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Near Surface 2009 - 15th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 07 Sep 2009 - 09 Sep 2009
- Location: Dublin, Ireland
- ISBN: 978-90-73781-72-6
- Published: 07 September 2009
121 - 139 of 139 results
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Seismic Monitoring of Slope Dynamics Caused by a Slow-moving Landslide in the Vorarlberg Alps, Austria
More LessOur work within the research unit ‘Natural Slopes’ is mainly focused on the monitoring of seismic signals caused by the movement of the Heumoes slope in the Vorarlberg Alps, Austria. The slope is set up by weak sediments and moves with several cm per year at the surface. During several field campaigns within the last three years we were able to detect and locate single fracture processes during the movement of the Heumoes slope by applying the method Nanoseismic Monitoring (Joswig, 2008) to the Heumoes slope. Dozens of fractures with magnitudes varying between -0.7 ≤ ML ≤ -2.4 have been detected and located. The spatial distribution of the epicentres correlates with slope areas of higher movement rates. The temporal occurrence of the detected fractures, up to 26 hours after intense rain events, seems to approve the assumption of a rainfall-triggered movement of the slope caused by fast subsurface water dynamics. The epicenters are mainly clustered in the western part of the slope, where the water saturation of the material varies with the season. We therefore preliminary assume, that the recorded fractures have been generated in dependence of the water saturation of the unstable sediments.
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Seismic Monitoring of Slope Dynamics Caused by the Mudslide at Super-Sauze in the Southern French Alps
More LessIn this study, we describe the application of Nanoseismic Monitoring to the fast moving mudslide at Super-Sauze in the southern French Alps. The unstable slope mainly consists of marls and shows an immense dynamic behaviour with velocities of more than 3 cm/day. During a 10 days field campaign in July 2008, we were able to detect and locate different signals (type ‘A’ – ‘C’) caused by the movement of the mudslide. While the events type ‘A’ are caused by rockfalls in the source area of the slope, we could identify different types of signals (type ‘B’ and ‘C’), which have been obviously generated by material failure within the unstable sediments. The spatial distribution of the epicenters (type ‘B’), respectively the estimated source area (type ‘C’), correlates well with parts of the slope moving with higher velocities. Most of these signals have been generated close to the “in-situ crests”, which are covered by the mudslide material today. Our preliminary assumption is that the events of type ‘B’ are generated by impulsive fractures within the sliding material, while the events of type ‘C’ are caused by by “scratching” and “grinding” at the boundary between the sliding material and the bedrock.
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DNAPL Detection and Monitoring Using Electrical Resistive Tomography – A Case Study in North Italy, Rho-Milan
Authors L. Orlando and P. ViottiThe study deals with the detection and monitoring of DNAPL using electric resistive tomography (ERT) on the topographic surface and in boreholes. The surveys were performed in an industrial area that had been contaminated by chlorinated solvents; the site, at Rho in north Italy, lies about 10 km NW of Milan. The ERT data, acquired on the topographic surface and in the boreholes, reveal a scenario of distributed resistivity variations in an area that was once associated with the movement of contaminants over the ground. The geophysical data interpretation was constrained by sedimentary stratigraphy, revealed by boreholes drilled in the area. The results validate the use of electrical resistive tomography for the detection and monitoring of DNAPL.
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Hydrodynamic of Leachate Plume in Bioreactor Landfill – Contribution of 3D Time-lapse ERT
Authors R. Clement, L. Oxarango and M. DescloitresLeachate recirculation in bioreactor waste landfills aims at increasing the moisture content to optimize biodegradation. Waste is a heterogeneous and anisotropic porous media and water recirculation process is poorly known at the scale of the site. To better understand recirculation hydrodynamics, we used 3D time-lapse Electrical Resistivity Tomography (ERT). After optimization using numerical modeling, time-lapse ERT is applied at the field scale, during an experimental injection. We compared ERT with injected volumes to evaluate the sensitivity of time-lapse ERT to delineate the plume migration. Results show that time lapse ERT can be used to characterize some hydrodynamics properties of waste.
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High Frequency Resistance and Capacitance Measurement for Archaeological Surveying
Authors X. Shang, K.S.K. Yeo and M.J. LancasterThere are many different ways to map underground archaeological features. The open-ended coaxial probe, described in this paper, is another new prospective method. The principle of this method is to measure the microwave reflection co-efficient at particular points on the surface to map out subsurface features. Apart from non-destructive and low cost, a major advantage of this method is the ease of implementation and the additional data provided. In this paper, the principle of open-ended coaxial probing is discussed, and some experimental results
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Physical-geological Modeling for Magnetic Study of Near-surface Archaeological Targets
Authors S.E. Itkis, M. Kats and B.E. KhesinNear-surface geophysical study of small archaeological targets in complex environments requires physical-geological modeling based on detailed characteristics of physical properties and geophysical fields, and analysis of preliminary archaeological data. The results of detailed survey in the vicinity of Tel Megiddo site in Israel show the effectiveness of magnetic prospecting. The interpretation of magnetic data was based on of physical-archaeological models which are appropriate for application in similar conditions.
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Freshwater Ground-penetrating Radar
Authors J.M. McKinley, R. Parker and A. RuffellThe dielectric permittivity, conductivity and velocity of fresh water causes GPR to work well on and in this media. Various publications, conference abstracts and personal comments quote different antenna frequencies as optimal for freshwater surveying: these are tested with some simple field experiments. Few publications comment on antenna orientation and these too are considered in this presentation. Following this, the effect of water temperature, conductivity, chemistry and suspended solid content is considered, with temperature showing a major imfluence on penetration and resolution. Freshwater GPR is ideal for the surveying of small, shallow (up to 15m depth) water bodies. Siltation, engineering and forensic searches can all benefit from our experiments in optimising the use of GPR on freshwater, which we show with case studies.
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Integration and Data-driven Zonation of Large Airborne Geophysical Maps Using Cluster Analysis
Authors H.C. Paasche and D.G. EberleSince the advent of modern desktop computers few attempts have been made towards rapid, automated and objective information extraction from large geophysical data suites. We employ fuzzy c-means (FCM) cluster analysis for the rapid and objective integration of disparate geophysical data sets comprising airborne radiometric and magnetic as well as ground-based gravity data covering a survey area of approximately 5000 km2 located 100 km east-south-east of Johannesburg, South Africa. After preparatory data processing and normalization, the three data sets are subjected to FCM cluster analysis resulting in the generation of a zoned integrated geophysical map delineating distinguished subsurface units based on the information the three input data sets carry. We refer the zoned geophysical map to all available geological information demonstrating that the zoned geophysical map as obtained from FCM cluster analysis outlines geological units which are related to Bushveld-type, other Proterozoic- and Karoo-aged rocks.
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The Interpretation of Aeromagnetic Survey in Eurajoensalmi, Olkiluoto, Finland (2008)
More LessIn 2001, Olkiluoto was selected as the site for the final disposal of spent nuclear waste in Finland. In April 2008, a high resolution aeromagnetic survey was carried out in the Eurajoensalmi inlet in order to investigate the sea and coastal areas north and west of Eurajoensalmi. The main objective of the study was to improve the magnetic image of Eurajoensalmi area, to locate the area’s most significant magnetic features, and by magnetic modelling, find the best geological explanations for them. Survey was conducted by Geological Survey of Finland and ordered by Posiva Oy. In order to fully interpret the magnetic data, various derivatives and visualisation techniques were used in addition to total magnetic field. In total four areas were modelled. Modelling was intentionally kept relatively simple using as large and continuous bodies as possible. Modelled areas demonstrate that magnetic modelling is a very suitable method for defining the properties of concealed structures causing magnetic anomalies and most importantly for finding reasonable geological explanations for them. A new airborne magnetic survey has been planned for the year 2009 to expand the survey area to the south.
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Joint Inversion of P and S Traveltime Tomography Data Using Poison Ratio and Cross-gradient Constraints
Authors P.N. Mpogiatzis, C.B. Papazachos, P.I. Tsourlos and G.N. VargemezisIn order to maximize the amount of information taken from a seismic tomography survey, P and S-wave travel times can be used under the same joint inversion scheme. In this work two different approaches have been used. The first inverts the two data sets subject to constant velocities ratio constraint, while the second uses the cross-gradients function constraint. In both inversion schemes, additional regularization is used and spatially variable Lagrangian multipliers are applied to the model parameters. The obtained solutions suggest that joint inversion can lead to improved results, stabilize the inversion process for both inversion strategies and reduce the non-uniqueness of the problem. However the constant velocities ratio method can create artifacts and reduce the resolution when P and S-waves velocities are uncoupled and vary independently. Therefore, it can only be applied when reliable a-priori information about the investigation area ensures that the models parameters are linked with a constant Vp/Vs ratio. On the contrary cross-gradient method is more robust and does not require any a-priori information, yet its effectiveness is increased when structural similarities exist.
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4D Active Time Constrained Resistivity Inversion
Authors M.C. Karaoulis, J.H. Kim and P.I. TsourlosIn this work an improved 4d technique for inverting monitoring resistivity data is examined and compared with the existing inversion schemes. Synthetic data tests illustrate that the time-lapse images of ERT data are affected by inversion artifacts.4D Active Time Constrained (4D-ATC) technique minimize those artifacts while producing realistic inversion models. Several comparative synthetic tests have been made between the techniques. Further, the presented algorithm is tested on field studies. The results demonstrate that this technique produces in most cases improved time-lapse images in relation with existing techniques.
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FEM Source Effect Investigation with 2.5D Numerical Modelling
By G. PethoLand-based controlled-source frequency domain electromagnetic methods (FEM) using horizontal electrical dipole (HED) source create a lot of interpretational problems associated with the finite transmitter-receiver distance and with conductivity inhomogeneities in the vicinity of the transmitter. For elongated conductivity structure the developed 2.5D finite difference forward modelling code makes it possible to investigate non-planewave-, source overprint- and shadow effects. These source effects are investigated both in broadside (TE mode) and in collinear configuration (TM mode). Because the investigations covered the transition zone to present these effects impedance amplitude and impedance phase sections were applied. For the investigated models the shadow effect develops similarly in the two modes and greater source overprint and non-planewave effect were observed in TM mode than in TE mode. These source effects manifest themselves characteristically in case of simple 2D structures and numerical modelling may help the interpretation of the multi-receiver FEM measurements using HED sources in the two modes.
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The Determination of Textural Parameters Using Interval Inversion of Borehole Geophysical Data
Authors N.P. Szabo, M. Dobroka and P. VassIn case of point-by-point inversion of new inversion unknowns like textural parameters cannot be taken up without facing the consequences of underdetermination and ambiguous solution. Our suggestion is a joint inversion method called interval inversion procedure, which was previously applied by the authors for determining cementation exponent in one inversion procedure with volumetric model parameters (Dobroka et al. 2007). In the paper, it is demonstrated how the method works for more textural parameters integrated to the joint inversion problem. The theoretical backgrounds are overviewed touching upon the questions of parameter sensitivity and then a case study is presented.
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Coupled Hydrogeophysical Inversion for Top Soil Hydraulic Properties Using Electrical Resistivity Tomography
Authors C.M. Mboh, S. Huisman, J. Sorg, J. Rings and H. VereeckenAccurate determination of soil hydraulic properties is essential for understanding subsurface flow and transport processes which can help mitigate soil and groundwater pollution.For a borehole infiltration experiment, it is numerically demonstrated in this study that surface Electrical resistivity tomography(ERT) contains enough information to physically constrain the inversion for top soil Mualem-Van Genuchten parameters provided there is a good knowledge of the soil petrophysics. This was achieved by coupling a forward geoelectric model to a vadose borehole infiltration model based on 2D axisymetric Richard's equation with the use of a suitable petrophyical relationship like the the Archie's law.The simulated water content is converted to bulk electrical resistivity of the soil using Archie's law.The simulated bulk electrical resistivity is then converted to simulated ERT-data(electrical transfer resistances) using the foward geophysical model based on the Poisson equation.The misfit between the simulated and measured ERT-Data is evaluated.The hydraulic parameters are perturbed and the whole process is repeated until a close fit is found between the measured and the simulated ERT-data.The uniqueness of the parameter estimates are improved when both ERT and Infiltration are used within this hydrogeophysical inversion framework.
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Nonlinearity of Soil Stress-strain Response from Laboratory Studies
More LessNonlinearity of clay soils based on the original data from laboratory studies is discussed. The presented new results of soil stiffness and damping ratio obtained in triaxial testing with small strain measurement using local Hall effect transducers, resonant column and torsional shear testing. Results of different tests are in good agreement demonstrating that threshold strain beyond which soil nonlinearity is triggered is about 0.02-0.03% and may be related to the displacement magnitude of the order of 0.01-0.03 mm which is comparable to predominant particle size in the soil. This idea can provide a basis to a new approach to understanding the nature and regularities of soil nonlinearity. It is also shown that damping is not only frequency dependent but is rather strain-controlled phenomenon.
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Geophysics in the Search for Homer's Ithaca
Authors G. Hodges, D. Kilcoyne, R. Eddies and J.R. UnderhillIdentifying the location of the island of Ithaca, legendary home of Odysseus, has been a problem for historians for centuries. The modern island of Ithaki, in the Ionian Sea, does not match the description in Homer’s epic poem. In 2003 Robert Bittlestone initiated a study of the Paliki peninsula in western Cephalonia in an attempt to determine whether this was the island that Homer called Ithaca, then separated from the rest of Cephalonia by a sea channel later described by the Greek geographer Strabo. Ground, airborne and marine geophysical surveys are being used to study the potential for a channel under an area now largely covered by colluvium from the adjacent mountains. Airborne EM and magnetic data provide a regional overview of ground conductivity. Ground EM, resistivity, gravity and refraction seismic surveys are being used to study the proposed channel zone in detail to determine the depth of fill and contours of the buried bedrock surface. Marine seismic has been employed offshore to analyze the drainage patterns at the low water levels of 3000 years ago. High resolution airborne LIDAR mapping provides detailed surficial information. All of these data sets are brought together to build a comprehensive geological model of the proposed channel area and to provide the ultimate test of the classical enigma.
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Estimating Debye Parameters from GPR Reflection Data Using Spectral Ratios
More LessIn the GPR frequency range, electromagnetic wave attenuation is largely controlled by dielectric relaxation processes. A primary relaxation commonly occurs in the 10 – 100 MHz range for many earth materials in which the GPR signal propagates effectively. This relaxation leads to strong nonlinearity in the frequency dependent attenuation and occurs in a frequency range that is often used for groundwater investigations. This non-linearity complicates data analysis but also may provide additional material property information. I investigate inversion for Debye relaxation parameters directly from GPR reflection data, including increasing the bandwidth of the signal by summing the response from 25 MHz, 50 MHz, 100 MHz, and 200 MHz radar antennas. I compute the timefrequency distribution using spectral decomposition, then use the method of spectral ratios to measure the attenuation vs frequency curve for significant reflection events. I then fit the curve with the multiparameter Debye model. Using synthetic and field data I show that this approach provides reliable estimates of the primary relaxation time for a variety of realistic subsurface models. This approach has the potential to improve our understanding of aquifer material properties.
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Application of Magnetic Susceptibility for Wetlands Delineation
Authors A.R. Lobred and J.E. SimmsWetlands are a natural resource that are protected under federal regulations, therefore the delineation of wetlands is necessary to ensure their protection. A study using magnetic susceptibility was undertaken in central Mississippi to identify the transitional zone between non-hydric (uplands) and hydric (wetlands) soils. The soils were clayey with a minor percentage of sand. A survey line that traversed an upland, transitional zone, and wetland on each end of a transect was revisited four times during a single year. The first survey occurred in the spring three weeks after the winter flood inundation (moderately wet soil conditions), and the other three were in the summer (one, dry soil conditions) and fall (two, dry and moderately wet soil conditions). There were measurable differences between the magnetic susceptibility values collected in the upland and wetland regions during each survey period. One transitional zone was easily identified using magnetic susceptibility, exhibiting a sharp decrease in susceptibility values between the upland and wetland. The other transitional zone contained an intermediate ridge, which made demarcation of the zone less obvious. The measured magnetic susceptibility values were comparable for the respective upland, transitional, and wetland regions, and the characteristics of the curves were similar for all time periods. A general increase in magnetic susceptibility values was observed in all regions as the site dried throughout the year. Overall, magnetic susceptibility proved to be a successful method for delineating a wetland in this area.
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A New Approach to Predict Hydrogeological Parameters Using Shear Waves from Multichannel Analysis of Surface Waves Method
Authors A.E. Cameron and C.C. KnappA high resolution P-wave seismic reflection survey was performed at the P-Reactor Area, Savannah River Site, South Carolina in order to delineate and predict migration pathways of a large contaminant plume including trichloroethylene that originates from the northwest section of the reactor facility located within the Upper Atlantic Coastal Plain
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