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Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 06 Sep 2010 - 08 Sep 2010
- Location: Zurich, Switzerland
- ISBN: 978-90-73781-88-7
- Published: 06 September 2010
51 - 100 of 153 results
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The Use of Seismic Noise for Assessing the Potential Failure of an Overhanging Column
Authors C. Levy, L. Baillet and D. JongmansRock falls are difficult to predict events, owing to the suddenness of the phenomenon and to the little information available about the internal structure of the mass. In this paper, we apply the seismic noise recorded by autonomous monitoring systems for evaluating the degree of coupling of a prone-to-fall column to the rock massif. From numerical studies and in-situ seismic noise measurements, we show that natural frequencies of a rock column drop with the breakage of rock and increase with the presence of ice in winter. This frequency evolution can be followed until the column fall, using seismic noise measurements.
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Investigating a Creeping Hillslope by Layer-based Inversions of Seismic First Arrival Times
Authors M. Rumpf, U. Böniger and J. TronickeDetailed structural and geological models of the shallow subsurface are essential to understand the complex processes associated with creeping mountain or hill slopes. In this study, we present two seismic profiles collected across a creeping slope in the eastern Vorarlberg Alps (Austria). We use a layer-based inversion procedure known as the generalized linear inversion (GLI) method to invert the first break arrival times from both profiles. The resulting P-wave velocity models illustrate that, at this site, four layers are required to explain the traveltime data and to result in reasonable geological structures. These models can be easily interpreted using available borehole information. Our results demonstrate that the major relevant subsurface structures (e.g., depth to bedrock), which are believed to be critical in order to understand the mass movement at this site, can be sharply imaged by the used inversion strategy.
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Integrated Seismic 3D-processing for Tunneling
Authors D. Krüger, R. Groschup and T. DickmannFor tunnel constructions in difficult geologic conditions a reliable prediction system is necessary. Reflection seismic measurements from within a tunnel are a suitable tool to predict important engineering parameters of the rockmass to be excavated. To use this technique efficiently in the tunneling environment an integrated system of tunnel suited recording equipment and processing software is essential. In tunnel seismology data is often sparse and results have to be available in short time after a measurement campaign. Thus processing has to be integrated in a semi-automated approach effectively perfoming algorithms such as dip-filtering, inverse Q-filtering, 3D velocity analysis and 3D migration. We present these steps as they are implemented in the novel TSP (Tunnel Seismic Prediction) System.
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A New Petrophysical Model for Describing the Pressure-dependent Acoustic Velocity in Rocks
Authors J. Molnar and M. DobrokaThe pressure dependence of the velocity and absorption coefficient of seismic/acoustic waves in rock is an extensively explored rock physical problem. Based on simple physical assumptions, a new petrophysical model is developed, through which a relationship between velocity and pressure is set up and explained. The material parameters of the model are determined by using laboratory measurements data and linearized inversion method.
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Linear Spread Configuration in the MASW Method
Authors Ü. Dikmen, M.O. Arisoy and I. AkkayaIn this paper, we evaluate the effect of geophone configuration on dispersion image for linear spreads in the MASW method. To reveal the spectral resolution of dispersion images, MASW records were collected using nine different types of linear spread configuration, then dispersion image for each record were calculated using phase velocity analysis technique, and were compared to present preferable linear spread type for a MASW survey. The results show that, better dispersion image resolution and dispersion curve in the wide frequency range can be obtained by using an SL-type geophone configuration. Besides this, in a survey in which higher frequencies are more important, an SII-type configuration may be preferable. With the SII-type spread configuration, the fundamental mode can easily be distinguished from the higher modes.
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Site Characterization of São Sebastião Volcanic Crater Using Geophysical Tests
Authors I. Lopes, M. Mendes, G.P. Deidda, C. Strobbia and J.A. SantosThe Azores archipelago is located at the North America, Eurasia and Africa triple junction showing high seismicity and volcanism typical of an active plate boundary. Terceira Island has been affected by several damaging earthquakes and the 1980 January 1st earthquake (M=7,2) was the strongest in Azores during the last century. The São Sebastião volcanic crater, located at SE of Terceira Island, is characterized by an amplification of ground movements inside the crater with respect to the surroundings. Inside this volcanic crater, is located the village of São Sebastião that has shown an anomalous seismic behaviour in a more local expression during the past earthquakes. The local geology and geotechnical characteristics of the soil deposit play a strong role in the seismic ground shaking at the site. Some previous geophysical studies with information merely qualitative were not able to perform a rigorous and well-defined model of the crater. A recent survey added new information that allows a better comprehension of the damage distribution, since it indicates inside the crater a basaltic lava layer controls the seismic behaviour of that area, serving as a protection to the buildings in the SW region of the crater.
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3D Cross-hole Electrical Resistivity Tomography to Control the Injection of Expanding Resins: A Case History in Venice
Authors G. Morelli, F. Fischanger, D. Gualerzi, M. Occhi, G. Ranieri and G. SantaratoInjection of expanding polyurethane resins has recently become a widespread technique to improve the geotechnical characteristics of shallow soils for consolidation purposes. Although resins injection practice has revealed to be a satisfactory solution for the above mentioned problems, a continuous monitoring of the process is needed to achieve optimal results. In order to control the injection of expanding resins a field procedure has been developed, based on the use of three dimensional Electrical Resistivity Tomography. The choice of electrical resistivity, as a parameter for designing, monitoring and validating the homogenisation task, is based on the basic assumption that such a property is the most sensitive to water content. During the injection stage, repeated ERT acquisitions allow to control the process and to modify the injection program/parameters, whenever necessary. In this paper the procedure and its results are illustrated, through a case history in Venice (Italy), where environmental problems, due to the presence of salt water bearing soils, had to be taken into account. Therefore a specific, careful analysis of arrays of electrodes and selection of the main parameters affecting ERT resolution had to be performed in advance.
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Multi-method High Resolution Geophysical & Geotechnical Quick Clay Mapping
Authors A.A. Pfaffhuber, M. Bastani, S. Cornée, M. Rømoen, S. Donohue, T.E. Helle, M. Long, P. O'Connor and L. PerssonQuick clay is highly sensitive, marine clay with an unstable mineral structure due to post glacial heaving and consequent leaching of saline pore fluids by surface- and groundwater. Extended quick clay layers pose a serious geo-hazard in Scandinavia and North America and need to be delineated in detail. Geophysical methods, especially resistivity methods, have been tested for small scale quick clay mapping at a research site close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Controlled Source Radiomagnetotellurics (CSRMT) and integrating them to geotechnical borehole data with the help of a resistivity logging tool (RCPT) we confirm the value of this integrated study for quick clay hazard zonation. ERT is an ideal tool to interpolate limited borehole results and thus to provide a more cost efficient and detailed result than with boreholes alone. Our resistivity data from ERT, RCPT and lab measurements are consistent and appear isotropic.
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Imaging of the Shallow Alpine Fault Zone (New Zealand) Using 2D and Pseudo 3D Seismic Reflection Data
Authors A.E. Kaiser, H. Horstmeyer, A.G. Green, F. Campbell, R.M. Langridge and A.F. McClymontThe Alpine Fault Zone in New Zealand's South Island is part of the boundary between the Australian and Pacific tectonic plates. We use ultra-high-resolution 2D and high-resolution pseudo-3D seismic reflection imaging to investigate the structure of the fault and adjacent subsurface in a northern part of the Island. Employing acquisition schemes that minimise field effort while yielding high-resolution data together with fine-tuned processing strategies, we derive high-quality images of the shallow subsurface down to about 200 m depth. Gravels with a maximum thickness of ~50 m are seen on the two sides of the fault. On the hangwall side, these gravels overlie up to ~90 m of glaciolacustrine sediments.The basement underneath is interpreted to be an erosional surface that has been folded and faulted. The principal strand of the Alpine Fault Zone is shown to have a dip of 75° - 80° from the surface to the basement at ~60 m depth. The dip at greater depths is not well constrained, but is likely to range from 50° - 80°. The ~25 m apparent vertical offset of the basement yields an average dip-slip rate of 2.0 ± 1 mm/yr.
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Reflection Seismic Surveying Across the Alpine Fault Immediately North of the Intersection with the Hope Fault
Authors S.F.A. Carpentier, A.G. Green, H. Horstmeyer, A.E. Kaiser, F. Hurter, R.M. Langridge and M. FinnemoreThe Alpine Fault in New Zealand is one of only a few active transform fault boundaries worldwide that offsets continental terrains. Although paleoseismological data suggest that large earthquakes have struck the Fault in the past, it has been quiescent for at least the past 200 years. We have acquired five high-resolution seismic reflection profiles across the Alpine Fault a little to the north of its intersection with the Hope Fault. Slip-rates on the Alpine Fault appear to decrease by ~50% in crossing the Hope Fault from south to north, and at our study site the faulting is distinguished by step-overs and/or multiple strands. Accordingly, the goals of our seismic investigation are to define the structure of the Alpine Fault in the top few hundred metres of the subsurface with emphasis on defining its step-over or multi-strand character at the study site. Our seismic images contain evidence for significant changes in geometry across one major fault strand and an astonishingly abrupt transition from a highly reflective region to a non-reflective region across another major strand.
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Geophysical Survey for Crypt Detection in the Church of Virgin St. Mary’s Birth in Horné Krškany, Slovakia
Authors J. Panisova, R. Pasteka, W. Rabbel, M. Bielik, T. Wunderlich and J. PapcoGeophysical methods are an essential part of non-destructive prospecting methods in archaeology. Geophysical measurements for crypt detection were performed in the interior of the Church of Virgin St. Mary’s Birth in Horné Krškany during the International course on ArchaeoGeophysics INCA 2009 (European Union LLP-Erasmus programme). The church is a valuable representative of Slovak historical heritage. Ground-penetrating radar (GPR) and microgravity techniques independently confirmed the presence of two crypts in the nave. Obtained results are very expressive by the character of the deformation of geophysical fields. Recommendations for video inspection or archaeological opening were also given to the archaeological site-excavators.
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Restoration of the Historico-architectural Plan of the Voskresensky New-Jerusalem Monastery by Geophysical Methods
Authors A.V. Urusova, A.A. Ivanov, V.V. Monakhov, N.G. Pudova and M.P. ShirobokovToday different geophysical methods are often applied in related fields of science. Thus application of geophysics in archeological exploration has actually separated into an individual branch characterized by specific methods of observations and data interpretation. Search and identification of archeological objects, especially those not distinguished in the relief of the daylight surface, is the issue of the day. Despite the fact that application of geophysical methods in archeology is half-century old, the surveyors are adding new effective methods and technologies to their stock. First of all we should mention GPR-position. However, it is good to remember well-behaved methods of resistance and magnetic exploration known since the middle of the last century. Russia as the rest of the world is currently favoring an integrated approach involving various geophysical methods which are jointly used for exploration of archeological monuments. Integration allows significant expansion of the circle of the tasks solved and increase in reliability of the results. In this report the authors intend to turn the searchlight on the matters related to geophysical mapping and data processing, and to demonstrate some results obtained during exploration of the archeological objects.
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Integration of Satellite Imagery, Airborne Radiometric and Regional Geochemical Data Sets for Mineral Exploration
Authors D.G. Eberle and H. PaascheCluster analysis algorithms enable the rapid and objective integration of multi-method data bases with unknown parameter relationship between the individual data types present in the data base. We are employing the fuzzy Gustafson-Kessel (GK) cluster algorithm to integrate a data base comprising 2D information from Landsat satellite imagery, airborne radiometric and regional geochemical data acquired over a survey area in the Northern Cape Province of South Africa. We are combining the structural information provided by satellite and airborne radiometric data with regional geochemical soil sample data to obtain an objective 2D classified zoned map reflecting sub-surface lithology. Ground truth control supports to ascribe the various clusters to lithology and generate mineral exploration target areas.
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Noise Sources for a Fixed Wing Airborne EM System, Quantified by Means of 3D Finite Element Modelling
Authors L. Rabenstein, S. Hendricks and C. HaasWith a 3D finite element model study the authors quantified noise occuring during aeroplane based electromagnetic measurements for the purpose of sea ice thickness determination. Namely, these are pitch and roll of the aircraft, electromagnetic coupling between aeroplane and ocean, and wing flexure. All effects are significantly amplified by the presence of the conducting sea water and for flight heights of 30 m over the ocean these effects can change the signal by about 10 percent or, in the case of wing flexure, by 100 percent. Roll, pitch and wing flexure signals have an inductive and a geometric contribution. For highly quantitative measurements such as sea-ice thickness, where a vertical resolution of 0.1 m is desired, all these effects must be taken into account. Most of the strong wing flexure signal appears on the in-phase component only, henceforth the quadrature component should be taken for sea ice thickness retrieval even when it encounters a weaker ocean response signal than the in-phase component.
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Validating SkyTEM Data Against Ground-based TEM Data at the Danish National Test Site by Upward Continuation
Authors N. Foged, A.V. Christiansen and E. AukenIn this abstract we show the results from a comparison of models from the SkyTEM-system with the ground based models at the extended test site. The comparison will also be carried out in data space by an upward continuation of the reference models to a SkyTEM-system response at nominal height. Furthermore, the repeatability of the SkyTEM-system at different heights is demonstrated. The national TEM test site was established to ensure that any TEM system used in the ongoing Danish groundwater mapping campaign can reproduce the reference models and match the upward continued responses of the reference model. The site was established in 2001. In 2009 the test site was extended from a point location with a known reference response and resistivity model to two crossing lines. This makes calibration and validation possible for airborne TEM-systems that cannot make hovering spot measurements. SkyTEM and VTEM have been on the new test lines so far. For the SkyTEM system we can conclude that it reproduces the ground-based TEM data and models very good and that SkyTEM data shows consistent results for different altitudes. In the data space we calculate an upward-continued dataset to perform the comparisons.
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Airborne EM Surveys Applied to Industrial Land-use Legacy Assessments in the UK
By D. BeamishRecent high-resolution airborne EM surveys in the UK have identified previously unknown, non geological, highly conductive environments in the near-surface. The surveys have covered a wide range of land-use contexts many of which have several centuries of industrial activities. Two case studies from southern Scotland are used to demonstrate the magnitude and scale of the high conductivity zones that may be encountered. The examples indicate that some of the most complex subsurface flow paths arise in the case of areas associated with coal extraction; here near-surface shafts and adits connect with shallow worked seams many of which are ancient and lost from modern records.
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Determination of Ground Thermal Properties for Dimensioning Large-scale Geothermal Heat Pump Systems
By L. RybachThe energetic performance of geothermal heat pump systems depends strongly on the local ground conditions. The key property dominating the performance is the ground thermal conductivity. Reliable values are needed for the design of large-scale systems. These can be determined in-situ. Site investigations, by specific equipment and procedures (wireless temperature logger, repeated measurements, numerical model simulations), provide the vertical thermal conductivity profile, along with the temperature profile. These are especially needed for systems foreseen for space heating and cooling.
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Estimation of Geothermal Reservoir Properties Using an Ensemble Kalman Filter Approach
Authors C. Vogt, C. Kosack and G. MarquartFor the assessment of a geothermal reservoir reliable knowledge of the physical rock properties as permeability, porosity and thermal conductivity are of primary concern. In our study the Ensemble Kalman Filter (EnKF) is used to estimate the permeability distribution in a geothermal reservoir. The EnKF is a stochastic technique based on a forward propagation of an ensemble of realizations. At successive instants in time, different kinds of data measured at various drill holes are collected in one data vector and used to update the system variables to improve the match between observation and simulation leading to a convergence of the ensemble. To demonstrate the method we studied a pumping test of a chemical tracer injected in one borehole and retrieved at a few others. As observation data we used bottom hole pressure and tracer concentration. We applied the method to a synthetic 2D test case and to the European EGS test site Soultz-sous-Forêts, France. We investigated the quality of the estimation depending on the data noise, the system error, a damping parameter and repeated application of the filter. Correct tuning of the filter allows a satisfying recovery of the permeability field with reliable error estimate.
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Microseismic Monitoring for Observation of Reservoir Processes – A Case study of the EGS Project Basel
Authors O. Genoni, A. Ebert and M.O. HäringDevelopment of the EGS reservoir in Basel was monitored by an array of six downhole geophone. An additional geophone was installed at 4422 m.b.s.l. depth for the initial phase of the stimulation. The calibration of the required velocity model was a crucial task prior to localization of events. The velocity model was derived directly from the microseismic data set. During a ten-days stimulation period, approximately 13’500 events were recorded, of which about 3’000 could be located in real-time with great spatial precision, allowing for a detailed study of the evolving reservoir. The event rate decreased within few months reaching a recent rate of approximately one event every two weeks. The monitoring network is still working with 4 stations.
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Pedophysical Study of Electrical Properties and Water Content, Laboratory and Field Experiments
Authors T. Wunderlich, S.A. al Hagrey, H. Petersen and W. RabbelWithin the iSOIL project we are developing a mobile multisensor platform to gain fast, accurate areal information of soils at the field scale. The measured geophysical attributes (e.g. electric, electromagnetic) need to be linked to soil parameters (e.g. clay and water content) via geophysical pedotransfer functions, GPTFs. These could either be empirical (e.g. Archie and Topp et al. Equations) or physically-based (e.g. mixing and effective medium models). Our interdisciplinary activities aim to develop robust GPTFs using diverse laboratory and in situ experiments. In laboratory experiments we investigated the influence of water content on di-/electrical measurements. The dried samples collected from different sites and depths were uniformly saturated with rain water in steps of ~2% pore volume. For each step we measured georadar reflections and geoelectrical resistivity as well as water content and temperature. Also independent permittivity measurements were conducted to validate GPTFs results. The geophysical results were complemented by a suite of standard soil parameters determined on the samples. From these lab and field data the relationship of water content with respect to both the dielectric permittivity and electric resistivity are established. Different empirical and mixing models are fitted to the data and discussed.
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Influence of Pore Fluid Conductivity and Water Content on the Complex Conductivity of Peat
Authors M. Ponziani, H. Vanhala, E.C. Slob and D.J.M. Ngan-TillardThis paper presents a new prototype cell that can perform simultaneous electrical and compression tests on peaty soils. Two different set-ups for measurements at high and low frequency are used. Results on the complex conductivity response of peat samples for different pore fluid salinities and degrees of compression are presented. This work presents reproducible results that can be utilized to improve interpretation of field measurements on peat.
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Bedrock Interface Detection for Sand and Gravel Mineral Reserve Assessment Using 3D ERT
Authors J.E. Chambers, P.B. Wilkinson, S. Penn, A.L. Weller, R.D. Ogilvy, O. Kuras, P.I. Meldrum and S.G. PearsonA study describing the use of 3D ERT to determine depth to bedrock, and hence mineral thickness, at a sand and gravel extraction site within river terrace deposits is presented. Two approaches are considered which automatically extract interface depths from 3D ERT models. The first assumes that the interface is located at the maximum slope of the resistivity-depth curve, and is therefore referred to as the ‘steepest gradient method’ (SGM). The second uses an intrusive sample point to calibrate the model by identifying the resistivity iso-surface associated with the interface, and is referred to here as the ‘known interface method’ (KIM). The results of these two approaches have been tested against direct GPS observations of the interface position that were made after the bedrock had been exposed by quarrying of the river terrace deposits. In this case, the use of intrusive data for model calibration (i.e. the KIM) was essential for recovering accurate depth information from the 3D ERT model. Reliance upon using the steepest resistivity gradient as an indicator of a geological boundary (i.e. the SGM) produced a severe overestimate of interface elevation.
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Prospecting Bauxite Horizons with GPR on Laterite Deposits at the Amazon Basin, Brazil
Authors M.A. Barsottelli-Botelho and J.L.G. GouveiaThe GPR was applied on areas with bauxite occurrences to see its capability of discriminate the different weathered horizons and, mainly, to detect the presence of bauxite horizon and also measure its width. The GPR was tested on two bauxite occurrences in Amazon Bazin, Brazil. The good quality of the radargrams was expected since the electric resistivity of the soil, evaluated by Vertical Electrical Sounding (VES) reveals a near surface resistivity varying from 450 to 1600 Ohm.m. The GPR data acquisition with constant-antenna-offset results in clear GPR sections where it was possible to separate four zones or horizons as well as its thicknesses. Each zone shows a different reflection pattern or radarfacies, which helps to differentiate them. The first horizon corresponds to the Belterra Clay, which is the most superficial layer, (AT); below, there is the laterite sequence composed by more three horizons, where the second one is a diffractions zone, which corresponds to a clay with concretionary laterites (ND); the third zone shows a homogeneous pattern and it corresponds to a massive ferruginous horizon (LT); an fourth zone shows discontinuous and small reflectors and/or chaotic pattern which corresponds to the main bauxite horizon (BX).
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Effective Dynamic Permeability for Seismic Waves in Inhomogeneous, Porous Media Using White's Patchy Saturation Model
Authors S.A. Greenhalgh, X. Liu and B. ZhouWe obtain an effective dynamic permeability model for a porous acoustic medium having mesoscopic heterogeneities of spherical inclusions by applying the non-self consistent theory to the scattered wavefield from a single spherical inclusion. At low frequency, our dynamic permeability tends to the real value of the effective static permeability, which depends on the host medium for small inclusion concentrations. This result is compared with the effective hydraulic permeability by replacing the permeability of the components with their dynamic values as determined from the JKD model For frequencies lower than the resonance frequency, these simple dynamic permeability models are very similar to those determined from the scattering model of this paper. However, the model which uses the arithmetic mean of the constituent phase permeabilities is not valid for inclusions having higher permeability than the host phase, and the model which uses the harmonic mean of the constituent phases is not valid for inclusions having lower permeability than that of the host material. In both cases, the effective dynamic permeability calculated from the static model of random spherical inclusions is in good agreement with our scattering model.
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Extraction of Permeability Variations from P-Wave Velocity Dispersion Data
Authors B. Milkereit, W. Pun and B. HarrisPermeability is a key parameter for the evaluation of subsurface formations in groundwater and hydrocarbon exploration. We utilize broadband full-waveform sonic data to exploit Pride's relationship between P-wave velocity dispersion and permeability for porous, fluid-filled media. Frequency dependent P-wave velocities are extracted from multi-channel sonic data during a two-step process: computation of semblance-based velocity spectra at two or more center frequencies followed by a 2D cross-correlation of the velocity spectra. A comparison with MRI-derived permeability logs confirm that P-wave velocity dispersion logs can be used to map permeability variations.
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Grounded Ice-sheets and Glacial Deposits within the Aberdeen Ground Formation of the Central North Sea
By F.A. BuckleyExploration and site survey seismic data from the Central North Sea have been analysed in a re-appraisal of the lithological characteristics of the Early to Middle Pleistocene Aberdeen Ground Formation. Several features, interpreted to result from grounded ice-sheets, are described and the implications for encountering a variety of geo-hazards in the top-hole sections of North Sea wells are considered. A regional seismic event, interpreted to represent near-base Quaternary is also described.
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Volumetric, Multi-fold Acoustic Interrogations of Complex Sub-seabeds
Authors J. Guigne, J.K. Welford and I. McDermottFor many marine applications in the offshore renewable energy and oil and gas industries, high resolution images of the shallow structures beneath the seabed are crucial for minimizing risks to infrastructure, personnel and investments. For both the emplacement and maintenance of piles and caissons for offshore wind turbines and oil platform supports, knowledge of drilling impediments like boulders and of the stratigraphic layering in which these supports are anchored can be invaluable to safe operations. We introduce a tandem approach of applying synthetic aperture sonar and high-resolution multi-fold seismic reflection techniques for obtaining such images in challenging marine site investigations.
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Evaluation of Properties of Morainic Soils Based on Shallow Seismic, Electrical Imaging Surveys and CPT Data
By V. LisinsThe evaluation of physical and mechanical properties of morainic soils based on shallow seismic, electrical imaging surveys, cone penetration testing (CPT) and standard penetration testing (SPT) data is considered. Shallow seismic investigations include the utilisation of reflected, refracted and surface wave techniques. The obtained correlation dependences were used for the prediction of the properties of morainic soils at a hardly accessible site of the river bed downstream the reservoir dam, drained during short-term stoppages of the operation of a hydroelectric power plant.
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Robust Weighted-mean Approach for the Evaluation of Ground Improvement Columns Using Surface Wave Analysis
Authors A. Madun, I. Jefferson, K.Y. Foo, P.R. Atkins, D.N. Chapman and M. CulshawThis paper demonstrates the application of surface wave analysis specifically for the evaluation of ground improvement columns, and considers the susceptibility of phase measurements to noise associated with this technique. This is addressed by introducing a robust approach of calculating the phase velocity based upon a weighted-mean approach that uses the signal-to-noise ratio (SNR) of the signal as a variable cost function to obtain an optimal phase velocity estimation. Experimental data was collected from a laboratory scale setup of a 0.1 m diameter x 0.15 m depth gravelly-sand column installed within a 0.6 m x 0.3 m x 0.3 m block of homogenous kaolinite-clay. Results from the measurements indicated Rayleigh-wave phase velocities of 59 m/s and 110 m/s for the clay and column respectively. The weighted-mean approach demonstrated consistently more accurate phase estimation as compared to the case of a rigid threshold approach.
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Difference in Resistivity Explained by Initial Stage of Weathering
Authors B.E. Danielsen and H.B. MadsenTwo core drilled boreholes in Southern Sweden were logged and the cores were inspected in order to correlate the results with the geological features. The resistivity log showed that the amphibolites in the two boreholes had very different resistivity. In one borehole the resistivity was high, whereas it was lower in the other. The visual inspection of the cores could not give an explanation as the amphibolites appeared similar. However, microscopy of the low resistivity amphibolites showed weathered pyroxenes, probably caused by introduction of water along the lithological contact between the amphibolites and the regional gneiss. The resistivity logs can thus detect even low grades of weathering of amphibolites which can be important for the mechanical properties of the rock.
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Dike Monitoring at Red River by Geophysical and Geotechnical Tools
More LessA dike monitoring system consisting of a permanent electrode array and geotechnical tools including tensiometers, frequency reflectometry sensors (FDR) and an observation well has been installed at a dike section of the Red River in Thai Binh province (Vietnam). Since the stability of the dike depends to a large extend on the seepage line the system is focused on data reflecting the water distribution inside the dike body caused by natural variations of the water level in the river and by precipitation. The conductivity data acquired during a two year period show significant temporal variations. Assuming that a conductivity increase is related to higher water content, the resulting 2D conductivity distribution can be transformed into a vertical cross-section displaying the water content in the dike body. The transformation of conductivity into water content requires reliable data from FDR sensors that are integrated in two multi-depth soil water monitoring probes “EnviroSCAN Plus”. Additionally, the water pressure or suction is determined by two tensiometers installed in the dike body close to the FDR sensors.
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Rock Percentage Estimation Using P-wave Tomography Method. A Case Study from Mochos, Heraklio, Greece
Authors N. Andronikidis, G. Kritikakis, C. Papathanasiou and A. VafidisThe diggability of soil materials is essential in urban sewage network development. This can be estimated using seismic methods. In this case study, the seismic tomography method was utilized in Mochos, Heraklio, Greece for the determination of P-wave velocity (Vp) at sallow depths. The seismic survey aimed to the estimation of rock percentage distribution using Vp from seismic sections. The seismic profiles covered a surveyed area of about 3.2 ha in order to map the distribution of average Vp and rock percentage at different depths. The surficial (0 – 2 m) formations consists of more than 83% of carbonate solid rock, which is present just under the street pavements in the 41% of the surveyed area. This indicated that hydraulic excavator is not suitable for the construction of sewage network and hydraulic hammer usage is recommended.
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The Use of 3D-3C Seismic Refraction Method to Detect Hidden Shear Zones
By R. NikrouzIn the last twenty-five years, there have been significant advances in the spatial resolution of most geophysical sets of data. Whereas geophysical data were once commonly acquired along widely spaced profiles, it is now more usual to obtain geophysical data along numerous closely spaced traverses. As a result, the spatial sampling is now comparable in both horizontal directions and the greater resolution has greatly improved the interpretation of geological features. A major benefit of high resolution geophysical data sets, is the ability to generate pseudo-geological maps. The geological map traditionally has been a one of the most important domains for visually summarizing geological information. The broad objective of this study was to use seismic refraction methods to map in detail a faults zone and shear zone, which there was no evidence of subsurface faults in study area. Seismic refraction methods were selected because of their potential ability to provide greater lateral resolution of the narrow vertical shear zone, than is currently the norm with electrical or electromagnetic methods. The results of the seismic refraction surveys show that the hidden faults occur as a narrow region with low seismic velocities and increased depth of weathering.
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Application of Surface Wave Method to Confirm Compaction of a Refurbished Embankment – Case Study
Authors W. Chwatal, W. Lienbacher, A. Freudenthaler and W. WuIn 2002 two extreme floods damaged the embankments of the river Kamp in Lower Austria nearby Vienna, because some parts of the dams are more than 100 years old. In 2009 it was decided to refurbish these embankments with the Impulse Compaction Method. To control if this compaction is successful surface wave seismic and dynamic probing was performed before and after the compaction. Results from refraction seismology indicate an improvement of the dam body after the compaction, but are not very accurate. Seismic surface wave results, which have a higher resolution near the ground surface, show a velocity inversion zone in 3-4m depth. This zone corresponds with the native soil upon which the dam was constructed. Furthermore there are in average higher velocities after the compaction down to the depth of 4m and no velocity changes below. Nevertheless the 2D-results of the seismic surface waves analysis indicates that the improvement of the packing density is laterally changing und goes locally deeper than the 4m depth. This is corroborated by the dynamic probing. Generally, it was demonstrated that the seismic surface wave analysis is a valuable tool to confirm the success of compaction and its penetration depth.
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Travel-time Inversion of Parallel-seismic Data for Foundation Length Estimation
Authors E. Niederleithinger and J. TronickeThe parallel-seismic method for foundation length estimation has proven to be one of the most reliable and versatile methods for foundation (mostly piles) length estimation. The interpretation is mainly based on simple graphical interpretation of the seismograms or first arrival travel-time curves. Recently introduced methods lead to better estimation but still contain several approximations. Intensive paramter studies have been carried out using the CEFIT simulation technique. The influence of certain geometry parameters as well as material changes in pile and soil have been studied. Based on theses studies improved algorithms have been developed to predict first arrival times. They are used as forward operator in inversion schemes based on the Levenberg-Marquardt method and Simulated Annealing. The advantages and limitations of both methods have been tested by simulated and field data. The developed methodology and software programs are currently used in field studies leading to improved foundation length estimations.
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Time-domain IP and Resistivity Sections Measured at Four Landfills with Different Contents
Authors V. Leroux, T. Dahlin and H. RosqvistFour IP and resistivity sections measured in four landfills with different contents but in comparable geological settings are compared. The normalized chargeability makes it possible to distinguish waste from natural geological materials and to solve ambiguities remaining when the sole direct-current resistivity is used. The amplitude on the IP response is more important on the two landfills containing large amounts of biodegradable organic matter and disseminated metals. It is weaker when these materials are absent. It also appears larger when the moisture content is high. Some uncertainties remain concerning the depth of investigation but high normalized chargeability seems to extend to possibly contaminated sediments below the landfills.
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Integrated Geophysical Characterization of a Hydrocarbon Contaminated Site
Authors G. Cassiani, A. Binley, A. Kemna, A. Flores-Orosco, E. Rizzo, V. Bruno, R. Deiana, H. El-Kaliouby, P. Dietrich, L. Zschornack and C. LevenThe characterization of contaminated sites requires that direct investigations be supplemented with a suitable set of less-invasive, and more extensive, measurements. A combination of geophysical methods and direct push penetrometric techniques has been recently proposed as the backbone of site characterization within the EU FP7 project ModelPROBE. Here we present the first results of the investigations conducted at a field site in Trecate (NW Italy) which was affected in 1994 by crude oil contamination from a well blowout. The investigations include surface GPR, ERT, IP, SIP and SP surveys, together with direct push sampling and EC logs and limited cross-hole measurements. Many of the geophysical measurements have been conducted in time-lapse mode in order to separate static and dynamic signals, the latter particularly linked to strong seasonal changes in water table elevations. The goal is to identify (a) the structural characteristics that controlled the contaminant penetration into the subsurface and its current possible movements, and (b) assess possible correlation between measured geophysical properties and contamination levels and/or biodegradation of contaminants. Our preliminary results help provide a reasonable description of the contaminant infiltration mechanisms into the subsurface, while further analyses are necessary to establish a direct link with contamination and biodegradation.
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Expected Discontinuous Terrain Deformation with the Microgravity Method in a Selected Area of Shallow Coal Extraction
By M. LojThe results of gravimetric research in a selected area of a former coal mine “Siersza” are presented in the paper. The microgravity methods were aimed at finding the loosened zones in the near surface part of the rock mass. The physical bases of the applied method were discussed. The geological and mining conditions of the study area are outlined. The interpretation of the results of microgravity observation revealed zones of high surface deformations hazard. In one of them a discontinuous deformation occurred. The final conclusion is that the consequences of mining activity on surface may be discovered even before the surface lowering if the microgravity measurements are performed in the right time.
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Near Surface Geophysical Methods Applying in Arctic Shelf
Authors I.E. Balanyuk, A.N. Dmitrievsky, S.M. Shapovalov and O.N. ChaikinaThe shelf of the Barents Sea is one of the most extensively studied with high resolution acoustic methods because of large-scale engineering and geological problems solved in process of its industrial development. Mainly, it is related to exploring and development of oil and gas fields, oil terminals and submarine pipelines construction and building up the whole infrastructure for their exploitation. A considerable attention is paid for research with seismic acoustic methods of the modern sedimentation of the Barents Sea region which is important for development of reliable criteria for the sea bed stability.
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Geological Characterization of the Dutch Wadden Sea Using Shallow Reflection Seismics
Authors B.F. Paap, C.W. Dubelaar and J.L. GunninkMapping of the Dutch Wadden Sea is a major challenge as it is an intertidal zone and therefore has limited accessibility by surveying vessels. Hence, the current subsurface model of the Dutch Wadden Sea is based on a sparse density of lithological information derived from core samples and seismic data. To improve the vertical and lateral detail of the current model of the shallow subsurface of the Wadden Sea area, additional seismic surveys were performed during 2007 and 2008. Four types of source and receiver systems were deployed simultaneously to provide subsurface information at different depth ranges. Results indicate that the amount of distorting interference by deploying different seismic sources simultaneously was insignificant for the major part of the data. Lithostratigraphic horizons, as defined in the existing subsurface model, were correlated to seismic horizons. The outcomes of the study showed that the use of seismic data, in addition to core samples, provides valued information for mapping and modeling of the shallow subsurface of the Dutch Wadden Sea. As a next step, this information derived from multiple shallow reflection seismics can be used to improve the current subsurface model.
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Investigation of the Bed Sediments Using a Suitable Processing Algorithm in GPR Method: A Case Study
Authors M. Mohammadi Vizheh, A. Ameri, A. Kamkar Rohani, F. Jafari and A. RazaviTar Lake has been created as a result of occurrence an extensive landslide in its southern heights. Hence, study of the bed sediments of the lake can be very useful in the age estimation of the landslide and possible earthquake that have caused the formation of the lake. In this study, the bed sediments of the lake have been investigated using a suitable processing algorithm in GPR method. The fine grain and young bed sediments of the lake have caused weak responses in the GPR sections due to the water saturation and slight compactness of these sediments. The processing algorithm, presented in this paper, consists of several filters and corrections that can be applied on the GPR data to enhance the detectability of the GPR responses. Using the processing algorithm, we have been able to detect and recognize the GPR weak responses of these sediments in the presence of high level of systematic noise and existing limitations. Also, from the processed GPR sections, we can recognize very well the sequence of sedimentation in the bed areas where considerable sediment load inter the lake. Furthermore, the coarse grain sediments can be distinguished from the fine grain sediments in the GPR sections.
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Geophysical Surveys for Delineating Salt Water Intrusion and Fresh Water Resources in the Oued Laou Coastal Aquifer
More LessBecause of its stable quantity and quality, groundwater has long been a reliable source of water for domestic, agricultural, and industrial users in the Oued Laou region (N. Morocco), but the establishment of management program that integrates groundwater and surface water use has been hampered by the lack of detailed information. In order to get a better knowledge on both the geometry of the formations and physical and chemical characteristics of the aquifer a combined study integrating the geophysical and hydrochemical data has been conducted. The existence of a zone with high chloride content detected by chemical analyses at the west of the river Laou, coincides with high electrical conductivity values obtained from a frequency domain electromagnetic survey that indicates the extension of saltwater intrusion in the Oued Laou aquifer. The joint interpretation of geophysical, geological and geochemical data allowed us to identify the geometry of the aquifer and estimate the limits between fresh and saltwater.
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Airborne Electromagnetic Surveys over Concealed Glacial Aquifers, United States and Canada
Authors B.D. Smith, J.D. Abraham and S.C. LundstromOver the last decade helicopter electromagnetic (HEM) frequency and time domain (HFEM and HTEM) electrical resistivity surveys have been performed over buried glacial features that constitute major aquifers in different regions of the central North America. These surveys have been conducted to map geology, contaminants, and hydrogeology for groundwater studies. This paper summarizes the applications in four different glacial terrains. The HFEM systems applied in the surveys described here used frequencies from 400 to 140,000 Hz domain methods that, in electrically conductive areas may be limited to mapping depths less than 60 m. The HTEM survey described here used gate times from 50 microseconds to 5.4 milliseconds and can map to depths of more than100 m. The HEM surveys provide an insight into subsurface mapping capabilities and into the hydrogeologic framework of buried glacial valleys that host important regional aquifers.
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Prospecting and Mapping of Aquiferous Stratums of Different Mineralization by Geoelectric Methods
Authors N.A. Yakymchuk, S.P. Levashov, I.N. Korchagin, D.N. Bozhezha and J.N. PischaniyMany years' using of the non-classical geoelectric methods of forming short-pulsed electromagnetic field (FSPEF) and vertical electric-resonance sounding (VERS) for the varied problems of near-surface geophysics solving has shown the possibility of their application for the aquifers searching and contouring. The multiple experiments testify that areal survey by FSPEF method allows to find and map operatively the zones of soils (rocks) moistening, the underground water flows of natural and man-caused origins and the aquifers. The bedding depths and thicknesses of water-saturated layers are defined in cross-section with high accuracy by VERS sounding. Works of such nature are carried out enough operatively and quickly. The preprocessing and interpretation of measurement data in field condition allows directly on place of the measurements undertaking to indicate the places of wells location. The FSPEF-VERS technologies efficiency is demonstrated by practical example of the optimum places determination for the water-supply wells locations.
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Optimization of Electrode Layouts for 3D-resistivity Imaging: Application to Water Infiltrations in an Experimental Dike
Authors Y. Fargier, S. Palma-Lopes, D. François, C. Fauchard and P. CôteEmbankment dikes can be subject to several phenomena such as internal erosion, which may lead to mechanical weakness and even a breach. Research needs include developing early detection methods to improve mitigation of hazard associated to these pathologies. Several international research projects aim at developing geophysical (non-intrusive or weakly-intrusive) methods as part of a toolbox for assessing changes occurring inside embankment dikes. More precisely, DC-Electrical Resistivity Imaging techniques (ERI) are known to permit monitoring of water content, clay content, and temperature changes within a porous medium. Until recently, there has been a research effort on the optimization of ERI data sets to increase the amount and reliability of information. In this paper, we try to take such approaches further as we apply and adapt them to 1) the fully 3D case (topography and physical properties) of an experimental dike and 2) the design of the electrodes positioning for increasing resolution and robustness ERI. This paper extends how to optimize electrode layouts to enhance the resolution capacity compared to a conventional layout. Results exhibit a significant improvement of the model resolution matrix, thus an expected enhancement of the robustness of future ERI images of the experimental dike.
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Effectiveness of Multielectrode Earth Resistivity Method for Characterizing Putting Greens of Golf Courses
Authors J.C. Tapias, R. Lovera, M. Himi, E. Marguí, I. Queralt and A. CasasThe main objective of this study is to provide evidences that near-surface geophysical methods can be useful tools for improving the maintenance and remodelling efforts of certain parts of golf courses, particularly greens. Typically, in order to determine the geometry and textural properties of soils and substrates samples should be removed from the different zone of the golf course and submitted to an accredited physical soil testing laboratory. The results from our study indicate that electrical resistivity tomography method is an efficient tool which can be used by golf course greenkeepers to assess lateral and vertical variability of soils and substrates and prevent salinity hazard from irrigation with reclaimed wastewater.The study has demonstrated that, although soil electrical resistivity depends simultaneously on many soil properties like moisture and salinity, the measurement of apparent resistivity in sandy substrates used for building putting greens can be easily correlated with his textural composition.
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Geophysical Measurements in Coastal Area of Nam Dinh Province (Vietnam) for Delineation of Aquifers
Authors T.V. Nguyen, A. Weller and D.N. TangThis paper presents new results of geophysical measurement in coastal area of Nam Dinh province. Resistivity imaging was carried out to map the resistivity distribution along a 2 km long profile. Two boreholes were drilled on the same profile. The resistivity of 26 soil samples was determined in laboratory. A comparison between the resistivity of well logging, resistivity imaging and samples shows a good agreement. The resulting resistivity section provides promising 2D image of geological structures. Regarding the resistivity values, fresh water bearing areas can be identified in the Holocene and Pleistocene aquifers. Resistivity imaging has proved to be an appropriate tool to investigate hydrogeological structures in coastal areas.
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Determination of Qualitative Characteristic of Aman Abad Aquifer Using Surface Geoelectrical Data
Authors F. Ghadimi Arousmahalleh, M. Mirzaei, V. Adeshteh and S. GholamiVertical electrical soundings were used to delineate the different subsurface geoelectric layers, aquifer units and their characteristic, the subsurface structure and its influence on the general hydrogeophysical investigation in the study area. In this study, the groundwater potential and protective capacity evaluation of the sediment units in the Aman Abad area was undertaken using 156 Schlumberger vertical electrical soundings. The computer assisted sounding interpretation revealed subsurface formation composing different alluvial. The results showed that alluvial with the higher content clay could lower the groundwater yields. The characteristic geoelectrical parameters of the delineated aquifer at each sounding station were used to produce the groundwater potential map of the area. The groundwater potential rating of the area is considered generally high except in the north part of the area. Results of the study also revealed that most parts of the area are underlain by materials of moderate to good protective capacity. Most of these parts coincide with zones of appreciable overburden thickness with medium or coarse grain size materials. The results also provided valuable information such as boundary indication between salt and fresh water and also salt and fresh water deploy in the area.
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Fully Coupled Hydrogeophysical Inversion of Salt Tracer Experiments Monitored by Electrical Resistivity Tomography
Authors D. Pollock and O.A. CirpkaGeophysical techniques have been used in various studies to monitor hydraulic experiments in order to obtain information about hydraulic conductivity. One of these methods is the use of electrical resistivity tomography during salt tracer tests. We use an approach in which the hydraulic conductivity distribution is directly related to the electrical potential differences measured during the experiments. This approach, which considers geoelectrics and hydraulics in a coupled way in order to avoid non-physical results, makes use of temporal moments of electrical potential perturbations. The latter can be computed by moment generating equations. We consider as measurements the ratio of the first over the zeroth moment, indicating the mean arrival time of the perturbation. In addition to that, also some head measurements are included. For inversion, the quasi-linear geostatistical approach is used. We apply the method to a two-dimensional synthetic case study, and compare the obtained hydraulic conductivity field and transient behaviour with the true conditions. The results show generally a reasonable agreement between the true and estimated field and also between the actual and estimated plume propagation within the domain.
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