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Near Surface 2007 - 13th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 03 Sep 2007 - 05 Sep 2007
- Location: Istanbul, Turkey
- ISBN: 978-90-73781-81-8
- Published: 03 September 2007
61 - 80 of 144 results
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Usefulness of Magnetic Resonance Soundings to Constrain Groundwater Modelling
Authors J.M. Vouillamoz, G. Favreau, S. Massuel, M. Boucher, Y. Nazoumou and A. LegchenkoThe unconfined aquifer in southwest Niger was intensively studied over the last decades. However, the transient numerical modelling that was conducted over the area suggested that additional field storativity and hydraulic conductivity measurements are needed. To check the capability of magnetic resonance sounding (MRS) to improve the knowledge of this aquifer, a survey was conducted in Niger in December 2005. We found that (1) the depth to the water table is estimated by MRS with an acceptable accuracy to survey poorly documented areas. (2) After a parameterization process, the transmissivity is accurately estimated from MRS. (3) MRS water content is linked to the total porosity calculated with the Bretjinski formula. However, MRS water content still needs to be parameterized with new measurements to quantify the specific yield and the total porosity. We finally found that MRS is useful to better constrain groundwater modelling of such large unconfined aquifer. (1) MRS depth to the water table can be used as head boundary in the numerical model (2) The range of realistic transmissivity and storativity values that could be used in the model can be estimated from MRS.
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Increasing Success Rates of Drill Holes for Water Exploration in Uganda Using MRS
Authors O. Mohnke, M. Agostini, J. Bernard and U. YaramanciMagnetic Resonance Sounding (MRS) is a geophysical method that allows direct determination of the distribution of the mobile water content and effective porosities by surface measurements. The amplitude of the MRS signal is directly proportional to the amount of mobile water. The supply of freshwater for local villages that are relocated after the end of the civil war in Uganda is a crucial effort of development aid to prevent spreading of infectious diseases such as cholera or hepatitis. Depending on the local geology the success rate of fresh water drill holes can ranges from 50 to 80 %. The yield of such wells is in the range of 300 l/h (hand pump) and more than 9000 l/h (production well). To increase the rate of successfully drilled boreholes for fresh water supply a feasibility study on the usage of MRS has been carried out. After each sounding predictions of MRS have been directly verified by drill holes. The results from MRS measurements were confirmed to a 100%, thus, making MRS a reliable tool for assessing the presence of producible water and increasing the success rates of drill holes for fresh water even in an intricate environment, as encountered in Uganda
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Determining an Appropriate Airborne Electromagnetic System for Groundwater Exploration
Authors A.D. Fitzpatrick and T.J. MundayThe social and economic viability of the southern Eyre Peninsula relies on access to sustainable groundwater resources. Determining the sustainable yield of these aquifers requires improved understanding of their characteristics, geometry and spatial (vertical and horizontal) variability. Airborne geophysics, particularly airborne electromagnetics (AEM) has the potential to address some of these issues. A forward modelling study was conducted to determine which AEM system, if any, would be best suited for mapping variations associated with the Quaternary aquifer systems in the Southern Basins Area of the Eyre Peninsula. The forward modelling suggests that the HOISTEM, SkyTEM and TEMPEST AEM systems are likely to yield comparable results, although SkyTEM is the best of the three. A recent TEMPEST survey was flown over the area. Variations in conductivity appear to be related to the hydrogeological characteristics of the area, including changes water quality, although this requires further investigation linked to a detailed study of available bore data.
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Mapping of Complicated Aquifer Structures – The BurVal-Project Area Ellerbeker Rinne
Authors R. Kirsch, E. Auken, G. Gabriel, H. Goetze, H. Rumpel, B. Siemon, W. Scheer and H. WiederholdWithin the scope of the EU-INTERREG project BURVAL (see poster Wiederhold et al.) the aquifer structure of the buried valley Ellerbeker Rinne (important for the water supply of Hamburg) was mapped with seismic, gravity and resistivity methods. Especially the combination of results from reflection seismic and airborne electromagnetic (SkyTEM) was successful to delineate sandy aquifers and clayey covering layers inside the valley.
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MRS Study of the Evolution of Aquifer in Sinkhole Hazardous Area, Dead Sea Coast
Authors A. Legchenko, M. Ezersky, C. Camerlynck and A. Al-ZoubiResults of investigations carried out by the Geological Survey of Israel and the Geophysical Institute of Israel reveal that the development of sinkholes in the Dead Sea coast is linked to subsurface salt formations and change of fresh – saline water interface caused by the drop of the Dead Sea level. Since groundwater is seen as the major reason for sinkhole development, it was decided to apply the Magnetic Resonance Sounding (MRS) method to investigate aquifers in potentially dangerous areas. In 2005 and 2007, two MRS surveys supported by NATO project SfP N° 981128 were performed in the area. Our results reveal a very heterogeneous aquifer with two orders of the transmissivity variation around the investigated area. It was observed with MRS that when sinkhole hazard takes place it causes 5-10 folds drop of the transmissivity. Formation of new caverns was not observed. Numerical modeling reveals that under Nahal Hever conditions the water table can be measured with MRS with the uncertainty of a few meters. However, accuracy of MRS is sufficient for monitoring of the transmissivity of aquifers in sinkhole hazardous areas where ground collapse may modify significantly the subsurface.
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Resistivity Inversion of Magnetic Resonance Sounding – Assessment of Sensitivity and Reliability
Authors M. Braun and U. YaramanciThe Magnetic Resonance Sounding (MRS) or Surface Nuclear Magnetic Resonance (SNMR) method is used for groundwater exploration and aquifer characterization. MRS provides useful information about water content and hydraulic conductivity of the aquifer. The excitation field of the NMR experiment is generated by a transmitter loop placed on the surface. Due to the electromagnetic attenuation, the magnetic excitation field depends on the resistivity of the subsurface. Therefore, the resistivity has to be taken into account when inverting the MRS data. Neglecting the resistivity causes a biased estimate of the water content distribution. The used resistivity information can be given by a priori information or, as newly introduced in the presented inversion scheme here, it can be obtained during the inversion process. Having also the resistivity along with the water content distribution as inversion result, MRS gives an information about the salinity of the aquifer and thereby, the quality of the aquifer can be estimated. This is of utmost interest for the further hydrogeological interpretation. This specific information cannot be achieved by geoelectrics alone, because of the nonuniqueness in resistivity concerning water content and salinity.
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Pore Size Distributions and Hydraulic Conductivity of Rocks Derived from Magnetic Resonance Sounding Relaxation Data
Authors O. Mohnke and U. YaramanciIn Magnetic Resonance Sounding (MRS) the hydrogen protons in the pore water are excited with an artificial magnetic field. This excitation field is usually generated by a circular/square or figure-of-eight antenna loop energized by an alternating current which oscillated with the local Larmor frequency of the hydrogen protons. The amplitudes and of relaxation constants of MRS signals are directly linked to the amount of pore fluid (e.g. water content) and the pore sizes in the subsurface. The fluid phase within the pore space exhibits well defined relaxation rates governing the decay of the nuclear magnetisation of the excited protons within a specific pore size, and thus can be correlated with the hydraulic permeability K of the material. Hydrological models using grain size analysis and NMR models using pore sizes are integrated and general relations between observed MRS decay times and derived hydraulic permeability K are reviewed and discussed and an in-situ K-estimation template is introduced.
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Nuclear Magnetic Resonance Relaxation at Water Filled Sand Samples in Earth Field
Authors S. Strehl and U. YaramanciNuclear Magnetic Resonance (NMR) decay parameters show a significant dependency on the Larmor frequency related to the strength of the static field of the method. A straight forward frequency downscaling of the results obtained with the higher field applications in the lab down to the very low frequency range of the Earth field (EF) is not possible. We investigated the EFNMR relaxation times of natural sand and different synthetic sand-clay-mixtures and compare these with relaxation times at high fields i.e. Larmor frequencies. We find, that the EF T2* decay time can be used as a measure for characterizing the water filled pore space in spite of the existence of small internal field gradients. This finding is very useful for instance for the application of the Magnetic Resonance Sounding method for groundwater assessment from the Earth surface. The field cycling of T1 measurements show that a downscaling from the higher field NMR down to the EFNMR is in principle possible, if the surface relaxivity of the material to be investigated is known.
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Discrimination of Rocks with Different Hydrodynamic Properties Using MRS, EM and Resistivity Methods
Authors M. Descloitres, L. Séguis, M. Wubda and A. LegchenkoA geophysical survey has been carried out on a watershed (10km²) in north Benin for contributing to a parameterization of hydrological modelling. For delineating rock and soils units we used Electromagnetic (EM) conductivity mapping. For estimating thickness of regolith we used resistivity methods. For characterising aquifers we used Magnetic Resonance Sounding method (MRS) implemented according to main geoelectrical units. We discriminated 5 MRS hydrogeological units: fresh rock with very low water content, low density fractured rock, clayey aquifer, sandy aquifers and fractured rock, high density fractured rock. A clear complementarity between resistivity and MRS methods is shown: MRS has advantage to discriminate low from high density fractured rock and solid rock with identical resistivity signature. Resistivity discriminates sandy aquifers from fractured rock with identical MRS parameters, and provides useful information for MRS inversion. Before integrating MRS parameters into hydrogeological model, identified units should be calibrated in boreholes with pumping tests. For the future, a detailed parameterization of hydrological model could be proposed using MRS results and EM map interpreted with geomorphological data. Keywords : MR Sounding, resistivity methods, hydrology, Benin
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Case Histories of Hydraulic Conductivity Estimation with IP at the Field Scale
Authors A. Hoerdt, R. Blaschek, F. Binot, A. Druiventak, A. Kemna and N. ZisserWe carried out Induced Polarization (IP) measurements to assess methods previously suggested to estimate hydraulic conductivity from complex electrical conductivity at the field scale. We discuss case histories from four selected sites, covering a lithological spectrum from gravel to silt, with a variation in hydraulic conductivity (k) over three orders of magnitude. At each site, hydraulic conductivity was estimated from the real and imaginary conductivity obtained from 2-D inversion of single-frequency IP data. We applied the constant phase model, where only one frequency, typically around 1 Hz, is used. For the purpose of this study, data from each area were reduced to about 2-3 values only. The IP-determined values are systematically too large when compared to data from grain size analysis or pump tests. For a more detailed investigation, real and imaginary conductivity were displayed vs. hydrogeologically determined k-values. Remarkably, the correlation between the real part and k is better than the correlation of the imaginary conductivity. Thus, the hypothesis that the imaginary part of single frequency IP data improves hydraulic conductivity estimation is not supported by our data.
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Fast-sampling EM Applied to the River Murray and Surrounding Floodplains in Australia
Authors M.A. Hatch, V. Berens, A. Fitzpatrick, G. Heinson, T. Munday and A. TelferAccurate, high resolution images of the top 10 to 50 metres of the earth’s surface potentially provide insights into the underlying hydrogeology that help us to understand, for example, the distribution of saline groundwater in the subsurface, or subtle changes in the distribution of clays in the top ten metres. This paper reviews the use of a commercially available fast-sampling TEM system in use in the surrounds of the River Murray system in southeastern Australia. This system has been used to collect data in a fairly standard TEM mode, i.e. discrete stations at fixed intervals along a line. Additionally results are shown using the system in a modified configuration, with much smaller transmitting and receiving antennae towed behind a boat, for example, with data collected in conjunction with a GPS in a “continuous” mode. This type of data provides a continuous electrical cross section with data points as close together as five metres apart, to a depth of at least ten metres.
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Assessment of Ground Water Aquifer at 10th Ramadan City Using 2-D Inversion of VES Data
More LessRecently Cairo metropolitan had been expanded and new urban had been inhabitated.10th of Ramadan City is one of main industrial zone in Cairo which attracts the attention of all investors. Water resources are very important for industrial and development projects. Therefore, in this study, the main aim is to investigate the groundwater aquifer in this area. Forty six vertical electrical soundings have been carried out using schlumberger array of electrode configurations. These VESes are interpreted using 1-D least squares method. The Results show that the Pleistocene aquifer in the area is subdivided into four geoelectric zones. The same data set has been inverted in 2-D using ABIC least squares method. The results of the 2-D were correlated to the 1-D inversion and other available geological data. In 2-D inverted geoelectric section, the geoelectric zones are the same as in 1-D interpretation. The third zone (zone C) is considered to be the water bearing horizon and composed of sands and gravels. The area is found to be affected by some normal faults, which may affect the extension, the thickness and the type of the groundwater. Pleistocene groundwater aquifer is considered as free to semi confined aquifer type.
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Application of Electrical Resistivity Tomography in Monitoring Recycled Water Injection
Authors G.N. Vargemezis, P. Tsourlos, J.H. Kim and M.J. YiThis work describes the installation and preliminary measurements of an electrical re¬sistivity tomography (ERT) system to monitor the injection of recycled water into a confined aquifer. The aim is to provide, through time-lapse electrical resistivity tomo¬graphy (ERT) measurements and processing, geoelectrical images of rather increased volumetric sampling around and between the holes and to obtain improved under¬standing of the flow and transport of the injected water. Preliminary measurements involving single-hole ERT measurements were obtained and processed with a 2D inversion algorithm to produce images of the subsurface. Results depict a very good correlation between ERT images and the lithology logs an indication of the reliability of the approach. Those images offer increased resolution and spatial coverage compared to traditional approaches. The entire ERT permanent installation is of low cost, easy to implement and can be used to understand and evaluate the effects of the water injection process.
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Joint Use of Geophysical and Hydrological Methods to Characterize Structures and Flow Geometry in a Complex Aquifer
Authors J. Hoareau, J.M. Vouillamoz, M. Beck, M. Reddy, M. Descloitres, A. Legchenko, M. Sekhar, M.S. Mohan Kumar and J.J. BraunCharacterizing the hydrogeological functioning of complex crystalline aquifers requires a precise assessment of their structural and hydrodynamic heterogeneities. This study shows how the joint use of TDEM, ERT and MRS enables to characterize both alterites and weathered-fissured zones. Moreover, with complementary data such as water table measurements and hydraulic tests, the characterization of the fractured zone is also possible. A conceptual hydrogeological model is then proposed from a case study in India, thanks to this joint use of geophysical and hydrological data.
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Geophysical Surveys to Monitor and Evaluate Environmental Management Initiatives
Authors V. Berens, A.D. Fitzpatrick, M.A. Hatch, T.J. Munday, M.G. White, N.J. Souter and K. CahillClark’s Floodplain, Bookpurnong, South Australia represents a pilot investigation site from which the outcomes will direct management of the larger 17,700 hectare Chowilla Floodplain system, one of six icon sites identified under the Living Murray Initiative. At Clark’s a number of technologies are being trialed including fresh water injection, saline water extraction and artificial surface flooding. The investigations across the sites are measured for success using a variety of vegetation health assessments and supported by investigations into water processes including numerical modeling, soil analysis, ground and surface water monitoring, and a suite of geophysical surveys (EM31, land and in-river TEM, and airborne FDEM and TEM).
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Problems and Possible Solutions of Geophysics in Eurocode 8 Based Soil Classification
Authors P. Tildy, L. Hermann and B. NeduczaThe 8th part of the European design standard series — Eurocode 8 (EC 8) was published in 2004. The structural Eurocodes are a set of unified international codes of practice for designing building, which will eventually replace national codes in the European Community. The base of the earthquake resistant structural design in EC 8 is the seismic action i.e. the response spectra of a single freedom system normalized by the ag value. The shape of the response spectrum of an earthquake motion is in close connection with the dynamic properties of the soil. Thus soil characterization is essential in defining the seismic action. In EC 8 the soil classification is based on the properties of the top 30 m of soil mainly on the average shear-wave velocity of this column. The characterization of this column is a near surface problem in geophysics, but need expensive ground investigations for civil engineers. In the last decades several geophysical methods and techniques, mainly (active and microtremor) surface wave ones were developed, which can help to avoid boring drills. However these methods have some limitations which can overcome by combining them, and by using geologic information and new processing techniques for archive geophysical data.
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Combination of Near Surface Geophysical and Geotechnical Methods for Exploring Construction Sites
Authors K. Marschall, U. Werban, E. Reboulet, S. Linder, M. Hirsch, H. Paasche and P. DietrichThe application of seismic, DC geoelectric and GPR technique in combination with CPT-surveys has been tested in order to describe the subsurface structures of construction sites for traffic and transportation systems more effectively and reliable. First field tests are carried out at a test site. 15 CPT surveys were done along a profile of 700 m length. The results show that the field is characterized by a channel structure of different sediment and a changing stiffness of material in the deeper underground. Afterwards a seismic survey was done with a land-streamer (4.5 Hz geophones). The data were generated into shear wave velocity (Vs) profiles by MASW (Multi Channel Analysis of Surface Waves). The analysis of shear waves display the layering in the shallow underground and the changing of stiffness in the deeper ground. The DC geoelectric and GPR are more sensitive for the anomaly caused by the channel. The results show that the applied geophysical methods are suitable to display geotechnical relevant structures at our test side. In future, geophysical investigations will be evaluated as a prerequisite for reliable and less expensive geotechnical surveys. Furthermore, the derivation of geotechnical parameters from geophysical investigation will be tested.
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Microtremor and Resistivity Studies for Evaluating Ground Conditions in Canakkale
Authors A. Demirci, M.A. Kaya, T. Bekler and Y.L. EkinciIn this study, we determined electrical and dynamic properties of surface geology by electrical resitivity at 42 stations and microtremor measurements at 46 stations. Interpretation of direct current resitivity data presents that a resistive structure (~150 ohm-m) exists between Canakkale strait and Sarıçay basin, extending towards western part of the study area. The resistivity values decrease towards the eastern side. On the other hand, according to microtremor measurements, predominant periods were determined in the range of 0.15-1 sec. and the period with the value between 0.5 and 1 sec., which points out soft alluvial layers in the middle of the Sarıçay basin. Additionally, small period values were observed at the northern side, where topography is high and consist of older geological unit than alluvium, and this indicates relatively compact zones.
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Comparison between Geoelectrical Imaging and Tunnel Documentation
Authors B.E. Danielsen and T. DahlinThe documentation from the use of a tunnel boring machine (TBM) is used for the evaluation of geoelctrical imaging. Rock type, RQD and weathering are visually compared with resistivity data extracted from different levels in the inversion result. The three types of resistivity, i.e. high, low and intermediate resistivity, can be correlated to certain types of rock mass conditions. The comparison shows that high resistivity corresponds well with good quality gneiss. The tendency is that low resistivity corresponds to a varying lithology with fractured contacts or merely rock with very poor quality (RQD < 25). The intermediate resistivity agrees often with areas with amphibolite having a dominating RQD of 50-75 (good quality). Even though there are exceptions from this tendency the evaluation has given valuable information about how to use the resistivity data in the future work in the tunnel. Previously there has been a large focus on the low resistivity zone in order to find poor rock conditions. But the comparison has shown that the high resistivity areas are interesting because they give a fair chance for good quality rock. This is just as important for the contractor to know as the location of rock with poor quality.
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The Resolution of Lithological Contacts through the Interpretation of Electrical Tomography Profiles
Authors E. Aracil, A.B. Espinosa, J.A. Porres, L. García-Castillo and S. IbáñezThe use of geophysical techniques as method for examining the subsoil has increased last decades. Probably, Electrical Tomography is the most versatile method of geophysical prospecting for making analyses of subsoil and it allows to work with variable depths and to carry out a lot of objectives.It is necessary to do a very simple and easy procedure for obtaining resistivity images of subsoil; nevertheless it is very difficult and complex to interpret them. In this case, the study is focus in the applicability of Electrical Tomography in the determination of lithological contacts by means of the interpretation of electrical tomography profiles. This abstract presents the experimentation that shows how with a correct interpretation of the electrical tomography profiles can be deduce from the surface, in a non-destructive way, the location, the depth and the approximate size of every material that make up the subsoil.
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