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Near Surface Geoscience 2013 - 19th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 09 Sep 2013 - 11 Sep 2013
- Location: Bochum, Germany
- ISBN: 978-90-73834-38-5
- Published: 09 September 2013
1 - 100 of 116 results
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3D Electrical Resistivity Tomography in Glacial Sediments’ Research
Authors A.M. Pavlova and V.A. ShevninThis article is about 3D electrical resistivity tomography (ERT) results in quaternary outwash and moraine sediments mapping and stratification. These investigations were conducted on Alexandrovka moraine plateau. We performed areal survey, using parallel 2D ERT profiles and following 3D inversion and as the result of our survey we obtained three-dimensional geoelectric model of the area. We studied and described technical and geophysical capabilities, efficiency and productivity of this technology. Moreover we estimated how parallel profiles' azimuth influenced investigation results, detected causes and proposed some methods of reducing this influence.
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GPR Indicators of Identifying the Boundaries of Asphalt Concrete and Cement Concrete Layers
Authors R.A. Eremin and A.M. KulishnikovIn order to realize the reinforcement of the existing road pavement it is essential to know its internal structure. One of the important tasks is the localization of concrete slabs under asphalt concrete pavement. The GPR method is used successfully in solving these problems [1,2]. The paper reviews the results of GPR surveying for determining old cement concrete slabs location under asphalt concrete pavement on section of the Highway М-2 “Crimea” in the Moscow Region of the Russian Federation. For survey the GPR technical equipment was used (“Logis” Ltd, Ramenskoe), that is the antenna assembly with the central frequency radiation of 1700 MHz and the maximum sounding depth up to 0,8 m. The works were held in October 2012 on wet road surface under damp weather conditions.
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Combined 1-D and 3-D Inversion of Helicopter-borne Electromagnetic Data
Authors A. Ullmann, B. Siemon, M. Scheunert, M. Afanasjew, R.U. Börner, K. Spitzer and M. MiensopustHelicopter-borne electromagnetic (HEM) surveys are an effective tool for investigating the spatial conductivity distribution in the subsurface, e.g. for groundwater or mineral exploration. As a standard procedure, the multi-frequency HEM data are inverted to resistivity-depth models using a 1-D inversion method. Since the footprint of the HEM system is rather small and smooth conductivity structures are close to 1-D settings this is a valid approach. However, conductivity structures with strong lateral variations (anomalies) are not reproducible by 1-D inversion and a multi-dimensional inversion is required. Our aim is to combine 1-D and 3-D inversion of HEM data. A 3-D inversion is only carried out for those parts of a HEM survey which are affected by an anomaly. For all other parts a 1-D inversion method is used. Thus, the knowledge where such anomalies occur in a HEM data set is crucial. We present a new method for identification, selection, and extraction of anomalies in HEM data sets. The identified anomalies are handed over to the 3-D inversion. The resulting 3-D inversion models are integrated in the quasi 1-D background. The technique is demonstrated on a synthetic HEM data set.
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Feasibility Study of Multi-dimensional HEM Forward Modelling - Example of Cuxhaven Buried Valley
Authors M.P. Miensopust, B. Siemon and A. UllmannHelicopter-borne electromagnetic data sets are extensive in size and multi-dimensional modelling and inversion of the entire data set is in many cases impractical. On the other hand, the commonly used 1-D approaches might result in models with artefacts or distortion when higher dimensional structures are present. A complex procedure is required to enhance the resistivity models for such areas. One has to identify those areas either to investigate if artefacts are the potential cause of ambiguous structures in 1D-based sections/pseudo-3D models or to model and invert those using higher dimensional approaches. Multi-dimensional modelling and inversion codes vary considerably regarding their advantages but also their restrictions. The latter mainly result from structure related requirements but also from memory usage and computation time. Thorough testing of various codes is required to find the most applicable one for the specific data set. Small scale higher dimensional models obtained represent the local structure but might have shortcomings on the regional trend. Hence, merging those models with the overall model of the entire data set is a challenge. The focus of this paper is the feasibility study of multi-dimensional forward modelling programs based on the example of the Cuxhaven buried valley.
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Aeromagnetics with an Unmanned Airship
Authors M. Petzke, P. Hofmeister, A. Hördt, K.H. Glaßmeier and H.U. AusterUnmanned aerial vehicles (UAVs) have a growing field of applications. Geomagnetic measurements in difficult terrain are one example where UAVs can be a useful tool. So far only a few case studies about ultra-light UAVs have been published. Most of them are based on the principle of rotary wings, e.g. helicopters or multicopters. The payload capacity of these systems is often limited to about 1 kg. Here, we introduce an alternative to the existing platforms: Our airship is able to carry a payload of about 3 kg and has a flight time of up to 60 minutes. Furthermore, we prove the feasibility of this system with the help of two field examples. These studies demonstrate the ability to detect magnetic anomalies and the challenges in operating an airship and interpreting the according data sets.
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Sensitivity of Seismic Interferometry and Conventional Reflection Seismics at a Landfil to Processing and Survey Errors
Authors L.A. Konstantaki, D. Draganov, T.J. Heimovaara and R. GhoseUnderstanding how sensitive the seismic method is to errors that can occur during a seismic survey or during the processing of the seismic data is of high importance for any exploration geophysical project. Our aim is to image the subsurface of a landfill, which is typically a heterogeneous system due to the presence of numerous scatterers. Therefore, in our case precision is of very high importance. Because of this, we test and compare the sensitivity of seismic interferometry (SI) and conventional reflection seismics survey (CRSS) to errors produced due to time-lapse surveys, migration-velocity errors and muting. Using numerically modelled data, we show that SI provides better subsurface information than CRSS.
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Semi-automatic Mobile Equipment Test for Detecting Holes in Geomembranes – the Prototypes Evolution
Authors R. Mota, L. Coimbra, M. Barroso, M. Matutino, M.G. Lopes, R. Dores and F. SilvaLandfills contain basal lining systems that include composite liners, among which a geomembrane (GM). The success of these systems depends mainly on the GM performance, which acts as the primary barrier to contaminants migration. GM performance is conditioned by the presence of holes, which represent preferential pathways for leachate migration. An equipment was developed to detect holes in geomembranes. It is based on the geophysical resistivity method and aims to overcome the main disadvantages of the existing methods for GM holes detection, mainly time spent to perform the tests and the associated high costs. Several prototypes were already developed and were tested at small scale in laboratory. The final version, which is the model for the equipment under construction, was already successfully tested at laboratory small scale pilot plant and in a large pilot plant at Laboratório Nacional de Engenharia Civil, I.P. (LNEC) campus. This paper presents the prototypes development and results so far obtained.
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Investigation in Sinkhole Terrains Using Complex of Geophysical Methods – Case Study in the Karst Area, Slovakia
Authors R. Putiška, D. Kušnirák, I. Dostál, R. Pašteka and A. MojzešComplex of geophysical methods was used to investigate small karst area for purposes to make of detailed geological mapping survey, to confirm geological localization of known sinkholes and pits and find possible continuation of caves and voids below the surface. The electromagnetic (the Electromagnetic Conductivity method – CMD) and radiometric (the Gamma-Ray Spectrometry method) mapping was applied to determine the spatial distribution of hard carbonate rocks and weathered valley-filled sediments. Detailed high-definition magnetometry was realized on selected sites of the studied region with the aim to distinguish between real sinkholes and man-made lime-kilns (unearthed pits, where limestone was heated and transformed into lime), which have been built at the site during the history. The microgravity and the electrical resistivity tomography (ERT) methods were used to create high-resolution images of underground cave. The result of ERT and geological survey was used as an entry model for the gravity modelling. The most important results from ERT and microgravity are two interpreted low density (low resistivity) structures - these can be important from the point of view of karst structures investigation.
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Geotechnical and Geophysical Properties of Soils in the Dead Sea Sinkhole Problem
Authors M.G. Ezersky and E. LivneGeotechnical and geophysical properties of Dead Sea (DS) sediments are of a great importance in the DS sinkhole problem. DS sinkholes are formed on the alluvial fans and mud flats. The first are composed of sandy-gravel sediments. Second ones are brine saturated lime carbonates. We reveal following features of the DS soil properties. Sandy-gravel sediments are presented by coarse sands, which are characterized by Vs = 300 – 650 m/s. Such increased shear wave values can be caused by increased the soil stiffness described in the literature. Brine saturated lime carbonates (DS mud) are characterized by low Vs values of 120 – 250 m/s. They do not exhibit cohesion. It is explained by mineralogical composition of the Dead Sea material, composed predominantly of calcite and not clay minerals, whereas other Israeli clays are comprised largely of montmorillonite (spectite) clay minerals. This feature allows considering of saturated lime carbonate as cohesionless soil like fine sands. Lime carbonates demonstrate also very low cation exchange capacity (CEC) that allows use of TEM resistivity method for determining of porosity of both sands and DS mud. Note also that significant difference of Vs in sands and lime carbonates allows mapping of different lithology along DS coastal areas.
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Resistivity Modeling of Sinkholes Effects Caused by Karst and Suffosion
Authors V.A. Shevnin, P. Cuevas Castellanos, K.V. Krivosheya, A.M. Pavlova, A.A. Ryjov and A.A. BobachevSinkholes caused by two different processes of karst and suffosion have some similarity and both create damages in cities, settlements and single houses in rural areas. Not once the problem was set to give forecast of sinkholes before their output on the earth surface. The authors with the help of mathematical modeling of anomalous effect above vertical cylinder and similar models tried to estimate possibilities of resistivity methods for forecast of dangerous underground sinkholes before their collapse.
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Detection and Mapping of a Palaeokarstic Network in a Quarry Work Using GPR
Authors C. Dubois, J. Deceuster, L. Dutilleul, B. Njinwoua, J. Poot, J. Tsongo and O. KaufmannA Ground Penetrating Radar (GPR) survey was conducted on the bench of a limestone quarry to detect and map a network of weathering. The investigated area is a rectangle of 16.60 m by 10.25m on the Carboniferous limestone from southern Belgium. Profiles are parallel and equally spaced of 25 cm from each other and the distance between each trace along a profile was 2 cm. The GPR survey was conducted using a 250 MHz antenna. After filtering, 2D-sections show very clearly weathered features to at least 10m in depth. A 3D model of the area was created to identify the branches of the weathered network. On time slices, these features are well located throughout the entire model and anthropic cuttings lines are recognized on their total height (~6m).
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Localising Fractures by Geoelectric Null-arrays
More LessThe term “geoelectric null-array” is used for direct current electrode configurations yielding a potential difference of zero above a homogeneous half-space. This paper presents a comparative study of the behaviour of three null-arrays, midpoint null-array (MAN), Wenner-g null-array and Schlumberger null-array in response to a fracture. The main objective is to determine which array(s) best localise vertical structures. Forward modelling of the three null-arrays revealed that the Wenner-g and Schlumberger null-arrays localise vertical fractures the most accurately. The numerical analysis served as basis to interpret the field results too. Field test measurements were carried out above a quarry in Les Breuleux (Switzerland) with the three null arrays and many classical arrays. The results were cross-validated with quarry-wall geological mapping. In real field circumstances, the Wenner-g null-array showed to be the most efficient and accurate in localising fractures. This study shows that geoelectrical null-arrays are better than classical arrays for localisation of fractures.
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A 3D GPR Time Slice Analysis of Paleo-reconstructions at Mound A, Ocmulgee National Monument
Authors D.P. Bigman and P.M. LanzaronePrehistoric societies from around the world constructed monumental mounded architecture or earthen pyramids for a variety of functions including the foundation for temples, leader’s residences, community stages, or cemeteries. Flat-topped earthen mounds often have complicated histories where the function, size, orientation, and summit architecture varied throughout time. This paper presents the results from a 500 MHz GPR survey conducted on the summit of Mound A, the largest prehistoric Native American mound at Ocmulgee National Monument located in central Georgia. Our study indicates that performing time-slice analyses of flat-topped mounds can effectively map successive construction stages and maintain sufficient resolution to identify summit architecture on earlier mound use episodes. The authors recorded four mound construction stages, the western expansion of the mound, and the shifting location and shape of summit architecture. Changes in summit architecture and mound size inform us of the shifting political landscape at Ocmulgee between approximately AD 900-1200.This study also resolved a discrepancy regarding the location of early archaeological excavation units from the 1930s. This may help archaeologists accurately provenience artifacts during future investigations.
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Biogeochemical Controls on Magnetic Susceptibility Variations across a Hydrocarbon Contaminated Site
Authors E. Atekwana, F. Mewafy, G.A. Aal, E. Atekwana, C. Beaver, S. Rossbach, L. Slater, D. Ntarlagiannis, A. Revil and D. WerkemaBiogeophysical investigations over the last decade have provided evidence documenting that geophysical technique can be used to assess in-situ biogeochemical processes. In this study, we investigated a site contaminated by crude oil at Bemidji, Minnesota where intrinsic bioremediation is known to occur and iron is the dormant electron acceptor process.
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Improving Resolution in Surface NMR by Transmitting Off-Resonance
Authors D. Grombacher, J.O. Walbrecker and R. KnightAn important development providing improved resolution in surface NMR images has been the advent of the inversion of complex NMR signals (Braun et al., 2005), instead of an amplitude only inversions. The signal phase is generally attributed only to conductivity effects, and a secondary geophysical survey is used to characterize the subsurface conductivity structure allowing the phase to be well constrained in the inversion. We propose a novel strategy for data acquisition motivated by similar reasoning as studies demonstrating improved resolution through inversion of a complex signal.
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Long-term Monitoring at a Levee of Red River in Vietnam
More LessA long term monitoring has been performed at a levee of Red River in Vietnam. The monitoring system consists of a permanent electrode array for resistivity imaging and geotechnical tools including tensiometers, frequency domain reflectometry sensors and an observation well. The components were installed after a detailed inspection of the levee and foundation by drilling, sampling, and a geoelectrical survey. The levee monitoring system provided time series of the sensor data that were related to the variation of pore water pressure and water distribution. The data clearly indicate the influence of variations of the water level in the Red River during the monsoon season and also the drying process of the levee thereafter. Furthermore, it was possible to observe the upward and downward movement of the seepage line within the levee. A two-dimensional transient finite element modelling of the seepage and groundwater flow was performed using the database of a single flood season. The resulting hydraulic model was used for a stability analysis. It could be shown that the stability calculations taking into account the transient development of the seepage line enables a more favourable safety factor in the stability analysis in comparison with the conventional steady state approach.
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Application of a Passive Surface Wave Method to Estimation of Deep S-wave Velocity Structure in San Francisco Bay Area
Authors K. Hayashi and D. UnderwoodWe performed two-station microtremor array measurements (2ST-MAM) at several sites in the South Bay of the San Francisco Bay Area. Two seismographs with three-component accelerometers were used for data acquisition. The two accelerometers were separated by 5 to 4125m and several different separations were used at each site. The total record length of microtremor data for each separation was about 10 to 60 minutes and measurements at one site took several hours. A spatial autocorrelation was used for calculating phase velocity and clear dispersion curves were obtained in frequency range from 0.2 to 10 Hz. A joint inversion of dispersion curves and H/V spectra was applied to observed data and S-wave velocity models to a depth of about 2km were obtained.
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Comparison of 1D Conventional and 2D Full Waveform Inversion of Recorded Shallow Seismic Rayleigh Waves
Authors L. Groos, M. Schäfer, T. Forbriger and T. BohlenWe present a successful application of a 2D elastic full waveform inversion (FWI) to shallow seismic Rayleigh waves. With FWI we are able to reduce the data misfit significantly during the inversion and to find a subsurface model that explains the recorded seismograms well. The reconstructed S-wave velocity model is predominantly depth dependent which is expected for the site under investigation. We observe no significant 2D artefacts although a 1D structure is not enforced by regularization. The field data were acquired on a test site that we chose because of its almost 1D subsurface structure. This gives us the possibility to additionally derive a 1D model by a conventional method that we compare to the 2D model obtained by the FWI. The 1D inversion is done by an inversion of Fourier-Bessel expansion coefficients which requires constant phase velocities along the profile. We observe differences between the 1D and the 2D model although both models produce seismograms with a similar waveform fit of the field data. This shows the ambiguity of the inversion. The 2D model explains the field data slightly better in some details of the waveforms.
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2D Full Waveform Inversion of Recorded Shallow Seismic Rayleigh Waves on a Significantly 2D Structure
Authors M. Schäfer, L. Groos, T. Forbriger and T. BohlenElastic full waveform inversion (FWI) of shallow seismic surface waves has the potential to infer lateral variation of the shallow subsurface which is important e.g. for geotechnical site characterization. First successful applications of a FWI to surface waves show the high potential of this method. In preparatory studies we have identified essential prerequisites for an inversion of field data: Viscoelastic damping and 3D to 2D correction filters. For the first time it is possible for us to apply FWI to shallow seismic field data. 2D full waveform inversion is reasonably applicable to field data acquired on 2D structures. We present a shallow seismic 2D survey on a transverse section of a vertical fault located on the southern rim of the Taunus (near Frankfurt on the Main, Hesse, Germany) which is a dominantly 2D feature of the subsurface. Next we apply the FWI to this shallow seismic field dataset. We are starting the inversion with strongly lowpass filtered data of 10 Hz. Within this frequency bandwidth we obtain reasonable inversion results. However, by adding higher frequencies the inversion is apparently trapped in a local minimum. We try to overcome this problem by preconditioning and smoothing.
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Multiple Reflected Refraction Method in the Application of Near Surface Modeling
More LessIn this paper, we have derived and analyzed the kinematics law of the multiple reflected refractions, which are abundant in mountains, uplift of basin and piedmont tectonic belt, and then based on its time-distance curve formula, we discussed its application in the near surface structure modeling, which was analyzed and verified in Junggar Basin compared with other near surface structure modeling methods. The successful application in Junggar Basin indicates that the multiple reflected refraction method has an important significance for the near surface structure modeling when combined with other near-surface exploration methods in complex exploration area.
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Very Low Frequency Method Combined with an Unmanned Aerial System
Authors R. Eröss, J.B. Stoll, B. Tezkan and R. BergersSo called Unmanned Airborne Systems (UAS) can be used beneficial for geophysical measuring campaigns. Their advantages reach from faster data acquisition compared to ground measurements to higher versatility (e.g. ground vicinity) to conventional aerial systems. The geophysical Very Low Frequency (VLF) method exploits this advantages especially good. This is because it is well suitable for mapping and therefore extremely powerful in combination with aerial systems. The system presented here is a combination of the unmanned helicopter Scout B1-100 from Aeroscout and the Super High Frequency induction coil Triple (SHFT) and Analog Digital Unit (ADU)-07 data logger from Metronix. The sensor and logger went trough a weight reduction process and had to be deployed on a special suspension developed by Aeroscout. With a payload of 18 kg and an operation time of one hour the system is ideal for mid sized measuring campaigns, i.e. several hundred meters. A proof of concept study was carried out in March 2012 on a test area close to Neuchâtel. Two man-made anomalies, a pipeline and a power cable were investigated to test the practicability of the system. The feasibility of airborne VLF measurements with an UAS to detect buried anomalies is demonstrated by first results.
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A Practical Field Experiment of Multiple Transmission Resistivity Profiling Using Code Division Multiple Access
Authors Y. Yamashita, F. Lebert, J.C. Gourry and B. BourgeoisWe have conducted field experiments for evaluating practicality of a multiple transmission DC resistivity profiling using Code Division Multiple Access (CDMA) technique. In a multi-transmission resistivity profiling, it is possible to inject current at multiple points simultaneously by using coded current waveforms among which covariance between mutual waveforms equal to zero. Multi-transmission resistivity profiling technique has the potential to enable very rapid measurement compared to mono-transmission conventional resistivity profiling. In this paper, we have conducted experiments for evaluating workability and noise-robustness. The results of field experiments show that the efficiency of a multi-transmission resistivity measurement is quite high regarding to current duration time. We have found that erratic low-frequency noise badly affects the measurement result but it was possible to improve this result by post-processing the measured potential waveform.
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Influence of Electrode Heights on 4-point and 2-point Measurements Based on Capacitively Coupled Electrodes
Authors A. Przyklenk and A. HördtTo survey resistive grounds and estimate electrical parameters, the capacitive resistivity imaging technique (CRI) can be used. It overcomes coupling problems that are known from DC techniques. Instead of metal stakes, thin plates are used to inject AC current into the subsurface. In field measurements capacitively coupled electrodes often do not have direct contact with the ground, because of vegetation or a rough surface. We are focusing on electrode height variations and its effect on 4-point and 2-point impedances theoretically and experimentally. To keep controlled conditions, we measured in the laboratory in a box (1.2m x 1.2m x 0.8m) filled with dry sand and gravel. We increased electrode heights from 1µm to 0.8m and repeated the measurements to verify the reproducibility. Results show, that the range of electrode heights can be separated into sections that are characterised by different properties with respect to sensitivity to height variations and electrical parameters of the subsurface. It is also beneficial to measure 2-point impedances. They can be used to assess 4-point impedance measurements. For investigations on unknown soils, it is recommended to use a wide range of electrode heights. This allows a reduction of non-uniqueness with respect to electrical parameters and electrode heights.
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Data Quality Quantification for Time Domain IP Data Acquired along a Planned Tunnel near Oslo, Norway
Authors T. Dahlin, E. Dalsegg and T. SandströmTests were done measuring resistivity and time domain induced polarisation using standard multi-core cable spreads and a special layout with separate cable spreads for transmitting current and measuring potentials. For both types of cables spreads both normal and reciprocal measurements were done in order to estimate the measurement errors. The tests were done along a planned tunnel stretch outside Olso in Norway. The electrode contact was variable with resistances in the range 0.6 - 25 kΩ. The results gave low median error levels for both types of cable spreads, but the single cable spread showed a significantly larger variation with more scatter in the IP data. Data for both types of spreads gave models that are consistent and appear to delineate the complex geology in a useful way. It is concluded that the single cable spread gives surprisingly good IP data considering the large layouts at this site, which is adequate for inversion of the integrated full decay. If on the other hand the data were to be used for spectral IP inversion of the decay curves for recovering the Cole-Cole parameters the extra effort of measuring with separated cable spreads would probably be well motivated.
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Management of Sinkhole Risks Using Long Term ERT Monitoring - A Laboratory Experiment
Authors J. Deceuster, J.E. Chambers, P. Goderniaux, O. Kuras, P.B. Wilkinson and O. KaufmannAccurate methodologies are required to manage risks linked to land-use planning in covered kart terrains, especially in densely urbanized areas. The main risk lies in the occurrence of sinkholes at the base of buildings or infrastructure. We conducted a laboratory experiment to evaluate the contribution of ERT monitoring in the long term management of such karstic risks. After presenting the design of the laboratory experiment, we detail the selected scenarios and the acquisition protocols tested. The methodology proposed to process the data and manage the inversion results relies on two steps: (1) we estimate the resistivity variations due to measurement and inversion errors based on Monte-Carlo simulations and (2) we define a resistivity changes index for every cell of the 3D model. The methodology is tested on a 3D surface survey including inline dipole-dipole, equatorial dipole and Wenner-Schlumberger arrays. A 10 cm in diameter plastic ball is used to model a 3.5 m sinkhole at depths ranging from the surface to 20 cm, using a water resistivity of 12 Ohm.m. Based on the proposed methodology, we show that this target can be suitably detected when its top reaches 10 cm or less in depth.
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Electromagnetic Monitoring in the Zone of Seismic Activisation
Authors A. Sanchaa and N. NevedrovaElectromagnetic monitoring is carried out in the epicentral zone of catastrophic Chuya earthquake (Mountain Altay, Russia) with the magnitude of 7.3. On the basis of transient electromagnetic method (TEM)soundings with central loops field data analysis the evolution of geological massive tension is investigated. Regular TEM measurements are carried out in several regime points that placed in the west part of the Mountain Altay Chuyskaya basin (fig.1). Geoelectrical parameters of rocks in that points carried out in low seismic activity period were estimated according to the 80th years archive data. Comparison of quantitative interpretation of up-to-date (2004-2011) and archive measurements results allow to trace the process of restoration of geoelectrical parameters of rock massive in aftershock period.
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Slidequake Generation at Softrock-landslides
Authors M. Walter, J. Gomberg, W. Schulz, P. Bodin and M. JoswigOur study describes the conditions for the generation of initial fracture processes, i.e. slidequakes, at three different creeping softrock-landslides: the Slumgullion landslide in the San Juan Mountains, Colorado, USA, the Heumoes slope in the Austrian Alps, and the mudslide in Super-Sauze, French Alps. Our investigations identified seismic and aseismic slip in each slide, with slidequakes focusing at significant bedrock structures or at lateral boundaries. At the Slumgullion landslide, the majority of slidequakes occurred at the lateral boundaries of the landslide, while no events were detected along the planar basal surface. At Heumoes slope, the slidequakes cluster in the slope area with the lowest surface displacement rates. A significant bedrock rise, oriented perpendicular to the direction of slope movement, divides the landslide geometry in two basins, and probably impedes motion, slows the slide, and leads to slidequakes. At the Super-Sauze mudslide, the slidequakes are preferentially generated in its centre where the deformation rates are highest. There, the slidequake generation is directly linked to in-situ bedrock crests that border several gullies oriented in line to direction of the entire slope movement. Synoptic comparison of these three scenarios underlines the importance of landslide-bedrock and landslide-lateral boundary interactions under gravitational loading.
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Near-surface Fracture Discovery from Landslides by Sonogram Screening
More LessFracture signals from near-surface processes like creeping landslides and pending rockfalls carry important information on formation stability and promise to enhance early warning approaches significantly. In principal, these fracture signals behave like any local earthquake signal. However, two aspects differ so much that for long time even the existence of these signals was unknown: (I) low energy of local magnitudes Ml = –2 and below, unknown source region and thus sub-optimal network layout with source-receiver distances of several hundred to thousand meter demand signal detection with SNR near one, (II) short signal duration, and a priori unknown signatures result in frequent confusion of fracture signals with noise bursts. In any field installation, noise forensics are crucial to differentiate between signals-of-interest and local noise. Classical detector approaches like STA/LTA are not suited to perform these tasks with sufficient rate of success due to very high false positive rates. Instead, we developed a scheme of visual inspection by seismologists, based on an optimum spectral display of detectable energy spots by combined sonograms of seismic mini-arrays. Applications from landslide monitoring in Austria and France and rockslide precursory detection in Austria demonstrate the appropriateness of our approach.
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Quick-clay Landslides in Sweden - Insights from Shear-wave Reflection Seismics and Geotechnical Integration
Authors C.M. Krawczyk, U. Polom, A. Malehmir and M. BastaniAs part of a joint project studying clay-related landslides in Nordic countries, we successfully tested the use of high-resolution shear-wave reflection seismics to survey shallow structures that are known to be related to quick-clay landslide processes. Three 2-D reflection seismic profiles were recorded in Frastad, southern Sweden, above the main slide area, using a 120 m long streamer of 120 SH-geophones at 1 m spacing, and the ELVIS micro-vibrator as source (10 s long sweep ranging from 20 to 160 Hz). The longest profile along a gravel road shows clear internal structuring of the up to 50 m thick marine sediments as well as strong undulations of top basement underneath. Different sedimentary sequences can be distinguished, and the quick-clay sequence is interpreted in 15-20 m depth, which correlates well with the height of the most recent scarp. The shear-wave velocities suggest extremely low values of 100-120 m/s, which geotechnically prohibits building areas. In addition, test measurements on a stubble field parallel to the gravel road showed the first time that the suppression of Love waves is not only restricted to paved surfaces. In addition, the gap between structural data from ground-penetrating radar and P-wave seismics can be closed.
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Estimation of Electrode Movements from Sparsely Distributed Reference Points for Time-lapse ERT
Authors S.S. Uhlemann, P.B. Wilkinson, J.E. Chambers, P.I. Meldrum, O. Kuras and L. OxbyThe improving capabilities of ground imaging techniques results in an increased application of geophysical techniques to the characterization of landslides. Time-lapse electrical resistivity tomography (ERT) can provide valuable data (i.e. changes in moisture content) to understand landslide processes in more detail. However, it relies on the exact knowledge of the electrode positions, which on a landslide will change over time. We present three different techniques to estimate electrode positions over time from a given set of reference points. The velocity approach is derived from a linear interpolation of directional movement of the reference points to the initial electrode positions. Another method, the general approach, assumes that the relative position of a point to two vectors remains constant over time. A third approach uses kriging to interpolate a ground movement to the initial electrode positions. The three different methods are applied to a synthetic example and real data, and their performance compared. The general approach is shown to be the most accurate one.
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Ground Truthing and Integrated Geophysical Surveying for the Safety Assessment of Dike Systems
By T. InazakiAn integrated geophysical technique we proposed has begun to be utilized as the vulnerability assessment of dike systems in Japan. Characteristic features of the technique are in combination of seismic and electrical methods, in multi-stage application of geophysical surveying, and in joint inversion of geophysical survey results with reference to geotechnical data. The proposed technique mainly consists of high-resolution surface wave method using Land Streamer, capacitively-coupled resistivity method using OhmMapper, multi-frequency Slingram electromagnetic survey method. The geophysical properties evaluated by these methods, S-wave velocity and resistivity, are used to evaluate permeability, stiffness and seismic resistance. We also demonstrate the usefulness of crossplot analysis as a simplified joint inversion based on the empirical relationship between geophysical and geotechnical data. Ground truthing is one of the most efficient ways to obtain the relationship mentioned above. Then we conducted the integrated geophysical surveying and several field measurements at a site where dike was to be partly excavated, and correlated the survey and measurement results obtained before and after the excavation. As a result, we could successfully identify anomaly structures in and beneath the dike systems at the excavation site. Furthermore, the surface geophysical survey results were concordant with the ground truthing data.
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Shear Wave Velocity Structure for Seismic Microzonation of Lorca town (SE Spain) from MASW Analysis
The average shear-wave velocity to a depth of 30 m (Vs30) has been obtained in the Lorca town (SE Spain). Measurements of Vs30 were made using the Multichannel Analysis Surface Waves (MASW) method. Those Vs30 values let us classify the urban area of Lorca town according to both the EC8 and the NEHRP site class. Then, these zones were related to the shallow geology of the town. Regarding the damage distribution in Lorca town due May 11th, 2011 Lorca earthquake, the most affected buildings were located on zones classified by the EC8 as B2 (from 360 m/s to 500 m/s) or C (from 180 m/s to 360 m/s). The geology in those zones is predominantly composed of shallow layers of Holocene unconsolidated sedimentary filling (including colluvial, aluvials and anthropogenic fillings) and layers of Pleistocene consolidated sedimentary fillings of glacis. Areas of the town composed of Tertiary (Neogene) medium hard rock were found Vs30 values above 500 m/s (zone B1) and where the building damages were not relevant.
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Wind Farm Site Characterisation using MASW Survey
By A. RichardThis study presents a non-intrusive geophysical investigation using Seismic Refraction and the Multi-channel Analysis of surface waves (MASW) techniques at the consented 10- wind turbine generator (WTG) locations at Tullo Wind Farm, in Aberdeenshire, Scotland. The aim was to provide geotechnical parameters including the ground stiffness profile at each turbine location. The results are intended to inform ongoing Site Investigation (SI) activity and inform detail design of turbine foundation across the site. At some turbine locations, the attenuation of relatively high frequency components of the transmitted seismic energy was recorded. This is most likely as a result of boggy nature of the near surface conditions. It is highlighted that there is a reduced confidence of the estimated P-wave velocity values for these cases. The S-wave velocity estimates including Vs30 were derived from MASW analyses. Data from most locations exhibited good dispersive characteristics that provided clear, laterally consistent phase-velocity frequency plots enabling 2D visualisation. However, a lack of dispersive behaviour at STI1, ST3 & TET3 locations limited the lateral extent of S-wave velocity determination. The deformation properties such as Elastic moduli were calculated based upon assumed densities of 1.8Mg/m3 for each geological units across the site.
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How to Explore Deep Geothermal Reservoirs in Populated Areas
Authors S. Bissmann, D.J. Orlowsky and B. LoskeWithin the framework of the high financial investment for deep geothermal projects it is especially important to include geoscientific work as possible. Consequently, comprehensive geological and geophysical investigations are needed to predict the reservoir development and its overburden. The application of 3D seismic data to predict reservoir conditions has become the standard approach having replaced initial surveying by way of 2D seismic line data in conjunction with reprocessed older survey data. Progress has been made in enhancing the quality of seismic imaging by use of state-of-the-art processing such as pre-stack depth migration and in deriving relevant factors (faulting/fracturing, facies, diagenesis, karstification) for predicting reservoir properties from this data. The ensuing results are of great help to significantly reduce exploration risks. Furthermore, the results allow planning a target orientated well path.
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Geothermal System Interpretation Based on 1D Magnetoteluric Model at Akesahu, North Maluku
Authors S Darmawan and G.M.L. JunursyahA region of east Indonesia with a big geothermal energy potential is Akesahu in the province of North Maluku. Akesahu is located on the island of Tidore. An integration investigation included geological, geochemical, and geophysical survey has been completed to determine of subsurface mapping in this area. There are seven hot springs are found in this area as a surface manifestation of geothermal system. Furthermore, Hg and CO2 mapping has been supporting an investigation due to of ability to indicate a heat fluid leak and structure zone mapping. The presences of these hot spring and also the other supporting data from geological and previously geophysical data could be help on magnetotelluric data interpretation. Based on these previous survey data, data compilation are consistent shown a reservoir prospect in Akesahu area is in the northeastern of Tidore Island. At least, there are three layers shown in LINE-01 and LINE-02 MT section. In addition, the presences of other structures or weak zones that appear on MT seem consistent with the interpretation based on the other data sets and it’s proved by 1-D magnetotelluric data which show the top reservoir is 900 m.
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GPR Monitoring on an Organic Target Decomposition
Authors E. R. Almeida and J.L. PorsaniIn this work the authors present GPR results of the monitoring of an organic target decomposition. The experiment consists in use a buried pig (Sus scrofa) as an organic target, which simulates a human body. The pig was buried at the University of São Paulo campus located in Pirassununga city (São Paulo State) in December, 2012 and has been surveyed periodically with the GPR method. The objective is to evaluate the effects of its decomposition on the GPR reflection data. The results show that the anomaly caused by the body as better viewed in the 270MHz radargrams, while depth slices made with 900MHz data were better to identify the trench borders. The research is still in progess, and the results are important for understand the use of GPR method in searching for human remains.
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ERT for Localizing 17th Century Tunnels at a Jesuit Mission in Buenos Aires, Argentina
Authors A. Osella, V. Grunhut, H.P. Martinelli, M. de la Vega and N. BonomoWe performed a geophysical study in a historical site of Buenos Aires, corresponding to a Jesuit Mission established during the 17th and 18th centuries. It consisted of a Church, the Cloisters, a School and a Procurator Office; also several tunnels were built, connecting the Mission with different public buildings of the town. In 19th century the Faculty of Sciences was constructed over one of the site, functioning until its demolition in 1973. At present, this space is a cobbled square. With the aim of preserving and restoring the buried structures, we worked in this square, looking for tunnels and also for remains of the basement of the old building. Taking into account both the conductive features of the soil, mainly formed by clays and silt, and the urban characteristics of the environment, we performed pre-feasibility studies to determine the penetration-depth of different methods. We obtained reliable data surveying with dipole-dipole profiles with apertures of 1.5 and 3 m, provided that the electrodes were buried at least 15 cm among the cobble stones. By correlating the resulting nine ERTs with the original plans, old structures and evidences of remains of tunnels were identified.
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Study of Industrial Disturbances Affecting Magnetic Prospecting Data in Archaeological Sites
By S.E. ItkisThe study of industrial disturbances was focused on “static” sources made of iron and reinforced concrete (e.g. water pipes, stakes), and “active” or electromagnetic-field producing elements (e.g. electrical power lines, generators and geophysical equipment). The measurements and forward modeling provided the patterns and main parameters (amplitudes, gradient and blind-zone sizes) for magnetic anomalies of static disturbances. Forward modeling was also applied to an analysis of the demagnetization effect of highly magnetic objects and the threshold, the latter being the rate at which the anomaly amplitude increases when that of the values begins to slow down. The disturbig effect introduced by a non-stable electromagnetic field cannot be eliminated by single-sensor magnetometer measurements but can be suppressed by the use of a gradiometer survey.
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Application of Areal Transient Electromagnetic Method Measurements Directed on Polymetal Prospecting
Authors N. Nevedrova, A. Sanchaa and S. BabushkinThe results of transient electromagnetic method soundings areal measurements directed on polymetal prospecting are presented in this paper. Measurements are carried out by transient electromagnetic method (TEM)soundings within Ergozhu ore field in the Irkutsk region, Russia. The way of excitement of electromagnetic field in a large size generating contour was chosen and measurements were executed by small receiver loops which were moved along on system of profiles with a detailed step. As a result of processing of TEM field data geoelectric parameters of rocks and ore objects which are allocated with abnormal values of the specific electric resistance are determined. Areal and spatial distribution of geoelectric characteristics of an ore field is received. Geoelectric sections and maps of the main structural horizons with allocation of ore objects are constructed. Areal survey and chosen sizes of loops provides necessary resolution of the TEM in the conditions of considerable lateral and vertical inhomogeneity of rocks. Thus, the purpose of work consists in identification of spatial placement of perspective ore zones according to the TEM on the basis of three-dimensional visualization of results of observation.
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Shallow Shear Wave Reflection Surveying on Alpine Glaciers – Comparison of Results from Gepatschferner and Colle Gnifetti
Authors U. Polom, A. Diez, C. Hofstede, M. Stocker-Waldhuber and O. EisenIn the summer season 2010, a shallow reflection seismic experiment using the small vibrator source ELVIS was carried out on the overburden firn and ice cover of the Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. This site is widely used for method testing, since the physical properties of ice are similar to those of polar regions. The unique experiment approved for the first time the shallow high-resolution vibroseis method using P- and S-waves for seismic targets on firn and ice masses at least to nearly 60 m depth. As a consequence of this successful experiment, the method was subsequently applied in April 2012 on the Gepatschferner, a glacier in the Austrian Alps, using S-waves only. In contrast to the commonly planted receivers at Colle Gnifetti, a land streamer modified for snow application was used as receiver system. The source was slightly modified by a ski to support operation on soft snow. The recorded data at Gepatschferner achieved clear reflections from the ice base boundary in nearly 150 m depth and from the glacier bed below.
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Comparison of High-resolution P- and SH-wave Reflection Seismic Data in Alluvial and Pyroclastic Deposits in Indonesia
Authors C.M. Krawczyk, W. Wiyono and U. PolomHigh-resolution, P- and SH-wave seismic reflection profiles were acquired at two different locations in Indonesia to test the capability of imaging in pyroclastic sediments. The seismic measurements for both locations were carried out using a 48-channel recording system, and sledge hammer and micro-vibrator ELVIS as sources. The results of seismic data in Semarang show fair to good seismic records for both P- and SH-waves. The raw data contain a high signal-to-noise ratio. Many clear reflectors can be detected. The P-wave data show reflectors down to 250 ms two-way time while the SH-wave records show seismic events up to 600 ms two-way time. This result is in strong contrast to the observations in the Tiris region. Here, the P-wave data show a very low signal-to-noise ratio, and there is no reflection signal visible, only the surface waves and the ambient noise from the surrounding area are visible. The SH-waves give a fair to good result which enables reflector detection down to 300 ms two-way time. Thus, the SH-wave reflection seismic seems to be the suitable method, which could be applied in Indonesia in both alluvial and pyroclastic regions.
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Increasing the Depth of Detectability of ERT Measurements
Authors K. Szokoli, S. Szalai and A. NovákThe issue of depth of investigation has always been a basic problem in geophysical studies. Nowadays, as more and more research should be carried out in built-up areas where space is limited, it became even more important. Since we think that the depth of investigation is basically determined by the detectability of small-size inhomogeneities, the so-called depth of detectability (DD) values, this parameter is investigated in this presentation for the 2D ERT. In the case of the studied prism and dyke models the not-yet investigated g11n arrays gave systematically higher DD values than the pole-dipole and dipole axial arrays which were the best conventional arrays from this point of view. The difference can be significant, up-to 2-2.5-fold in comparison even with these arrays. This makes possible to obtain useful information even where the measuring area is limited, eg. in settlements, in less time and from greater depth. Moreover, due to the imaging properties of arrays having larger DD values are generally better it is easily possible that these arrays have also better imaging properties than those of the traditional arrays.
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First Results from the New Multi-purpose Instrument CapGeo
By T. RadicHigh quality geophysical equipment is expensive. Therefore, people repeatedly ask for instruments, which are adapted to measure more then one physical parameters. An example for this is the idea to combine Transient Electromagnetic Method (TEM) with Magnetic Resonance Sounding Method (MRS). The MRS method delivers the water content and the TEM method delivers the resistivity distribution. Unfortunately, this idea is not really practicable. The loop may be useful for both methods but the electronics must be quite different. Here we present and discuss first results measured with new type of equipment called CapGeo. This instrument was developed for the geophysical institute of TU Braunschweig (Prof. Hördt). The main purposes are capacitive resistivity measurements. But the severe requirements make this instrument eminent suitable for other geophysical exploration methods. Till now we have made some first field test measurements for four different methods: - Capacitive Resistivity (CR) - Spectral Induced Polarization (SIP) - Radiomagnetotellurics (RMT) - Very Low Frequency (VLF), Z/H
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3-D SNMR Inversion of Water Content
Authors W. Warsa, H. Grandis, W. Parnadi and D. SantosoSurface Nuclear Magnetic Resonance (SNMR)is the geophysical method, which allows to determine water content and pore size distribution directly from the surface. A 3-D forward modeling code for SNMR amplitudes and decay times has been developed; after that improved 2-D and 3-D inversion algorithm has been investigated consist of model parametrization regularization schemes. Giving a short review of general inversion schemes used in geophysics, the special properties of SNMR or MRS (magnetic resonance sounding) inversion are evaluated. This inversion will be applied for the SNMR field of water content and decay time. At the end, the SNMR can be applied to dectect subsurface water in suitable geological formation and more depending on the presence of natural and cultural electromagnetic noise.
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Optimizing Distribution of SNMR Pulse Moments and Stacking Strategy
Authors E. Dalgaard, M. Müller-Petke and E. AukenIn this study we are analyzing the balance between number of data points and data quality in SNMR survey design. In SNMR the number of data points increase with the number of pulse moments and data quality increase with the number of records at each pulse moment (stack size). The premise for our analysis is that the total amount of records is fixed to 1500 and the stack size and number of pulse moments are dependent on each other. Thus, for a high number of pulse moments the stack size and thereby data quality are low and for a low number of pulse moments the stack size and data quality are high. From our model uncertainty analysis we show that it is better to discretize the data space densely, by having a high number of pulse moments with a lower data quality compared to fewer pulse moments with a higher data quality. Furthermore, we analyze and compare the logarithmic pulse moment distribution scheme with an optimized pulse distribution scheme. We show that the determination of the model parameters at a certain depth can be improved by adding more pulse moments at corresponding interval.
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Surface-NMR in Urban Area - A No Go?
Authors M. Mueller-Petke and S. CostabelSurface-Nuclear-Magnetic-Resonance (surface-NMR) has shown a continuous development towards a frequently used and established hydro-geophysical tool during the last decades. However, the measured signals are as low as few tens of nanovolts and therefore often contaminated by significant amounts of electromagnetic noise. Consequently, surface-NMR has been used on remote area with great success but is also known to be a technique that needs to be chosen with respect to low noise conditions. Several developments both on instrumentation and processing allow overcoming these strong restrictions on applying surface-NMR. Multi-channel devices use remote references to cancel harmonic noise. High sampling rates of the used analog-to-digital converter allow for advanced de-spiking schemes. Finally, the combination of formerly known figure-of-eight loop layouts with remote references provides access to areas close or even within the borders of settlements. We give several examples demonstrating these developments and highlighting the new capabilities of the technique.
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Geophysical Investigations of a Rural Water Point Installation Program in Nampula Province, Mozambique
Authors F. J. Chirindja, E. Sjöstand, O. Enkel and T. DahlinThere are many projects in Mozambique for poverty reduction. One of these projects is funded by Millennium Challenge Account (MCA) and is aimed to install a total of 600 rural water points in the provinces of Nampula and Cabo Delgado. Each water point consists of a drilled well, a water pump and a communal washing basin. Vertical Electrical Sounding (VES) was used for assessing the suitability of the drill sites but despite this many boreholes have come out with an insufficient yield and the failure rates in certain areas are as high as 50 %. Continuous Vertical Electrical Sounding (CVES), also known as Electrical Resistivity Tomography (ERT), was carried out in an attempt to explain the high failure rate. In total 11 sites were investigated, including seven boreholes with sufficient yield and five boreholes with insufficient yield. A perpendicular cross with two 400m survey lines were made over 7 boreholes and single 400m survey lines were made over 5 boreholes. Due to lateral variation the geology in study area is best described in 3D therefore ERT appears to be a suitable method for groundwater exploration and could probably lower the failure rate.
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Spectral Induced Polarization Measurements on New Zealand Sands - Dependence on Fluid Conductivity
More LessSpectral induced polarization (SIP) measurements have been made for different fluid conductivities on a sample of sand typical of the host material in New Zealand coastal aquifers. Fitting the data with a Cole-Cole model shows that neither the mean relaxation time nor the shape exponent depend upon the fluid conductivity. This is consistent with measurements of the hydraulic conductivity made on the same sample which also show no dependence on fluid conductivity. Simple predictions of permeability based on the Kozeny-Carmen relationship are of the correct order of magnitude but tend to overestimate the permeability. These results lay the groundwork for future measurements aimed more directly at drawing correlations between the SIP response and hydraulic properties of New Zealand aquifers.
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Electric and Electromagnetic Measurements on the North Sea Island of Langeoog
Authors S. Costabel, T. Günther, U. Noell, B. Siemon and W. VoßElectric and electromagnetic methods were conducted on the North Sea island of Langeoog to investigate its freshwater lens that is managed by the local water works to supply the island with drinking water. Helicopter-borne electromagnetic (HEM), as well as surface geophysical methods were applied, i.e., electrical resistivity tomography (ERT), transient electromagnetic (TEM), and magnetic resonance sounding (MRS). We found that, beside the depth of the fresh-saltwater interface, the combined interpretation of TEM and MRS allows a reliable identification of lithological layering, whereas ERT or TEM alone exhibit a high degree of ambiguity regarding subsurface lithology and groundwater salinity. Future research is focused on the joint inversion of the surface methods, as well as on the combined inversion of surface and HEM data. In this way, 3-D multi-parameter models will be generated as additional input for numerical modeling of groundwater dynamics inside the freshwater lens.
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Comparison between the Hydraulic Properties of Two Low-permeability Alluvial Systems Supported by Geophysical Methods
Authors A. Guinea, W. Timms and I. AcworthThe mechanisms for vertical flow through clayey alluvial deposits are poorly understood. This type of deposits are usually studied as sets of separated permeable (aquifers) and less permeable (aquitards) strata assuming water flow is negligible over the short to medium term. Nevertheless, in some clayey deposits this simplification proves to be inadequate as there is evidence of vertical groundwater flow. This study was aimed at improving the understanding of hydrological mechanisms for vertical flow, determined by the degree of homogeneity or heterogeneity of the alluvial deposit. In order to study the vertical hydraulic connection through clay-silt dominated deposits, near surface and borehole geophysics have been combined with other hydrological techniques at two different sites with a contrasting degree of heterogeneity in the Liverpool Plains within the Namoi River catchment (NSW, Australia). At the Breeza DPI farm site, situated in an area of groundwater extraction for irrigation studies indicates active deep drainage through heterogeneous clayey strata and hydrochemical changes occurring in the underlying aquifer systems. By contrast, at the Cattle Lane site, located upstream in a dryland farming area with no groundwater extraction, there appears to be little significant vertical flow through a more homogeneous clayey deposit.
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A Heat Injection and Pumping Experiment in a Gravel Aquifer Monitored with Crosshole Electrical Resistivity Tomography
Authors T. Hermans, S. Wildemeersch, P. Jamin, P. Orban, S. Brouyère, A. Dassargues and F. NguyenThermal tracing experiments are becoming common in hydrogeology to estimate parameters governing heat transport processes and to study geothermal reservoirs. Electrical resistivity tomography (ERT) has proven its ability to monitor salt tracer tests, but few studies have investigated its performances in thermal tracing experiments. In this study, we monitor the injection and pumping of heated water using crosshole ERT in a panel crossing the main flow direction. Difference inversion time-lapse images clearly show the heterogeneous pattern of resistivity changes, and thus temperature changes, highlighting the existence of preferential flow paths in the aquifer. Comparison of temperature estimates from ERT and direct measurements in boreholes show the ability of ERT to quantify the temperatures in the aquifer and to draw the breakthrough curves of the thermal tracer with a relative accuracy. Such resistivity data may provide important information to improve hydrogeological models. Our study proves that ERT, especially crosshole ERT, is a reliable tool to follow thermal tracing experiments. It also confirms that ERT should be included to in situ techniques to characterize heat transfer in the subsurface and to monitor geothermal resources exploitation.
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Reliability of ERT-derived Temperature - Insights from Laboratory Measurements
Authors T. Robert, T. Hermans, G. Dumont, F. Nguyen and D.E. RwabuhunguWe performed laboratory measurements on fully saturated sand samples in the context of deriving reliable temperature from time-lapse electrical resistivity tomography (ERT). The experiment consisted in monitoring an increase of temperature in sand samples with electrical resistivity measurements. We neglected the effect of surface conductivity since experiments showed two orders of magnitude between surface and fluid conductivities. We show that using simple linear relationship between fluid electrical conductivity and temperature alone does not allow reliable temperature estimates. Indeed, chemical analyses highlight the importance of accounting chemical reactions occurring when temperature changes, including dissolution/precipitation processes. We performed two experiments based on typical in-situ conditions. We first simulated the injection of a less conductive tap water and second, the injection of heated formation water. In the second case, minerals solubility decreases and precipitation occurs, leading to an increase of bulk resistivity. This mechanism competes with dissolution of minerals when tap water is injected, since tap water is not in equilibrium with the medium. In any case, further research is needed to fully understand the mechanisms and to develop a fully integrated law to derive better temperature estimates.
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3-D Time-lapse Electrical Resistivity Monitoring of Injected CO2 in a Shallow Aquifer
Authors J. Doetsch, E. Auken, A.V. Christiansen and G. FiandacaContamination of potable groundwater by leaking CO2 is a potential risk of carbon sequestration. With the help of a field experiment, we investigate if surface electrical resistivity tomography (ERT) can detect dissolved CO2 in a shallow aquifer. For this purpose, we injected CO2 at a depth of 5 and 10 m and monitored its migration using 320 surface electrodes on a 126 m × 20 m grid. A fully automated acquisition system continuously collected data and uploaded it into an online database. The large amount of data allows for time-series analysis for data quality and noise estimation. A baseline inversion reveals the geology at the site consisting of aeolian sands near the surface and glacial sands below 5 m depth. Time-lapse inversions clearly image the dissolved CO2 plume with decreased electrical resistivity values. We can follow the CO2 plume as it spreads and moves with the groundwater, and the spatial distribution of the plume agrees well with direct measurements of the groundwater resistivity. Future work will include improved time-series analyses and quantitative comparison with the groundwater resistivity measurements.
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Joint Analysis of Lunar Surface Waves - The Apollo 16 Dataset
By G. Dal MoroUnderstanding the near-surface structure of the Moon is important not only for scientific reasons, but for possible future technological applications related to mining, geotechnical activities, oxygen production etc. The Lunar regolith is represented by a layer of unconsolidated debris constituted by fine soil (average density around 1.5-1.8 gr/cm3) occasionally including breccia, rocks and boulders from the bedrock. Its thickness generally seems to range from about 5 m on younger maria to about 20 m on older highlands and appears highly stratified with many buried soil horizons. Although most of the Lunar literature necessarily dates back to the 70's, the opportunities provided by recent technical developments allow the re-processing of Lunar data capable of providing a deeper understanding of the formation and evolution of our satellite. The current study focuses on the re-processing of some data acquired in the framework of both the active and passive experiments of the Apollo 16 mission (April 16-27, 1972). In particular, a joint analysis of Rayleigh-wave dispersion (ASE data) and Horizontal-to-Vertical Spectral Ratio (HVSR) (LSPE data) is performed also considering the effects of attenuation and with the aim of retrieving the local shear-wave velocity profile.
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Unambiguous Determination of the Vs Profile via Joint Analysis of Multi-component Active and Passive Seismic Data
Authors G. Dal Moro and L. KellerThe determination of the shear-wave velocity profile is a major request in site characterisation studies for geotechnical and seismic-hazard studies and it is often accomplished via surface-wave analysis according to Multichannel Analysis of Surface Waves (MASW). In order to overcome non-uniqueness of the solution and possible interpretative issues, for the present study we consider the joint analysis of different and independent multi-component data: Rayleigh and Love waves (from MASW and frequency-wavenumber analysis), Horizontal-to-Vertical Spectral Ratio (HVSR) and p- and s-wave Seismic Refraction Tomography (pSRT and sSRT). Obtained Vs profile is eventually compared with the data obtained from a Vertical Seismic Profiling (VSP).
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Detailed Digital Terrain Models for Supporting the Analysis and Interpretation of Near-surface Geophysical Data
Authors J. Tronicke and U. BönigerToday's near-surface geophysical acquisition approaches readily allow for combining geophysical measurements with high-resolution positioning solutions based on the global-positioning system (GPS) or self-tracking total stations (TTS). Under favourable site conditions, such surveying strategies allow for efficiently acquiring high-quality geophysical data with a positioning accuracy in the centimetre range. In addition to accurately locating geophysical measurements, such positioning data can also be used to generate a detailed digital terrain model (DTM) of the surveyed area. Here, we present two case studies where a detailed DTM supports and complements the interpretation of near-surface geophysical data. While the first case study comprises magnetic and electromagnetic induction data collected to locate buried utilities, the second case study presents a 3D ground-penetrating radar survey recorded to detect archaeologically relevant structures. In both cases, the topographic data resulting from the employed TTS-based surveying strategy are analyzed using different topographic attributes supporting the geophysical data interpretation. We conclude that, from our experience, inclusion of a detailed DTM into the geophysical data processing flow, can provide additional valuable information for data analysis and interpretation.
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Combined Geophysical Survey at the A2 Tunnel Maastricht
Authors O. Brenner and D. OrlowskyIn advance of the construction of the A2 tunnel in Maastricht a geophysical survey was necessary in order to investigate the position of layers, faults, flint layers, occurrence of rock deposits and potential cavities within the limestone layer. For this purpose three different geophysical methods were combined: CMP refraction seismic, refraction tomography, reflection seismic. The whole survey was performed with three different sources to get the best results for each depth: air sound source, sledgehammer and accelerated weight drop source. After processing and during the interpretation process the results of 18 boreholes were included. Finally all relevant layers could have been detected and corresponded to the results of drilling. Furthermore minor and major faulting in the survey area could be detected. Potential cavities in the limestone were not detected.
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Geophysical Investigation for the Detection of Liquefaction Phenomena in an Archaeological Site, Lechaion, Greece
More LessA geophysical survey was designed in order to detect ground liquefaction phenomena in a wrecked Early Christian Basilica in the archaeological site of the ancient harbour of Lechaion, located in the southeastern coast of the Corinth Gulf, Greece. EM, GPR and ERT measurements led to the detection of stratigraphy and resistive zones of coarse material which were possibly used in ancient times for geotechnical purposes and could be related with the liquefaction phenomenon characteristics. ERT sections have detected the liquefaction zones successfully suggesting that the method can be a useful tool for such applications. EM and GPR methods add valuable information regarding lateral and vertical changes in sedimentation.
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The High Resolution Short Length P&S Wave Velocity Mesurement
Authors T. Kobayashi and H. AzumaThe shorter seismic velocity measurement is currently required to obtain more detailed and precise velocity for the assessment of the surface of rock, such as the rock tests sites. The physical properties that are surveyed with rock test, such as Unconfined Compression Test and Shear Test, are important to calculate earthquake-resistant building designs. To achieve high precision with the short length measurement, we should use the high speed sampling data logger, the wide range sensors, and the high frequency seismic source, compared to the existing seismic measurement systems. In this paper, we demonstrated the importance of three components for accuracy in the short range velocity measurements: estimating the errors before beginning an investigation, selecting the optimum measurement system, and assessing the velocity result of errors. We will present the new measurement system developed for increasing precision and these successful results in the field.
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Detecting Fracture Anisotropy Using Geological Mapping and Vertical Electrical Resistivity Survey
Authors G.O Mosuro, K.O. Omosanya, I.O. Kehinde and V.U. UkaegbuThe two areas were studied and have shown different trend of fracture orientations at surface and in the subsurface. Area 1 showed overlap between fractures oriented at shallow subsurface reveal by electrical resistivity anisotropy polygon plot and that of fracture orientation on the surface rock exposures in the study area; suggest that fractures are penetrative and hard-linked. Thus, the fractures at both levels are produced by similar tectonic. Whereas, lack of overlap between fracture orientations at both surface and subsurface at Area 2 suggests that the surface fractures are not deep seated, and that fractures at both scales are thought to be produced by different tectonic events
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Shear Wave Aquiclude Survey and Interpretation
By H.P. MuellerExamining, processing and mapping of an aquiclude zone at some 10 to 15 m target depth in an urban environment by SH-wave recording using a mini vibrator as a source is presented. The aquiclude's top depth, only indicated before at a monitoring well, had to be mapped throughout the seismic line. The layout in the street demanded 2D crooked-line processing. In the seismic section a remarkable two-sided step-down of some 2 m height within 20 m distance was observed which made interpretation difficult. This result could only be validated by combining surface profiling with VSP survey information at the well location. Evidence can be stated now, as two additional wells, recently drilled on the very surface line, being the subject of this presentation, found the aquiclude as predicted by the seismic forecast. The seismic model constitutes concluding evidence of pre-glacial scouring here. Setting out the necessity to benefit from both methods in the attempt to identify the aquiclude's layer surface, is claimed here. To gain optimized constraints in designing the SH surface line to follow, it is proposed to perform SH VSP first if wells in the vicinity were available.
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Seismic and Geoelectric Investigation of Historical Mining Structures - A Case Study
Authors U. Swoboda, S. Uchtmann, K. Limbrock, R. Elsen, D. Orlowsky, K. Telenga, A. Koschare and F. WollnikThe Ruhr area is the biggest urban agglomeration and is well known for its coal mining activities. In consequence of the former underground mining numerous artificial caves near to the surface can be found. These caves lead to earth drops and sinkholes which form a safety hazard for the environment. In this presentation a case study of a geophysical investigation will be shown whereat seismic and geoelectric methods were applied close to an area where several historic coal mines were placed. Six survey profiles were planned whereas five were mainly parallel and one perpendicular to a main geologic fault. The survey lines had an overall length of approx. 1000 m and covered a region of 14 000 sqm. The seismic data were analyzed using the following methods: standard refraction processing (wavefront method), refraction tomography, single shot analysis for gaps in the refracting waves, reflection processing and tomographic velocity analysis of the transverse travelling surface waves. The geoelectric data were processed using a 2-D inversion approach. All processing methods showed a number of anomalous zones. It became visible that the anomalies form several clusters which can be interpreted as second order fault positions with a typical appearance for wrench faults.
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Geophysical Investigation of a Burning Dump in Estonia – A Case Study
Authors S. Uchtmann, O. Brenner, U. Swoboda, R. Elsen and D. OrlowskyA burning semi-coke dump site at Kohtla-Järve, Estonia has been investigated by geophysical methods. The aim of the project was to test the capability of seismic and electromagnetic surveys to detect former or active burning zones. The results of the seismic and electromagnetic measurements show different characterizations of the investigated area. As both geophysical methods are based on distinct physical principles, the results are different. The seismic investigation is based on the propagation of elastic waves in the subsurface, which is mainly influenced by discontinuities in the ground like cracks and weak zones, changes in the texture of the ground or cavities. The seismic results, processed with a focus on surface waves, show approximately 20 anomalous positions with a punctual appearance. The results of the electromagnetic survey correlate with the observations at the surface. It is assumed, that hot spots of the burning dump can be detected with electromagnetic measurements. During the field survey two of the reported seismic anomalies were mechanically excavated. At both positions the results of the survey are verified.
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Integrated Geophysical Investigation at a Construction Site in a Highly Urbanised Area
By F. BahloulTechniques to investigate the shallow underground with the help of a combination of several geophysical methods increased rapidly during the last decade. Nevertheless, the use of different geophysical methods results often in different interpretations of the investigated area. This paper deals with the integrated interpretation of CMP-refraction seismic, refraction tomography, traditional refraction seismic, electric resistivity tomography and borehole logging data acquired at a construction site in a highly urbanized area (Makkah, Saudi Arabia). Objectives of the geophysical measurements were mainly to determine und characterise the thickness of the saturated zone, the depth of the groundwater as well as the depth of the bedrock. All geophysical results (seismic tomography, resistivity and borehole logs) are combined and joint interpreted together with the data from the given boreholes in order to calibrate the final interpretation result. Interpretation has been performed with the state of the art interpretation and modelling software PETREL (Schlumberger Information Solutions) according to international industry standards. The chosen interpretation system ensures the generation of exact and consistent spatial images of the underground conditions.
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On Inversion of Frequency Domain Electromagnetic Data in Salt Water Problems - Sensitivity and Resolution
By T. GüntherFrequency domain electromagnetics (FDEM) is a fast investigation method for large scales and can particularly be helpful in salt-water problems. Sensitivity studies and simple models show the dependence on geometric factors and noise levels. Methods of resolution analysis can be successfully applied to reveal uncertainties of the model parameters and interdependence of the data. A combination of electromagnetic and direct current data significantly improves the results and their reliability. The methods are demonstrated on synthetic data and field data from a coastal aquifer system.
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1D Joint Inversion Analysis of VES and TDEM Soundings at Termas de Ibirá Region, Paraná Basin, Brazil
Authors M.A. Couto, J.L. Porsani, C.A. Bortolozo and D.N. LeiteIn this work, the authors present results of 1D joint inversion of Vertical Electrical Soundings (VES) and Time Domain Electromagnetic (TDEM) soundings at Termas de Ibirá region, countryside of São Paulo State, Brazil. The objective is the analysis and comparison of the results of individual inversion and joint inversion. The VES results can solve the shallow layers (until the depth of 100 m) and the TDEM results solve the deep layers (between 100 - 1000 m depth). Therefore, the joint inversion explore the best of each method and it allows to make a more reliable interpretation about the local geology. Keywords: TDEM, VES, Joint inversion, Paraná basin, Termas de Ibirá, Brazil.
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Geoelectrical Characterization by VES/TEM Joint Inversion at Urupês Region, Paraná Basin, Brazil
Authors J.L. Porsani, D.N. Leite, C.A. Bortolozo and M.A. CoutoThis paper shows the results of joint inversion of VES/TEM for Urupês region, Paraná sedimentary basin, São Paulo State, Brazil. The objectives of this research were to map the Adamantina Formation (sedimentary aquifer) and to locate fractures zones in the basalt layer of the Serra Geral Formation. The results indicate the tick of the Adamantina Formation (Bauru aquifer) and the fracture zones in the basalt of the Serra Geral Formation. Keywords: VES, TEM, Joint inversion, Paraná basin, Urupês, São Paulo State, Brazil.
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Automated Electrical Resistivity Tomography Monitoring of Quarry Dewatering
Authors J.E. Chambers, P.I. Meldrum, P.B. Wilkinson, B. Matthews, O. Kuras, D.G. Gunn, S. Uhlemann and L. OxbyA study has been undertaken to demonstrate proof-of-concept for the use of time-lapse ERT for the monitoring of changing groundwater conditions at mineral extraction sites undergoing active dewatering using a well-point system. Monitoring was undertaken using a permanently installed geoelectrical imaging system, which comprised an advanced data logger (referred to here as “ALERT”) capable of the autonomous collection of geoelectrical data, and a network of surface electrodes. ALERT monitoring is demonstrated as an effective means of monitoring groundwater rebound and drawdown, although a time-lag is observed between hydraulic head levels recorded in the piezometers and the bulk saturation information provided by ERT.
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Long Electrode ERT for Salt Water Monitoring - Modelling, Sensitivity and Resolution
Authors T. Günther, M. Ronczka and C. RückerThe upward surge of saltwater due to a high extraction rate of water plants, tectonic weakness zones or eroded clay layers threatens the fresh water supply in some regions of northern Europe. For monitoring the movement of salt water, the usage of borehole measurements is insufficient as only point information can be retrieved. Our aim is to develop a monitoring system that uses boreholes as electrodes (Long Electrode Electrical Resistivity Tomography - LEERT) in conjunction with surface-electrodes. As a first step we conducted synthetic modelling using the Finite Element Method and the Complete Electrode Model in order to obtain a better insight on the influence of long electrodes ERT measurements. Sensitivity and resolution measures show a reasonable capability of imaging saltwater intrusions if surface electrodes are used in combination with boreholes.
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ERT Survey for Characterising the Subsurface of a Golf Course Build over the Edge of a Volcanic Caldera (Canary Islands)
Authors A. Casas, E. Estévez, R. Lovera, J.C. Tapias, M.P. Palacios-Díaz, M. Himi and M.C. CabreraKnowledge of geometry and hydraulic properties of the unsaturated zone is essential to assess the dynamics of the subsurface drainage of water that seeps from the ground surface, whether it is rain water as excess of irrigation. This interest increases when irrigation is conducted with reclaimed water, as it happens on the golf course of Bandama which since 1976 is being watered with reclaimed water from the wastewater treatment plant (WWTP) of the city of Las Palmas de Gran Canaria. The golf course of Bandama is located on the edge of the volcanic caldera of the same name and occupies an area approximately 30 hectares of which some 14.5 ha are irrigated by sprinkling. This work arises to determine the characteristics of the subsoil under golf course by electrical resistivity tomography method, with particular interest in trying to delimit the areas of preferential drainage affecting vulnerability to contamination of the underlying aquifers.
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Electrical Resistivity Tomography at the Search of Groundwater on the South of Russia
Authors V.A. Shevnin, D.A. Kvon, B.A. Nikulin, A.A. Ryjov and A.D. SkobelevDue to shortage of fresh drinking water near Anapa town (near Black Sea), geophysical investigations were performed for searching and mapping aquifers in the area, where, according to rare wells exist probability to find fresh underground water. Geophysical survey was carried out with Electrical Resistivity Tomography (ERT) and water resistivity measurements. The resistivity of fresh groundwater is 15 Om.m, its salinity is 0.4 g/l. We calculated rocks resistivity taking into account groundwater salinity and concluded that basement rock is limestone not marl. This conclusion was supported by roentgen-radiometric measurements of rock samples estimating 85% of CaCO3 content. When petrophysical model of rocks was determined we found boundary values of resistivity for fractured water-saturated limestone, gravel and sand and these isolines put on geoelectrical cross-sections to mark probable water-saturated zones and plan boreholes for their verification.
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Estimating Recharge Potential and Preferential Flow Paths in a Meander Close to a Recharge Pond Using ERT
Authors A. Casas, A. Sendrós, M. Himi, R. Lovera and I. CasadoThis study was launched to obtain more detailed understanding of the internal varieties of hydraulic conductivities within a meander located in the Cubeta de Sant Andreu aquifer that will be used for infiltration through a rechange pond. The spatial distribution of unsaturated hydraulic conductivity in the subsurface is of importance for hydrological modeling. Conventional methods to determine unsaturated hydraulic properties in the field are invasive and typically have a poor spatial resolution. In order to overcome these drawbacks, geophysical methods have received much attention in the last decades. The task required means to identify the architectural units affecting the groundwater flow, and to define the precise flow paths and residence times of the infiltrated water. The work was carried out by electrical resistivity tomography profiles that were used to create a detailed sedimentological model of the area.
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Geochemical and Geophysical Study of Beach Las Glorias, Guasave, Sinaloa, Mexico
We performed hydrogeochemical and geophysical study in the area Playa Las Glorias, Guasave, México. With Vertical Electrical Sounding (VES in modification ERT having high space resolution) we characterized the interface between slightly saline and saline water. Chemical analysis and granulometry supported conclusions about groundwater salinity along ERT profile and with depth.
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Investigating the Effect of NaCl Content of Pore Water on the Electrical Resistivity of Sand Samples in the Laboratory
Authors A. Hojat, S. Karimi Nasab and S. KosariGeophysical laboratory measurements are important to provide data base for better interpretation of field surveys as well as developing new applications of these methods. The purpose of this research is to study the effect of NaCl content of pore water on the electrical resistivity of saturated samples of sand. To do this, water samples of different salinities were first prepared and their resistivity values were determined. Then, laboratory measurements of electrical resistivity were carried out on saturated sand samples of different grading. The results of all the tests showed that as the salinity of the pore water is increased, the resistivity of the sand samples decreases. Moreover, it was observed that using samples of equal NaCl content, the coarser the soil, the higher resistivity is measured.
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Experimental and Theoretical Studies of the Temperature Dependence of Spectral Induced Polarization (SIP)
Authors K. Bairlein, M. Bücker, A. Hördt and S. NordsiekThe temperature dependence of spectral induced polarization (SIP) parameters is of interest for measurements near the surface where seasonal and meteorological temperature variations are present. Also measurements at greater depths, where the temperature increase with depth dominates, are influenced by the temperature. We analyze the temperature dependence by calculating the polarization parameters with an extended membrane polarization model, and by measuring the SIP-response of a limestone sample in a temperature range of 0°C to 60°C. Both simulations and measurements show a temperature dependence of the complex electrical conductivity. The phase shift slightly changes with temperature, depending on the pore geometry of the material.
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Marine ERT Modelling for the Detection of Fracture Zones
Authors P.I. Tsourlos, G.A. Tassis and J.S. RønningThe applicability of marine ERT surveys into the detection of underwater fracture zones is studied in this work. Various marine fracture zones models and measuring modes were tested using synthetic data which were subsequently subjected to inversion. In particular factors such as the effect of the sea water layer, the resistivity contrast, the overburden layer in both floating and submerged electrode modes are examined and compared. Among others, results indicate that the exact inclusion of the water layer into the inversion models is extremely important and that ERT surveys can be successful only for the first 10 meters of sea water.
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Detailed Study of Near Surface Resonance Properties Using Low Channel Microtremor Measurements - Physical Modelling Data
Authors Y.I. Kolesnikov, K.V. Fedin and A.F. EmanovThe method for detailed study of resonance properties of near-surface section using low channel microtremor measurements was tested on three-dimensional physical model with known characteristics. The model of layer overlying the vertical boundary of two mediums was investigated. It is shown that for measurements made with low-channel equipment and relocatable receivers the conversion of nonsimultaneous data to “common” time using synchronous data from fixed receiver can improve considerably the results of seismic microzonation. Experimental results are in good agreement with the data of finite element simulation.
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Magnetotelluric and Seismic Noise Techniques Combination for the Cerdanya Basin Characterization (Eastern Pyrenees)
Authors B. Benjumea, A. Macau, A. Gabàs, J. Ledo, P. Queralt and S. FiguerasA sedimentary Tertiary basin filled with unconsolidated sediments located in eastern Pyrenees has been subjected to a combine passive geophysical survey in order to increase knowledge of bedrock geometry and sediment properties. The methodology consists of combining and integrating both seismic noise techniques (H/V and array) and audio-magnetotelluric (AMT) and magnetotelluric (MT) data. AMT and MT data have been recorded along a NW-SE profile crossing the basin. Four 2D seismic noise arrays and 79 single station measurements were deployed at this profile. H/V technique has been suitable to obtain a soil fundamental frequency. Bedrock depth assessment can be obtained using S-wave velocity profiles derived from seismic noise array analysis. This information will allow establishing a soil fundamental frequency-bedrock depth relationship suitable for this basin. On the other hand, interpretation of the electrical resistivity model obtained from an AMT- MT survey will be constrained by the seismic results. This electrical resistivity model provides a detailed imaging of structural features such as the Cerdanya fault. This methodology, based on sediment characterization from shear-wave velocity and resistivity information, can be an attractive option for basin characterization essential for projects such as reservoir imaging.
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Application of the Controlled Source Radiomagnetotelluric Method in Remote Regions
Authors A.E. Simakov and A.K. SaraevThe new RMT-C system, based on measurements of the electromagnetic field of a grounded electric dipole as a controlled source, provided fulfillment soundings in remote regions, where is possible to measure only VLF radio transmitters signals by standard RMT instruments. Realization of CSRMT surveys using several main frequencies of the controlled source (CS) and their odd harmonics from the band 1-1000 kHz can provide short time of measurements at each sounding station. The distance from the source to measurements sites can reach several kilometers that can fulfill CSRMT measurements of wide areas using one CS installation. In receiving group ungrounded electric antennae can be used what is very important during measurements with bad conditions of groundings (on snow, ice, gravel, concrete etc.). Geoelectric sections obtained from 2D inversion results have good correlation with borehole data. The developed RMT-C system can be successfully applied in the near-surface geophysics for the solution of engineering and other tasks.
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Illuminating the Internal Structure of Two Monogenetic Volcanoes by ERT (La Garrotxa Volcanic Field, Spain)
Authors A. Casas, S. Barde-Cabusson, X. Bolós, D. Pedrazzi, R. Lovera, M. Himi, G. Serra and J. MartíEruptive activity of individual monogenetic volcanoes usually lasts few days or weeks. This short lifetime is not ever synonym of simplicity in terms of dynamics and structure. The construction of monogenetic cones is rarely witnessed from the beginning to the end and conditions for observing their internal structure are hardly reached. We provide high-resolution electric resistivity tomograms (10 m electrode spacing) of three monogenetic cones from north-eastern Spain. Details of their structure that could not be observed through other geophysical methods are revealed. We compare our results to geological observations, interpreting their underground continuation. With a maximum depth of exploration of about 100 m, we provide images of almost the entire edifices, highlighting the relationships between Strombolian and phreatomagmatic deposits in two multi-phases edifices. A main observation is a column of lower resistivity at the centre of the Puig d’Adri Strombolian cone, which we interpret as the eruptive conduit. This method provides information on the past volcanic dynamics of the monogenetic volcanic fields in general, which has real implications for the forecast of future activity.
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Investigation of Landslide Kinematics Derived from UAV- and Ground-based Imagery - Precision Analysis of 3D Point Clouds
Authors M. Walter, R. Rothmund, U. Niethammer, J.P. Malet and M. JoswigDuring an eight week field campaign at the Super-Sauze landslide (South French Alps), several UAV- and ground-based photographs have been acquired. Additionally, high-resolution terrestrial laser scanning (TLS) has been performed in order to capture a precise reference data set. The High-resolution and multi-temporal, but also disordered and oblique images were used to generate 3D point clouds from Structure-from-Motion and Multi-View Stereo (SfM-MVS) algorithms. These multi-temporal digital surface models (DSMs) were required for a precise ortho-rectification, as well as for morphometric and kinematic investigations. In order to determine the quality of the point clouds, all photogrammetric data were compared to the time corresponding TLS-based DSMs. The analysis of DSMs and orthophotos is one part of a field experiment that has the objective to characterize the relationship between fracture processes (slidequakes and local tremor events), slope movements, fissure developments and pore water pressure build-up. The main focus of this contribution is a comprehensive evaluation of the SfM-MVS-based point clouds in order to demonstrate the potential but also the limits of such approach.
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Combined Active and Passive Seismic Investigations on Railroad Embankments
Authors F. Koellner, E. Pomponi, C. Sens-Schönfelder and A. SchuckThe present study introduces combined active and passive seismic investigations on a railroad embankment that is constructed on a soft layer of peat clays. This type of sediments causes the embankment to be in a dynamic movement whenever trains are passing. As long term consequence the stability of the embankment is affected. Based on the concept of seismic interferometry temporal changes in the medium due to the heavy dynamic force load induced by the train can be identified. Furthermore, velocity information of travling body waves that have been retrieved by cross-correlating the signals of train passages are compared to results from seismic crosshole measurements.
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Imaging Subsurface Cavities Using Geoelectric Tomograpgy, Ground-penetrating Radar and Microgravity
By M. SobhIn the past few years, construction extended to the southeast of Cairo, Egypt, where limestone caves occur. The existence of caves and sinkholes represents a hazard for such new urban areas. Therefore, it is important to know the size, position, and depth of natural voids and cavities before building or reconstruction. In this paper, geoelectric-resistivity tomography, ground-penetrating radar (GPR) and microgravity have been applied to image shallow subsurface cavities. Microgravity measurement was done to detect density variations in rock strata. Several gravity anomalies were found in the residual map. Semi-automated interpretation techniques including the Euler deconvolution, analytic signal have been used to investigate the depth and size of anomalous sources. Resistivity survey was conducted along three profiles over an exposed cave with unknown extensions. Results from 27 GPR profiles obtained by a SIR-2000 instrument equipped with a 200 MHz antenna were visualized in the form of horizontal time slices and vertical time sections. As a result, the cave at a depth of about 2 m and a width of about 4 m was detected using the geophysical data, which correlates with the known cave system.
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Optimised Sequential Experimental Design for Geoelectrical Resistivity Monitoring Surveys
Authors P.B. Wilkinson, S.S. Uhlemann, J.E. Chambers, P.I. Meldrum, L.S. Oxby and O. KurasSequential experimental design methods use previous data and results to guide the choice and design of future experiments. This paper describes the application of a sequential design technique to produce optimal resistivity imaging surveys for time-lapse geoelectrical monitoring experiments. These survey designs are time-dependent, and are optimised to focus a greater degree of the image resolution on the regions of the subsurface that are actively changing than static optimised surveys that do not change over time. The sequential design method is applied to a synthetic 2.5D monitoring experiment comprising a well-defined cylindrical target moving along a trajectory that changes its depth and lateral position. The data are simulated to be as realistic as possible, incorporating survey design constraints for a real resistivity monitoring system and realistic levels and distributions of random noise, in order to match a forthcoming experimental test of the method. The results of the simulations indicate that sequentially designed optimal surveys yield an increase in image quality over and above that produced by using a static (time-independent) optimised survey.
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Computation of Optimized Arrays for 3-D Electrical Imaging Surveys
Authors P.B. Wilkinson, M.H. Loke and J.C. ChambersThree-dimensional surveys and inversion models are required to accurately resolve structures in areas with very complex geology where 2-D models might suffer from artifacts. Many 3-D surveys use a survey grid where the number of electrodes along one direction (x) is much greater than in the perpendicular direction (y). Frequently, due to limitations in the number of independent electrodes in the multi-electrode system, the surveys use a roll-along system with a small number of parallel survey lines aligned along the x-direction. The 'Compare R' array optimization method previously used for 2-D surveys is adapted for such 3-D surveys. Offset versions of the inline arrays used in 2-D surveys are included in the number of possible arrays (the comprehensive data set) to improve the sensitivity to structures in between the lines. By using PCs with modern graphics cards incorporating a fast Graphics Processing Unit (GPU) and using an improved single-precision 'Compare R' algorithm, the 3-D optimized arrays can be calculated within a reasonable time despite the comprehensive data set possibly have millions of arrays. A comparison with data sets using inline measurements made using conventional arrays show that structures located between the lines are much better resolved with the optimized arrays.
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Source and Receiver Array Simulations for Near Surface Seismic Investigations
Authors A. Tognarelli and E. StucchiWe present a procedure for enhancing the data quality of shallow seismic data based on two different steps: 1) an acquisition step that requires the recording of closely spaced common source records with standard source and receiver equipment and 2) a processing step where weighted and/or un-weighted source and/or receiver arrays can be simulated on the basis of required needs for source related noise attenuation and depth penetration. The data acquisition can be carried out employing single source – single geophone recordings, with a standard 24 or 48 channels equipment. Simple energy sources such as weight drops or sledgehammer are considered. The design and application of the spatial filters in the processing phase is very flexible and can be tailored to the specific needs: in fact, the simulated source and/or receiver arrays can be time and/or space variant and, if needed, can be weighted to provide the desired responses. Optimal weights can be determined by means of Chebyshev polynomials. Real data examples show the increase in the data quality in terms of better coherent noise attenuation and of enhanced depth penetration.
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Software for Development of Seismic Models of Rock Masses Employing 2D-3D Seismic Exploration
Authors O.O. Glukhov, A.I. Kompanets, A.V. Antsiferov, V.A. Antsiferov and L.A. KamburovaA vital task of seismic exploration in Ukraine is prediction of tectonic structure of coal-rock masses in conditions of thin-bedded near-surface (up to 1,500 m) geologic section taking into account low-amplitude tectonic faulting. In our paper, software developed in UkrNIMI of the National Academy of Sciences of Ukraine (NAS of Ukraine) for construction of models for such coal-rock masses based on the results of 2D – 3D seismic exploration is briefly described. Software units enable to execute reading and visualization of initial seismograms; to estimate record quality and preprocessing of seismic traces, input of acquisition configuration and seismic data tying; to make estimation of 3D survey quality, computation and visualization of seismic sections and cubes, computation and visualization of the surfaces of geologic structures.
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Estimation of the P-wave Velocity Profile of Elastic Real Data Based on Surface Wave Inversion
Authors A.V. Ponomarenko, B.M. Kashtan, V.N. Troyan and W.A. MulderRecently, we proposed an analytical approach to invert for a smoothly varying near-surface P-wave velocity profile that has a squared slowness linearly decreasing with depth. The exact solution for such a velocity profile in the acoustic approximation can be expressed in terms of Airy functions and leads to a dispersion equation. The method was successfully applied to synthetic elastic data with small Vs/Vp-ratio. Here, we apply the method to land data. The result agrees with that of multi-layered inversion, confirming its potential to provide an initial P-wave velocity model for acoustic full waveform inversion. Compared to multi-layered inversion, the method is simpler to use and produces a smooth model characterized by three parameters. In some cases, having a smooth rather than a blocky initial model for full waveform inversion is more appropriate.
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Relationship between Surface Conductivity and Induced Polarisation Parameters
Authors A. Weller, L. Slater and S. NordsiekInterpretation of resistivity measurements is often uncertain as resistivity is controlled by pore fluid chemistry, moisture content and lithology. Induced polarisation measurements may help to reduce this uncertainty as they are primarily controlled by lithology and, relative to resistivity measurements, weakly dependent on pore fluid chemistry. In order to exploit this potential, we have investigated the relationship between the surface conductivity appearing in petrophysical models for resistivity and the imaginary conductivity measured with induced polarisation. It has previously been suggested that imaginary conductivity can be used to estimate the surface conductivity if the ratio of surface polarisation to surface conduction (l) is known. However, a detailed investigation of this ratio has not hitherto been reported. We performed a study on an extensive database of sandstone and unconsolidated samples where resistivity and induced polarisation measurements were acquired as a function of salinity. The surface conductivity was estimated assuming a simple parallel conduction model where conduction through interconnected pores and conduction along interconnected pore surfaces add in parallel. The surface conductivity exhibits a weak salinity dependence that is similar in shape to the salinity dependence of the imaginary conductivity. This finding suggests that l is only weakly dependent on salinity.
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The Salinity Dependence of Spectral Induced Polarization Studied with an Extended Model of Membrane Polarization
More LessIn order to estimate hydraulic parameters from SIP measurements, it es essential to understand the dependence of the electrical impedance not only on structural parameters describing the pore space, but also on electrochemical properties. Here, we investigate the influence of fluid salinity on the IP parameters based on an analytical model to simulate membrane polarization. The model was originally developed for a one-dimensional pore system, where the membrane effect is postulated to be caused by varying ion mobilities in two different pore types. The model was recently extended to explicitly include finite pore radii and the properties of the electrical double layer, and the membrane effect comes out naturally as a result of varying ion concentration with distance from the pore wall. We carry out simulations to study the influence of salinity on the IP effect and compare the results with experimental observations. While our results are consistent with several trends observed in experiments, such as an increasing imaginary conductivity with fluid salinity, some features, such a decrease of the imaginary conductivity at high salinities, were not yet reproduced.
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Estimation of van Genuchten-Mualem Parameters from Spectral Induced Polarization Measurements
Authors S. Nordsiek, A. Hördt, E. Diamantopoulos and W. DurnerThe application of spectral induced polarization (SIP) surveys to estimate hydrological properties of the subsurface is promising. For the interpretation of the SIP data, the knowledge of the relationships between soil hydraulic properties and parameters obtained from SIP measurements is essential. Therefore, we performed hydrological and SIP measurements in the laboratory on seven natural soils and a medium sand sample with a narrow grain size distribution. The SIP spectra were interpreted with the Debye decomposition, which yields integrating parameters for the resulting relaxation time distribution. To derive soil hydraulic parameters from the hydrological experiments, we fitted the van Genuchten-Mualem model, which is widespread in soil physics, to the measured hydrological data. In this study, we concentrate on the relationship between two parameters: the normalized chargeability from the Debye decomposition of SIP spectra and the parameter alpha from the van Genuchten-Mualem model. We present an approach to estimate the parameter alpha from the normalized chargeability.
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Prediction of GPR Performance in Soils Using Broadband Dielectric Spectroscopy
More LessOne of the soil properties influencing the sensing depth of ground-penetrating radar (GPR) is intrinsic attenuation. Especially in high-frequency investigations, as for explosive ordnance and landmine detection, it plays a major role. In many cases attenuation is the limiting factor that determines the applicability of the GPR system. In order to investigate the frequency-dependence of electromagnetic soil properties, different soil samples from Germany and Afghanistan were analyzed. For this purpose, we used a coaxial transmission line together with a vector network analyzer in the laboratory. Two coaxial line cells were used in order to determine the complex dielectric permittivity in the 1 MHz to 10 GHz frequency range. The complex permittivity curves were fitted by a generalized model, which accounts for different dielectric relaxation mechanisms. The inversion of the model was carried out by means of the Geophysical Inversion and Modelling Library (GIMLi). Splitting the measured complex dielectric permittivity data using the generalized model made it possible to investigate the different energy loss contributions to the intrinsic attenuation, which are in turn attributed to certain soil components. Based on the laboratory results, we intend to create a simple method for the prediction of the GPR performance in the field.
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Study on Fractal Structure of Pore Space of Sandstones Based on NMR and Capillary Pressure Curves
Authors Z. Zhang, W. Debschuetz and A. WellerWe apply the fractal equation of pore distribution and NMR spectrum to 25 sandstone samples from several boreholes in China, and we show the results of the relations between NMR, fractal dimension and capillary pressure. The fractal dimensions can be calculated from NMR and capillary pressure data, and the value of correlation coefficient could indicate whether the pore structure shows fractal features. For the same core sample, there are different fractal dimensions in different pore radii ranges, indicating that core pore space has multiple fractal structure. The fractal dimensions determined from NMR data is lower than the values determined from capillary pressure data, this is due to the different pore throat space reflected by these two methods. The fractal dimension from NMR data can better represent the rock’s physical properties.
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Lab-scale Tests in the Detection of a Multi-phase DNAPL
Authors L. Orlando and B. RenziIn order to improve the understanding of the sensitivity of geophysical investigation to different phases of contaminant, GPR, electric resistivity and TDR measurements performed to detect and monitor DNAPLs spilled in a saturated media are analyzed. The experiment was performed in a Plexiglas cell filled with saturated grain glasses, simulating a permeable sand media, and hydrofluoroether (HFE-7100) simulating the contaminant. The results showed that GPR is more sensitive to the separate phase of contaminant because the dissolved phase does not significantly influence the electric permittivity. These results were confirmed by TDR measurement. On the contrary the electric resistive measurements resulted sensitive to all phases of contaminant (gaseous, separate and dissolved). The water flow and the variation of the water table contribute to the dissolution and migration of the DNAPL, even if the lab experiment showed that it cannot be easily removed within a saturated medium because its separate phase remains trapped within the pores and, very slowly, it moves downwards, also in the presence of a water flow.
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Integrated Investigation of the Instability-prone Landfill Site
Authors V Frid, A. Averbakh and G. LiskevichThe study was motivated by the landfill surface fracturing and grate doubts regarding to the landfill body stability. Three geophysical methods were used for the site study as follows: electrical resistivity tomography, ground penetration radar and seismic refraction. Application of the geophysical methods was combined with borehole drilling campaign, DPSH testing and inclinometer measurements. As a results of common geophysical geotechnical data processing 3D model of the landfill body was built, the regions of leachate concentration were found and waste physical properties were studied. The weak zone observed in the landfill bode and very a low values of P-wave velocity certainly indicated intensive waste disintegration and hence confirmed our work hypothesis on the lower waste body stability that was aggravated by the leachate location at the landfill body bottom. Inclinometer measurements indeed corroborated high values of waste instability. Application of the integrated methodology allowed one to assess waste and leachate condition and not only to evaluate the landfill stability in general, but also to locate the most sensitive its parts.
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Using Geophysical Tomographical Techniques for the Weak Point Analysis on a Potash Mining Dump
Authors K. Dünnbier, T. Schicht and F. BörnerThe objective of the presented research project is to develop a combination of geophysical and petrophysical methods for the non-invasive characterization of the potash mining dump material. It includes tomographic methods such as geoelectrics and seismics as well as ground penetration radar (GPR), spectral induced polarization (SIP) and nuclear magnetic resonance (NMR) methods. The resulting geophysical models should detect and display weak areas due to, for example, abnormal strengths, hydraulic permeability and water content. Previously acquired data has given first impressions of a part of the waste dump. The project aims to give the operating company a tool to characterize the different areas of a potash waste dump. The results could help the company to decide whether to reuse the material (backfill material for stabilization of salt mines) or to cover the waste dump effectively.
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Geophysical Characterisation of a Former Waste Disposal Site in the Context of Landfill Mining
More LessIn the context of landfill mining, we conducted electrical resistivity tomography (ERT), borehole electromagnetics (EM39), and magnetometry campaigns to characterise the subsurface of a waste deposit. The electrical resistivity structures in the landfill appear tabular with a first layer (12-15 m) characterised by higher resistivities (> 10 ohm.m; with spots > 30 ohm.m) whereas the saturated zone (below 15 m) presents lower resistivities (< 4 ohm.m). Based on ERT images, we positioned 6 drillings, collected waste samples, and recorded in-situ measurements (distributed temperature and EM39) to confirm the ERT interpretation and bring out some limitations of the method. Magnetic field data were recorded in the same area. We noticed a clear distinction between the inside (high magnetic field gradient) and the outside (low variation) of the landfill. The large scale variation of the total magnetic field is compatible with the existence of a clinker dam below the landfill bottom membrane (known from historical information). Therefore, magnetometry could be used to detect variation in the waste composition (clinker, metallic drums, or municipal waste) but is also a cheap and powerful tool to map unknown landfill boundaries. Both aspects are crucial in the context of landfill mining.
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3D ERT Monitoring of the Reactivation of Waste Biodegradation with Fresh Leachate Injection
More LessThe aim of this study is to monitor (bio) physical processes occurring in a landfill. The experiment consists in injecting leachate towards a drain in unsaturated and not yet digested waste to reactivate (or activate) waste biodegradation. The target is the first 15 meters of the studied landfill subsurface. The visualisation of the wet front arrival (short term effect) is crucial because we want to ensure that waste is entirely humidified to allow the reactivation of waste digestion. The second process is a long term effect consisting in the increase of the internal temperature of the landfill which is synonymous of the reactivation of biodegradation processes. We currently use 3D time-lapse ERT on a monthly basis to capture the decrease of electrical resistivity related to the increasing temperature. We also collect ground truth data, including distributed temperatures in a borehole to validate results. For short term effects, we monitored the wet front arrival with three 2D ERT profiles during an entire day. Preliminary results, corroborated by ground truth data, show that leachate flow is anisotropic (more rapid horizontally than vertically). So far, waste was completely humidified but only slight changes of temperature occurred.
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Landfill Leakage Detection Using ERT
Authors P. Tsourlos, G. Vargemezis, I. Fikos and A. GiannopoulosIn this work we applied the electrical resistivity tomography technique in an attempt to locate areas of leakage in a lined landfill. A systematic ERT survey was carried out using dipole-dipole and pole-dipole forward and reverse arrays. Based on the explanation of the different inversion images obtained over the same line for the the different electrode arrays we propose an interpretation approach which can be used for identifying leakage within a geomembrane liner landfill. Further, the systematic survey with dipole-dipole and pole-dipole (forward and reverse) arrays and the combined graphical interpretation can be used to locate potential leakage area.
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Combining Redox Potential Measurements and IP Signatures to Improve the Monitoring of a Chlorinated Solvents Plume
Authors A. Etienne, J. Deceuster and O. KaufmannRemediation strategies for contaminated sites require accurate mid and long term monitoring methods. This is especially the case when groundwater is polluted by soluble organic contaminants such as chlorinated solvents or LNAPL (Light Non-Aqueous Phase Liquids). In general, the monitoring of remedial actions is based on water samples analyses from which geochemical parameters are derived. However this information is often limited in time and space for technical and economical reasons. As a result, a spatiotemporal uncertainty may remain on the evolution of the plume. Complementary methods, such as geoelectrical methods, can be used to reduce this uncertainty. A field experiment was conducted on a former industrial site mainly contaminated by trichloroethylene (TCE) and exhibiting natural bioremediation phenomena. A carbon source was injected to enhance the reductive dechlorination process. Geoelectrical and geochemical monitoring were carried out on the test site during this experiment. Geochemical data were integrated to geophysical results. A linear correlation between the redox potential and the IP (Induced Polarization) signatures was observed during this study.
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