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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
41 - 60 of 127 results
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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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