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Near Surface Geoscience 2012 – 18th European Meeting of Environmental and Engineering Geophysics
- Conference date: 03 Sep 2012 - 05 Sep 2012
- Location: Paris, France
- ISBN: 978-90-73834-34-7
- Published: 03 September 2012
81 - 100 of 194 results
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3D Geological Modelling Improvement with Airborne Time Domain Electromagnetism
Authors P.A. Reninger, G. Martelet, J. Perrin, J. Deparis and Y. ChenStandard geological modelling based on boreholes and geological maps can be strengthened using geophysical data. Constrains such as gravity, magnetic and seismic data have already been used. We propose a novel method combining boreholes and the resistivity model resulting from inversion of airborne time domain electromagnetic data. First, the “geophysical” top of the chalk has been identified in the resistivity model after a detailed cross-analysis of resistivities versus boreholes. Then, we jointly interpolated slopes extracted from this geophysical surface together with the top of the chalk in boreholes. Comparison of uncertainties between this model together with pure geological and geophysical models shows that the joint modelling yields the most accurate top of the chalk. A cross-section, intersecting five boreholes (two used as control boreholes) and displaying each of the three surfaces, highlights the usefulness to take into account “geophysical slopes” when modelling. The proposed joint modelling improves what is commonly obtained with geological or geophysical data. This makes the method very attractive for detailed 3D geological modelling.
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MRS Parameter Estimation – Improvement by Joint and Laterally Constrained Inversion of MRS and TEM Data
Authors A. Behroozmand, E. Auken, G. Fiandaca and A.V. ChristiansenWe present a new scheme for joint and laterally constrained inversion (LCI) of magnetic resonance sounding (MRS) data and transient electromagnetic (TEM) data, which greatly improves the estimation of the MRS model parameters. MRS is a non-invasive geophysical technique which directly quantifies the water content distribution from surface measurements. The resistivity information of the subsurface is obtained from a complementary geophysical method such as TEM or DC resistivity methods. The conventional inversion of MRS data assumes the resulting resistivity structure to be true and considers a constant MRS kernel through the inversion. We show that this assumption may introduce an error to the forward modeling and consequently result in erroneous parameter estimations. We discuss the advantage of TEM for the joint inversion compared to DC resistivity. A fast and numerically efficient MRS forward routine makes it possible to invert the MRS and TEM data sets simultaneously along profiles. As results, a more reliable and robust estimation of all parameters is achieved. We examine the approach through a field example in Denmark where good agreement with borehole data is demonstrated with clear correlation between the relaxation time T_2^* and the grain size distribution of a sandy aquifer.
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Model-based Noise Cancelling of Multichannel Magnetic Resonance Sounding Measurements
Authors J.J. Larsen, E. Dalgaard and E. AukenThe signals recorded in a multichannel magnetic resonance sounding measurement are often heavily dominated by noise, in particular spikes and powerline harmonics. The efficiency of the standard techniques for noise cancelling; multichannel adaptive and Wiener filtering are limited by the fact that they fail to properly account for the presence of multiple noise sources. In this paper, we address this problem through a model-based noise cancelling approach. A model of the powerline harmonic interference is constructed using a weighted least squares method to estimate the fundamental powerline frequency and the amplitude and phase of all relevant harmonics. The model is subsequently subtracted from the signal records. The proposed method efficiently removes the large contribution from powerline harmonics in the MRS signals. Simulations with synthetic signals embedded in real noise records show that the synthetic signals can be recovered with a fidelity similar to or exceeding the standard noise cancelling methods.
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ERT Monitoring of Tide-induced Water Flow in a Shallow Beach Aquifer
Authors M.C. Chouteau, M. Labiau, C. Taviau, R. Chapuis and C.T.K. ChouA joint geophysical-hydrogeological experiment was performed in 2011 to monitor groundwater level changes in a shallow aquifer caused by tidal loading. Electrical resistance tomographies (ERT) were recorded in time-lapse mode for two days over a 5 hour period and consisted of a 1-m spacing profile of 41 electrodes. Eleven piezometers were installed with an average separation of 10m; pressure and temperature data was recorded at 2 min interval. Other hydrogeological data included water electrical conductivity, and porosity estimations from laboratory and in-situ measurements. The ERT monitoring experiment has shown that it was possible to monitor cyclic seawater intrusion into the shallow coastal aquifer driven by tide loading, and the subsequent hydrodynamic regime of the aquifer. Salinity changes with times caused by the telescoping salt water and freshwater flows and tide loading generate lateral subsurface resistivity changes that can then be monitored. The hydrogeology study has shown that water head data displayed minute changes up to 30 m inland; however, ERT is not sensitive enough to monitor those small vertical changes. The combination of spatially coarsely sampled piezometer data combined with geophysical monitoring allows yielding a more complete picture of the hydrodynamic system occurring in this shallow coastal aquifer.
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Coastal Aquifer Characterization Based on Seismic and Resistivity Imaging Methods: a Case Study from Costa Brava, Spain
Authors B. Benjumea, A. Gabàs, A. Macau, S. Figueras, J. Piña and I. TubauCoastal aquifer formations located in Northeast of Spain affected by saline intrusion has been the focus for a multi-technique geophysical study. Two aquifers are present: a shallow aquifer in the quaternary sediments and a deeper karstic aquifer. The objectives were: (1) estimate quaternary sediment thickness and characterize weathered/fractured rock (2) qualitative assessment of water degradation changes within the study area. The methodology consisted of combining and integrating seismic and resistivity imaging techniques: seismic refraction tomography (SRT), electrical resistivity tomography (ERT) and H/V microtremor technique. Three ERT and SRT coincident profiles have been acquired from sea to inland. Comparison of velocity and electrical models has allowed obtaining quaternary and weathered/fractured bedrock thickness. Electrical resistivity differences from one to another profile help mapping changes in saline intrusion. H/V technique has been suitable to obtain bedrock depth in the profile close to sea. However, strong attenuation of the microtremor signal has precluded its application in the rest of the study area. This attenuation could be related to the dissipation of seismic energy in moving fluids through a thick zone of fractured/weathered bedrock. All this information is critical in order to establish preferential water flows and groundwater degradation of the aquifers.
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High-resolution 2D Landstreamer Vibroseis Applied in Groundwater Mapping of Miocene Deposits
Authors T. Vangkilde-Pedersen, E.S. Rasmussen and M. KristensenThe Miocene succession in central and southern Jylland was formed during four phases of shoreline progradation into the basin that covers present-day Denmark. The deposits comprise several aquifers with potential drinking water resources and have been investigated by drilling and acquisition of seismic data integrated with sedimentology and biostratigraphy. The multi-disciplinary approach and an exclusively large dataset of 1200 km high-resolution 2D onshore seismic data and 150 boreholes, 100-400 m deep, has formed the basis for an extensive mapping of the Miocene succession. One of the cornerstones has been the generally very high data quality of the seismic data acquired in the past 10 years using the landstreamer vibroseis method. A detailed interpretation of the seismic data has been performed, focussing on mapping the extent of sand-rich bodies, by identifying top and base of separate delta lobes and fluvial deposits. The result of the investigations is a dynamic 3D geological model that reflects the basin development and the depositional processes as well as the palaeogeographical development during the Miocene. The model is intended to serve as a geological database of lithological and stratigraphical information and provides a better understanding of the geological architecture of the aquifers than traditional lithofacies models.
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Observing Sedimentary Deposition in an Artificial Lake using 3D Ground-penetrating Radar
Authors G. Hamann, J. Tronicke and M. WeberIn February and 2010 and February 2012, we have collected 3D ground-pentrating radar (GPR) data across the lake of the artificial water catchment Hühnerwasser (“chicken creek”) situated in the post-mining landscape of the opencast mine Welzow-Süd (Germany). This study shows, that the GPR technique can provide information relevant for analyzing fluvial erosion processes within the artificial catchment. We employed a conventional GPR surveying approach based on cart-mounted100 MHz antennae and a combination of the GPR instrument with a self-tracking total station on the frozen lake. Clear reflection from the lake bottom in the entire processed data cube are visible. Furthermore, deeper reflections can be recognized. These are interpreted to represent sedimentary structures associated with fluvial sedimentary input into the lake. In addition to detailed 3D images, our GPR data allow to quantify the amount of sedimented material. I am a PhD student and I would like to be considered for the AGAP-Quality Award.
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Applicability of MRS Soundings for the Characterisation of the Unsaturated Zone of Karst Systems
Authors N. Mazzilli, K. Chalikakis, M. Boucher, A. Legchenko, H. Jourde, H. Guyard, A. Chevallier and S. CarriereThis study aims to assess Magnetic Resonance Soundings (MRS) potential for the characterisation of the unsaturated zone of karst systems and thereby groundwater vulnerability assessment. The issues addressed are: (i) does water storage within the unsaturated zone of karst yield quantifiable MRS measurements ? if so, (ii) do different hydrogeological contexts yield different MRS signals ?, and (iii) what are the key factors influencing MRS signal ? The proposed methodology is based on a careful characterisation of the experimental sites, involving both hydrogeological and geomorphological analyses. MRS measurements have been performed on a total of 19 sites with varied hydrogeological and geomorphological setting. Analysis of the measured MRS signals shows that : (i) water storage within the unsaturated zone of karst does yield quantifiable MRS measurements, (ii) the first-order determinant of the MRS signals is the nature of the geological formation. These results confirm MRS applicability for the characterization of the unsaturated zone of karst aquifers and thereby vulnerability assessment. Future work will focus on the variability of the MRS response within a given geological formation.
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Identification of Past and Present Groundwater Recharge Pattern in the Lake Chad Basin with Magnetic Resonance Soundings
Authors M. Boucher, G. Favreau, A.M. Moussa, M. Ibrahim, M. Descloitres, M. Le Coz and A. LegchenkoLarge changes in the Lake Chad surface area occurred during the Holocene and had an impact on aquifer recharge, yet little is known on past and present pattern of the aquifer hydrodynamics. Two complementary MRS field surveys (23 soundings) were conducted in 2008 and 2010 for accessing of the free water content and hydraulic properties of aquifers in the semiarid northwestern part of Lake Chad Basin. High MRS water contents (10-35 %) and permeability values (10-3 to 10-2 m/s) were shown below present-day Lake Chad. At distance, below the former clayey deposits of the Megalake Chad, lower MRS water content (8-13 %) and permeability (10-4 to 10-3 m/s) values were estimated for the unconfined aquifer. These low porosity and permeability values suggest limited aquifer fluxes, in accordance with the occurrence of isotopically determined palaeo-groundwaters below the clayey plain. Intermediate and homogenous hydraulic properties (MRS water content and permeability values ranging from 16 to 25% and 1.10-3 to 3.10-3 m/s, respectively) were found in the valley of the Komadougou River which currently represents the main source of recharge for the aquifer. This study proved the decisive contribution of MRS for quantifying past and present surface-water/groundwater exchanges interactions in flat sedimentary basins.
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An up-scaling Approach to Groundwater and Aquifer Characterisation in the Australian Outback
Authors T.J. Munday, C. Sorensen and K. CahillAustralia's sustained economic growth and development over the past decade has been accompanied by an increased demand for water, particularly groundwater. Drivers for that demand include new industries, new mines, the need to secure urban and regional water supply. With reference to an example from the Murchison region of Western Australia we examine an approach that could use AEM to aid the scaling-up from a local to catchment-scale hydrogeological framework for groundwater assessment. It draws on local scale AEM surveys and hydrogeological studies, along with a regional scale model of the landscape geomorphology to determine an optimal approach to AEM data acquisition at the catchment scale, as an aid to groundwater resource assessment. Local scale studies using TDHEM data, linked to both water resource investigations and mineral exploration have demonstrated application in defining the extent and quality of groundwater resource in the sedimentary palaeovalley aquifers of the study area. This knowledge was used in combination with terrain indices to define an optimal AEM survey resolution given a limited, fixed budget.
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Magnetotelluric Method in the Exploration of Deeper Aquifers
By F. SumanovacElectrical methods are commonly used in groundwater investigations but in case of deeper aquifers the electrical tomography cannot achieve deeper targets and the electrical sounding cannot ensure a complete and dense space coverage. In such a case the use of the magnetotelluric method can provide for great depths of research, as well as a good space sampling. The capabilities of the MT-method are shown in two examples of deeper groundwater investigations carried out in the north-western Croatia. In the Gotalovec area the aquifers are fractured carbonates (Triassic and Miocene) underlying thick series of Neogenic clays and marls. On the basis of all results a conclusion can be drawn that the MT-method should be applied first in similar model in order to define potential aquifers and indicate the most favourable micro-locations where reflection seismics can be applied as the latter allows for a more precise mapping of aquifer depths and faults. The second geological model, investigated in the Apatovac area, is very complex consisting of clastic and magmatite bodies. The MT-method provided for a complete and clear solution to the structural relationships in the area and gave a precise definition of the deep narrow diabase body which is mineral water aquifer.
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Evolution of Electrical Resistivity Measurements during Process of Waste Biodegradation
Authors M. Sylvain, M. Bergeron and R. ClementElectrical Resistivity Tomography (ERT) is applied in bioreactor landfill for many years to monitor leachate variation in waste body (Moreau et al., 2007, Rosqvist et al., 2006), mainly during recirculation events when moisture content variations induced by liquid diffusion are the most important. Influence of moisture content and temperature is recognised to be significant on waste electrical resistivity (Clément et al., 2011). Nevertheless, impact of waste biodegradation should not be forgotten (Moreau et al., 2011): the assumption of a medium, electrically neutral, could not be considered in waste studies. Influence of the different steps of waste mass biodegradation on electrical resistivity measurements was studied with laboratory tests. Resistivity of well known waste composition was compared to biogas production to achieve this goal. Resistivity device was evaluated using simulation software developed by Clément (F3D-Lab, 2011) and the interpreted resistivity was calculated using BERT software package (Guenther, 2011. The resistivity variation observed on one cell can be linked to biogas production, which can not be confirmed by the tests on the other cells for the moment. Field capacity seems limiting to induce a fast biodegradation of waste mass which can be problematic for landfill generally operated with that hydraulic condition.
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Application of Electrical Resistivity Tomography Technique for Investigating Subsurface Flow Constructed Wetlands
More LessConstructed wetlands are widely used for removing pollutants from wastewater in small communities because their simplicity and low operation costs. Nevertheless, with time the cleaning process can result in gradual clogging of the porous layer by suspended solids, bacterial film, chemical precipitates and compactation. The clogging development causes decrease of hydraulic conductivity, reduced oxygen supply and further leads to a rapid decrease of the treatment performance. As the investment involved in reversing clogging can represent a substantial fraction of the cost of a new system it is essential to assess in advance the evolution of clogging process and detect potential failures in the system. Since there is a lack of experiences for monitoring the functionality of constructed wedlands a combination of non-destructive geophysical methods have been tested in this study. With this purpose electrical resistivity tomography and ground probing radar have been conducted at different horizontal subsurface flow municipal wastewater treatment wetlands of Catalonia (Spain). The obtained results have shown that the applied geophysical techniques may delineate the clogging expansion and help take the preventive measures for enlarge the lifetime of the treatment system.
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Innovative Project for In-situ Geophysical Instrumentation of Bioreactor Landfill: Short and Long Term Monitoring
Authors J. Grossin-Debattista, R. Clément, S. Loisel, A. Poulain, J.L. Böhm and S. MoreauOptimizing landfill operated in bioreactor mode is to date important challenge. Moisture content and temperature are two key parameters of the anaerobic biodegradation process. In order to improve the waste degradation and the recovery of biogas (renewable energie), more specific bioreactor studies are requiered. Indeed, waste is a very heterogeneous porous media. The water recirculation and temperature repartition is poorly known at the scale of the bioreactor site. Started on January 2012, as part of a multi-stakeholder project, experiments are being prepared: at the laboratory to optimize recirculation strategies and at the scale of the cell with a multi-instrumentation methods. For this purpose, a combination of innovative methods is being installed to allow has both a short-term follow-up during episodes including recirculation and a long-term monitoring of biodegradation process waste. Innovative methods implemented across the cell include ERT to study the distribution of moisture within the solid waste, optical sensor to measure temperature all accross the cell and moisture sensor. The objectives of this project are multiple: optimization of methods implemented, acquire a better understanding of the biodegradation process and leachate reinjection influence, achieving a long-term monitoring with a multi methods and allowing an optimization of bioreactor operating methods
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Pilot Study of using Resistivity-IP, Magnetometry and Slingram for Separation of Industrial Waste
Authors T. Dahlin and K. HellmanA controlled experiment was carried out by doing geophysical measurements on a purpose-built heap consisting of two different industrial waste fractions, called NF ( non-ferrous metal) and SLF (shredder light fraction), from the recycling plant of Stena Metall in Halmstad, Sweden. The purpose of the experiment was to see if it was possible to distinguish the different types of material using geophysics. Resistivity-IP, magnetics and slingram were measured over the test heap. The results gave distinctly different signatures for the different materials. In conclusion the test shows a large potential for the tested methods for material characterisation of buried waste in connection with landfill mining.
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Complementarity of Electrical Resistivity Tomography and Ground Penetrating Radar to Study Karst Unsaturated Zone
Authors S. Carriere, K. Chalikakis, C. Danquigny, G. Sénéchal and A. ChapeletThe structure of karst hydrosystems is complex and the related hydrodynamic functioning mechanism can be highly heterogeneous in both unsaturated and saturated zones. The Low-Noise Underground Laboratory of Rustrel (LSBB) is located in unsaturated zone (UZ) of Fontaine de Vaucluse (FdV) karst hydrosystem. In this way, the LSBB is a unique place to apply surface-based geophysical surveys due to numerous and various geological and hydrogeological knowledge under the investigated zone. We exhibit in this paper complementarity and efficiency of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR). Both GPR and ERT are well suited for the first phase of our study, in order to characterize the general structure of the investigated chosen zone. GPR results highlight the tectonic features which are also, but less accurately, seen by ERT apparent resistivity maps. ERT 2D inverted results have shown the presence of moderate resistivity zones within the limestone. The provided information allowed carrying out following step of our study to enhance understanding of local karst UZ e.i. apply Magnetic Resonance Sounding (MRS), Gravimetry and Low Frequency Electromagnetics (LF-EM).
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Detection of a Fracture Network Near a Cluster of Sinkholes using 3D Electrical Resistivity Tomography
Authors N. Dupont, M. Attali, J. Deceuster and O. KaufmannOver the last two decades, 2D electrical resistivity tomography (ERT) surveys have been conducted to detect and map karstic features as the expected electrical resistivity contrast is high between sound limestone and weathering residue as well as between sound limestone and cavities filled with clayey or sandy sediments. However, karstic features often show complex 3D geometries which are not well reconstructed through 2D surveys. In this paper, the efficiency of 3D ERT in the identification of a fracture network located at the top of limestone bedrock is assessed through a field experiment. The results of the 3D ERT interpretations were confirmed by visual observations of an outcrop located 100 meters away. Moreover, former sinkholes which occurred near the investigated area are aligned following the directions pointed out with the 3D ERT survey. This indicates that local karstification probably affect fractures located at the surface of the bedrock leading to the formation of a cryptolapiaz.
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Electrical and Electromagnetic Resistivity Imaging Environmental Applications and Engineering Study
Authors M. Djeddi and F. KhaldaouiA frequency-domain electromagnetic (FDEM) and electrical resistivity imaging (ERI) surveys can be used for delineating underground cavities and characterizing pseudo-karst dissolution of limestone sandstone on the northwest of Algiers. This site has complex geology still requires a detailed survey prior to the execution of civil works. The FDEM survey was performed using both a very low frequency resistivity (VLFR) and a controlled source EM system (Geonis EM31). The electrical resistivity imaging has been conducted using dipole-dipole array. The results of this study have identified a number of karst cavities in the calcareous sandstone associated with carbonate dissolution by groundwater from a depth of about 4 m, some of these cavities and fracture zones were identified by boreholes at the site. The networks of cavities are organized along a N110 ° and N60 °.
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The Use of In-river Resistivity to Classify Recharge Rates in Losing Disconnected Streams in the Murray Basin, Austr in
Authors A.C. Davis, T.J. Munday, S. Lamontagne and A.J. TaylorMany of the waterways in the lowlands of the Murray-Darling Basin are rivers and creeks that are typified by having long and meandering channels that are underlain by a clogging clay layer of varying quality and thickness. It is the presence and thickness of these clay layers, along with depth to water table, which are the primary parameters in the determination of the rate loss from the watercourse to the underlying aquifer. Using an in-river resistivity survey, corroborated by along-shore VES and hand auger profiles, we have produced a geo-referenced recharge/loss map for a 2 km section of Billabong Creek near Jerilderie, NSW. We have shown that in-river resistivity surveys are a possible geophysical technique that can be used to estimate recharge rates and clay thickness along the river bed at a river reach scale. Maps such as the one produced here are useful for groundwater flow modelling, and may assist in the determination of water allocations in the future.
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Geoelectrical Measurements for Agricultural Canal Seepage Detection
Authors C. Comina and L. SambuelliThe protection of water resources is a current problem at a global scale, especially in regions where this resource is poorly available. In this respect there is an increased pressure on water managers to improve water use efficiency. The use of geophysical tests as a tool for managing the efficiency of irrigation networks is well documented in literature since they have an high potential for canal seepage identification and quantification. In this respect, the present study is focused on the use of electrical resistivity methods on some sample canals of the Piedmont Region network. The results obtained have underlined the usefulness of geophysical methods in detecting local anomalies in the lining, potentially related to seepage losses, by means of waterborne CVES and in imaging the passage of saline tracer tests plumes, to be used for a quantification of the seepage losses, using cross-flow high-speed ERT.
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