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Near Surface 2006 - 12th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 04 Sep 2006 - 06 Sep 2006
- Location: Helsinki, Finland
- ISBN: 978-90-73781-62-7
- Published: 04 September 2006
41 - 60 of 142 results
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Electrical Resistivity Tomography (ERT) as a Tool for Monitoring Moisture Dy-namics in Soil Filled Containers
Authors O. Mohnke, K. Prokoph and U. YaramanciThe geophysical subproject of the multidisciplinary research group INTERURBAN (DFG 409), formed to investigate the water- and solutedynamics at urban locations with particular attention to spatial heterogeneity and biological transformation processes in the unsaturated zone, aims at the development of a noninvasive conception to map the water distribution and dynamics at small scales. Hydrophobic areas have a major impact for the transport of water and matter through preferential flow in soils. Soil animals, e.g. earthworms, modulate the composition of soil solutions in time and have a species-specific impact on the release pattern of nutrients and pollutants from plants and soil. To assess the possibilities and limitations of 3D ERT in the cm-dm range to monitor the long and short term dynamics of water repellent zones in the presence of soil fauna activity a feasibility study was carried out using soil filled mesocosms (MC) inoculated with earthworms. Results indicate a sufficiently high spatial resolution and sensitivity to reliably monitor in-situ changes of soil moisture on the relevant scales, Earthworm inoculated MC indicate a more homogenous decrease of resistivity throughout the MC whereas in the reference MC the changes are confined to the area below the irrigation zone.
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Criteria of Gas Hazard Assessment in the Areas of Closed Mines of the Wałbrzych Coal District, Southwestern Poland
Authors H. Sechman, M. Dzieniewicz, M. J. Kotarba and A. KorusThe mine closures in the Wałbrzych Coal District, and particularly the cessation of drainage, resulted in the recovery of the groundwater table in Carboniferous multi-aquifer system which, in turn, intensified the flux of coalbed gases into the near-surface zone.
Surface geochemical surveys run in the years 1997-2001 and 2004-2005 in the Wałbrzych District demonstrated general increase in methane (up to maximum 49.6 vol. %) and carbon dioxide (up to maximum 17.4 vol. %) concentrations in soil gas samples collected at depth down to 2 m. The forced influx of methane and carbon dioxide into the near-surface zone measured with the modified, static chmaber method was up to 620 dm3/m2h i 330 dm3/m2h, respectively. Studies on coalbed methane and carbon dioxide flux to the near-surface zone supported by geochemical, geological and mining-engineering criteria, enabled the determination of a three-degree scale of near-surface gas hazard. Recognition of gas hazard mechanisms in the near-surface zone of post-mining areas is significant for restructuring of the mining industry in Poland and prediction of gas hazard caused by coal mine closure.
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Shallow 2D and 3D Seismic Tomography on an Unstable Mountain Slope
Authors H. Horstmeyer, B. Heincke, H. R. Maurer, A. G. Green, H. Willenberg and T. SpillmannAs transport routes and population centres in mountainous areas expand, risks associated with rockfalls grow at an alarming rate. As a consequence, there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and non-invasive manner. For this purpose, we have developed a 3-D tomographic seismic refraction technique and applied it to an unstable alpine mountain slope, a significant segment of which is moving at 0.01-0.02 m/year. First-arrivals recorded across the exposed gneissic rock mass have extraordinarily low apparent velocities. Inversion of their traveltimes produces a 3 D tomogram that reveals the presence of a huge volume of very low quality rock with ultra-low to very low P-wave velocities of 500-2700 m/s. Such low values likely result from the ubiquitous presence of dry cracks, fracture zones and faults at a wide variety of scales. They extend to more than 35 m depth over a 200 x 150 m area that encompasses the mobile segment of the slope and a large part of the adjacent stationary rock mass. Although hazards related to the mobile segment have been recognized since the last major rockslides in 1991, those related to the adjacent low quality stationary rock mass have not.
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Geophysical Characterization of the V.F. De Xira-Lisbon Fault System, Portugal
Authors J. P. Carvalho, T. Rabeh, F. Carrilho, J. Cabral and M. MirandaThe Lower Tagus Valley (LTV) has suffered large historical seismic events which originated important material damages and loss of lives. Besides the plate boundary activity, the study area has been recently considered an important seismic source to the seismic hazard of the LTV area. The V.F. Xira-Lisbon, was selected as a priority target for investigation, based upon its near-surface expression on the oil-industry seismic reflection profiles, its significance in the Cenozoic basin structural pattern and the apparent relationship to the regional seismicity, its closeness to Lisbon and its seismic potential. Using aeromagnetic, seismic reflection and seismicity along with geological data, new insights into this fault system have been established. Its deep rooting into the paleozoic basement, the unknown northward and southward prolongations and its connection to relocated seismic epicentres are a few examples. The expected total length of the fault will be more than 90km, increasing the estimated magnitude of the maximum credible earthquake and the seismic hazard of Lisbon significantly. Acquisition of high resolution seismic data will help confirming estimated vertical offsets of the fault in Quaternary times, allowing an improved assessment of the seismic potential of the V.F Xira-Lisbon fault system.
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The Effect of Vibrations from Wind Turbines on Seismically Sensitive Critical Installations
Authors P. Styles, R. England, I. G. Stimpson, S. M. Toon, D. Bowers and M. HayesIn order to meet Kyoto targets, the UK has set the challenge of reducing carbon dioxide emissions by 60% by 2050. Renewable energy, especially wind power, will be an important contributor with a target of 10% of energy from renewables by 2010. The Southern Uplands of Scotland offer a prime wind resource because of a large region of high topography, appropriate wind conditions and 2 Gigawatt of generating capacity is planned. However, the United Kingdom Comprehensive Test Ban Treaty seismic monitoring site (CBTB) is situated at Eskdalemuir in the Scottish Borders. Concern was expressed that vibration from wind farm developments might prejudice the detection capability of this facility. The Ministry of Defence placed a precautionary blanket objection to wind farm developments within 80 km of Eskdalemuir. In order to assess vibration levels a 10 station broadband seismic network and a 4 station infrasound network were established for 6 months at distances out to 20 km from a 26 turbine wind farm situated on very similar geology and topography to Eskdalemuir and the planned wind farm developments. The study has permitted the identification of the principal propagation mode for ground vibrations from wind turbines and enabled their characterisation.
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Progress and Problems in Near Surface Gravity
By S. EloThe paper describes three applications of near-surface gravity: (i) an attempt to estimate in-situ weathering of a dimension stone deposit, (ii) improvements in 3D gravity inversion of overburden thickness and (iii) new results of monitoring the subsidence and density of a landfill. The examples demonstrate the state-of-art in near-surface gravity: mean error of gravity measurements ±0.010 mGal or better, advanced modelling techniques and a demand for highly accurate terrain models.
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MRS Study of Groundwater Flow and its Relationship with Sinkholes Development in the Dead Sea Coastal Area
Authors A. Legchenko, M. Ezersky, C. Camerlynck, A. Al-Zoubi and K. ChalikakisA geophysical survey was performed in the Dead Sea costal area of Israel and Jordan. It was carried out in framework of NATO project SfP N° 981128. The goal was trying to understand the processes of sinkholes development caused by the fresh water flow towards the Dead Sea. For that, the Magnetic Resonance Sounding (MRS) method was applied.
Our results reveal a very heterogeneous aquifer with two orders of the transmissivity variation around the investigated area that could be divided into two parts. In the northern part, the aquifer allows intensive circulation of groundwater. It was identified as a karst aquifer thus validating the salt dissolution mechanism of sinkholes development known from the literature. In the southern part, the subsurface is composed of compact clay-type material with low hydraulic conductivity. Development of the karst was not observed.
Basing on MRS results, we suggest that development of the karst in the northern part have changed the local hydraulic gradient and consequently, smaller amount of water is flowing towards the south. Change of the water path may explain why these sinkholes are slowing down their development while in the northern part they continue growing up.
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Using ERT and CPTR in Geotechnical Investigations
Authors I. M. Vaara and A. MattssonIn this study the applicability of the electric resistivity tomography and the CPTR-bore logging method was studied in geotechnical soil investigations. Also the induced polarisation effect is discussed. Usually the geotechnical soil investigations utilize lots of drillings and the number of them is set in advance. The aim of this study was to show that by using the ERT-measurements the number of the bore loggings could be diminished and they could also be situated in the most relevant places.
The most important conclusions drawn from the results are that the ERT-method is very useful in geotechnical soil investigations. Also the IP effect gives information on the quality of different soil types. The CPTR gives the point like value of the resistivity of the ground which is important in evaluating the corrosion risk for steel structures. Together with ERT –measurements the CPTR loggings can be very informative about the depth of peat and clay layers.
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GPR Investigations in Coastal Sambaqui Archaeological Site in Santa Catarina, South Brazil - Preliminary Results
Authors J. L. Porsani, S. I. Rodrigues, P. C. F. Giannini and P. A. D. DeblasisIn this paper, preliminary results from GPR Investigations are presented, accomplished at Encantada-III coastal sambaqui, in Jaguaruna, Santa Catarina state, South Brazil. This sambaqui consists of sand mound covered by decimetre black sediment layer with carbonatic shells, dated of 4970-4830 years AP (Beta 189713), and could be constructed by societies that inhabited this region during pre-colonial period. GPR profiles show anomalous reflectors, which can be related to targets of archaeological interest. Numeric modelling studies, simulating propagation of electromagnetic wave at coastal sambaquis, presented good accordance with real data obtained in the field. Synthetic model was construct in order to evaluate the performance of GPR method in relation to materials found in that environment, besides helping interpretations of real results. So, the results function as reference to direct excavation activities in burying quest that will be the next step of this searching.
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Mapping Ice-Bonded Permafrost with Electrical Methods in Sisimiut, West Greenland
More LessPermafrost delineation and thickness determination is of great importance in engineering related projects in arctic areas.
In this paper, 2D geoelectrical measurements are applied and evaluated for permafrost mapping in an area in West Greenland.
Multi-electrode resistivity profiles (MEP) have been collected and are compared with borehole information.
It is shown that the permafrost thickness in this case is grossly overestimated by a factor of two to three. The difference between the inverted 2D resistivity sections and the borehole information is explained by macro-anisotropy due to the presence of horizontal ice-lenses in the frozen clay deposits.
It is concluded that where the resistivity method perform well for lateral permafrost mapping, great care should be taken in evaluating permafrost thickness based on 2D resistivity profiles alone. Additional information from boreholes or other geophysical techniques is needed for correct interpretation.
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Electromagnetical Investigation of a Dune Valley in Oostduinkerke (Belgium)
Authors K. Martens and K. WalraevensAlong several profiles, an electromagnetic prospection (horizontal dipole mode) has been performed in a draining dune river valley. Different factors are at the basis of the variability of the apparent ground conductivity: groundwater flow, lithology, topography and sea spray.
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Spatializing Water Tension in Heterogeneous Sandy Soils with Surface ERT During Rain-Evaporation Cycles
Authors M. Descloitres, O. Ribolzi, Y. Le Troquer and J. P. ThiebauxDunes are almost exclusively the support of herbaceous vegetation in the Sahel. There is a need to better understand their hydrological functioning during the rain cycles. This paper evaluates the interest of using surface Electrical Resistivity Tomography (ERT) for spatializing water tension. A representative micro-dune has been studied in the field using 3 artificial rain cycles. ERT inversion was corrected from temperature and optimized using a comparison with in-situ resistivity measurements, giving an estimate of ERT accuracy. A porosity-independent relation between resistivity ratio (final / initial state) and water tension differences (final – initial) is found for the first 2 rains with demineralised water. When using salted tracer (third rain), the slope of this relationship changes. We found that the ERT image smoothing doesn’t allow us to track accurately some localized phenomena. Moreover, the interest of spatializating water tension using non-destructive surface ERT is limited because inversion has to be constrained using a priori information given by destructive measurements at depth. We conclude that surface ERT imaging reconstruction needs improvements to reliably spatialize resistivity variations linked with processes in heterogeneous sandy soils.
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Non Destructive ERT Survey at the South Wall of Akropolis of Athens, Greece
Authors P. Tsourlos, G. N. Tsokas, N. Papadopoulos, V. Manidaki, M. Ioannidou and A. SarrisResistivity tomogaphies were carried out on the south wall of the Akropolis in Athens. The electrode lines were deployed vertically and horizontally on the wall plus on the top of the hill. Further, some tomogarphies were conducted encompassing the volume of the material between the wall and open pits (baulks). Bentonite mud was employed instead of metal stakes in order to comply to the non destructive character of the whole operation. An existing algorithm was modified to cope with the particular requirements of this unconventional survey. Areas of increased moisture were revealed in the prospected volumes. Further, the thickness of the wall was assessed in particular locations. It is showed that this type of surveys are perfectly feasible and can produce results informative and elucidating images to archaeologists and restoration scientists.
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Estimation of Subsurface Pipe Radius Using Ground Penetrating Radar Data
Authors A. Dolgiy, A. A. Dolgiy and V. ZolotarevIn this paper, we consider three techniques for radius estimation of subsurface metal pipes using Ground Penetrating Radar (GPR) data. The GPR acquisition system was Zond-12c with shielded antennae of frequency 900 MHz. Seven buried pipes from 0.01 m up to 0.36 m in radius were studied. For GPR data processing were applied the methods of weighted least squares, the recursive Kalman filter and the maximum likelihood. The recursive Kalman filter method shows the minimum value for average error of 3.08% for radius estimation. The maximum value for average error of 13.3% for all considered methods corresponds to pipe with the minimum radius of 0.01 m. For pipes with radius from 0.024 m up to 0.36 m the average errors are varied in the range of 0.74 - 5.17%. We studied the influences of random errors of measurements on the accuracy of radius estimations for the metal pipe of radius 0.36 m. We also inspected the influence of the angle between the antennae scan direction vector and the pipe axis on the accuracy of radius estimations by recursive Kalman filter technique for all studied pipes.
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The Effect of Surface Polarization in Galvanic Modelling
More LessSurface integral equation solution, where surface polarization is included to model galvanic measurements. The model and the corresponding software are best suited in forward modelling. The problem is formulated for finite conductivity contrast.
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Applying Syscal-Pro Resistivity-Meter in Fast Measurement Mode to Soil Investigation
Authors A. A. Bobachev, I. Cousin, H. Robain and A. A. GorbunovThe paper deals with geophysical aspects of studying soils by resistivity method.
Electrical resistivity measurements have recently come to use in field experiments to explore soil structure. Rapid resistivity-meters like 10-chanels Syscal-Pro (Iris Instruments) are able to record up to six hundreds of measurements per minute. Such units open new possibilities for soil investigation.
The authors fitted Syscal-Pro for soil exploration. All 96 electrodes were fixed in 8 plastic slabs with 10 cm inter-electrode spacing. Fast and exact setup of array is thus provided. Three different types of measurements were implemented: 2D and 3D resistivity imaging and the infiltration monitoring. Some valuable geophysical results are discussed.
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Surface-to-Hole FEM Modeling in Conductive Half Space
More LessThis work presents several modeling examples for frequency-domain electromagnetic (FEM) system. Measuring system consists of a large surface transmitter loop and a borehole receiver probe. In EM methods a typical target is a conductive body in a resistive host. The primary magnetic field at the borehole receiver stations is first calculated theoretically and then reduced from the measured response. This residual field is used for the interpretation. This method provides also visual interpretation, but if the surrounding rock is not very resistive, the method leads to difficulties and may cause false results. In conductive subsurface, currents inside the conductive body are induced, instead of a magnetic dipole, by more complicated electromagnetic field, known as the "smoken ring" in the time-domain EM method. On the contrary, when a conductive body is lying in a resistive environment, the induced currents are flowing and decaying inside the body. That gives information about a shape of the body, but in the conductive environment the current flows freer trough the interface of the body. Interaction between the body and the host increases and the shape of the body becomes more difficult to interpret from the data.
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DC, EM and Direct Push Resistivity Surveys for Pollution Pathways Delineation – A Case Study from Poland
By M. RudzkiThe paper presents the results of the geophysical multi-method survey that was carried out in order to map the contamination migration pathways in the vicinity of waste disposal ponds of one of the biggest Polish chemical plant, located in central Poland. Three geophysical methods were adopted for this survey: EM profiling, ERT resistivity surveys and direct push resistivity logging. While EM profiling was chosen for surface area mapping, the application of ERT method was aimed at vertical characterization of the contamination distribution. Finally, the direct push resistivity logging was performed at two locations, with the aim of verifying the ERT interpretation. As a result, several extremely high conductivity/low resistivity anomalies has been identified, that can be attributed to the migration pathways from waste ponds. The survey results for each method applied here are shown, and emphasis is placed on comparison between the results obtained with each individual method. Differences in the resistivity/conductivity image of the area can be explained as arisen from different spatial characteristics of each method applied.
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DERT and Magnetotelluric Investigation of Bradano Foredeep Deposits
Authors E. Rizzo, M. Balasco, P. Harabaglia, V. Lapenna, A. Siniscalchi and G. TamburielloGeoelectrical and magnetotelluric surveys have been carried out to better understand the deep water circulation system in the Venosa area (Southern Italy) located in the frontal portion of the southern Apenninic Subduction. In this area, measurements undertaken in some deep wells give a water conductivity of 3mS/cm and a temperature of about 35°C. A deep dipole-dipole geoelectrical tomography has been carried out along a profile of 10.000m and an investigation depth of 900m. Furthermore a broad band magnetotelluric profile consisting of six station was performed in order to infer the resistivity distribution up to some kilometerers of depth. The MT profile was almost coincident to the geoelectrical outline. The applied methods allow us to obtain a mutual control and an integrated interpretation of the data. The high resolution of the data was the key to reconstruct the structural asset of buried carbonatic horst which top is located at about 600m deep. On the basis of data wells, geothermal analysis and geophysical data, it results that the horst is saturated with salted water and an anomalous local gradient of 60°C/km is present. The proposed mechanism is that of a mixing of fossil and fresh water.
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A General Approach for Introducing Information into Inversion and Examples from DC Resistivity Inversion
Authors T. Guenther and C. RückerWe present a more general inversion and regularization method based on constrained minimization on arbitrary grids.
Weighting matrices are used to control both model cells and their boundaries.
This allows for involving additional information as bore hole data, known boundaries or other geophysical methods, e.g., in form of a joint inversion.
We show different examples, each with additional information that is incorporated into the inversion and, thus, enhances the interpretation significantly.
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