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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
1 - 100 of 144 results
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Shear Wave Reflection Landstreamer Technology Applied to Soil Response Evaluation of Earthquake Shaking in an Urban Area
Authors A. J-M. Pugin, J.A. Hunter, D. Motazedian and G.R. BrooksShear wave seismic test locations and boreholes have outlined a buried bedrock valley in the Ottawa, Ontario, suburb of Orleans. This region is in a significant high seismic hazard zone, and the surficial materials are primarily high water-content, poorly compacted Holocene-age Champlain Sea sediments. Since the in-filled sediments exhibit an extremely low average shear wave velocity (~200 m/s) and the bedrock beneath exhibits shear wave velocities on the order of 2500 m/s, it is suspected that the buried valley may give rise to three-dimensional ground motion amplification phenomena in the event of significant earthquakes. In order to better define the buried valley and to prepare for future three-dimensional shake modeling, shear-wave reflection landstreamer lines were acquired, to confirm the depth to bedrock, and to detail its shape. Despite the large broad-frequency band of ambient traffic noise, it was possible to observe reflections from the bedrock as well as additional infra-overburden reflectors. The valley shape and its internal structure, as determined by these surveys, will form a vital contribution to the three-dimensional interpretation for soil response to earthquake shaking in the Ottawa area.
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Non-contact Surface Wave Testing While Moving
More LessWe present experiments with a rolling multichannel microphone array where measurements can be taken continuously while moving. Leaky air-coupled surface waves are measured with ordinary non-directional audio microphones. Results show that microphones can be successfully used to produce a realistic phase velocity spectrum even while moving along the surface. The paper presents some theoretical background along with initial experimental results using the moving microphone array.
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Combining Multiple Seismic and Ground Penetrating Radar Techniques to Analyze Geologic Controls of Riparian Meadow Complexes in the Central Great Basin, Nevada USA
Authors G.S. Baker and K. SturtevantRiparian meadow systems in the Central Great Basin are of interest because they support the majority of ecosystem diversity in the region. The riparian meadows are highly dependent upon groundwater levels, thereby making them vulnerable to fluctuations. These systems are actively degrading due to incision of the streams, resulting in a lowered water table and modification of the associated ecosystems. Geologic controls, such as bedrock geometry and sediment variability, are important in the meadows because of their control on the overall system. The current hypothesis is that the sediments associated with side-valley alluvial fans and fault-related bedrock steps interact to constrict ground water flow. Seismic reflection data and seismic refraction tomography data were collected to analyze bedrock structure and topography from 10 to 80 meters depth, while ground penetrating radar (GPR) data were collected to determine the stratigraphic variability in the upper 10 meters. These data were integrated to provide a comprehensive interpretation of the upper 80 meters of the subsurface. Seismic reflection data were processed to identify the bedrock surface. This surface was then correlated with the seismic refraction tomography to extend the bedrock surface across the meadow complexes. The large volume of GPR data were interpreted by classifying radar facies based on the characteristics of the radar reflectors. These facies (in conjunction with borehole information) confirm and extend areas of alluvial fan related sediment distribution. Integration of these three geophysical techniques is advantageous because they provide more information than could be obtained with the individual techniques.
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Mapping Salt-loads of the Murray River, Australia, Using Airborne and In-river Electromagnetic Methods
Authors A.D. Fitzpatrick, T.J. Munday, V. Berens, M.A. Hatch and A.L. TelferThis paper examines the potential of using the FDHEM RESOLVE system as basis for mapping reaches of the river that contribute to elevated salt loads in the Murray River to the south east of Mildura in Victoria. The advantages of the airborne systems become more apparent when data coverage and acquisition costs are considered, particularly in a situation where a parallel swath approached is employed. This entails the acquisition of adjacent lines of EM data along the centre and along the margins of the river. We suggest this approach provides for a better understanding of recharge and discharge processes and links between the floodplain and the main-river channel. Compared with data acquired along the river alone, this study demonstrated our ability to use Helicopter EM data to map losing and gaining (from a salt load perspective) stretches of the river and to provide insight into which parts of the groundwater-floodplain system were significant contributors to river salt loads. The rapid acquisition of airborne EM data makes these systems more suited to providing temporal snapshots of a river-floodplain environment during dramatic climatic events, such as flooding.
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Efficient Datasets – An Alternative Approach Analysing the Data Space
Authors M. Mueller-Petke and U. YaramanciThe quest and need for optimisation of geophysical data sets either to improve the resolution or the field work (or both) is of growing interest for geophysical applications. So far, the approach to solve the task is to analyse measures derived from the model resolution matrix and adopt the dataset iteratively. Here we present a direct method to estimate an efficient dataset. The approach is based on analysing the data space, i.e. calculating the data resolution matrix. Essentially variable quantities of data errors are taken into account to define the necessary amount of independent unique measurements. Applied to the technique of Magnetic Resonance Sounding (MRS) we show that not only the singular value distribution of the forward operator is improved but also the resolution, i.e. the inversion result of the efficient dataset compared to a logarithmic and linear spaced.
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Preserving Horizontality Using Topography Corrected Laterally Constrained Inversion
Authors A.V. Christiansen, E. Auken, H.Ø. Andersen and K. SørensenGeophysical dataset from areas with substantial topography cause problems to many inversion routines. Commonly, the cells or layers of the model follow the topography which then favors layers boundaries of the inverted model that are topography-parallel. In many cases the geological layering is not connected to the overall topography, e.g. an erosional environment over a horizontal sub-stratum. Inverting data from such areas are penalized if the topography is not taken into account when dealing with the constraints We present a simple way of correcting the constraints entering the roughness matrix of the inversion problem so that topography is neutralized and horizontal layering is favored. Synthetic modeling confirms the superiority of the topography neutralization over traditional depth referenced inversion. Field examples will be shown in the talk.
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Geophysical Investigation of a Diatreme Structure Near Ebersbrunn/W-Saxony in Germany
Authors H. Schütze, C. Kroner and T. JahrFrom 2003 to 2006 an extensive geophysical survey, including gravimetry, magnetics, and electromagnetics, of the structure characterized as a tuff chimney was carried out. This fossile fluid migration zone which is located near Ebersbrunn/West-Saxony in Germany is afresh of geoscientific interest as it is near an area in which unexpectedly swarm earthquakes occurred in the last years. From the gravimetric survey an elliptical Bouguer-anomaly emerges with a gravity minimum of -3 mGal and a diameter of about 1000 m. In agreement with the meas-urements of the vertical intensity in the 1960s a dissected anomaly of the magnetic total field intensity is found. The anomalies are characterized by variations of several 100 nT with al-most no recognizable minima. The degree of weathering appearently affects the presence of maxima in the total field since visibly reduced magnetic susceptibility values were found in areas in which strong weathering occurred. From the conductivity map the diatreme border could be derived which beforehand was deduced from near-surface wells. By a 3d gravimetric and magnetic modeling including the results from the other geophysi-cal surveys as well as information from geological and petrographic studies, core analyses, and well logging, details on the diatreme structure could be determined.
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Cooperative Inversion Based on Fuzzy c-Means Cluster Analysis – Application to Field Data
Authors H. Paasche, J. Tronicke and P. DietrichIn many near surface geophysical applications it is now common praxis to collect co-located disparate geophysical data sets. The advantage of such multi-method based exploration strategies lies in its potential to reduce ambiguities and uncertainties in data analysis and interpretation. To reconstruct the physical parameter distributions underlying geophysical data requires in many cases the application of tomographic reconstruction techniques. To improve the reliability of tomographically reconstructed physical parameter models, the information content of all co-located data sets should be considered during the model-generation process. Here, we apply a novel approach based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of crosshole seismic P-wave and S-wave traveltimes. The approach results in a single zoned two-parameter model outlining the major subsurface units and provides additionally P-wave and S-wave velocity fields.
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Auxiliary Results of Collection and Classification of Surface Geoelectric Arrays
More LessRecently, we have made a classification of more than one hundred various surface geoelectric arrays ever published in geophysical literature (Szalai and Szarka 2007a, 2007b). The classification is based on three divalent parameters (as “superposition” of measurements, “focusing” of currents and “colinearity” of the array), thus we set up eight groups of geoelectric arrays. One further group was separated for about 10 so-called “composite” arrays, which cannot be classified in the aforementioned way. Here we present some application examples of the classification results. Namely, we call the attention to some hidden relationships among geoelectric arrays: (1) we give an illustration how various arrays can be derived from their root array (besides the Schlumberger-related arrays several other examples will also be given in the presentation); (2) we provide a summary of arrays, capable to measure various partial derivatives of the electric potential. Among the 21 arrays 14 are already published arrays, but there are seven possible, but not-yet-applied arrays. In this way, such missing links in the genealogic trees may lead to creation of reasonable and purposeful new arrays.
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Modelling and Measuring Shallow Inductive Electric Field Response with Capacitive Sensors
Authors C.H. Adams and J.C. MacnaeModelling software developed shows that a new Capacitive Array Resistivity with Inductive Source (CARIS) method has potential for detecting both conductive and resistive buried objects. Detecting buried resistive objects has possible application in UXO, archaeology, civil or engineering applications, void detection, pipe delineation, or fracture detection and near surface geology. A prototype CARIS instrument has been designed and built. The prototype CARIS instrument has been tested with highly repetitive success under laboratory conditions, showing good comparison with expected results from modelling. The CARIS system has also been tested in the field. Initial results from field testing show high repeatability but also high lateral variability. This appears to be due to sensitivity to near surface moisture and consolidation contrasts.
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Incorporating Topography into 2D Resistivity Modeling by Using Finite-element and Finite Difference Approaches
Authors E. Erdogan, I. Demirci and M.E. CanadansayarIn this study, we examine the topographic effect in two-dimensional dc resistivity forward solution. A new modeling algorithm is developed which is capable of solving Poisson’s equation by using finite-difference (FD) and finite-element (FE) approaches. In this algorithm, topography can be incorporated into the model by using three different methods with FE approach and by using two different methods with FD approach. We compared these various methods on different models of surface geometry and resistivity distribution regarding their accuracy and CPU time.
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Incorporation of Topography into 2D Resistivity Inversion Models by Using FD Modeling with Triangular Discretization
Authors I. Demirci and M.E. CandansayarWhen interpreting direct current resistivity data collected on rough terrain topographic effects should be incorporated. We developed a new 2D inversion algorithm for dc resistivity data in which topography is incorporated in to the model using a finite difference technique.
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Efficient 2D Inversion of Long ERT Sections
By P. TsourlosIn this work a new algorithm for the efficient 2D inversion of long ERT lines was introduced. The algorithm incorporates an experimental procedure to avoid calculation and storage of the entire Jacobian matrix. This approach speeds up the Jacobian matrix calculations and also reduces the required memory resources to store it. The efficient storage of the sparse Jacobian and Smoothness matrices and the efficient inversion using the LSQR method increase significantly the inversion speed. The application of the new algorithm to synthetic and real data sets resulted in reconstructed models of comparable accuracy to the standard approach.
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Roll-along Inversion – A New Approach for Very Long Dc Resistivity Profiles
By T. GuentherIn some cases elongated dc resistivity profiles are measured either continuous or by a roll-along technique. The interpretation in one step is usually a time consuming numerical task with large memory requirements. We present a new approach for a subsequent solution using segmentation and sophisticated constraints. It proves to be very efficient for large segment numbers and can be used on standard pc's. We show two field data examples: First, data from a flood-protection measured with the roll-along technique are treated. Thus it is possible to distinguish cohesive and sandy zones in the dike as well as in the substratum. The second examples uses data collected by a sea-bottom towed multi-electrode cable with continuous data acquisition. The result provides valuable insight into the sedimentation structures.
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3D Effects on 2D Resistivity Imaging – Modelling and Field Surveying Results
More LessToday resistivity surveying plays an important role in many large-scale area investigations. The existence of 2D effects on 1D resistivity modelling is a well known problem; however, former studies show that 3D effects in 2D surveying are less evident. The results presented here show that, nevertheless, there is an advantage in performing 3D inversion. A comparison between 2D inversion and 3D inversion has been made with analysis of data from two different synthetic models and three field datasets. From the synthetic study it is clear that 3D inversion give higher contrast, less inversion artefacts and better model recognition than 2D inversion. From the field studies it is also evident that 3D inversion gives models with higher contrast. With only limited ground truth data it is not always possible to determine which model is closest to the true one; however, where ground truth data is available it is clear that the 3D inversion gives a better result. In addition we show that the choice of array configuration have a significant influence on the result, with gradient array generally giving better results than the other options.
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Application of 3-D ERT for Enhanced Subsurface Imaging
More LessIn the geotechnical and environmental engineering, high resolution imaging technologies are required and 3-D ERT method is commonly used to obtain 3-D image of subsurface with quite high resolution, especially when the vertical structures such as fault or fractures are main target. In this paper, we show several examples of 3-D ERT imaging. The survey areas in the examples are (1) Granite quarry mine, (2) Abandoned lead-zinc mine, and (3) Test boreholes for fracture system evaluation. In the first example, by combining 3-D ERT with borehole radar reflection imaging, we could obtain 3-D attitude of fractures in a granite quarry to help the future mining plan. When the subsurface shows strong electrical anisotropy, we could obtain reasonable 3-D image of subsurface, which was used in safety analysis of subsurface. In the last example, by including the 3-D borehole deviations, we could obtain more accurate image of subsurface in the 2-D ERT case with the help of 3-D ERT program. These successful 3-D ERT results demonstrate that 3-D ERT is a very powerful method that can be used for wide application areas to obtain reliable 3-D earth images.
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Time Domain Electromagnetic (TEM) Measurements on a Buried Subglacial Valley in Northern Germany
Authors B. Tezkan, H. Mufor, S. Helwig and R. BergersBuried subglacial valleys are important groundwater reservoirs in Northern Germany. However, relative little is known about their exact locations, their sizes and the internal structure of their sediment fills. Due to the presence of a conductive clay layer at about 40m depth within the Bremerhaven-Cuxhaven valley, no information about the structure below this layer could be obtained by conventional ground TEM, SkyTEM and helicopter electromagnetic measurements. To overcome this problem, the TEM method in the central loop configuration was used with a large transmitter size of 400m by 400m and a high current of 12.7A for early times and 20A for late time measurements. Five soundings could be realized and the data were interpreted by means of OCCAM and Marquardt type 1D inversions. The inversion results were compared with SkyTEM, HEM and seismic sections of the buried valley. A conductive clay layer was found between 30 and 83m within the valley and the bottom of the valley was determined to be at a depth of 285 to 309m
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Frequency or Time Domain HEM Systems for Defining Spatial Processes of Salinisation?
Authors T.J. Munday, D. Sattel, A. Fitzpatrick and J. ReidAirborne EM methods have considerable potential for providing detailed spatial information on the distribution of salinity in soils and groundwater, across the floodplains of the Lower Murray River in southern Australia. This potential is examined along with the relative merits of high resolution airborne electromagnetic technologies, specifically the RESOLVE frequency domain helicopter EM (FDHEM) and the SkyTEM time domain helicopter EM (TDHEM) systems. For a coincident area, these two data sets were inverted for conductivity and depth using a smooth model Occams inversion, a conductivity depth transform (CDI) obtained using EMFlow and a Laterally Constrained Inversion (LCI) technique for defining variations in near surface conductivity and sediment salt load . Results from the two systems are comparable, both indicating the presence of an extensive flushed zone adjacent to the Murray River, and identifying finer scale variations between losing and gaining groundwater on the floodplains adjacent to the Murray River. Both systems effectively map the near surface conductive Blanchetown Clay in the adjacent highlands and the high saline groundwater system at depth
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3D Inversion of Time Domain Data with Integrated Application to Mineral Exploration
Authors D.W. Oldenburg, E. Haber, S. Napier and R. ShekhtmanWe present a 3D time domain inversion algorithm. The forward problem is solved using finite volume methods in the spatial domain and an implicit method (Backward Euler) in the time domain. A modified Gauss-Newton strategy is employed to solve the inverse problem. The practical modifications include the use of a Quasi-Newton method to generate a preconditioner for the perturbed system, and implementing an iterative Tikhonov approach in the solution to the inverse problem. In addition, we show how the size of the inverse problem can be reduced through a corrective source procedure. We invert UTEM data at San Nicolas and compare the results to previously obtained images from gravity, magnetics, and IP and with geologic information from drill holes. A consistent interpretation for the location of a massive sulfide is achieved.
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An Approach to Calibrate EMI Multifrequency Sensor
Authors E.V. Balkov and Y.A. MansteinNowadays electromagnetic induction (EMI) methods are common approach to solving a wide range of ecological, geotechnical, archaeological and agricultural shallow depth problems. For the determination of apparent and true values of ground resistivity the device calibration should be performed. Usually calibration is a single experiment above a known conductive half-space or its simulation by metal coil. But such a way is insufficient for a multifrequency sensor calibration which action can not be explained in the terms of low frequency approximation. The paper explains the new calibration approach. It uses a minimization procedure for oversaturated system of experimental data to make possible finding and correcting quite a large amount of device parameters. Its values are needed to be precisely known for the quantitative data processing. The practical application for the existing portable multifrequency sensor NEMFIS is shown as well.
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Model Analysis of Cross-hole Resistivity Tomography for Void Detection
Authors K. Leontarakis, G. Apostolopoulos and C. OrfanosModel experiments in the laboratory were used to find the best measuring parameters which affect the quality of results for the improved application of resistivity tomography between boreholes. Plastic tubes of various diameters were used to simulate the boreholes and the tunnel’s void in a big plastic tank with non-ionized water. The experiments have shown that a void can be detected using various diameter sizes and depths, and that the resolution is dependent on the distance of the target from the drillings, the distance between the drillings, the electrode array, the use of various and certain dipole lengths and distances, the measuring protocol, and the extended resistivity model. In general, sensitivity analysis is the main guide.
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Resistivity Monitoring for Assessing Cement Grouting Effect
More LessIt would be important to investigate the actual grouting performance and to examine whether the original purpose of the grouting work would be achieved. Geophysical imaging technique has a high potential in the evaluation of the grouting effect as a non-destructive and cost-effective way. In order to study the applicability of resistivity monitoring technique to the assessment of cement grouting effect, we carried out DC resistivity monitoring before, during and after grouting work conducted for the reinforcement of weak ground. In accordance with the field monitoring, we also monitored the resistivity change of cement mortar specimens in a laboratory. The laboratory monitoring results clearly showed that electrical prospecting methods are good to locate the cement grouted zone. The field monitoring data were inverted by the four-dimensional (4-D) inversion algorithm and difference images were reconstructed. Owing to the 4-D inversion algorithm, we could more easily and clearly identify the decrease of resistivity in the region where the grout material was injected and infiltrated. Through the monitoring experiments at a field site as well as in a laboratory, we were able to know that DC resistivity monitoring technique is an effective way for evaluating the cement grouting performance.
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Detecting Linear Features Using the Directionality of the HLEM Response
By D.C. NobesThe horizontal-loop electromagnetic (HLEM) response is sensitive to the orientation of the instrument relative to buried objects. Linear features have maximum responses when they are are oriented perpendicular to the long axis of the HLEM instrument, especially for features with relatively sharp boundaries, such as pipes and archæological targets. Thus, HLEM responses are often acquired using two orientations, parallel and perpendicular to the survey line direction, and the responses are then combined, usually by summing or averaging the bi-directional responses. If the ratio or the difference between the two responses are used instead, the contrast between the directionality of any linear anomalies is emphasised. Two examples are presented to illustrate the utility of this approach: (1) an early to mid-19th century archæological site; and (2) a soccer pitch where shallow drainage pipes and an old stream channel are located. In both examples, the quadrature response for the two orientations, separately and averaged, shows indications of linear features, but the response from other features, such as buried stream channels, obscures the linear anomalies. When the ratio or the difference are used, the object locations become clear and correlate well with results from other methods.
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Imaging the ROOT Cause of Subsidence Using Electrical Resistivity Tomography
Authors G.M. Jones and N.J. CassidyThroughout the world, ground movement caused by moisture content variation in expansive clay soils is a major problem. In southern areas of the UK particularly, where major urban areas coincide with outcropping clay, subsidence and the resulting structural damage caused by clay shrinkage is at the forefront of both the public and insurers concerns. The principal cause of such damage however, has not only been attributed to expansive clays but more increasingly to the influence urban trees have in promoting clay subsidence through water uptake, especially during times of drought. At present the diagnosis of tree-induced subsidence (TIS) involves verifying clay desiccation via testing soil index properties (i.e. Atterberg Limits) and comparing soil water and strength profiles at points proximal and distal from the tree. These methods however, are costly, difficult, and sometimes unreliable and ultimately provide a very limited and often incomprehensive understanding of tree induced subsidence, especially in the long term. As part of an interdisciplinary study of this system, a geophysical approach is taken to bridge the knowledge gap by using Electrical Resistivity Tomography (ERT) to provide a more comprehensive understanding and visualisation of the interactions between trees, moisture movement and volume change in clays.
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Delaunay Triangulation-based Spatially Constrained Inversion for Quasi 3D Modeling of Airborne TEM Data
Authors A. Viezzoli, A.V. Christiansen and E. AukenThe spatially constrained inversion (SCI) is a robust methodology for quasi-3D modeling of geoelectrical and EM data of varying spatial density, using a 1D forward solution. Information migrate horizontally through spatial constraints applied between nearest neighboring soundings, and allow resolution of layers that would be locally poorly resolved. The constraints are built using the Delaunay triangulation, which ensures automatic adaptation to data density variations. In this study the SCI was applied to airborne Time domain EM soundings, but it can be implemented also with other data types. The field case study proves that the SCI produces laterally smooth results that respect the 3D geological variations of sedimentary settings, while suppressing the elongated artifacts commonly seen in profile oriented data sets.
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Deriving Biophysical Products from Airborne Electromagnetic Data
Authors K. Cahill and A.D. FitzpatrickConcerns by the Angas Bremer Irrigators over the future expansion and sustainable use of water resources in the Angas Bremer Plains of South Australia has driven a number of recent initiatives aimed at trying to understand the complex interaction between surface and ground water and, in particular, the effect this has on salinity of these resources and the potential for irrigation-induced salinisation of the region. Among these initiatives is the development of the Angas Bremer Integrated Systems Model which aims to better define strategies for a truly conjunctive approach to water management from the paddock to catchment scale. Key to the successful development of that model is the availability of good biophysical data which describe the spatial variability of salinity and water quality across the region, most particularly in the near surface (<30m). Airborne electromagnetic data have the potential to assist in delivering this information, but they require careful, constrained interpretation and analysis, if they are to deliver products with this information.
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Spatial Patterns and Processes of Floodplain and River Salinisation – Sunraysia, Victoria, Australia
Authors A.D. Fitzpatrick and T.J. MundayThe Sunraysia irrigation region of Australia supports a nationally significant citrus, vegetable production and horticulture industry. This industry is dependent upon the Murray River supplying water for irrigation, household use, livestock consumption, and industrial water use. Salinisation of the Murray River is a significant threat to the sustainability of the irrigation resource for this region. Natural inflows of highly saline groundwater to the Murray River have been exacerbated by irrigation development and adjacent clearing of native vegetation and the development of dryland agriculture within the vicinity of the river. Airborne electromagnetic methods have the capacity to provide detailed spatial information to assist in the prediction of the impact of current and future irrigation developments, the design of salt interception schemes and protection of floodplain values. These issues are central to the regional and state strategies for the River Murray. The RESOLVE survey of the Sunraysia region has demonstrated that floodplains can exhibit varying conductivity, both laterally and with depth, over fine scales. The data from this airborne survey is being used to produce key land use management tools.
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Influence of the Parameterization and a Priori Information on the Inversion of Surface Waves – Parametrical Study and Application
Authors M. Wathelet and D. JongmansInversion of Rayleigh dispersion curve does not provide one unique solution and the choice of the parameterization (number of layers, range of velocity and thickness values for the layers) is of prime importance for obtaining reliable results. The number of solutions can be reduced by a priori information (geologic log), as well as by the use of other sets of data, which can be easily introduced in the inversion process using direct search inversion techniques. The method is illustrated on both synthetic and real cases.
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Determination of Dispersion Curves from 3D Seismic Surveys
Authors B. Neducza, E. Törös, P. Tildy, Z. Pronay, A.I. Kanli and L. HermannThe presented method is a modification of the well-known SASW method. The algorith is tested on a real dataset measured above a refilled mine pit. Phase velocity maps are generated directly from shot records. Local dispersion curves are determined by the most frequent values instead of using tomographic inversion. Finally, the phase velocities are plotted on velocity maps at each frequency.
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Modeling of R-wave Dispersion Curve by Using Fast Delta Matrix Method
Authors E.E. Ekincioglu and S. KadiogluThis paper is based on computation of fundamental mode Rayleigh wave (R-wave) dispersion curve depending on layered earth model by using fast delta matrix method. In addition, this paper indicates the changes in the dispersion curve and phase velocity according to the model parameters, thickness, density, P and S-wave velocity. In this study, we introduced the mathematical solution of the fast delta matrix method which computes free mode dispersion of a layered earth model. We modelled dispersion curves for two earth model and figured out only the fundamental mode. First, we calculated dispersion curves for by increasing each parameter by 25% for a six layered earth model. Secondly, we examined changing in the thicknesses, densities, P and S-wave velocities for a two layered model. Result showed that variation in S-wave velocities has an important effect on R-wave phase velocities when it is compared with other effects of model parameters. However, this doesn’t mean that other parameters are not affective. When individual changing of the parameters examined, it is seen that all parameters have a role in changes of phase velocity and dispersion curve according to the frequency range.
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An Integrated Wave Equation Datuming Scheme for the Overthrust Data Based on the One-way Extrapolator
Authors K. Yang, F. Jiang, J.B. Cheng and Z.T. MaThe wave equation datuming (WED) is a necessary technique to deal with datuming correction in areas where the surface-consistent assumption is not valid. The previous applications of WED usually downward or upward continue the data from the surface to a planar datum. But for overthrust-belt data, it is always hard to find a planar base for weathering layer (WL). So the wave-equation layer replacement has to be done under the assumption that the base of WL is non-planar. In this paper, an integrated WED scheme applicable to the overthrust data is proposed. Firstly, the wave-field is downward continued to a non-planar base of the WL to remove the unexpected effects caused by the WL. Then, an upward continuation is applied to extrapolate the wave-field to a final planar datum with the replacement velocity. Either the downward or the upward continuation are accomplished by the one-way extrapolator such as finite-difference (FD) operator in a recursive way which constitutes an integrated wave-equation layer replacement scheme.
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Imaging of Different Moisture States Inside an Undisturbed Soil Monolith with Georadar Tomography
Authors J. Schmalholz, M. Heigel, U. Yaramanci, J. Köstel and A. KemnaFour different moisture states inside an undisturbed soil monolith, in our case a lysimeter, (cylindrical vessel of 1.2 m diameter and 1.5 m height) are imaged with georadar tomography. Two of the moisture states correspond to different steady state flow conditions which were adjusted by means of an irrigation device. Due to the all-around accessibility of the investigated lysimeter high resolution horizontal tomographic planes can be acquired. In this way, the lateral heterogeneity inside the lysimeter is investigated non-invasively, which is new for soil physical investigations. Based on the tomographic planes distinct lateral and vertical heterogeneous volumetric water content distributions are identified. According to the tomographic results lateral volumetric water content differences of up to 0.1 m3/m3 are encountered. In the vertical direction, volumetric water content changes of more than 0.1 m3/m3 are detected. This reflects the presence of different soil layers. Due to the different response of specific regions inside the lysimeter subject to the moisture state, supplementary information on the soil hydraulic properties is gathered.
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Attenuation and Velocity Characteristics of Nano-to-micro Scale Magnetite Mixtures – Implications for Near-surface GPR
By N.J. CassidyFor many higher-frequency (500-2000MHz) near–surface ground-penetrating radar applications the magnetic properties of the subsurface are considered unimportant or negligible. However, if significant amounts of magnetically lossy minerals are present, such as magnetite, hematite, maghemite and/or iron in its free state, the influence of these individual components on the GPR signal attenuation or propagation velocity cannot be ignored. In an attempt to obtain an element of quantitative control on the attenuation/propagation characteristics of such materials, the results of a dielectric spectroscopy investigation into mixtures of quartz and micro-to-nano scale magnetite is presented where the relative attenuation enhancement (in decibels per metre – dB/m) and velocity retardation (in percent velocity difference) is given for a range of volume percent, ‘generic’ magnetite/quartz mixtures. The results indicate that even small amounts of magnetite can produce significant degrees of loss and velocity reduction and at the higher frequencies associated with near-surface GPR, any observed dispersion behaviour will be predominantly controlled by the magnetic loss characteristics rather than frequency-dependent variations in propagation velocity.
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GPR Survey in Coastal Sambaqui Archaeological Site in Santa Catarina State, Brazil
Authors S.I. Rodrigues, J.L. Porsani, P.A.D. DeBlasis and P.C.F. GianniniIn this work some GPR results obtained in coastal sambaqui Jabuticabeira II are presented. Jabuticabeira II is located in Santa Catarina State, South of Brazil. GPR profiles show interesting anomalous reflectors, which can be related to targets of archaeological interest. 2D FDTD modelling studies presented good accordance with real data obtained in the field. Synthetic model has been constructed in order to evaluate the performance of GPR method in relation to materials found in that environment, besides helping interpretations of real results. So, GPR results function as reference guide to direct excavation activities in buried archaeological, and geological structures, a task that will be the next step of this research.
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Numerical Modelling of Commercial GPR Antennas
Authors C. Warren and A. GiannopoulosThe use of Ground Penetrating Radar (GPR) has become widespread in a range of fields - civil engineering, geophysics, environmental engineering, archaeology, and remote sensing. This has led to the development of increasingly sophisticated GPR models to help progress understanding of how to detect, interpret and identify buried objects. The transmitter and receiver antenna(s) are a key component of any GPR system, but are often not accurately described and therefore not well modelled in many GPR simulators. This research aims to improve understanding and interpretation of GPR signal attributes, by incorporating accurate descriptions of commercially used antennas into GPR models. A methodology is demonstrated for creating 3D FDTD models of two real GPR antennas. It is shown that when the, often complex, geometry is accurately represented in the FDTD model, a good agreement can be obtained between the simulated results and those measured directly using real GPR transducers.
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Geophysical Study of Salinity Distribution in Northeastern Thailand Paddy Soils
Authors J.-P. Montoroi, O. Grünberger, N. Bouzid, Y. Enet, S. Sukchan, K. Srisuk and S. NadeeSalinization process is a worldwide constraint for soil management especially in Northeast Thailand, region characterized by widespread, deep and thick continental salt deposits. The near-surface distribution of salinity is performed using ground geophysics methods including time domain electromagnetic, frequential domain electromagnetic, vertical electrical soundings and self potential. The vertical and horizontal mapping of the electrical resistivity indicate a very conductive anomaly, situated at nearly 10 m depth owing to a 3-4 m thick clayey layer. Electrical conductivity mapping made in rainy season gives better details than the one made in dry season. Self-potential contrasts are clearly shown and are mostly due to electrodiffusion potentials. A conceptual model of the hydrosystem functioning was proposed at a local scale including two groundwater systems namely a deep saline groundwater flowing up through fractures and a shallow saline groundwater overlying a low-permeable clayey layer.
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Characterization of Contaminated Sites by Resistivity and IP Methods
Authors Z. Nyari and B. NeduczaA consortium with three partners from different fields of environmental sciences (Geophysics, Hydrogeology, Geotechnics) was formed for a three-year-long project to improve hydrogeophysical methods in order to detect and characterize special subsurface contaminants. Contaminated sites were chosen in Hungary as study-areas to improve and calibrate special geophysical methods to provide remediation experts and hydrogeologists with necessary information for reliable transport modeling. A strong collaboration between the geophysicists and hydrogeologists evolved protocols and techniques to carry out successful site assessment and remediation schemes of contaminated lands. In this multi-disciplinal project the results provided by the geophysicists were immediately applied and controlled by the end users. This study presents two case histories on detailed geophysical survey of two contaminated areas. At both sites high resolution techniques providing reliable data were applied. The results were validated and accepted by hydrogeologists and applied during the remediation processes.
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Geophysical Investigation of the Bosherston Lily Ponds, South Wales, UK
By C. HusbandThe Bosherston Lily pond is a man made water body situated in Pembroke, South Wales, UK. Created in stages between the late 18th and mid 19th century, the lily ponds were formed by damming a river valley. The ponds are now maintained by the National Trust and have been classified under the EC Habitat Directive as a Special Area of Conservation (SAC). The lily ponds are of great importance not only due to the diverse ecology but because of the significant financial income generated in the area by tourism. The lily ponds exist due to a delicate balance of influxes and outflows; any change could lead to the lily ponds demise. It has been noted that the summer level of the lake has decreased over the last 80 years. It is believed that the water is seeping away through cracks and crevices in the underlying carboniferous limestone. The aim of the geophysical survey was to locate possible conduits which would explain the decrease in water level. A combination of geophysical techniques were employed in order to comprehensively survey the subsurface; these included electrical resistivity arrays, ground penetrating radar and self potential surveys.
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Using GPR to Monitor Fluid Channelization in a Subhorizontal Bedrock Fracture by Tracking a High-salinity Tracer
Authors G.S. Baker, K. Burns and J.T. PardyTo observe flow channeling in situ, surface ground-penetrating radar (GPR) was used to detect a saline tracer moving through a sub-horizontal bedrock fracture. The tracer is mapped using amplitude variations resulting from the significant difference in electromagnetic properties between the tracer and natural ground water within the fracture. Results show that hydrogeophysical investigations using GPR can successfully image tracer movement in a variety of flow configurations in three spatial dimensions through time (4D). Observed tracer travel paths demonstrated meter-scale spatial variability that may be attributed to heterogeneity in fracture aperture. Subsequent controlled experiments yield empirical relationships between GPR reflection amplitudes and fracture aperture and tracer concentration.
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Realistic GPR Simulation of a Modelled DNAPL Migration – A Novel Link between Two Numerical Models
Authors C. Power, V. Wilson, J.I. Gerhard and A. GiannopoulosIn this paper we present some initial results of our investigations in creating a meaningful and realistic link between two numerical models that can be used to enhance our understanding of the process of migration of DNAPL and of the GPR’s potential for its detection. The aim is to simulate realistic GPR scans using an electromagnetic simulator from realistic subsurface DNAPL spills as obtained using a hydrogeological fluid flow model.
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CR Behaviour of Fully and Partially Brine Saturated Building Stones and FE Modelling of Microstructural Features
More LessIn order to assign the value of complex resistivity measurements to non-destructively infer salt and water damages of building stones an experiment series has been conducted on brine saturated samples varying in type, fluid concentration and water content. CR parameter are evaluated and compared to previous results by other authors. A new image based FE modelling approach has been started to discriminate chemical from microstructural implications on CR spectra.
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Geophysical Exploration of Mining Ponds – An Effective Tool for Their Geochemical Characterization
Authors E. Aracil, P. Martínez-Pagán and Á. FazIn the Murcia Province, there are eighty five mining silt ponds resulting from the treatment and exploitation of ancient metallic sulphur mines. These silt ponds are the potential focus of environmental pollution. The Technical University of Cartagena (Murcia, Spain) and Análisis y Gestión del Subsuelo Company (for geophysics) have been carrying out an integral study of these structures from the year 2003 to the present. In this work, studies from the Brunita and El Lirio mining silt ponds (La Union, Murcia) are described. These have allowed the characterization of different horizons, combining the results obtained through geochemical analysis with data derived from electrical tomography (2D and 3D) techniques. Geochemistry has demonstrated that oxidation and salinization processes are more active in the sediment surface layers than in deeper layers. Acidification has also been found to be more active in upper layers, with analyses showing pH values slightly less acidic, combined with high concentrations of Zn. The results were analyzed with the different electrical profiles, and correlated and corroborated with the geochemical sampling analysis.
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GPR Data Interpretation Using Self Organizing Maps
Authors N.S. Spanoudakis and A. VafidisA systematic workflow is proposed which includes ground penetrating radar data pre-processing and attribute calculation. The attributes are classified using a competitive artificial neural network, namely Self Organizing Maps (SOM). This workflow is applied to 3-D ground penetrating radar datasets acquired with the PulseEkko1000 system from a tank model and the archaeological site of Aptera, Chania, Crete, Greece. This proposed methodology, proved to be useful for distinguishing different pipes from the surrounding medium with high resolution and buried relics.
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GPR Characterization of Concrete Tubes and Electrical Cables Buried in the Test Site at São Paulo University – Applications to Urban Planning and Infrastructure
Authors J.L. Porsani, V.R.N. Santos, M.G. Oliveira and F.Y. HiodoThis paper presents some GPR (Ground Penetrating Radar) results on the first Brazilian test site. The test site was constructed close to Institute of Astronomy, Geophysics, and Atmospheric Science (IAG) at São Paulo University campus (USP), in São Paulo city. Different materials and object sizes were installed at several depths along seven lines. The targets buried simulate geotechnical, environmental, and archaeological studies. 200 MHz and 400 MHz GPR profiles were acquired along two lines containing empty concrete tubes, and electrical cables buried that are usually used in the great cities of whole world. Obtained results show typical hyperbolic reflections pattern characteristic for each target, and permit to estimate target dimension. These results can be used as reference pattern, and can be extrapolated to areas in which information about surface is lacking.
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Subsurface Pipe Detection with an Array of Inductive Broadband Vector Receivers
Authors S. MacIntosh, R. Birken and Q. ZhuAn electromagnetic induction survey was conducted using an array of inductive receivers (AIR) to map and verify a network of water lines in a western city in the USA. Data was collected on a 30 cm by 30 cm grid covering 2800 m2. The array utilizes 16 vector broadband electromagnetic sensors arranged in two rows of eight sensor pods. The position of the system was measured using a robotic laser tracking system which records position information with sub-centimeter accuracy. Multiple active “clamp-on” transmitters were used to induce signals into the water lines and other nearby utilities. Each transmitter operated at an individual frequency in the range of 1 kHz – 83 kHz. In addition, ambient signals, such as electric power transmission frequencies were measured. Detailed magnetic field maps are created for each particular frequency of interests and for any combination of the three magnetic field polarizations. Qualitative positions and depth of current carrying lines can be done by directly assessing the magnetic field maps. Magnetic field inversion methods were used to quantitatively determine positions and depths of current carrying lines.
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2D Inversion of Gravity Data Using Artificial Neural Networks in Order to Detect of Subsurface Cavities
Authors A. Hajian, H. Zomorrodian, C. Lucas, H. Sarooee and N. AminiWe aim to estimate the depth of subsurface cavities from gravity data by a new method of 2D inversion through a Multi Layer Perceptron(MLP) neural network.Infact, this method is an intelligent way to interpret gravity data and gain an estimation of depth and shape. The MLP neural network was trained for two main models of cavities: sphere and cylinder in a domain of radius and depth. We tested different MLP’s with different number of neurons in the hidden layer and obtained the optimum value for number of neurons in the hidden layer. Then it was tested in present of 10% noise(S/N=.1), and also tested for real data. It presented good results for depth estimation of subsurface cavities. Keywords: gravity, 2D inversion, subsurface cavities, artificial neural networks, Multi Layer Perceptron
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GPR and Magnetic Methods Applied to Characterize an Archaeological Site in Central Mexico
Authors R.E. Chavez, D.L. Argote, M.E. Camara-Moral, G. Cifuentes and P. LopezA geophysical survey was carried out in an archaeological site called Los Teteles (bunch of rocks), which covered several areas within the zone. This investigation included high resolution total magnetic field and ground penetrating radar (GPR). Previous archaeological work carried out on a northern terrace in 1974 discovered an ancient tomb. Magnetic and GPR data were collected in this site. Results depicted a very nice N-S oriented dipolar magnetic anomaly. GPR data displayed features related to the location at depth of walls and floors. Also, remains of stairways and platforms were also inferred. The magnetic study revealed interesting magnetic trends related with ancient walls and platforms. An interesting inverted magnetic dipolar anomaly found could be interpreted as the location of a tomb. GPR profiles were taken over some of those anomalies, and a correlation was performed among the interpreted profiles confirming previous interpretation. A 3D visualisation of the GPR data along with the application of an enhancement filter helped to define characteristic features related to archaeological artefacts, as the magnetic results defined. The location of the possible tomb could also be confirmed.
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Geophysical Imaging of an Early 19th Century Colonial Defensive Blockhouse
Authors D.C. Nobes and L.R. WallaceIn 1845, the French navy built three blockhouses as part of their defence of French settlers in the Akaroa area, located on Banks Peninsula, near Christchurch, New Zealand. In the 1860’s, the blockhouses were removed and the timber used for other purposes. Two of the blockhouses were situated at either end of the Akaroa township; the locations are well known and documented. The position of the third, in the neighbouring village of Takamatua, is not well known, but is thought to have been sited in what became a public reserve, first known as the Blockhouse Domain and more recently as the Takamatua Domain. To aid local archaeological studies, non-invasive, non-destructive geophysical imaging was carried out across the Takamatua Domain. We expected that little if any of the blockhouse itself remained. However, the nature of the construction was such that we expected to find the defensive trench that enclosed the blockhouse. Using horizontal loop electromagnetic (HLEM), total field magnetic, ground penetrating radar (GPR), and electrical resistivity tomography (ERT) techniques, we identified coincident linear anomalous responses. The clear and unequivocal results of the geophysical surveys suggest that we have indeed found the blockhouse and its surrounding trench or moat.
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Electrical and Seismic Imaging of Ancient Wall Foundations in Thessaloniki, North Greece
Authors P. Tsourlos, G.N. Tsokas, C.B. Papazachos, G. Vargemezis and P. BogiatzisThis work describes the application and results of an ERT and Seismic survey which was conducted at locations of the western part of the Thessaloniki ancient City Walls (N. Greece) aiming to estimate the extend of the walls’ foundations. The borehole ERT results are in agreement with the existing information but are not in full agreement with the seismic tomography results regarding the exact depth estimate of the foundations. It is possible that a joint tomographic interpretation would be able to resolve this bias and create a single interpretation model for both data sets.
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Amplitude Inversion in Seismic Traveltime Tomography – An Application in Subsurface Imaging at a Historical Site
More LessSeismic amplitude is an important parameter for it carries information about the structure of the subsurface which complements that provided by the traveltimes. However, both the need for a special pre-inversion processing of amplitudes and their complex relationship to subsurface features, has often resulted in a preference to traveltimes. Nevertheless, for relatively confined datasets and use of independent information, amplitudes can readily contribute to a better imaging and interpretation of subsurface features. The present work is concerned with the application of seismic first arrival amplitude inversion using ray theory, on a set of seismic tomographic data acquired at a historical site. Amplitude inversion results are compared with those of traveltime inversion. Results show that amplitudes and traveltimes are sensitive to different features of the subsurface. Attenuation distance is more related to the geological bedrock and deeper and broader (low wavenumber) structure, whereas velocity is more related to shallower and detailed (high wavenumber) structure. Amplitude inversion contributes thus to the improvement of the overall subsurface imaging. Seismic tomographic exploration of historical and other near surface sites of historical and engineering interest can benefit from the use of amplitude inversion.
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Monitoring In-situ Horizontal Stress in Soil Using Shear Waves
By R. GhoseWe have developed a laboratory facility where very small change in stress in soft soil can be accurately implemented. The effect of such small stress change on seismic velocity has been measured. Our observation suggests that change in shear-wave velocity can be used to monitor change in in-situ horizontal stress, even if the magnitude of stress change is small. A properly constrained stress-porosity model can successfully predict the observation in most cases.
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Imaging by Scattered Surface Waves – Preliminary Results From Seismic Field Data
Authors A. Kaslilar and G.C. Hermancontrast of the heterogeneity by using scattered surface waves. In the method three dimensional elastodynamic wave propagation and scattering in a laterally invariant embedding is considered. The scattered wavefield is expressed as a domain type integral representation. Weak contrast in density is considered and the Born approximation is used. Due to the lateral invariant medium assumption, the radial symmetry of the medium is considered and the Green’s tensor elements are calculated in an efficient way. The method is verified by numerical tests and by a scale model experiment. Preliminary results of the seismic field data are obtained. The location and the size of the tunnel are reasonably well estimated and the density contrast is qualitatively estimated. The estimation of the actual density contrast is in progress.
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Linearized Inversion of MASW Data Using Inequality Constraints
By M. CercatoInequality constraint formulation of least squares inversion is a flexible method to insert physical constraints (as well as a priori information) into the inversion process. In MASW (Multichannel Analysis of Surface Waves) inversion this has proved to be useful to stabilize convergence when applied to fundamental-mode dominated data as well as to data containing higher modes of propagation. A reliable S-wave velocity profile can be obtained from inversion of surface wave data if all available information is inserted into the inversion algorithm.
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Weight Coefficients for Representing Data Uncertainities in Refraction Microtremor (ReMi) Inversion
Authors K. Yanik and A.T. BasokurIn the ReMi (refraction microtremor) method, the phase velocities are determined at the frequencies where a slope or a peak occurs in the spectral ratio map. Once, a dispersion curve is obtained as a velocity values versus periods, then S-wave velocities of the subsurface can be derived by using some inversion methods. We use the damped least-squares method with the singular value decomposition for the matrix manipulations. An appropriate weighting scheme is suggested for the ReMi data for describing data uncertainties. First, the measured data are smoothed by a combination of simple fitting functions that simulates the behaviour of a dispersion curve. Then the weight coefficients are derived from the differences between the measured and the smoothed data sets in order to make the interpretation algorithm stable.
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Geophysics Assisting the Needs of Soil Stabilisation
By P. ValastiSoil stabilisation is an increasingly popular method to improve foundation conditions in areas of soft soils where direct construction is not possible because it would result in heavy and uneven settlements detrimental to infrastructure. In order to improve the quality of soil stabilisation, subsurface conditions of target area must be known a priori. Especially water content is the single most important factor determining the amount of binding agent to be injected. Various geophysical measurements have been carried out for determining water content independently. A method to transform measured resistivity to water content in clayrich areas where simple law of Archie will fail to function will be presented.
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A CFP Approach to Estimate the Shear-wave Velocity Distribution in the Shallow Subsoil
Authors J.W. Thorbecke and R. GhoseThe CFP approach has been used to estimate Green functions for a high- resolution shear-wave reflection field dataset. This data set contains lateral variations of the near surface and has very low velocities. To estimate a velocity model for this kind of data is not an easy task. The CFP approach gives one-way Green function times which are accurate as indicated by the flat events at t=0. These Green functions contain all the information needed to build an accurate velocity model.
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Geoelectrical Monitoring of Seasonal Moisture Content Changes in an Earth Embankment
Authors J.E. Chambers, P.B. Wilkinson, D.A. Gunn, R.D. Ogilvy, S.G. Pearson, O. Kuras, P.I. Meldrum, G. Ghataora and M. BurrowEarth structures, such as embankments, require ongoing monitoring and maintenance to identify potential failure zones and to compensate for the effects of settlement. Extreme weather events leading to prolonged periods of desiccation or saturation are becoming more frequent and threaten embankment stability. In this paper we develop electrical resistivity tomography (ERT) as a non-invasive tool for characterising and monitoring earth embankments. A study is described in which ERT was applied alongside conventional intrusive techniques to investigate and monitor a section of Victorian Great Central Railway embankment, near Nottingham, UK. A number of modes of deployment were considered including linear 2D ERT arrays both parallel and perpendicular to the long-axis of the embankment and 3D imaging arrays. The resulting ERT images, when calibrated using intrusive geotechnical testing and core samples, revealed the spatial variability of the embankment soils. Parallel ERT sections were used to identify major discontinuities between material types at locations associated with poor track geometry. Perpendicular ERT sections also revealed significant internal heterogeneity, and were used to monitor seasonal changes in the moisture content within the embankment.
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Influence of Water Content and Dry Density on Electric and Electromagnetic Properties of a Compacted Fine-grained Soil
Authors Y-L. Beck, V. Ferber, S. Palma-Lopes and P. CoteIn geotechnical engineering, there is a need for efficient techniques which could measure soil density and moisture content, volume integrating and in a non-destructive manner. Electrical resistivity and dielectric permittivity depend on both these soil parameters and it is hoped that these properties could help addressing this geotechnical issue. In order to evaluate this possibility, the influence of water content and dry density of a compacted fine-grained soil on its resistivity and its permittivity is being studied. As suggested by previously published models and experimental data, electrical resistivity decreases when dry density or gravimetric water content increases, and dielectric permittivity increases with an increase of water content or dry density. The influence of water content is the most significant on permittivity whereas both dry density and water content have a significant influence on resistivity.
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Combining 3D GPR and Subsurface Utility Engineering to Create Accurate Utility Maps in Istanbul
Authors R.A. Birken, R. Stearns, Q. Zhu and S. MacIntoshA pilot project was commissioned by the Istanbul Metropolitan Municipality to assess the combined capabilities of traditional Subsurface Utility Engineering (SUE) and Ground Penetrating imaging Radar (GPiR) to accurately map and identify the utilities underneath the historic streets of Istanbul. The array based GPR technology combined with cm accuracy land surveying and advanced imaging software allows for an efficient and complete mapping of large-areas (1000’s of square-meters) with radar traces collected on a 10 cm grid. The resulting high-resolution 3D radar images are interpreted for utility lines, which are then identified and calibrated with standard SUE techniques. The combination of SUE and GPiR is a powerful technology that produces more accurate utility maps than by each method alone. The complementary technologies were successfully applied in the Eyup district of Istanbul. Typically we achieved good radar penetration down to a depth of approximately 1.5 m, after that a clay layer attenuates the radar signal significantly. The Municipality received accurate CAD drawings with the 3D location of each pipe as determined by GPiR and SUE.
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Peat Dike Stability Monitoring with a Semi-permanent Electrical Resistivity Set-up
Authors W.A. van Leeuwen, E.C. Slob and G. de VriesOne of the results of the failure of a peat dike near Wilnis in the summer of 2003 was that peat dikes needed monitoring during dry periods to observe their sensitivity for heave. It was thought that this could possibly be done combining geotechnical and geophysical techniques. This research focuses on the possibility of electrical resistivity imaging to aid in determining the stability of peat dikes with respect to heave. To achieve this, laboratory experiments, cone penetrations tests, and electrical resistivity measurements were made. After data analysis, a relationship between volumetric weight and electrical resistivity of soil samples was found. This has resulted in a procedure that will allow the successful monitoring of peat dike stability.
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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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Flow Zone Index for Improving Permeability and Porosity Relation and Facies Description
Authors E. Puskarczyk, J. Jarzyna, J. Krawiec and M. BalaA well-defined relation between permeabilty and effective porosity is a way for calculating permeability in all cases where direct lab measurements are impossible. In homogeneous rock formations, correlations between discussed reservoir parameters are always high. In the group of lab data from the Rotliegendes formation from the Polish Lowland, correlations between permeablity and porosity showed not very good R coefficients and a great dispersion. The data originated from 78 wells, over a dozen of them was chosen for analysis. The whole research area was divided into smaller ones. We assumed the smaller data groups would be more homogeneous than the whole data set as regards lithology and facies composition. The relations permeability vs. porosity were determined for them and the results were better than those obtained for the whole data set, but still they were not satisfied. The Flow Zone Indexes were calculated for the units of the same hydraulic parameters and relations permeability vs. porosity were separately calculated for each such unit. The correlation coefficients were good and the prediction equations for permeability were constructed. The obtained formulas can be applied to calculating the continuous permeability log in the borehole profile on the basis of the porosity curve.
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On the Reduction of Noise-sensitivity in Seismic Tomography
Authors M. Dobroka, T. Ormos and A. GyulaiThe Iteratively Reweighted Least Squares (IRLS) procedure is combined with the Conjugate Gradient method in defining a robust tomography method (W-CGRAD). In order to make the Simulated Annealing (SA)-based tomography algorithm more robust, a generalized weighted norm is introduced, as generalized energy function. Robustified versions of the SIRT method are also introduced. The generalization is made both in the data space (weighted average using Cauchy weights) and in the model space. A robust SIRT algorithm can also be defined by introducing alpha-trimmed mean instead of the arithmetic mean in the well-known updating formula of the traditional SIRT method. Using synthetic traveltime data the generalized (robust) tomography algorithms are tested in a medium sized tomography example. It is proved that the weighted tomography methods are more robust and less sensitive for noise (especially outliers) then their original versions.
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Application of Well Logging Data to Determine a Model of Elastic Parameters for the Ksiezpol Gas Reservoir
By M.J. BalaThis paper presents effects of gas- and water saturation on elastic parameters of rocks. Gas present in the pore space of rocks significantly decreases velocity of P-wave propagation whereas it is much smaller on S-wave velocity. Hence, a small VP/VS ratio can be an indicator of gaseous hydrocarbons in the deposit. The analysis was based on theoretical relationships for porous media, known as the Biot-Gassmann’s and Kuster and Toksöz’s models, and on well-logging data and results of the quantitative interpretation of wells logs. The relationships between P-wave and S-wave velocities and reservoir parameters may contribute to solving some problems associated with seismic interpretation of wave forms in autochthonous Miocene deposits.
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Improving the Data Quality of Earth’s Magnetic Field NMR Measurements in a Noisy Laboratory Environment
By T. RadicNMR (Nuclear Magnetic Resonance) measurements on rock samples provide important information about the structure and the fluids of the pore space. Normally these measurements are done in fields of strong permanent magnets. However, in some cases it is important to do this in the weak earth’s magnetic field. Unfortunately the high laboratory noise level in combination with the very low NMR signal amplitudes complicates the NMR measurement and reduces the data quality. For this reason we have updated a former NMR instrument. The new hardware concept considers three unwanted noise signal types: 1) Mains noise (50/60 Hz), railway traffic (162/3 Hz) and harmonics, 2) wideband noise from switched power supply and electronic instruments, 3) low-frequency magnetic fields (1 Hz) from DC motors. Three noise reduction techniques were implemented for routinely use: a) Gradiometric receiver coils to suppress up to 97-99% of the lab noise, b) Remote Reference Receiver Loops to predict and eliminate residual noise in the time series, c) Method for compensating the influence of rapidly changing Larmor frequency while stacking. As long as the lab noise is moderate, these three methods provide a good NMR data quality as tests at different locations have demonstrated.
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International Cooperation for the Mapping of Buried Valley Aquifer Systems – The Project BurVal
Authors H. Wiederhold and The BurVal Working GroupThe project BurVal has been established to study systematically buried Pleistocene subglacial channels valleys - or in short buried valleys - of the North Sea Region. The aim of the project BurVal is to develop tools for the mapping of buried valleys under the aspect of groundwater exploration and protection, to test and demonstrate these tools at selected Pilot project areas, and to show how these mapping results can contribute to a sustainable use of the water resources. Nine partners from Denmark, Germany and the Netherlands cooperated in this project.
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Integrating Multi-scale Geophysical Data for the 3D Characterization of an Alluvial Aquifer
Authors H. Paasche, T. Günther, J. Tronicke, A.G. Green, H.R. Maurer and K. HolligerUnderstanding groundwater flow and contaminant transport within alluvial aquifers requires the detection and detailed characterization of preferential flow paths. We present an integrated interpretation of a 3D geoelectric data set, 2D crosshole radar and seismic tomograms and the results of sparse direct-push slug tests that allows us to identify potential preferential flow paths within an alluvial aquifer in northwest Switzerland. The 3D electrical resistivity model is divided into clusters of high and low electrical resistivity using a fuzzy c-means cluster analysis technique. Despite significant differences in resolution, dominant structures identified in a 2D cross-section extracted from the clustered 3D resistivity model largely coincide with those obtained from an independent fuzzy c-means cluster analysis of crosshole radar and seismic tomograms. By linking the 3D electrical resistivity clusters to hydraulic conductivity measurements obtained from a limited number of slug tests, we derive a 3D aquifer model distinguished by zones of increased hydraulic conductivity that may act as preferential flow paths.
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The Role of Geophysical Methods in Salt-water Intrusion Mapping for Strongly Karst Formations, a Case Study at Stylos, Crete
Authors H.A. Hamdan, G. Kritikakis, A. Vafidis and E. ManoutsoglouGeophysical methods are useful tools for mapping the boundary between fresh and saline water. A geophysical survey was conducted in order to study the mechanisms of saltwater contamination in a strongly karstic coastal area. The region of interest is located in Western Crete about 3 Κm from the seashore. Seismic and electrical resistivity methods, along with detailed geological mapping have been employed to study the complex geological structure of the area under investigation. Electrical tomography detected a low resistivity zone which is attributed to a saline water layer. From the combination of the geophysical and geological data, a depicted major normal NE-SW fault zone mainly causes the groundwater salinization.
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Characterization of Electrical Anisotropy for Cuttability of Rocks Using Geophysical Resistivity Measurements
More LessThe rock hardness, strength, fracturing and anisotropy property parameters are the main factors controlling cuttability of the rocks. The orientations of elongated grains cause considerable variations in hardness, anisotropy and layering parameters of the rocks. The resistivity sounding technique (DES) is used to determine anisotropy resulted by the rock's fracturing, pressure and metamorphism properties. We calculated typical polar diagrams by modelling using various anisotropy values and electrode arrays for comparing with field measurements. Based on the synthetic data it is clearly seen that dipole azimuthal sounding was most determined electrical anisotropy. To understand the reason of these effects and to estimate suitable direction for easily cuttability of the rock, resistivity soundings using dipole azimuthal array were applied on karstified limestones covered with alluvial units around Kozyatağı quarry, Istanbul. Dipole azimuthal soundings were made on 10°, 55°, 100°, 140° azimuths with spacing r=10-90 meters. The azimuthal diagrams in the ranges of 88-950 ohm-m were indicated characteristic orientations as the anisotrophy indicator at various depth levels within limestone. Based on the azimuthal diagrams, the direction ~120° (from north) where apparent resistivit considerably decreased to lower values towards deep levels (>15 m) within limestone is interpreted as more suitable extension for easy cuttability.
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Assessment of Silicified Wood Potentiality Using Geoelectrics and GPR Applications in New Cairo City, Egypt
More LessNew Cairo City is a new urban area which has been developed on the periphery of Cairo. A natural protectorate area of Silicified woods is located inside this city. Accordingly, a Conflict between the Ministries of Tourism and Buildings about this protectorate area is arisen. As an attempt to assess the existence of silicified wood in this natural protectorate area, 2D and 3D electrical resistivity and GPR measurements have been conducted. The survey has been conducted at two sites in the percorate area. At site 1, in which there are no traces of silicified wood on the surface but in site 2, there are traces of silicified wood on the subsurface. The data of geoelectric in show accumulation of burden traces of silicified wood. The data obtained from Geolectric are well in range and fits with those from GPR data. This survey approves that the study area should be maintained as protectorate area and should not be used for further constructions.
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Magnetic Detection of Buried Oil and Gas Metallic Pipes
Authors D. Ioane, F. Chitea and M. MezincescuMagnetic detection of burried metallic pipes is a well-known application of this geophysical method, the “pipe” magnetic anomaly being easily recognised. Recent applications carried out in Romania aimed at locating abbandoned oil transportation pipes or gas transportation network in view of land “clearing” for new industrial or commercial facilities. The Ploiesti city area, hosting refineries and dense networks of oil trasportation pipes, enabled the authors to find processing methods for separating the “pipe” magnetic effects from other magnetic noises due to neighboring industrial facilities and electric lines. The Sibiu city area hosting buried gas transportation network at its limits offered good possibilities for a clear location of the pipelines, but also information on the subsequent modifications of the buried pipe network.
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Gravity Effects of Deformation Zones Induced by Tunnelling in Soft and Stiff Clays
More LessWe calculated gravity response of geotechnical models of deformation zones induced by tunnelling in two environments - soft clays and stiff clays. Approximately elliptic zones of deformation above the tunnels arise in both cases, but internal structures of the zones are different, and different are also theirs gravity effects. In soft clays, the deformation zone above the tunnel is actually zone of compaction and causes decrease of amplitude of gravity anomaly over the tunnel. In stiff clays a zone of dilatancy occurs next to the tunnel, but an overall gravity effect of deformation zone is insignificant and doesn’t affect the amplitude of gravity anomaly measured above the tunnel at the surface.
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Liquefaction Risk Assessment by the Use of Seismic Techniques – The Test Area of Nafplion City, Peloponnesus, Greece
Combined seismic surveys of P and S waves (seismic reflection, refraction, modelling, MASW and crosshole investigations) with geotechnical borehole testing effectively evaluated the land where the Nafplion city, Greece is expanding. Since the area was suspected for high liquefaction potential, a study was performed using all the modern tools for this evaluation. The study area was too large to be examined by the standard laboratory and borehole testing, therefore the aid of geophysics was proved to be valuable. We present the methodology we followed and the main results of the study for the test area of Nafplion city. Evidence was found indicating a low factor of safety against liquefaction at specific sites within the area of interest.
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New Results of High Resolution Seismic Measurements at Lake Balaton
Authors Z. Pronay, T. Cserny and R. MiskolcziInvestigations of Lake Balaton by the Geological Institute of Hungary started in 1981. Between 1987 and 1989 within the framework of Cuban–Hungarian cooperation a total of 373 km boomer seismic section was registered covering the entire lake in a quasi uniform network. As a result of the profiles’ evaluation a picture was drown up about the spatial distribution of lacustrine sediments, the mud structure as well as the variable morphology and geological setting, till 30 m depth from the lake bottom. During 2003 and 2004, in the cooperation of Geophysical and Geological Institutes the original thermo-paper records were digitally archived, re-evaluated and the recent records were integrated as well. The re-evaluation gave the opportunity for a more detailed spatial determination of the geological structure of the bed (layers, faults). This gave an opportunity to plan the research of Pleistocene sediments, deposited before the lake formation.
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Shallow Reflection Seismic Shear-wave Velocity Analysis on Paved Soils Using a Land Streamer Unit
Authors I. Arsyad, U. Polom and S.T. WiyonoAs part of the Project “Management of Georisk” of the Federal Institute for Geosciences and Natural Resources (BGR), Hannover, high resolution shear-wave reflection seismic was applied in the Indonesian province Nanggroe Aceh Darussalam in cooperation with the Government of Indonesia and local counterparts. Target was earthquake site effect classification in the city of Banda Aceh and the surrounding region of Aceh Besar. Using a land streamer system was advantageous in the populated area and for an easy recording operation. Additionally, paved roads, compacted dirt roads or concrete surfaces suppress Love waves and refracted waves, if the soil layer below the surface is of lower seismic velocity. The recorded wave field of single records often includes direct and reflected SH shear-waves only in such cases, allowing RMS-velocity-depth determination on site. Because RMS-velocity is higher than mean velocity, this gives a good estimation of the mean shear-wave velocity-depth function. Results show that shear-wave reflection seismic in combination with Land Streamer is a useful tool to evaluate the soil stiffness in populated urban areas. The specialized seismic system for compacted soil surfaces allows a wide range of applications within cities, industrial sites, paved roads and also on small dirt roads.
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The Influence of the Surface Conditions on Reflection Seismic Data – A Joint Study of Reflection Seismic Data in Denmark
Authors E. Nørmark, H. Lykke-Andersen and K.H. EsbensenIn Denmark reflection seismic data are intensively used for mapping ground water resources. However, due to variations in the near surface layers, significant variations in the data quality are encountered. Seismic modelling may be carried out, in order to understand the influence of the surface layers. However, in lack of reliable geological models of the surface layers, this will normally not be feasible. In this context we seek another approach, where the seismic data quality is evaluated and compared to borehole information, ground water potentials and other kinds of surface information. By use of multivariate data analysis, a regression model has been established, which reasonably well predicts the seismic data quality. By using this approach up to 60 % of the variations in the data quality can be predicted. The model is only valid in the region where it is estimated or in regions with the same acoustic properties. However, the procedure can be applied anywhere and is also believed to be relevant in other similar cases.
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Joint Interpretation of P and SH Refraction Data
Authors T. Ormos, M. Dobróka and A. DaragóIn this paper we propose the use of a fast kinematic refraction inversion method for P and SH arrivals for interpretation near surface structures. The basic aim for this method is that lateral changes both in layer thicknesses and propagation velocities in the media are described by adequately chosen functions expanded in series.With the method we can estimate the lateral changes of the P and S velocities and layer thicknesses. With the joint interpretation of both inversion results we can recognize in near surface structures the water table and low velocity contrast too. In the paper we show a practical example.
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Comparison between Inversion Algorithms of Surface Refraction Tomography for Cavities Detection
Authors M. Dwornik and A. LesniakThe analysis of popular algorithms of the inverse problem dedicated to surface refraction tomography is discussed in the paper. Surface refraction tomography is a method, which is able to estimate a velocity of elastic wave on a layer boundary. This work presents comparison between popular methods of inversion of seismic data: Singular Value Decomposition (SVD), genetic algorithm and ART and SIRT iterative methods. Stability of results and error of estimated solution were analysed. Numerical experiments show, that the most effective method is SVD (it gives acceptable solution in shortest time). Genetic algorithm, which is one of the basic methods for non-linear inversion, gives the best inversion result, but its computation time is the longest one. Significant improvement of convergence was obtained by using some iterations of SIRT in single iteration.
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Numerical Simulations of NMR Responses in Fractal Pore-space with the Finite Element Method
Authors J. Lehmann-Horn, S. Strehl and U. YaramanciA finite element technique for simulating NMR responses in a complex porous media is developed to investigate the effects of rough inner surfaces to the NMR response of fluids in rocks. The influence on the NMR response (i.e. decay times and intensities) by changing physical and geometrical parameters of the inner surface is investigated. Attention is focused to the effect of increasing inner surface whereas volume maintains constant. The finite element program allows to investigate the NMR response of any complex two-dimensional pore-space. Simulations are designed and conducted for fully water saturated rocks. It shows that roughness shifts the T2 (distribution) to lower values and the intensities of higher modes are not negligible in rough, high surface relaxivity but still fast diffusion condition. A multi-exponential function describes the decay. Commonly used permeability vs. T2 (distribution) relationships vary in rough inner surface conditions. Estimations with the logarithmic mean of T2 (distribution) give a realistic trend for the permeability. The NMR response of fluids in a pore with rough inner surface in the fast diffusion regime between 0.1 and 1 is adequately described only by a multi-exponential function.
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The Strategy of Joint Inversion Using Function Series
Authors Á. Gyulai, T. Ormos, M. Dobróka and R. KavandaThe paper presents the results of investigations on the choosing of the optimal number of coefficients (unknown) in the function inversion. Using synthetic noisy geoelectric data we show, that it can be find an optimum number of these coefficient using in the joint inversion. The method can be used also in case of field data the optimum of the coefficient’s number is at the minimum of the main model parameter estimation error and the relative data distance as a function of the number of expansion coefficients.
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Geophysical Application of Power Lines EM Fields
More LessAs it has been shown by using numerical forward modelling and experimental field measurements, the electromagnetic fields due to power lines can give geological information even in the vicinity of powerlines. For the localisation of relatively near-surface lateral inhomogeneities, we recommend to construct a map about the ratio of electric field components measured in two neighbouring sites, along profiles parallel to the power line. Such a survey can be carried out easily, quickly and cheaply, and by means of a very simple instrumentation. This method may become a useful complementary tool of traditional geoelectric soundings. The stratigraphic identification and the depth position of the inhomogeneities are given from related traditional measurements. It is possible to obtain a vertical resolution from several upper harmonics of the fifty hertz, too. For such a frequency-sounding a simultaneous measurement of the electric and magnetic field components is suggested, which requires multichannel, frequency-selective voltmeters of higher sensitivity -possibly with digital recording - as well as additional theoretical and field examinations.
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