill site with methane gas accumulation shows near surface resistivity changes that are probably due to surface temperature variations. The temperature variations cause changes in the gas volume and moisture content in the near surface landfill materials.
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Joint Inversion of Seismic Refraction and Resisitivity Data Using Layered Models
Authors N. Juhojuntti and J. KammWe present a layer-based joint inversion of refraction seismic and geoelectric data and show its utility by applying it to a field example from western Sweden. The joint inversion couples the two data sets by assuming the electrical and acoustic parameters to have a common layer structure, but otherwise leaving them to vary freely (laterally within those common layers). The lateral parameter variations are constrained as previously proposed for pure resistivity inversion (Auken & Christiansen 2004). We assume our assumptions are justified for sediment bedrock situations in ground water exploration. We compare the joint inversion results with those obtained from single inversions, and conclude that it is mostly the seismic data constraining the structure. The particular situation under consideration exhibits a pathological hidden-layer situation, where mere seismic exploration may be inaccurate. In cases like these, the joint inversion allows an improved determination of the structure compared to seismic inversion alone. The resistivity distribution, and also to a lesser degree the velocity distribution, benefit significantly from being constrained by the improved structure.
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Ground Conductivity Meters Considered as Elevated Dipoles
By D. BeamishGround Conductivity Meters (GCMs) are used across a wide range of application areas and the measurements are spatially assembled and correlated with data from many other earth science, environmental, soil and land use investigations. They are understood, by users, to operate under a Low Induction Number (LIN) condition. This paper provides a new assessment of the subsurface conductivity measurements obtained by GCMs when the source is considered as an elevated dipole. The theoretical behaviour of the common systems is examined in relation to both the prevailing level of subsurface conductivity and the instrument elevation. It is demonstrated that, given the inherent high level of accuracy of modern instruments, the prevailing LIN condition may require operation in environments restricted to very low (<12 mS/m) conductivities. A simple correction procedure that can be applied to the measured data obtained from any of the LIN instruments is developed. The correction procedure would, in the limit of a uniform subsurface, return the same (correct) conductivity, irrespective of the ground conductivity meter used, the prevailing conductivity or the measurement height.
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Airborne Geophysical Data Used to Characterise Groundwater Dependant Wetlands
Authors D. Beamish and G. FarrThis paper provides an assessment of existing airborne baseline geophysical data in relation to the hydrogeological characterisation of protected groundwater dependant wetland areas found on Anglesey, Wales. Radiometric data appear to identify the main areas of water saturation together with the degree of saturation in the very near surface. The radiometric data also show regional scale transition from limestone in the east, important for water supply to the wetlands, to older more impermeable strata to the west. The conductivity data appear capable of mapping the lateral extent of clay accumulations, concealed below peat cover and acting as a confining layer. The data therefore provide a non-invasive spatially-continuous, characterisation of the sites, improving the understanding of their dynamic water balances and, potentially, guiding further invasive investigations.
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Spatially Mutually Constrained Inversion of Airborne Hydrogeophysics Survey with Ground Geophysics and Borehole Data
Authors K. Martinez, O.F. Nielsen and J.A. MendozaThis work presents examples from processing and interpretation of 2576 km of SkyTEM data in the Ringsted-Suså groundwater mapping area located southeast of Denmark. The mapping area is approx. 422 km2 and includes approx. 75,000 SkyTEM soundings. Additionally, diverse ground based geophysical data (>260 km Multi-Electrode Profiling (MEP), > 130 km Pulled-Array Continuous Electrical Soundings (PACES) and 200 ground-based TEM soundings) were utilized in a pseudo-3D inversion. Furthermore prior information was added from 39 borehole logs and lithological information from approx. 735 boreholes in the final inversion. The objectives of the work were to map the near subsurface for units of hydrogeological significance and to evaluate the available inversion schemes. Three types of inversion were used and compared; Laterally Constrained Inversion (LCI), Spatially Constrained Inversion (SCI) and Spatial Mutually Constrained Inversion (SMCI). SMCI is seen to produce results more consistent with both geological data of other geophysical data. Furthermore, the SMCI is less influenced by the degree of lineaments compared with LCI and SCI. This is due to the use of different geophysical data sets represented in one inversion result, which in addition allows gathering the geophysical inversion into one result.
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Aquifer Contamination at Roorkee/India Studied by Spatially Contrained Inversion of TEM Inloop Data
Authors B. Tezkan, M. von Papen and M. Israil46 inloop soundings were carried out close to Roorkee/India on a contaminated area and on a noncontaminated site used for reference. Due to the inaccessibility of some parts of the survey area the TEM soundings could not be carried out on profiles and therefore were realized on an irregular measuring grid. First of all, the data were interpreted by a conventional 1D inversion and afterwards the TEM soundings were simultaneously inverted by a spatial constrained inversion to derive a quasi 3D conductivity model. The TEM-models show a relative low resistive aquifer (8-15 Ohm-m) at 5-15 m depth indicating a possible contamination which was also observed (radiomagnetotellurics, DC) and confirmed by other geophysical measurements on the same area. The other object of the study was the second deep aquifer at 40-70 m depth. Its resistivity was determined as 20-40 Ohm-m. However, the TEM inversion results couldn’t clearly indicate a possible contamination of this deep aquifer.
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Investigating Heterogeneous Costal Aquifer Using Magnetic Resonance Tomography and TDEM
By A. LegchenkoMagnetic Resonance Tomography (MRT) and Time Domain Electromagnetic (TDEM) methods were applied to investigate a costal aquifer in the Dead Sea area of Israel. The subsurface in this area is highly heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. Variation of the water salinity is poorly known. TDEM is an efficient tool for locating electrically conductive targets like saline water, but it is also sensitive to the porosity of rocks. MRT, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface (through the relaxation time T1). We used MRT for a 3D imaging of the water content in the subsurface and 2D mapping of the T1. 3D distribution of the electrical resistivity was obtained using interpolation of 1D TDEM results. We show that joint use of the MRT and TDEM enables us to resolve the aquifer and to demonstrate that the salinity of water in the investigated area is fairly constant.
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Improving Groundwater Flow Model Conceptualisation and Calibration with ERT and Self-potential Methods
Authors T. Robert, R. Therrien, J.M. Lemieux and F. NguyenThe self-potential (SP) method relies on passive measurements of the ambient electrical potential at the ground surface or in boreholes. When the electrokinetic effect is the dominant contribution, the resulting signal is called the streaming potential and contains information about groundwater fluxes that can be useful for calibration of groundwater flow models. The streaming potential forward equation was implemented in the HydroGeoSphere model, which simulates 3D groundwater flow and solute transport in porous media, including fractured geological formations. HydroGeoSphere is able to calculate the streaming potential given a distribution of Darcy velocity and electrical resistivity. Since groundwater flow modelling relies on a conceptual model, prior information on the distribution of the geological units and hydraulic conductivity at the site is mandatory. However, this information is often scarce or missing. In this work, we use the electrical resistivity tomography (ERT) and the SP methods as an additional source of information for building the groundwater flow model. ERT is used to identify the location of fractured zones in a fractured and karstified calcareous aquifer of South Belgium. The SP signal is used with PEST in order to calibrate the groundwater flow model and better constrain the hydraulic conductivity of the fractured zones.
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Geoelectrical Imaging for Groundwater Exploration in the Shallow Zone in an Area of Pesquería, Nuevo León, México
Authors D. Garza-Rocha and J.J. Castellanos-FloresIn this paper we would like present the results of hydrogeological and geophysical investigations that were carried out in an area located in the northeastern of Mexico. The area of investigation is found in the city of Pesquería, Nuevo León. The main goal of this study was the exploration of groundwater. In this region the pluvial precipitation is erratic and concentrated. On the basis of the superficial geology we decide to carry out several electrical profiles, and several vertical electrical soundings to try to obtain one model of the lithology of the underground. Measurements of the phreatic level of existing wells were also carried out. This helped us to determine the location of the vertical electrical soundings and the electrical profiles. The interpretation of the obtained resistivity models was made with the aid of information of the geology of the area, and lithology of the materials of some existing wells. The information, about the depth and direction of the flow of groundwater, was also obtained from the hydrogeological and geophysical measurements. The results, of the distribution of the measurements of the electrical resistivity, show locations suitables for future work to explore for groundwater.
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An Extended Model for Predicting Hydraulic Conductivity from NMR Measurements
Authors R. Dlugosch, M. Müller-Petke, T. Günther and U. YaramanciThe prediction of hydraulic conductivity (K) from NMR measurements (Phi, T2) can be applied successfully on sandstones. For hydrological applications in the near surface unconsolidated material is more common. This material generally shows less variability in porosities but a larger range of pore sizes compared to sandstones. The known (semi-)empiric K-T2 relations have often be extended to this field of application but their validity needs to be verified. In this work we present a simple model based on tube pores, which is valid for the whole range of laminar flow from silt to gravel. By considering the slow diffusion regime we are able to estimate a maximum K from a measured T2 time. The model replaces the empiric factors in known K-T2 relations with (petro-)physical parameters. This enables to separate effects caused by variations of the surface relaxivity, a material characteristic that links T2 to the inner surface of the material, from variations of other parameters. This may help to reduce the range of the predicted K-values from NMR measurement on similar materials and enable to incorporate results from ofter methods.
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3D very High Resolution Shallow Seismic Data from Carbonate Reservoir - A Case Study
Authors M. Toqeer, H. Perroud and D. RoussetShallow seismic surveys are becoming a considerable way for detecting and characterizing shallow subsurface for environmental, geological, geophysical and geotechnical applications. Shallow 3D seismic survey acquisition and data processing is not an easy task. This is partly due to survey cost and due to near subsurface heterogeneities and several type of noises which partly depends upon the particular site. We present the complete processing and preliminary interpretation results of shallow 3D seismic survey carried out in Campos, Mallorca, Spain. This project is aimed to characterize the carbonate reservoir down to 100m that is composed of hard carbonate rocks. This rock sequence is karstic and fractured. Due to this factor energy penetration is a problem and severe noises are produced due to diffractions. Furthermore Rayleigh waves pose another serious problem for processing. By efficient processing sequence and application of CRS method zero offset stack data is obtained which is further tuned by post-stack depth migration. The objective is to process the severely noisy data to get the subsurface image for further interpretation and seismic attribute analysis. Here preliminary interpretation results of the data also are presented that will be analysed in detail through seismic attributes.
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Experimental Correlations Between Geophysical and Hydraulic Parameters from Different Inversion Procedures
Authors G. Boehm, R. Brauchler, D. Nieto, G. Soncin, L. Baradello, M. Pivetta and F. BottiThis work is aimed to test the possibility of integrating different geophysical method used in near surface application and in particular in studies devoted on a better characterization of the hydraulic system and aquifer structure. We used different inversion procedures to correlate the geophysical and hydraulic parameters and to extrapolate them in those areas not covered by the corresponding survey. The parameters involved in this study were: seismic p-wave velocity from seismic tomography; resistivity, electrical conductivity and induced polarization from electrical tomography (ERT); diffusivity, hydraulic conductivity and specific storage from hydraulic tomography. The study area is located in the Friuli plain, east of Italy. The dimension of the investigated area was approximately 2000 m². We also drilled two wells, 8 m apart, used for the hydraulic cross-well acquisition. The comparison of the specific correlation functions showed that the geophysical parameter p-wave velocity and the hydraulic parameter diffusivity feature the highest correlation. The derived correlation function between the two parameters allowed us to transform the p-wave velocity tomogram into a diffusivity tomogram with high accuracy. The proposed approach has the potential to provide a problem adapted scale and accuracy for the prediction of contaminant transport in the subsurface.
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Calibration of Density Driven Flow Model for the Freshwater Lens beneath the North Sea Island Borkum by Geophysical Data
Authors H. Wiederhold, H. Sulzbacher, M. Grinat, J. Igel, T. Burschil, T. Günther, B. Siemon and K. HinsbyWith the aim to be able to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea a numerical model is set up for the North Sea island of Borkum. The work is done in the frame of the EU Interreg project CLIWAT. The database includes information from drillings, seismic measurements, an airborne electromagnetic survey (HEM) and monitoring data from two vertical electrode chains as well as measurements of groundwater table, pumping and slug tests, water samples. The groundwater model has been set up by the finite-element programme FEFLOW. The density driven flow model can be calibrated successfully on the base of hydraulic, hydrological and geophysical data. Verification runs with the calibrated model show a good agreement for the measured and computed hydraulic heads and the measured with computed TDS-data. This is the case as well on a large scale for the whole freshwater lens as also on a small scale in the area of the well fields.
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An Integrated Geophysical/Geological Model of the CLIWAT Project Area Föhr as Basis for Groundwater Modelling
Authors T. Burschil, H. Wiederhold, W. Scheer and R. KirschIn the EU project CLIWAT sixteen institutions investigate the groundwater situation in the North Sea coastal areas from Belgium to northern Denmark to simulate consequences of climate change for the groundwater systems and water supply. The North Sea island of Föhr is part of a pilot area. For a better understanding of the current groundwater situation several geophysical surveys were carried out including an airborne survey with the transient electromagnetic system SkyTEM and high resolution reflection seismic surveys from surface and in boreholes with P- and S-waves. The results reveal glacial structures as Quaternary buried valleys as well as a glaciotectonic thrust fault complex. They were located with a combined interpretation of geophysical results and geological information of boreholes. Both of these structures influence the groundwater systems. An integrated geological-geophysical 3D model supports groundwater modelling.
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Hydrogeophysical Characterisation of Artificial Recharge Basins on the Llobregat River Barcelona, Spain
Authors A. Casas, A. Sendròs, H. Gallardo, M. Himi, R. Lovera and J.C. TapiasOver the past ten years there has been growing interest in the use of the subsurface for water storage using shallow ponds, either natural or excavated, for the infiltration of water into the subsurface and then uses wells to recover the water. The design and operation of these subsurface systems require an understanding of the hydrogeologic structure and properties that control both the movement and storage of water. In our study, we have performed a detailed hydrogeophysical characterization about the subsurface of the pond from electrical resistivity tomography.
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Statistical Method to Delineate Resistivity Anomalies Associated with Contamination from a Controlled Dump - N Portugal
Authors M.J. Fontoura, F.J. LINO, S.P. RIBEIRO, S.M. RIOS, R.M. MOURA and A.G. DIASThe assessment of contaminants dispersion in Controlled Dumps (CD) of Municipal Solid Waste (MSW) is possible through the combination of geophysical, geochemical and geostatistical methods as described in this paper. The methodology applied in this study will contribute to evaluate the environmental consequences of the Matosinhos CD (N Portugal), which is set in a granitic crystalline geological context with a permeability controlled by differential weathering associated with fracturing. The statistical methods here described (Lepeltier, 1969; Matschullat et al. 2000) are adapted to use in geophysical data obtained from the resistivity profiles performed in the CD surroundings by the electrical resistivity method. Groundwater samples were collected in piezometers; upstream and downstream of the CD, with the aim of directly confirming the presence of contaminants indirectly detected by the geophysical and geostatistical methods. The combination of these different approaches allows an approach to the detection and delineation of contaminant plumes from these deposits.
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Contributions of TDEM Survey in a Seismic Activity Area Triggered by Wells in Paraná Basin, Brazil - Preliminary Results
Authors J. Porsani, E. Almeida, C. Bortolozo and F. SantosThis article present TDEM results on a seismic activity area triggered by groundwater exploitation from deep wells. The study area is situated at Paraná basin, São Paulo state, Brazil. Stations were individually arranged next to wells and along profiles. Preliminary result showed the geoelectrical stratigraphy of Paraná basin. 1st geoelectrical layer is constituted by sedimentary aquifer, followed by top of basalt layer that occur ~60-80 m depth. A fractured zone filled with water within of the basaltic layer was found between 100-300 m depth, which agree to the depth estimated for more than 3000 micro earthtremor hypocenter in the studied region. The bottom of the basalt layer was estimated between ~400-650 m depth. The last geoelectrical layer is characterized by Botucatu sandstone. Results suggest that the high water flow in some wells seems to be related to the presence of fractured zones filled with water founded in the middle of basalt layer. Moreover, there are a good correlation between the seismic activities area and the fractured zones filled with water, which some wells presented the high flux of groundwater. This is a good result which suggest that the shallow seismic activity, can probably is being triggered by water wells drilling.
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Revealing Potential Flow Pathways within the Pow Catchment Using Geophysics - Initial Results and Conceptualisation
Authors L. Mejus, A. Binley and B. SurridgeIn order to help protect groundwater resources we need to understand the pathways of potential contaminants. Whilst the threats from localised sources may be more damaging (locally), the impact of contaminants from diffuse sources is often more difficult to determine because of the spatial extent and complexity of subsurface transport pathways. The first stage in any assessment of threat to groundwater is the establishment of a conceptual model of the groundwater system. Here we present initial results from a study of a sub-catchment of the River Eden in Cumbria, UK. Our focus is on improving the hydrological conceptual model of the catchment, through the use of geophysical measurements.
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Noise Cancellation for Surface NMR - Derivation of Time and Frequency Domain Approaches
Authors M. Mueller-Petke and U. YaramanciEven though surface nuclear magnetic resonance (surface NMR) is the only geophysical technique that provides hydrogeophysical rock properties on the base of direct sensitivity to subsurface water, in many cases surface NMR measurements suffers bad signal-to-noise ratio, and measurements can be carried out only far from sources of electromagnetic noise. To overcome these restriction some approaches using a reference loop based system were developed during the last years (Radic, 2006; Walsh, 2008; Müller-Petke and Yaramanci, 2010; Neyer, 2010). All approaches have demonstrated useful capability to improve S/N. But a comparison that allows for determining properties, i.e., pro and contra of each approach is missing. Thus, to compare time domain with the frequency domain approach, we developed an own frequency domain code beside the existing time domain code. We present a complete derivation of both the time and frequency domain reference technique. Both allow for single and arbitrary number of reference loops. This gives insight into the common idea of both but also the differences in final equations and realization. Finally, this development enables to compare important properties such as applicability at different noise conditions or numerical costs (Müller-Petke and Yaramanci, 2011).
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Two-layer Inversion of Post-calibrated Multi-configuration Electromagnetic Induction Data
Authors A. Mester, E. Zimmermann, J. van der Kruk and H. VereeckenCommon electromagnetic induction (EMI) devices are capable of collecting apparent conductivity values which represent a single value that is assigned to the cumulatively sensed electrical conductivity over a certain depth range. Here, different sensing depths are obtained for different coil orientations, different coil offsets, and different frequencies in the given order of significance. We introduce an inversion scheme that uses post-calibrated EMI data and inverts for a two-layer earth. The inversion minimizes the misfit between the measured and modeled magnetic field by a combined global and local search and does not use any smoothing parameter. Joint inversion of EMI data from horizontal coplanar (HCP) and vertical coplanar (VCP) loop configurations, coil offsets of 1 and 1.22 m, and frequencies of 8 and 15 kHz provides lateral and vertical conductivity variations very similar as observed in an elaborate ERT experiment. Application of this method enables the fast mapping of true conductivity distributions over large areas. This approach can be easily extended for multi-layer inversion when the appropriate multi-configuration EMI data is available.
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1D Joint Inversion of DC and TEM Data for Hydrogeological Applications - Preliminary Results
Authors C.A. Bortolozo, J.L. Porsani, E.R. Almeida and F.A.M. SantosElectrical (DC) and transient electromagnetic (TEM) soundings have been used in great number of hydrological,environmental, and mining exploration studies. The data interpretation usually is done individually by 1D models resulting in ambiguous results. This fact can be explained by how the two different methodologies observe the medium below the surface. The vertical electrical sounding (VES) is good at marking very resistive structures, while the transient electromagnetic sounding (TEM) is very sensitive to map conductive structures. Another difference is that VES is better to detects shallow structures, while TEM soundings can reach deeper layers. In this way, a Matlab program for joint inversion of VES and TEM soundings was developed aiming explore the best of the both methods. Initially, the algorithm was tested with synthetic data and after was used real data from Paraná Sedimentary Basin in Bebedouro region, São Paulo State, Brazil. The geoelectrical model obtained from joint inversion of VES and TEM data are more similar to the real geological condition and ambiguities were minimized. The results with synthetic and real data shows that joint inversion of VES/TEM is better for recovering the simulated models and shows a great potential in geological studies, particularly in hydrogeological studies.
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On the Improvement of Inverse Modeling in Geoelectric Exploration
Authors N.P. Szabo, Á. Gyulai and A.N. ParipásIn the paper, some problems of using conventional inversion methods in geoenvironmental exploration are studied. The series expansion based inversion procedure is presented as a joint inversion method that performs reliable parameter estimation by decreasing efficiently the number of unknowns of the inverse problem. Quality improvement in parameter space is demonstrated by applying the series expansion based 1.5D and 2D CGI (Combined Geoelectric Inversion) inversion methods on DC geoelectrical data. The CGI inversion method was further developed by weighting individual data sets in order to improve inversion results. It is shown that the series expansion based inversion technique is able to resolve near-surface structures such as rapidly varying layer boundaries, laterally inhomogeneous formations and pinch-outs.
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Imaging of a Fault Zone by a Large-scale DC Resistivity Experiment and Seismic Structural Information
Authors T. Gunther, G. Schaumann, P. Musmann and M. GrinatWe present a large-scale resistivity experiment to image a fault system down to a depth of a few hundred meters. A strong transmitter and continuous registration of potential differences is used to acquire time series on a profile of more than 2,km length. The time series' are processed in frequency domain to obtain about 300 apparent resistivities. An independent inversion reveals the course of the fault line but shows a smoothed resistivity distribution. Seismic data from a reflection survey are able to significantly improve the resistivity image by adding structural information to the inverse problem.
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Ambiguity Question in Kinematic Multilayer Refraction Inversion
Authors A.N. Paripas and T. OrmosFor the description of realistic geological models it is important to develop an inversion method that is able to calculate slow lateral changes in the model parameters. But laterally varying parameters of a multilayered structure can cause ambiguity in the inversion process. This is a problem in the use of the kinematic multilayer refraction inversion method developed in the University of Miskolc, because it can interpret laterally changing model parameters (layer thicknesses, and propagation velocities) (for a maximum of 5 geological layers in the same process) described by continuous basis functions expanded in series. The reduction of the arising ambiguity question could be reached by searching for the optimal number of coefficients in the inversion method and it is also important to use different coefficient numbers or different basis functions for the description of the ambiguous parameters. Thus no joint or constrained inversion techniques are needed for the interpretation; our inversion method with simple, quick ray tracing forward modelling is able to handle the ambiguity problem.
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Seismic Horizontal Component Measurement - Reduced Scale Modeling and Benefits in Full Waveform Inversion
Authors R. Valensi, F. Bretaudeau, D. Leparoux, O. Durand, R. Brossier and P. CoteThe study concerns the investigation of the benefits of the horizontal particle displacement component for the seismic velocity parameters imagery using the Full waveform inversion method. A geological consistent model has been firstly numerically modeled. A comparison of the FWI inversion results using either the vertical component measurements or the horizontal component shows that the two components provide complementary information to recover the P wave velocity parameters even if the benefits are less visible for transversal wave velocity. Furthermore, a novel laser interferometric measurement process allowing to measure simultaneously the vertical and horizontal components has been implemented in a reduced physical scale measurement bench previously designed to validate experimentally seismic inversion strategies with a great accuracy: the laser interferometer records the seismic particular displacement, the source is a 100 KHz piezoelectric sensor, the minimum incremental displacement of the sensors is 100 µm with an accuracy of 10 µm. Experimental measurement on the same geological consistent model has been conducted and data are compared to numerical ones. The results show a great accordance between numerical and experimental signals concerning both of the components and suggest that this new measurement method can be applied to a whole experimental campaign at reduced scale.
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Geostatistical Approach for Shale Volume Estimation in Water-bearing Formations
Authors N.P. Szabo and M. DobrokaIn the paper, factor analysis as a geostatistical method used for the interpretation of borehole geophysical data is presented. The result of the procedure is a factor log, which contains valuable information about the lithology of the sequence of strata. We found a strong connection between one special factor and shale content. Analogously to gamma ray index, the so-called factor index was defined, which proved to be a good shale indicator. The method gives consistent results providing reliable information in water-bearing sedimentary structures.
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Pressure Dependence of Seismic Q – A New Petrophysical Model
Authors J. Somogyi Molnar and M. DobrokaThe pressure dependence of quality factor of acoustic waves is an extensively explored rock physical problem. To reasonably evaluate and interpret laboratory measurements, a quantitative model of the mechanism of pressure dependence of quality factor is needed. A new petrophysical model based on simple physical assumptions was developed which describes the relationship between quality factor and pressure as well as acoustic velocity and pressure. The model is based on the idea that microcracks are opened and closed besides different pressure states. The model was applied on quality factor and acoustic velocity data set measured by Prasad and Manghnani or rather Yu et al. The material parameters of the models are determined by using joint inversion method and the laboratory measurements were compared to the theoretical data. The theoretical data matched accurately with measured data proving that the petrophysical model applies well in practice. Studying this phenomenon is relevant because we can infer to the changes in rock pressure by right of seismic/acoustic measurements.
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Microgravity Estimation of Filling up the Near Surface Mineshaft - Example of the Gravity Modelling Usage
By L. SlonkaThe study presents an example of application of the microgravity method in estimation of the degree of filling up old mineshafts, whose existence may be dangerous for the near surface and the infrastructure located on the area. The field data was collected in the area of mining excavations located near the city of Olkusz (Lesser Poland province). The relaxations spreaded in the surrounding of an unfilled object caused emergence of the near-surface deformations, futhermore it causes damages of buildings. An essential part of this study was to discover which objects had been totally filled up and do not create any danger for surrounding infrastructure, and which mineshafts are filled to insufficient extent. One of useful methods of quantitative interpretation of gravity image is gravity modelling. It consist in the calculation of the gravity effect for the customary models and its comparison with measured gravity anomaly, in order to obtain compatibility between field and teoretical curve (with margin of RMS error).
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New Method for Calculation of the Building Effect Correction in Microgravity Technique
Authors J. Panisova and R. PaštekaMicrogravity surveys for archaeological applications conducted in the interior of the buildings require careful data acquisition and processing. We have developed modern, effective and fast data processing procedure for calculation of the building effect correction in microgravity technique. Polyhedral body of historical building is created from images in Eos System’s PhotoModeler Scanner software which is based on close range photogrammetry principles. Comparison of equations for calculation of the gravitational effect of polygonal bodies is presented on a simple test model. The case study from small Slovak church shows that close range photogrammetry offers a possibility to improve microgravity data processing procedures. Our new method provides high accuracy of calculated building effect corrections, thus improving the ability of microgravity technique to detect more difficult identifiable cavities which are situated in a close vicinity to man-made structures.
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Imaging very Shallow Sand Deposits with High Resolution SH Wave Reflection Seismic
Authors A. Almholt, F.W.G. Olsen, R.B. Jørgensen and R. WisenThe main objective for this case study was to map the geological structures along a railway ramp of the great belt tunnel in Denmark. Special focus was put on mapping a thin near surface sandy deposit situated between two clay till units. The purpose for the survey was to provide a detailed geological model as basis for improving the groundwater pumping in the ramp. A SH-wave reflection seismic survey provided a high resolution imaging in the very shallow subsurface and was also the primary source for the geological interpretations. Refraction seismic analysis of the reflection records, surface wave measurements and VSP measurements in two boreholes were also carried out and together with the SH reflection result and available borehole data a detailed geological model was created.
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Comparison of Dominant Frequency and Shear Wave Velocity in Haji Abad City
Authors M. Shahnazi and S. Seyed MohadesFor estimation local response and recognition vulnerable media in urban area, we need to know shear wave velocity and dominant frequency in that area. In some media cannot be preformed seismic refraction operation. But microtremors measurement can be used for acquire dominant frequency. The study preformed in Haji abad city. This city stand on Hormozgan state in Iran. We obtain relevance between two parameters. This result showed that the value of correlation coefficient between shear wave velocity and dominant frequency is about 1. Therefore, shear wave velocity value was obtained from a polynomial function of dominant frequency with high certainty
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Stochastic Analysis of Cross-hole GPR Data for Subsurface Characterization
Authors M. Rossi, A. Binley and G. CassianiGround penetrating radar (GPR) is a well-established geophysical technique, used in particular via specific relationships to estimate hydrological parameters in vadose zone, i.e. moisture content, both from the surface and in boreholes. The principal aim of this work is to have a more complete view of how boreholes GPR ZOP measurements are informative of the subsoil geometry and distribution of relative permittivity. For this purpose an electromagnetic (EM) wave simulator has been applied within a stochastic Monte Carlo framework. In this manner both averaging and critically refracted wave effects are taking into account. Results from synthetic and real ZOP datasets are statistically analysed to deduce what kind of subsoil εr-distributions are resolvable and well defined with a degree of uncertainty. The results illustrate how the often employed direct-wave approach is not able to take into account the complexity of the system. It usually reconstructs a smooth profile, sometimes underestimating the real permittivity: particularly a high εr layer might be underestimated, when it is enclosed between low εr media. Results show that care must be used inverting ZOP data for physical parameter estimation, subsurface stratification could be more complex than appears from direct ZOP evidences.
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Real-time Automated Detection of Buried Utilities for 3D GPR Investigations
Authors P. Mazzucchelli, D. Molteni, E. Cottino and N. di BuonoThe realization of network infrastructures with lower environmental impact and the tendency to use digging technologies less invasive in terms of time and space of road occupation and restoration play a key-role in the development of communication networks: non-destructive investigation and mapping of buried services must match the improvements of new digging equipments. The development of a fully automated 3D GPR processing system allows to avoid processing steps that cannot be carried in-field, highly improving the productivity of optical network deployment by the "low impact mini-trench" technique. The computation and process of the pre-stack reflection angle gathers, built while focusing 3D GPR data, is the core issue of the proposed approach, allowing to detect ducts in real time just while acquiring GPR data, without the need of specific high performance hardware. GPR investigation can be carried on just ahead trenching equipment, as the detection results are immediately available and can be easily understood even by unskilled personnel.
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Practical Results of a Research Project
More LessWe present the practical results we got in the frame of the OTKA project K49604. At first we calculated the consequences of incorrect positioning of the electrodes for various multielectrode systems. In practice these effects were found to be negligible. The only exception is the case of rocky surface, where it is impossible to put the electrodes in the desired positions. From the tensorial geoelectric measurements around a Cistercian Monastery we concluded that areal measurements are able to provide more detailed and unambiguous anomalies than 2D profile measurements. We tested the applicability of 3-D electrical resistivity tomography (ERT) technique to detect landmines in different soil conditions and at various depths. Metallic and non-metallic landmines buried in wet and dry soils had been synthetically modeled. A geoelectrical procedure was elaborated which is able to map multidirectional fissure systems by combining geoelectrical profiling and geoelectrical azimuthal measurements. Results received by using both the so-called null-, and traditional arrays were jointly interpreted. At last we investigated the so-called standardized pricking probe (PP) surveying technique and demonstrated its usefulness in an archaeological study. The optimum PP parameters: horizontal interval, pricking depth, observable quantity and its way of presentation were optimized through field experiments.
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Application of the Controlled-source RMT Method for the Solution of Engineering Tasks in Chukotka Region
Authors A.E. Simakov and A.K. SaraevEquipment of controlled source radiomagnetotelluric method (CSRMT) operates in frequency band 1-1000 kHz and experience of its application for the solution of engineering tasks in Chukotka region (far North-East of Russia) are described. The CSRMT method allows us to work in remote areas where there are not enough radio transmitters. A new RMT-C system includes transmitter and receiver groups with a grounded electric dipole as a controlled source. Using of 3-4 main controlled source frequencies from the range 1-150 kHz and their several odd harmonics allows us to realize CSRMT soundings in frequency band 1-1000 kHz. Application of such technique does not require the emitting of a big number of frequencies and a measurement at one station takes little time. Results of investigations by the RMT-C system in Chukotka region has been confirmed by drilling.
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Geophysical Investigation of Track Defects
More LessThe article presents possibilities of application of selected geophysical methods in railway engineering. The text below is supported mainly by the measurements performed under the INNOTRACK projects (6th FP EU) at the sites in the Czech Republic, France, Spain and Sweden. The geophysical testing has proved that the geophysical methods can reliably, quickly, in detail and a relatively low cost inform about problematic zones on the track. The effect of the geophysical application can be enhanced by long-term monitoring track segments. Virtually at all sites subjected to the measurements, the methods of refraction seismics, seismic tomography, resistivity tomography and gravimetry were applied.
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