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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
61 - 80 of 127 results
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Importance of Transmitter Waveform and Receiver Transfer Function Modelling in Time Domain Induced Polarization
Authors G. Fiandaca, E. Auken, A.V. Christiansen and A. GazotyIn the computation of the forward response for Time Domain Induced Polarization the incomplete description of the transmitter waveform causes dramatic errors in the estimation of the magnitude and time characteristic of the IP phenomenon. In fact both the duration of the current pulse and the sequence of pulses used for the stacking procedure have a strong effect in the magnitude and shape of the IP decays. Furthermore, it is important to model, if present, the low-pass filters of the receivers, in order to use all the information contained in the acquired data. For these reasons, a new 1D forward and inversion algorithms have been developed using the full time decay of the IP response and the receiver transfer function to reconstruct the distribution of the four Cole-Cole parameters of the earth. The waveform implementation in the forward response for TDIP is a significant improvement that allows moving from a qualitative interpretation of TDIP data for recognition of anomaly patterns towards a quantitative analysis, able to discriminate soil lithotypes and, if present, some contamination patterns.
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2D Time Domain Inversion of Induced Polarization Data
Authors J. Ramm, G. Fiandaca, A. Binley, E.A. Auken, A. Gazoty and A.V.C. ChristiansenWe have developed a new, robust 2D inversion approach for modelling time domain induced potential data. Instead of considering the integral chargeability, the full IP waveform is modelled allowing inversion for spectral content in terms of the Cole-Cole parameters. The method is based on a finite element complex resistivity forward response, which is transformed to the time domain using a Fast Hankel Transform. The forward response is implemented in a general inversion code where constraints can be applied on all model parameters to allow migration of information along the profile. A priori information from e.g. boreholes may also be included to constrain the inversion.
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Minimisation of Electrode Polarisation Errors when Using Optimised or Generic Resistivity Imaging Arrays
Authors P.B. Wilkinson, P.I. Medlrum, M.H. Loke, J.E. Chambers, O. Kuras, D.A. Gunn and R.D. OgilvyPolarisation potentials are caused when metallic electrodes are used to transmit current in resistivity imaging surveys. If these electrodes are subsequently used to measure potential differences, the decaying polarisation potentials can be a source of significant error. In this paper we describe a general method to minimise the impact of polarisation errors by rearranging the resistivity measurements to maximise the time between any electrode injecting current and later measuring potential. This method does not rely on the existence of a natural ordering of the measurements and can therefore be used with arbitrary resistivity imaging arrays, specifically including those generated by automated optimisation schemes. The method uses a global minimisation algorithm ("simulated annealing") to attempt to avoid local minima without performing an exhaustive search of the configuration space. We determine the control parameters and permutation types for the method from the results of a series of numerical experiments on a randomly generated measurement sequence. We then demonstrate the efficacy of the method using real data measured with a permanently installed resistivity monitoring system on an active landslide. The results show that polarisation errors can be effectively eliminated when using optimised resistivity imaging arrays.
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Finite Difference Resistivity Modeling on Unstructured Grids with Large Conductivity Contrasts
Authors S. Penz, H. Chauris and D. DonnoThe 3-D geo-electrical forward problem solved with a finite difference approach faces several difficulties. Besides the singularity at the source location, major issues are caused by the definition of the computational domain to match a particular topography, and by high conductivity contrasts. To address these issues, we combine here two methods. First, we implement a specific finite difference method that takes into account specified interfaces in elliptic problems. Here, the contrasts are defined along grid lines. Second, we extend the method to unstructured meshes by integrating it to the generalized finite difference technique. In practice, once the conductivity model is defined, the approach does not need to explicitly specify where the large contrasts are located. Several numerical tests are carried out for various Poisson problems and show a high degree of accuracy.
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Constrained Time-lapse Inversion of 3-D Resistivity Surveys Data
More LessThree-dimensional surveys and inversion models are required to accurately resolve structures in areas with very complex geology where 2-D models might suffer from artefacts. 3-D data sets collected at different times are inverted simultaneously using a least-squares methodology that uses roughness filters in both the space and time domains. The spatial roughness filter constrains the model resistivity to vary smoothly in the x, y and z directions. A temporal roughness filter is also applied that minimizes changes in the resistivity between successive temporal inversion models. This method can accurately resolve temporal changes in the resistivity even in the presence of noise. The use of the L1 and L2 norm constraints for the temporal roughness filter are examined using a synthetic model. The L1 norm temporal constraint produces significantly more accurate results when the resistivity changes abruptly with time. A test with field data from a landfill site with methane gas accumulation shows near surface resistivity changes that are probably due to surface temperature variations. The temperature variations cause changes in the gas volume and moisture content in the near surface landfill materials.
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Joint Inversion of Seismic Refraction and Resisitivity Data Using Layered Models
Authors N. Juhojuntti and J. KammWe present a layer-based joint inversion of refraction seismic and geoelectric data and show its utility by applying it to a field example from western Sweden. The joint inversion couples the two data sets by assuming the electrical and acoustic parameters to have a common layer structure, but otherwise leaving them to vary freely (laterally within those common layers). The lateral parameter variations are constrained as previously proposed for pure resistivity inversion (Auken & Christiansen 2004). We assume our assumptions are justified for sediment bedrock situations in ground water exploration. We compare the joint inversion results with those obtained from single inversions, and conclude that it is mostly the seismic data constraining the structure. The particular situation under consideration exhibits a pathological hidden-layer situation, where mere seismic exploration may be inaccurate. In cases like these, the joint inversion allows an improved determination of the structure compared to seismic inversion alone. The resistivity distribution, and also to a lesser degree the velocity distribution, benefit significantly from being constrained by the improved structure.
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Ground Conductivity Meters Considered as Elevated Dipoles
By D. BeamishGround Conductivity Meters (GCMs) are used across a wide range of application areas and the measurements are spatially assembled and correlated with data from many other earth science, environmental, soil and land use investigations. They are understood, by users, to operate under a Low Induction Number (LIN) condition. This paper provides a new assessment of the subsurface conductivity measurements obtained by GCMs when the source is considered as an elevated dipole. The theoretical behaviour of the common systems is examined in relation to both the prevailing level of subsurface conductivity and the instrument elevation. It is demonstrated that, given the inherent high level of accuracy of modern instruments, the prevailing LIN condition may require operation in environments restricted to very low (<12 mS/m) conductivities. A simple correction procedure that can be applied to the measured data obtained from any of the LIN instruments is developed. The correction procedure would, in the limit of a uniform subsurface, return the same (correct) conductivity, irrespective of the ground conductivity meter used, the prevailing conductivity or the measurement height.
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Airborne Geophysical Data Used to Characterise Groundwater Dependant Wetlands
Authors D. Beamish and G. FarrThis paper provides an assessment of existing airborne baseline geophysical data in relation to the hydrogeological characterisation of protected groundwater dependant wetland areas found on Anglesey, Wales. Radiometric data appear to identify the main areas of water saturation together with the degree of saturation in the very near surface. The radiometric data also show regional scale transition from limestone in the east, important for water supply to the wetlands, to older more impermeable strata to the west. The conductivity data appear capable of mapping the lateral extent of clay accumulations, concealed below peat cover and acting as a confining layer. The data therefore provide a non-invasive spatially-continuous, characterisation of the sites, improving the understanding of their dynamic water balances and, potentially, guiding further invasive investigations.
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Spatially Mutually Constrained Inversion of Airborne Hydrogeophysics Survey with Ground Geophysics and Borehole Data
Authors K. Martinez, O.F. Nielsen and J.A. MendozaThis work presents examples from processing and interpretation of 2576 km of SkyTEM data in the Ringsted-Suså groundwater mapping area located southeast of Denmark. The mapping area is approx. 422 km2 and includes approx. 75,000 SkyTEM soundings. Additionally, diverse ground based geophysical data (>260 km Multi-Electrode Profiling (MEP), > 130 km Pulled-Array Continuous Electrical Soundings (PACES) and 200 ground-based TEM soundings) were utilized in a pseudo-3D inversion. Furthermore prior information was added from 39 borehole logs and lithological information from approx. 735 boreholes in the final inversion. The objectives of the work were to map the near subsurface for units of hydrogeological significance and to evaluate the available inversion schemes. Three types of inversion were used and compared; Laterally Constrained Inversion (LCI), Spatially Constrained Inversion (SCI) and Spatial Mutually Constrained Inversion (SMCI). SMCI is seen to produce results more consistent with both geological data of other geophysical data. Furthermore, the SMCI is less influenced by the degree of lineaments compared with LCI and SCI. This is due to the use of different geophysical data sets represented in one inversion result, which in addition allows gathering the geophysical inversion into one result.
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Aquifer Contamination at Roorkee/India Studied by Spatially Contrained Inversion of TEM Inloop Data
Authors B. Tezkan, M. von Papen and M. Israil46 inloop soundings were carried out close to Roorkee/India on a contaminated area and on a noncontaminated site used for reference. Due to the inaccessibility of some parts of the survey area the TEM soundings could not be carried out on profiles and therefore were realized on an irregular measuring grid. First of all, the data were interpreted by a conventional 1D inversion and afterwards the TEM soundings were simultaneously inverted by a spatial constrained inversion to derive a quasi 3D conductivity model. The TEM-models show a relative low resistive aquifer (8-15 Ohm-m) at 5-15 m depth indicating a possible contamination which was also observed (radiomagnetotellurics, DC) and confirmed by other geophysical measurements on the same area. The other object of the study was the second deep aquifer at 40-70 m depth. Its resistivity was determined as 20-40 Ohm-m. However, the TEM inversion results couldn’t clearly indicate a possible contamination of this deep aquifer.
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Investigating Heterogeneous Costal Aquifer Using Magnetic Resonance Tomography and TDEM
By A. LegchenkoMagnetic Resonance Tomography (MRT) and Time Domain Electromagnetic (TDEM) methods were applied to investigate a costal aquifer in the Dead Sea area of Israel. The subsurface in this area is highly heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. Variation of the water salinity is poorly known. TDEM is an efficient tool for locating electrically conductive targets like saline water, but it is also sensitive to the porosity of rocks. MRT, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface (through the relaxation time T1). We used MRT for a 3D imaging of the water content in the subsurface and 2D mapping of the T1. 3D distribution of the electrical resistivity was obtained using interpolation of 1D TDEM results. We show that joint use of the MRT and TDEM enables us to resolve the aquifer and to demonstrate that the salinity of water in the investigated area is fairly constant.
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Improving Groundwater Flow Model Conceptualisation and Calibration with ERT and Self-potential Methods
Authors T. Robert, R. Therrien, J.M. Lemieux and F. NguyenThe self-potential (SP) method relies on passive measurements of the ambient electrical potential at the ground surface or in boreholes. When the electrokinetic effect is the dominant contribution, the resulting signal is called the streaming potential and contains information about groundwater fluxes that can be useful for calibration of groundwater flow models. The streaming potential forward equation was implemented in the HydroGeoSphere model, which simulates 3D groundwater flow and solute transport in porous media, including fractured geological formations. HydroGeoSphere is able to calculate the streaming potential given a distribution of Darcy velocity and electrical resistivity. Since groundwater flow modelling relies on a conceptual model, prior information on the distribution of the geological units and hydraulic conductivity at the site is mandatory. However, this information is often scarce or missing. In this work, we use the electrical resistivity tomography (ERT) and the SP methods as an additional source of information for building the groundwater flow model. ERT is used to identify the location of fractured zones in a fractured and karstified calcareous aquifer of South Belgium. The SP signal is used with PEST in order to calibrate the groundwater flow model and better constrain the hydraulic conductivity of the fractured zones.
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Geoelectrical Imaging for Groundwater Exploration in the Shallow Zone in an Area of Pesquería, Nuevo León, México
Authors D. Garza-Rocha and J.J. Castellanos-FloresIn this paper we would like present the results of hydrogeological and geophysical investigations that were carried out in an area located in the northeastern of Mexico. The area of investigation is found in the city of Pesquería, Nuevo León. The main goal of this study was the exploration of groundwater. In this region the pluvial precipitation is erratic and concentrated. On the basis of the superficial geology we decide to carry out several electrical profiles, and several vertical electrical soundings to try to obtain one model of the lithology of the underground. Measurements of the phreatic level of existing wells were also carried out. This helped us to determine the location of the vertical electrical soundings and the electrical profiles. The interpretation of the obtained resistivity models was made with the aid of information of the geology of the area, and lithology of the materials of some existing wells. The information, about the depth and direction of the flow of groundwater, was also obtained from the hydrogeological and geophysical measurements. The results, of the distribution of the measurements of the electrical resistivity, show locations suitables for future work to explore for groundwater.
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An Extended Model for Predicting Hydraulic Conductivity from NMR Measurements
Authors R. Dlugosch, M. Müller-Petke, T. Günther and U. YaramanciThe prediction of hydraulic conductivity (K) from NMR measurements (Phi, T2) can be applied successfully on sandstones. For hydrological applications in the near surface unconsolidated material is more common. This material generally shows less variability in porosities but a larger range of pore sizes compared to sandstones. The known (semi-)empiric K-T2 relations have often be extended to this field of application but their validity needs to be verified. In this work we present a simple model based on tube pores, which is valid for the whole range of laminar flow from silt to gravel. By considering the slow diffusion regime we are able to estimate a maximum K from a measured T2 time. The model replaces the empiric factors in known K-T2 relations with (petro-)physical parameters. This enables to separate effects caused by variations of the surface relaxivity, a material characteristic that links T2 to the inner surface of the material, from variations of other parameters. This may help to reduce the range of the predicted K-values from NMR measurement on similar materials and enable to incorporate results from ofter methods.
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3D very High Resolution Shallow Seismic Data from Carbonate Reservoir - A Case Study
Authors M. Toqeer, H. Perroud and D. RoussetShallow seismic surveys are becoming a considerable way for detecting and characterizing shallow subsurface for environmental, geological, geophysical and geotechnical applications. Shallow 3D seismic survey acquisition and data processing is not an easy task. This is partly due to survey cost and due to near subsurface heterogeneities and several type of noises which partly depends upon the particular site. We present the complete processing and preliminary interpretation results of shallow 3D seismic survey carried out in Campos, Mallorca, Spain. This project is aimed to characterize the carbonate reservoir down to 100m that is composed of hard carbonate rocks. This rock sequence is karstic and fractured. Due to this factor energy penetration is a problem and severe noises are produced due to diffractions. Furthermore Rayleigh waves pose another serious problem for processing. By efficient processing sequence and application of CRS method zero offset stack data is obtained which is further tuned by post-stack depth migration. The objective is to process the severely noisy data to get the subsurface image for further interpretation and seismic attribute analysis. Here preliminary interpretation results of the data also are presented that will be analysed in detail through seismic attributes.
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Experimental Correlations Between Geophysical and Hydraulic Parameters from Different Inversion Procedures
Authors G. Boehm, R. Brauchler, D. Nieto, G. Soncin, L. Baradello, M. Pivetta and F. BottiThis work is aimed to test the possibility of integrating different geophysical method used in near surface application and in particular in studies devoted on a better characterization of the hydraulic system and aquifer structure. We used different inversion procedures to correlate the geophysical and hydraulic parameters and to extrapolate them in those areas not covered by the corresponding survey. The parameters involved in this study were: seismic p-wave velocity from seismic tomography; resistivity, electrical conductivity and induced polarization from electrical tomography (ERT); diffusivity, hydraulic conductivity and specific storage from hydraulic tomography. The study area is located in the Friuli plain, east of Italy. The dimension of the investigated area was approximately 2000 m². We also drilled two wells, 8 m apart, used for the hydraulic cross-well acquisition. The comparison of the specific correlation functions showed that the geophysical parameter p-wave velocity and the hydraulic parameter diffusivity feature the highest correlation. The derived correlation function between the two parameters allowed us to transform the p-wave velocity tomogram into a diffusivity tomogram with high accuracy. The proposed approach has the potential to provide a problem adapted scale and accuracy for the prediction of contaminant transport in the subsurface.
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Calibration of Density Driven Flow Model for the Freshwater Lens beneath the North Sea Island Borkum by Geophysical Data
Authors H. Wiederhold, H. Sulzbacher, M. Grinat, J. Igel, T. Burschil, T. Günther, B. Siemon and K. HinsbyWith the aim to be able to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea a numerical model is set up for the North Sea island of Borkum. The work is done in the frame of the EU Interreg project CLIWAT. The database includes information from drillings, seismic measurements, an airborne electromagnetic survey (HEM) and monitoring data from two vertical electrode chains as well as measurements of groundwater table, pumping and slug tests, water samples. The groundwater model has been set up by the finite-element programme FEFLOW. The density driven flow model can be calibrated successfully on the base of hydraulic, hydrological and geophysical data. Verification runs with the calibrated model show a good agreement for the measured and computed hydraulic heads and the measured with computed TDS-data. This is the case as well on a large scale for the whole freshwater lens as also on a small scale in the area of the well fields.
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An Integrated Geophysical/Geological Model of the CLIWAT Project Area Föhr as Basis for Groundwater Modelling
Authors T. Burschil, H. Wiederhold, W. Scheer and R. KirschIn the EU project CLIWAT sixteen institutions investigate the groundwater situation in the North Sea coastal areas from Belgium to northern Denmark to simulate consequences of climate change for the groundwater systems and water supply. The North Sea island of Föhr is part of a pilot area. For a better understanding of the current groundwater situation several geophysical surveys were carried out including an airborne survey with the transient electromagnetic system SkyTEM and high resolution reflection seismic surveys from surface and in boreholes with P- and S-waves. The results reveal glacial structures as Quaternary buried valleys as well as a glaciotectonic thrust fault complex. They were located with a combined interpretation of geophysical results and geological information of boreholes. Both of these structures influence the groundwater systems. An integrated geological-geophysical 3D model supports groundwater modelling.
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Hydrogeophysical Characterisation of Artificial Recharge Basins on the Llobregat River Barcelona, Spain
Authors A. Casas, A. Sendròs, H. Gallardo, M. Himi, R. Lovera and J.C. TapiasOver the past ten years there has been growing interest in the use of the subsurface for water storage using shallow ponds, either natural or excavated, for the infiltration of water into the subsurface and then uses wells to recover the water. The design and operation of these subsurface systems require an understanding of the hydrogeologic structure and properties that control both the movement and storage of water. In our study, we have performed a detailed hydrogeophysical characterization about the subsurface of the pond from electrical resistivity tomography.
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Statistical Method to Delineate Resistivity Anomalies Associated with Contamination from a Controlled Dump - N Portugal
Authors M.J. Fontoura, F.J. LINO, S.P. RIBEIRO, S.M. RIOS, R.M. MOURA and A.G. DIASThe assessment of contaminants dispersion in Controlled Dumps (CD) of Municipal Solid Waste (MSW) is possible through the combination of geophysical, geochemical and geostatistical methods as described in this paper. The methodology applied in this study will contribute to evaluate the environmental consequences of the Matosinhos CD (N Portugal), which is set in a granitic crystalline geological context with a permeability controlled by differential weathering associated with fracturing. The statistical methods here described (Lepeltier, 1969; Matschullat et al. 2000) are adapted to use in geophysical data obtained from the resistivity profiles performed in the CD surroundings by the electrical resistivity method. Groundwater samples were collected in piezometers; upstream and downstream of the CD, with the aim of directly confirming the presence of contaminants indirectly detected by the geophysical and geostatistical methods. The combination of these different approaches allows an approach to the detection and delineation of contaminant plumes from these deposits.
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