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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 - 50 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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