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Near Surface 2008 - 14th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 15 Sep 2008 - 17 Sep 2008
- Location: Kraków, Poland
- ISBN: 978-90-73781-56-6
- Published: 17 September 2008
1 - 50 of 130 results
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Automated Dispersion Mapping of Surface Waves
Authors R.S. Westerhoff and R.P. NoorlandtWith the increasing amount of innovative geophysical sensors and sensor networks there is a need for faster and more controlled data processing and interpretation in order to cope with the abundance of data coming from monitoring systems. In this article, we are presenting a possible method to deal with automated processing of multi-station Rayleigh wave data. We use the strong points of the visualization process of the MASW method (Park et al. 1999) and combine them with an automated dispersion curve mapping procedure. By doing this, we are presenting a possible alternative for fast and repeatable real-time processing of surface wave data. By presenting a case study on a dike the operational speed of the method and the potential for use in monitoring studies is demonstrated. There is a short evaluation of alternative inversion methods that are able to perform the calculation to a subsurface layer model.
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Combination of Active (MASW) and Passive (Microtremor) Source Based Geophysical Approach
Authors A.I. Kanli, T.S. Kang, A. Pinar, P. Tildy and Z. PronayThe microtremor and the MASW (Multi-channel Analysis of Surface Waves) survey were carried out to determine the shear wave velocity-depth information, resonance frequencies and the depths of the sedimentary layer in the Dinar city, Southwestern of Turkey. The shear-wave velocity profile of the basin sediments was estimated from the inversion of the microtremor horizontal-to-vertical spectrum based on surface waves from seismic noise at each site using a genetic algorithm. The average shear-wave velocities estimated from the multichannel analysis of surface waves experiments were given as constraints in the inversion. A new relationship was derived between the resonance frequency ( ) and thickness ( ) of the overlaying layer, which is given as
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A Comparison of Widely-used Seismic Methods for Shear Wave Profiling on Soil Sites with High Stiffness Contrast
Authors M. Cercato, E. Cardarelli, G. Di Filippo and G. MilanaThe Shear wave velocity estimation of near surface materials is of primary importance in several engineering and environmental problems due to its direct relationship with the stiffness in-situ properties of soils. Within the framework of a seismological national project aimed at estimating the influence of the surface geology on seismic ground shaking, several sites of the Italian seismic network were selected for detailed S-wave profiling, with application of both borehole and surface-wave seismic methods. In such sites, the presence of relevant stiffness contrasts within the soil column has proven to influence deeply the surface wave modal superposition, with “mode jumps” more frequently present than previously thought also in normally dispersive sites. In these cases, mode misidentification can be avoided by investigating the effective phase velocity distribution using active data sets and full waveform seismic modelling. When a correct modal identification is achieved, the surface-wave inversion results are well consistent with borehole data.
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The Parallel Seismic Technique for Determination of Foundation Pile Length
More LessThe length of existing foundation piles is unknown in many cases due to missing files and drawings. The investigation methods used in most cases today have certain limitations that require the application of new techniques. The parallel seismic method uses first arrivals of elastic waves generated by an impulse on the pile top registered by sensors in a nearby borehole. The travel time curves can be interpreted in terms of velocities and foundation depth. Numerical simulation (ray-tracing and full waveform modelling) helps to understand the results achieved at non-standard foundations or under complex subsurface conditions. The method is successfully applied in research and commercial projects.
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4-D Inversion of 3-D Resistivity Monitoring Data from LNG Storage Pilot Plant
More LessGeophysical methods are commonly used to obtain ground condition changes in various application fields. These monitoring of ground condition needs accurate data measurements as well as proper interpretation method to obtain or image the change of the physical property of the subsurface with time. In this study, we extended the three-dimensional (3-D) resistivity imaging algorithm to four-dimensional (4-D) imaging algorithm, where measured resistivity data and geoelectric structure of subsurface is assumed to be 4-D (i.e., 3-D in space and varies with time). To demonstrate the enhanced imaging capability by our new 4-D imaging algorithm, we revisited the 3-D resistivity monitoring data set acquired at the LNG storage pilot plant to delineate the zones of ground condition changes due to cryogenic environment due to the LNG storage. Compared to the conventional time lapse imaging based on the separate inversion of data set acquired at different time of monitoring survey, 4-D inversion algorithm provided much clear image of the change of the ground with much less inversion artifacts, which suggests that our new 4-D imaging algorithm provides more quantitative means to evaluate the change of the ground conditions and their causes.
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Numerical Modelling for Improvement of the Interpretation of Geoelectrical and Induced Polarization Measurements
Authors B.E. Danielsen and T. DahlinWith a general knowledge about the geological setting it is possible to use numerical modelling of geoelectrical and IP data to obtain useful experience for the interpretation of real measurements. In this case the vertical extension of a dolerite dyke has been modelled and compared with real data. It is obvious from the resistivity modelling that it in this particular case not is possible to say anything about the vertical extension of the low resistive zone in the real measurements. Based on the experience from the IP modelling it is much clearer that the dyke extends to the bottom of the model. By combining different methods which exploits different physical properties a better and more reliable interpretation can clearly be done.
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Applications of New Technologies of Ground-based Interferometric Radar to the Study of Cultural Heritage Buildings
Authors M. Marchisio, G. Ranieri, G. Bernardini, L. D'Onofrio, P. Farina, M. Manunta and L. PiroddiDifferential Synthetic Aperture Radar Interferometry is a technique that, by making the phase difference of SAR image pairs, allows to extract the information relevant to the earth surface displacements. The result of the phase difference, referred to as Interferometric Phase or Interferogram, contains different terms: the deformation phase term, a topographic phase contribution, an atmospheric phase term and the noise contibution. Applying an algorithm referred to as Small Baseline Subset (SBAS) proposed by I.R.E.A. - C.N.R. it is possible to know the displacement of buildings and constructed parts (roads, bridges ecc.) which have a dimension of about 10 meters The results demonstrate that the DInSAR technique is a fast and precise method to detect and follow the temporal evolution of deformations, so it is a powerful technique for monitoring the territory and to map its vulnerability.
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Conclusion of POHVA I Project – Ground Improvement by Soil Stabilisation
By P. ValastiThis extended abstract will concentrate on results of POHVA I which lasted from May 2005 to the end of 2007. POHVA I consisted of assessing the means and their reliability to measure subsurface conditions where water content distribution was the single most important factor. A model for transforming resistivities to water contents was established. During the project a new index testing method for soil stabilisation testing was developed and an extensive soil stabilisation database was constructed and based on this database statistical models for shear strength improvement to be obtained by given amount of binder in given environmental conditions was created.
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The New Inversion Scheme for Magnetic Resonance Sounding – q-t-Inversion of Complete Data in a Single Step
Authors M. Mueller-Petke and U. YaramanciBasically the data for Magnetic Resonance Sounding are in two dimensions: For every pulse moment i.e. excitation intensity q, there is a time t dependent relaxation signal i.e. record. These are to be inverted into depth distribution of water content and rock decay time respectively. Inversion schemes available so far handle the dataset in two steps, either deriving the parameter of each record i.e. amplitude and decay time by fitting, and then inverting these parameter into the subsurface properties or inverting the unfitted records at any time step to subsurface water content and then fitting for the decay times. We present a new scheme that handles the inverse problem in one step using complete data. We assess and examine this new scheme, called q-t-inversion due to the parameter variables, using a synthetic example with realistic field conditions of noise and dead time. The new scheme leads to a higher resolution and is consideably more stable as expected because it is more appropriate for space parameter, which are in physical terms not independent and therefore the resulting observations too.
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The Role of Transform Functions in Geophysical Inversion
Authors T. Guenther, M. Müller-Petke, M. Hertrich and C. RückerWe give an overview on possible transform functions (TF) used in geophysical inversion. For three different geophysical methods - dc resistivity sounding, surface nmr sounding and travel time tomography - we investigate how TF's affect the results of synthetic data, the singular value spectrum and resolution properties.
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Averaging and Lateral Resolution for Continuous TEM Methods
Authors N. Foged, E. Auken and A.V. ChristiansenFor continuous methods there is a trade-off between lateral resolution and the size of the data stack, or average width. A narrow average (small stack) results in a high lateral resolution, a wide average (large stack) results in a better signal-to-noise ratio but reduces the lateral resolution. We present inversion results of synthetic TEM-data with different averaging widths to explore the influence on the lateral resolution. The study shows that for the deeper parts of the model it is possible to use a very large average width without loosing lateral resolution. Averaging airborne data adds another issue, because the data entering the average interval originates from different flight altitudes (aircraft movement). The question is whether the error introduced by assigning the average flight altitude to the average dataset is acceptable? This is relevant because the altitude enters the inversion scheme as a fixed or strongly constrained parameter. We show that a linear correlation between altitude and data response for TEM-data exist. Due to the linearity the error introduced by using average data with the average altitude is very small. In the worst case, the error is 3% and negligible for the late time gates.
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Efficient Finite-difference Time-domain GPR Modeling Using Subgrids
Authors N. Diamanti and A. GiannopoulosThe well-known finite-difference time-domain (FDTD) approach to the numerical solution of Maxwell’s equations for ground penetrating radar (GPR) modeling has a conditionally stable nature and this is one of the major limitations of the FDTD scheme. For the stability condition to be fulfilled, the maximum allowable time-step for the numerical computations is limited by the minimum cell size in the computational domain. Therefore, when detailed structures need to be simulated and/or when there are small features or regions of limited extent in the overall computational mesh with values of high dielectric constant - supporting propagation of waves at very short wavelengths - the spatial-step should be small enough so as all the objects in the computational domain are adequately resolved. For such cases, the FDTD solution employing a small, uniform spatial-step in the whole computational domain would lead to substantial computer memory requirements and execution time increase. This problem can be overcome and efficiency can be increased by introducing subgrids, into the FDTD method. Examples of areas of applications of these subgrids include the efficient modelling of fractures and other small targets as well as targets with high values of dielectric constant.
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Multi-mode Inversion for P-wave Velocity and Thick Near-surface Layers
By F.E. ErnstIn seismic data for hydrocarbon exploration, the near surface is often a major source of wavefield distortion, reducing the quality of the image at the exploration target level. Adequate knowledge of shallow subsurface P-wave velocities and layer thicknesses allows to mitigate these distortions. Dispersion curves can be used to estimate near-surface properties. However, the fundamental mode of the Rayleigh wave is not sensitive to P-wave velocities and has only a limited penetration depth. I show that considering higher modes in the inversion allows to reconstruct subsurface models with thick shallow layers. Furthermore, the higher modes also resolve the P-wave velocities. Higher modes are included in an error norm based on the Wronskian. This has as additional benefit that there is no need to know the number of the mode a pick belongs to. I show the improvement over using only the fundamental mode both on synthetic data as well as field data from Oman.
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Weighted Multipath Summation as a Tool for Imaging Localized Near-surface Inhomogeneities
Authors V. Shtivelman and S. KeydarWe present a new approach to imaging shallow subsurface objects in 3D space based on weighted multipath summation of seismic waves diffracted by the target objects. The method is a further development of the recently proposed multipath summation technique, where the imaging is performed by stacking the diffracted waves along all possible diffraction time surfaces having a common apex at the given point. The multipath approach does not require any explicit information on velocities since the summation is performed for all possible velocity values within a wide specified range. We show that the images obtained by the multipath summation can be considerably improved by applying a proper weighting function. Moreover, the weighted multipath summation can also produce a reliable estimate of velocity above the diffractor. The obtained velocity can be used to further improve the image quality. Application of the proposed method is demonstrated by synthetic and real data examples.
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Robust and Accurate Seismic/Acoustic Ray Tracer
By W. DebskiRecent development of high resolution seismic tomography and an increasing necessity of high precision seismic (acoustic) source locations calls for robust and very precise numerical methods of an estimation of seismic (acoustic) wave travel times and propagation ray paths. In this contribution I present an algorithm based on a parametrisation of ray paths by series of the Chebyshev polynomials. This pseudo-spectral method combined with the accurate Gauss-Lobbato integration procedure allows to reach a very high relative accuracy of travel time calculation of order of 1e-6 or better. Employing of the Genetic Algorithm based optimizer to seek for the shortest travel time path assures an extreme robustness of the algorithm which allows the method to be used in complicated 3D geological structures.
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Evaluation of an Hybrid Refraction Tomography Code
Authors M.S. Mendes and T. TeixidóSeismic refraction tomography is an attractive strategy for modeling near surface presenting lateral and vertical gradients and also producing static corrections, where traditional seismic refraction fails. We evaluate a hybrid inversion method that combines a global search with a local optimization method to build the velocity model from first break pickings without ray tracing. In the initial stage of the inversion, the Monte Carlo technique searches for the region of the model space where the solution is. This solution is then further improved by using a simultaneous iterative reconstruction technique (SIRT). The travel times and ray paths required for the inversion are calculated by a highly efficient grid ray tracing technique based on the Fresnel volume approach. First, we evaluate the method with synthetic data and show that the hybrid optimization method satisfactorily reconstructs the model shape and the velocity range. Then, we analyse the method with real seismic data to verify that, in both cases, the code performs at a level similar to the commercial software RAYFRACT
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Resolution and Quality Assurance in Near-surface Geophysical Surveys – A Multi-method Field Trial and It's Results
Authors I.A. Hill, N. Linford and J. UpwoodWe have conducted multiple repeat geophysical surveys over areas containing known near-surface structures. Here we discuss the comparison of these geophysical data sets, and their relationships to the control data. We consider multiple magnetic data sets collected over an area of known archaeology, and compare the details of the data quality. We also compare the geophysical data to other methods of locating the archaeological features. While there are interesting differences between the geophysical data sets in terms of spatial resolution and noise, in each case the main archaeological features are detected very clearly.
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Integrated Full 3D Geoelectrical and GPR Tomographies in the Ambulatory of the Roman "Villa Del Casale", Piazza Armerina
Authors P.L. Cosentino, P. Capizzi, G. Fiandaca, R. Martorana, P. Messina and I. Razo AmorozThe “Villa del Casale” is a wonderful Roman Villa excavated in the archaeological site of Piazza Armerina (Sicily) in 1929. Its interest is mainly to be referred to the floor mosaics, among the largest and most beautiful ones of Roman times, of superlative quality. The floor of the Corridor is covered by a wonderful mosaic depicting the hunt and capture of wild animals, so that it is called the “Corridor of the Great Hunting Scene”. The Villa is now under restoration, and during the planning of the restoration a 3D electrical resistivity tomography was performed on the corridor by using an acquisition grid composed by 704 electrodes. Furthermore 25 GPR profiles with 400 MHz antenna were acquired. The 3D ERT survey was carried out by utilizing a new 3D array, named “Maximum Yield Grid” (MYG), that allows a great decrease of the acquisition time and of the invasivity of the ERT investigation in respect of classical arrays. The results show shallow anomalies, that correspond to the canalizations for water drainage and to the concrete basement built below the corridor during a previous restoration intervention, and a diminution of resistivity with depth, related to the aquifer below the Villa.
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Ground Penetrating Radar Trace Shape Classification Using Self Organizing Maps – An Example from Ancient Aptera, Greece
Authors N.S. Spanoudakis and A. VafidisIn this paper we propose and apply a GPR trace shape classification technique, for imaging a Roman cistern in the ancient city of Aptera in Greece. We classify the trace shape of reflected waves from the roof of a Roman cistern. The classification is based on an artificial neural network namely, Self Organizing Maps (SOM). The same technique classifies reflected and diffracted waves from metal and plastic pipes buried in a sandbox.
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Archaeomagnetic Provinces in Israel as a Basis for Magnetic Prospecting of Archaeological Sites
Authors S.E. Itkis, S. Feinstein and B.E. KhesinMagnetic prospecting is a rapid, economic and noninvasive method for studying buried near-surface structures of archaeological sites. A new generation of magnetic equipment for field data acquisition and advanced methodology for their data analysis allows to reveal a broad range of buried archaeological targets: walls, columns, foundations, caves, tunnels, tombs, water pipes, kilns, furnaces, ovens, and other objects. However, magnetic survey at archaeological sites is commonly affected by a number of natural and artificial “noise” factors. Such complex conditions require an optimization of current methodology of magnetic survey. The implication of magnetic prospecting for the study of archaeological sites is based on the difference (magnetic contrast) in magnetic properties of archaeological targets and surrounding medium. Two archaeomagnetic provinces were defined according to the magnetic characteristics of archaeological elements, background soils and rocks, and pattern of magnetic anomalies. This allowed us to develop an optimized methodology for high-resolution and reliable archaeomagnetic prospecting in each archaeomagnetic province.
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Geophysical Survey of Salpausselkä Glacial Deposit near by the City of Hanko in Finland
Authors T.M. Hokkanen, H. Rainio and E.L. LaineIn Finland we have lots of esker and end moraine deposits originated from the time of glacial period and from the time of glacial rebound. The purpose of this study is to investigate those deposits with different geophysical methods. We carried out ground penetrating radar, refraction seismic, and resistivity imaging measurements when investigating Salpaussekä end moraine near by the city of Hanko in Southern Finland. Moreover, we bored four reference boreholes and exploited all geological data in available when making the final interpretation. As a result of these studies, we identified different soil types and the topography of bedrock. We are also able to evaluate better the genesis of certain parts of Salpausselkä end moraine in the vicinity of the city of Hanko.
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Three Step Depth Focussed Inversion as a Tool to Resolve Small Resistivity Contrasts by ERT
Authors U. Noell, T. Günther and M. FurcheIn more recent times Electrical Resistivity Tomography (ERT) has become a valuable tool for many environmental research themes. Within these topics the resistivity contrasts of the different layers and zones are of importance, i.e. sand, clay, water salinity and preferential infiltration pathways are quite small. Inclusions with such small resistivity contrasts to the surrounding are difficult to resolve by ERT inversion and typically the following problems occur: 1.) the true resistivity contrasts are underestimated 2.) the size of the anomalous zone is overestimated 4.) “fake” anomalies arise beside and between the true anomalous zones. 4.) surface heterogeneities infer “fake” anomalies into the subsurface. A two step inversion method is suggested to overcome these problems at least partly. Firstly the near surface heterogeneities are reconstructed by inversion. A forward model comprising the near surface structures only is extracted and used to remove their effect from the data by the application of a reference inversion method. Forward modelling is used to estimate the true sizes and the true resistivity contrasts of the inclusions. These methods are applied to array electrical resistivity tomography measurements at a sandy site with distinct heterogeneities at a depth of 30 – 60cm and show improved resolution capabilities.
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Dynamic Dilatancy in Saturated Sands
Authors E.S. Kushnareva and E.A VoznesenskyNew experimental aspects of saturated sands dynamic response are discussed. It is demonstrated that shear straining in saturated sands with any density occurs in several stages. Incremental changes of pore pressure and effective stresses at the shear zones triggering point are determined by initial sand porosity relative to its critical value. Sand porosity before shearing is considered as critical, if triggering of shear zones does not result in porosity alteration, which in undrained conditions is physically equivalent to zero change of mean effective stress and pore pressure. Type of sand response is revealed as incremental changes of pore pressure and mean effective stress at the stage of shear zones triggering, indicating negative or positive soil dilatancy. The notion of critical porosity is not applicable in dynamic loading conditions since in different phases of a single cycle changes of pore pressure and effective stress are observed. Progressive strain accumulation under dynamic loading of saturated sands with any density results in the development of a specific state with zero sum changes of pore pressure and mean effective stress called zero dynamic dilatancy. It is assumed to relate to the development of shear zones with constant thickness where subsequent straining is further located/
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Petrophysical Analysis of Resistivity Data
Authors V.A. Shevnin, A.A. Mousatov, A.A. Ryjov and O. Delgado-RodriguezRyjov developed forward problem of soil resistivity for sand-clay models. It uses 9 parameters (clay and sand porosity and capillary radii, water salinity and humidity, cation exchange capacity, temperature and clay content) to calculate soil resistivity. We applied these calculations for petrophysical analysis of sand clay soils using soil resistivity obtained from VES interpretation. It became clear that dependence of resistivity from clay content, porosity, cation exchange capacity, etc. allows solving inverse problem (estimation of petrophysical parameters on soil resistivity), that was fulfilled as algorithm and software in 2003. It is evident that we can’t determine all 9 parameters by using only soil resistivity; we need to find and use some additional information to have quantity of known parameters more than unknown ones. Temperature and groundwater salinity can be determined in each field site. We take soil samples to measure soil resistivity versus pore water salinity in laboratory to obtain soil model and recalculate resistivity data (cross-sections and maps) into cross-sections and maps of petrophysical parameters: clay content, porosity, cation exchange capacity, filtration coefficient, etc. After 2003 this technology was probed at more than 20 field sites in Mexico and Russia and demonstrated its efficiency.
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A Study of Hillslope Hydrologic Dynamics Using Irrigation Tests and Time-lapse 3D Electrical Resitivity Tomography
Authors G. Cassiani, S. Stocco, A. Godio, A. Villa, P. Frattini, M. Rossi, R. Deiana and G.B. CrostaThe description of catchment response is a fundamental issues in geoscience. However, some of the basic governing processes, particularly the role of the subsurface, remain poorly understood. In this paper, we present the results of non invasive time-lapse monitoring of a controlled infiltration test at a site in the Italian Alps. The key hydrologic question is whether rainfall infiltrates mainly into the underlying bedrock or flows in the soil layer towards the stream channel a few hundred meters downgradient. We applied 1000 mm of artificial rain on a 3 m x 3 m slope box over about 18 hours. The soil moisture variation and the underlying bedrock was monitored via a combination of electrical resistivity tomography (ERT), TDR probes and tensiometers. A small-scale 3D cross-hole ERT was performed via 2 m deep borehole purposely drilled and completed with electrodes in the irrigated plot. As a result, we observed a fast vertical infiltration through the soil cover followed by infiltration into the fractured bedrock. The lateral transmission of infiltrating water was negligible. This experiment confirms that the fractured bedrock has a key role in controlling the fast hydrological dynamics of the small catchment system under study.
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Joint Interpretation of Magnetic Resonance Sounding and Borehole NMR Data
Authors TH. Hiller, U. Yaramanci and J.P. DelhommeThe main objective of the joint research project between the Technical University of Berlin and Schlumberger Water Services was to assess how high resolution borehole NMR can be used to enhance the interpretation of Magnetic Resonance Sounding (MRS) measurements. Therefore a profile measurement on a ground water test site in the desert of Abu Dhabi was carried out. With new and extended processing and inversion techniques we were able to interpret the well near MRS soundings together with the information from the borehole. On the one hand the borehole data was used to directly improve the inversion by implementing the resistivity-log data. On the other hand we could verify the results of our surface measurements when comparing the inverted water content and the belonging decay times with the equivalent CMR Bin porosity classes from the borehole.
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Resolution Limits of NMR-based Estimation of Pore Size Distributions
Authors S. Strehl and U. YaramanciNuclear Magnetic Resonance (NMR) methods offer the determination of pore size distributions of loose sediments, which is the basis of investigations concerning ground water dynamics, i.e. water content and hydraulic conductivity of the subsurface. The NMR resolution range of pore radii depends on many parameters such as the technology of the applied NMR system, the Larmor frequency and the properties of the pore fluid and the pore surface, respectively. Different NMR methods do not necessarily show the same results at the same samples. At core samples from a shallow aquifer we compare Earth’s field NMR measurements with conventional lab NMR measurements and show the influence of the dead time and the Larmor frequency on the resolution limits of the pore size distribution.
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Well-logging and Near-surface Seismic Methods for Aquifer Detection
Authors G.M. Suarez, S. Miong, J. Wong, R.R. Stewart, A.D. Alcudia, H. Lu and K. Al DulaijanWell-logs, high resolution shallow seismic, and VSP surveys were conducted in a experimental well near Priddis, Alberta, to test their feasibility in characterizing the near-surface stratigraphy and hydrogeological characteristics of the Paskapoo Formation. According to the analysis of well logs, the well penetrates interbeds of sandstone-shale layers and five porous zones were identified at depths of 28, 39, 50, 62 and 120 m, ranging from 0.34 to 0.58. Among these porous zones, there were three water-bearing zones at depths of 28, 50 and 120 m. Based on the P-wave velocities calculated from the sonic log, surface-wave analysis was carried out to estimate S-wave velocities. Up to depths of 40 m, the respective P-wave velocities for the sandstone and shale units were 2250 m/s and 2080 m/s, and 1220 and 1150 m/s for the S-wave, respectively. Using a microphone-geophone analysis, the airwave was characterized by high frequency and broadband. The integral interpretation of all the datasets and well-logs show a good correlation between all of them. The water-bearing zones (28 and 50 m) and the strong reflector at 76 m were resolved by the surface seismic and VSP data.
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Ground Water Detection by Means of Electrical and EM Methods – The Case of Paliouri, Northern Greece
Authors G.N. Vargemezis, P. Tsourlos, A. Stampolidis, I. Fikos and D. BallasIn this study the resuls of the application of electrical methods (Electrical Resistivity Tomography and Self Potential) and VLF method are presented. Since underground water resources were expected mainly in faulting zones of the ophiolithic outcrops, initially the VLF method has been applied in order to detect faulting zones. In a second phase SP and ERT measurements have been conducted in order to further study the characteristics of the faults. Finally, positions for the wells to be drilled have been proposed.
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Photometric Flowmeter for Measuring Slow Vertical Water Flow in Wells
Authors J. Urík, S. Mareš, M. Kobr and S. RezaninaThe logging tool is equipped with injector enabling to create in the streaming water a colour mark using the food dye Brilliant Blue (E155) as a tracer. A pair of photometric sensors is located in the axis of the flowmeter in a distance 5 and 10 cm above and below the special outlet nozzle of the injector. The probe is capable to measure vertical flow velocities in the range 10E-4 to 10E-2 m/s or vertical flow rates in the range 10E-6 to 10E-4 m3/s. The proper function and reliability of the logging tool were checked in the model of the well under laboratory conditions.
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Recovering Old Geophysical Documents for Deep Aquifer Research
Authors J.L. Plata, M. Riveira and P. IbarraMost of the geophysical field data do not depend on the objectives and date of their measurement, and can be used again with another purpose, whenever its quality and the acquisition parameters allow it. The Geological Survey of Spain (IGME) is involved in the development a new database gathering most of the geophysical surveys carried out within the institution in the last thirty-forty years and is now incorporating data from other sources as the hydrocarbons exploration. For ancient surveys the most normal situation is that original field data no longer exists, and only paper copy documents are available. Two examples of recovering such information are given. They belong to multidisciplinary projects for the development of a methodology for hydrogeological research. In both cases the reprocessing and/or reinterpretation of old documents has provided fundamental information and served as an excellent guide for optimizing the design of new geophysical surveys.
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Quantitative Appraisal of Noise in Time Domain Induced Polarization Data
Authors A. Viezzoli, J. Pedersen and A. PytlichThe possibility to acquire rapidly time domain IP data with multi channel systems is spreading the application of this technique. The quality of such IP data measured in the field is an important, yet not well documented matter. We present a systematic study of noise levels and sources in time domain IP data, carried out with repeated measurements, and different instrumental set ups, in two distinct geological settings. The results show that relative (random) noise to signal values are usually in excess of 10 % at early times, higher at late times. Measuring too early after placing the (stainless steel) electrodes in the ground seems to affect the early times of the decay. Periodic sub harmonics of the 50 Hz can be found in some occasions superimposed to the IP data. It is very important to obtain good electrical contact between electrodes and ground in order to lower both random and coherent noise, in particular capacitive coupling. Inductive coupling is reduced to the minimum using separated cables and electrodes for sending the current and measuring the potential. Full analysis of the whole IP decay curve is crucial for assessing data quality.
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Detection of Hydrocarbon Contamination with 3-D Resistivity and IP Method
Authors B. Neducza and E. TörösThe abstract shows a case history of a 3-D geoelectrical measurement on a contaminated site. It illustrates the advantage of IP measurement with non-polarizable elecrodes furthermore a resistivity/chargeability crossplot is shown for quality control prior to inversion.
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Integrated Interpretation of Geophysical Data on Metalliferous Mining Waste Deposits
More LessThe geophysical investigation of waste dumps has registered a huge progress in the last two decades, benefiting implicitly of a significant improving until present times of the data acquisition, processing and interpretation techniques. The integrated interpretation of many datasets, each belonging to a different investigation method, proved to be the optimum solution for high quality results in the condition of relatively small investments in the data acquisition stage. The resistivity, conductivity and magnetic surveys are some of the cheapest near-surface investigation methods. In this study we present an approach of the integrated investigation of metalliferous mining waste deposits using these three methods.
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Geophysical Imaging of a Tailings Lagoon at an Abandoned Lead-zinc Mine in the Central Wales Orefield, UK
Authors O. Kuras, V. Banks, B. Palumbo-Roe and B. KlinckAs part of ongoing research into the environmental impact of abandoned metalliferous mines in the Central Wales Orefield in the UK, a geophysical study was undertaken in order to characterise mine tailings at the Frongoch Mine near Devil’s Bridge, Ceredigion. A combination of ERT and GPR surveys over a tailings lagoon has revealed the extent of the tailings deposits as well as the presence of underlying strata, including peat and glacial till. Observed resistivities are in accordance with the properties of samples taken from trial pits and with the previous extent of the lagoon indicated on historic maps. It has been demonstrated that GPR can successfully map peat and glacial till layers below the conductive tailings deposits. Under the circumstances, GPR data proved more useful to estimate the thickness of the tailings layer, while ERT may hold more information about superficial cover and bedrock topography. The potential of non-invasive geophysical techniques as a rapid and cost-effective tool in the context of the appraisal and environmental impact assessment of mine waste has been highlighted.
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Observations of the Bioremediation of Oil Contaminated Soils – Application of EM and Resistivity Imaging Methods
Authors B. Zogala, R. Dubiel, M. Robak, W.M. Zuberek and K. WzientekThe survey has been carried out in the area the former military underground fuel base situated in the firing ground Borne Sulinowo (NW Poland). The oil contaminations were revealed in 7 zones in the studied area. In 2003 and 2005 yeast strain Yarrowia lipolytica A-101 suspensions were introduced into shallow bore-holes to intensify the bioremediation process. In succeeding seasons the bioremediation was observed and monitored using EM and resistivity imaging measurements. The study showed the significant measurable change in soil electrical properties after several months since the yeast suspension had been introduced into hydrocarbons contaminated soil. The bioremediation can be efficiently observed and monitored with geoelectrical methods such as resistivity imaging or electromagnetic induction ones.
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Potential of Geoelectrical Imaging Techniques for Detecting Subsurface Gas Migration in Landfills – An Experiment
Authors T. Dahlin, H. Rosqvist, V. Leroux, M. Svensson, M. Lindsjö, C.H. Månsson and S. JohanssonWe measured eleven regularly spaced resistivity profiles over a ten by ten meter experimental plot where high landfill gas (LFG) emissions had been recorded on the surface. The measurements were repeated seven times during four days which made it possible to trace the development of resistive areas. The LFG emissions at the surface of the landfill were estimated with laser absorption spectroscopy and static chamber measurements at the same time as resistivity surveying. By studying the changes of resistivity with time it was indicated that the gas migration in the waste mass was a relatively fast process, changing within only a few hours. Our presentation will focus on details regarding temporal and spatial changes of measurement.
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Airborne Geophysical Studies around Landfill Areas – Case Histories from Finland
Authors J. Lohva, T. Jokinen and I. SuppalaGeological Survey of Finland (GTK) had completed an airborne low altitude geophysical mapping survey covering the whole Finland. In recent years airborne data have been used in environmental applications related to landfill, groundwater and soil contamination. In this paper we present aerogeophysical studies of landfill areas in Finland. The first examples are from closed landfills, Mäntyvaara in north Finland and three landfills in Helsinki region. Mäntyvaara landfill was the first case in Finland, where AEM data were used for mapping contaminated areas. The other examples are monitoring cases. The airborne measurements have been conducted four times over the Ämmässuo and twice over the Metsä-Sairila landfills. Using GTK's three-in-one airborne system to locate possible leakages is a useful method. The AEM out-of-phase component is sensitive to detect small changes in conductivity. AEM modelling and inversion are useful tools for estimating detailed conductivity changes. Magnetic data can be used to interpret linear features of bedrock to detect possible path of leakages. Radiometric data can estimate soil thicknesses and the changes of moisture. The changes in the environment can be observed more reliably from natural condition if repeated measurements are carried out.
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Geophysical Investigations of a Chemical Munition Dumpsite in the Baltic Sea
By T. MissiaenVery high resolution seismic and magnetic investigations were carried out over a chemical munition dumpsite in the south-western Baltic Sea. The shallow geology was imaged in great detail on the seismic data and seven seismic-stratigraphic units were identified, related to different stages in the Holocene and late-glacial history. A large number of diapir-like features were observed that most likely represent fluid expulsion phenomena. Seismic and magnetic data indicate the presence of a large number of buried objects. In most cases there is a good correlation between the two data sets. The objects are buried no deeper than 1-2 m. Their size varies between 1.5 and 5 m, occasionally up to 10 m. The data confirm the wide variety of dumped war material ranging from bombs and shells to encasements and containers. The distribution of the buried objects seems heterogeneous with locally high object concentrations surrounded by areas of lower object density. The results of this case study demonstrate the benefit of complementary, concurrent geophysical investigations for munition dumpsite research. This will yield a better assessment of the current status of the dumpsite and possible ecological risks related to the dumped war material.
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Monitoring Changes in Bentonite at the Grimsel Test Site Using Crosshole Seismic Tomography
Authors S. Marelli, H.R. Maurer, E. Manukyan, S.A. Greenhalgh and A.G. GreenWe have been exploring the feasibility of monitoring the swelling of bentonite using non-intrusive crosshole seismic tomography. Our study has included numerical simulations and extensive field experiments at the Grimsel Test Site in the central Swiss Alps. Numerical simulations have demonstrated that changes in bentonite's elastic properties resulting from variations in water-saturation conditions should yield measurable effects in crosshole seismic data. Based on the simulation results, a relatively thin block of encased bentonite at the end of a tunnel was surrounded by six inclined boreholes. Seismic energy sequentially generated in three of the holes was detected by hydrophones in the other holes. The recorded seismic wavefields, characterized by high signal-to-ambient-noise ratios and coherent frequencies of 1-4kHz over propagation distances exceeding 20m, have provided sufficient information for meaningful high resolution tomographic inversions. Varying the water-saturation conditions resulted in marked changes in the transmitted and reflected wavefields that were readily observable in the raw data. Source and receiver coupling effects also varied significantly during the course of our experiments, and unusual phases due to wave propagation in the source and receiver cables and to borehole-related phenomena were prominent features of the seismic sections.
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Underground DC-geoelectrical and Seismic Surveys to Investigate Weakness Zones in Salt Rock Formations
Authors C. Schuetze, A. Just, C. Ruecker, H. Zoellner and T. SchichtThe results of a research project concerning the safety assessment of underground waste disposal sites using non-destructive subsurface geophysical surveys within salt rock are presented here. The geophysical techniques were methodically adapted to detect possible weakness areas, like fault zones, disturbed rock zones and fractures with brines. The recent results concern DC-geoelectrical and seismic investigations within disused potash mines in Central Germany where brine reservoirs were supposed in the fractured anhydrite layers. The geoelectrical investigations served to determine areas with higher water content, the seismic surveys were useful to image layer boundaries of the geological units. The combined interpretation of the methods leads to more reliable information on the occurrence of potential weakness areas within the salt rock.
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Comparison of Profiling Results of Attenuation and Velocity of Refracted P-wave in Coal-seam
Authors Z. Szreder, Z. Pilecki and J. KlosinskiIn the paper the problem of effectiveness of seismic profiling of attenuation and velocity of refracted P-wave in coal-seam along mining excavation in conditions of coal-seam edge impact has been presented. For practical use of seismic attenuation profiling, special measurement and interpretation methodology has been prepared. The measure of calculated attenuation is the value of attenuation coefficient based on determination of amplitude of refracted P-wave. In the paper examples of attenuation coefficient and velocity of refracted wave changes in the zones of exploitation seam edge in one of the Polish coal mines have been described. The usefulness of attenuation coefficient for such analysis has been underlined due to its greater sensitivity for changes of the state of stress and deformation in coal-seam comparing to changes of refracted P-wave velocity. It is shown that attenuation can be an interesting parameter complementary to refracted wave velocity in coal-seam, especially in high stress conditions.
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Intensity Scale of Mining Seismic Events – GSIGZW
Authors G. Mutke, J. Dubinski, A. Baranski and A. LurkaIn Polish hard coal mines seismic events occur very often and reach value of magnitude up to 5,0. In the epicentre areas strong mining tremors cause slight damaging effect in buildings. The new criteria of empirical assessing influence of mining tremors on the surface are presented. The so far applied scales, mainly borrowed from the earthquake science or worked out for paraseismic vibrations, have not delivered an authoritative assessment of consequences of vibrations caused by mining tremors. For the purposes of assessing the influence a conception was adopted, that is based on the parameters of ground velocity amplitude, and vibration duration of velocity. As a result of two-year’s science work a new empirical intensity scale of mining seismic events, GSIGZW for assessing the impact of mining tremors on buildings has been worked out. It was elaborated by correlating observed consequences with recorded parameters of vibration velocity and time duration of ground motions. A very good fit are observed between the vibration intensity estimated by means of the GSIGZW scale and the macroscopic observations of their consequences at the Coal Company’s collieries – Poland. The scale GSIGZW is successful used in Polish hard coal mines.
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Dynamics of Rupture Process of Selected Seismic Events at Rudna, Poland Copper Mine
Authors W. Debski and B. DomanskiUnderstanding the physics of the rupture processes of the mining induced seismic tremors has two important aspects. The first one is purely scientific. The induced seismicity provides the bridge between well controlled laboratory experiments on rock samples and natural earthquakes. It concerns both the size of the induced tremors as well as the measurement conditions. On the other hand, the analysis of the rupture processes of induced events is very important for an improving the safety of the mining as it is a key factor for the accurate estimation of the seismic hazard. In this contribution we present the preliminary results of an analysis of the dynamic stress drops for selected events at Rudna, (Poland) copper mine.
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Embankment Dam Seepage Evaluation from Resistivity Monitoring Data
Authors T. Dahlin, P. Sjödahl and S. JohanssonMethods for monitoring seepage are important for dam safety of embankment dams. Increased seepage may be associated with internal erosion in the dam, and internal erosion is one of the main reasons for dam failures. Internal erosion progresses inside the dam, and is difficult to detect by conventional methods. Therefore, there is a need for new or improved methods. The resistivity method is a non-destructive method that may accomplish this task. A method for evaluating the seepage from resistivity monitoring data is theoretically described and tested for four selected areas in the foundation of the Sädva dam. Seasonal resistivity variations are apparent in the reservoir as well as inside the dam. The four selected areas represent areas with low, via intermediate to high variations in the seasonal resistivity variation. The areas are compared qualitatively and thereby permeable zones within the dam may be identified. Quantitative assessment of the seepage flow is also carried out as an initial test of the described method. It is concluded that the experiences from the Sädva dam are valuable for the application of the resistivity method on embankment dams. The presented method is a promising first step for quantitative assessment of seepage.
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Geophysical Investigation of Earth-fissures by Means of ERT
Authors P. Tsourlos and G. VargemezisEarth fissures tend to be an issue of increasing concern, especially at the margins of sedimentary basins at semi-arid areas, since they cause damage to houses, commercial buildings, or infrastructure. In this work we present case studies involving relatively high resolution ERT measurements carried out at areas of earth fissures at the margins of sedimentary basins in Greece. From the results presented in this work, it is obvious that the application of the technique of electric tomography provides important information on the structure of subsoil in the investigated regions and is capable to locate regions in which earth fissures are likely to take place.
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Detection, Localization and Backfill Assessment of Old Mining Shafts in the Upper Silesia Coal Basin with Geophysical Me
By A.M KotyrbaThe old shafts can create serious hazards for safe use of post mining lands. They complicate engineering conditions and need special geotechnical solutions to be applied when have to serve as structures bedrock. The paper describes experiences from application of various geophysical methods for old shafts detection and characterization. It concerns mostly gravity surveys that are compared with supplementary resistivity and GPR surveys, conducted on areas of abandoned coal and ore mines in the Upper Silesia Coal Basin (UCSB - Poland.). As an introduction to applications several geophysical models of post mining subsurface structures are described and discussed.
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Tunnel Seismic Investigations – 3D Imaging
Authors J. Tzavaras, K. Groß, S. Buske and S.A. ShapiroThe project OnSITE (Online Seismic Imaging for Tunnel Excavation in hard rock) aims at developing an integrated approach for seismic imaging around the tunnel and prediction ahead of the tunnel face during construction work. We present the recent results of our subproject which is concerned with the determination of the spatial locations of faults, fractures and heterogeneities by advanced seismic imaging techniques. So far we have extended existing 2D imaging techniques to 3D. We have implemented 3D versions of Kirchhoff prestack depth migration(KPSDM), Fresnel-Volume-Migration (FVM) and Reflection-Image-Spectroscopy (RIS). The application of the mentioned imaging techniques to data from the Gotthard base tunnel show significant improvements compared to standard (KPSDM) processing. The 3D RIS approach suppresses scattering effects in the low frequency band and increases resolution in the high frequency band so that some of the geological structures are much better visible. The 3D FVM technique uses slowness and polarization based estimates of the emergence angles at the receivers and restricts the imaging to the region around the actual reflection or diffraction point. We observe less spatial ambiguity and a higher resolution of most structures in the stacked section as well as in common receiver gathers.
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Geophysical Anatomy of the Hollin Hill Landslide, North Yorkshire, UK
Authors J.E. Chambers, A.L. Weller, D.A. Gunn, O. Kuras, P.B. Wilkinson, P.I. Meldrum, R.D. Ogilvy, G.O. Jenkins, A.D. Gibson, S.J. Ford and S.J. PriceGeophysical methods are playing an increasingly important role in the investigation and monitoring of landslides; such methods are proving to be particularly effective for revealing the 3D structure, failures surfaces, and the hydrogeological regimes associated with rock and earth slides. In this paper we present the results of a geoelectrical reconnaissance survey of the Hollin Hill landslide, UK. This work was undertaken in advance of the installation of a permanent geophysical and geotechnical monitoring system, and was designed to assess the suitability of resistivity (resistivity mapping and 2D/3D ERT) and self-potential methods (profiling and mapping) for investigating and monitoring this site. In particular, we were concerned to assess the electrical property contrasts and the magnitude of SP response across the study area. The surveys revealed that there was a good resistivity contrast between the slipped material and sandstone bedrock, which allowed us to use resistivity mapping data and ERT models to define the geometry of the landslide. An SP signature consistent with the movement of groundwater through the landslide was observed at the site, and was used to identify seepage patterns associated with the slipped material.
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Application of GPR Scanning for Landslide Investigations in Polish Carpathians
More LessLandslides in recent years became a serious problem and were causing economic losses in Poland. They are formed on steep Carpathian slopes of flysch sediments. Built from many thin layers of marine sedimentation, soil-rock type deposits were difficult for in-situ and laboratory tests. It required effective techniques of investigations. Traditional drillings were very important but time consuming, costly and not always gave answer for the entire question in mass movement investigations. Some types of geotechnical tests were proper for soils some other only for rocks. Therefore good quality core drillings always should be connected with other types of field measurements and monitoring techniques chosen with respect for the landslide type and its internal structure. Ground penetration radar GPR measurements were found to be a one of very useful methods in conjunction with other geotechnical engineering tests. Using GPR surveys landslide depths were recognized at 1-20m below the natural terrain level. Over 10 km of GPR measurements was performed on 24 landslides inside the project for Polish Roads Authorities, financed by European Investment Bank. In the paper conclusion from using GPR, together with diamond impregnated core drillings, ground movement measurements, groundwater monitoring and geotechnical laboratory tests are described.
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