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Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 06 Sep 2010 - 08 Sep 2010
- Location: Zurich, Switzerland
- ISBN: 978-90-73781-88-7
- Published: 06 September 2010
101 - 150 of 153 results
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Method for Comprehensive Computing of Water Flows Geodynamics in the Dead Sea Basin
Authors L.V. Eppelbaum, Y.S. Yakubov and M. EzerskyThe Dead Sea (DS) is a lowest place at the Earth with a unique physical-chemical water composition. The most important physical factor of this basin is the vertical gradient of water density: from 1150 kg/m3 at the DS surface up to 1430 kg/m3 at the depth of 310-330 m. The new mathematical solution obtained for stratified, rotating and compressible liquid (with utilization of gravity, density, temperature and acoustic data) permits to create a solvable system of differential equations and utilize it for development of exact physical/mathematical/geological model of the DS basin.
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Spectral Induced Polarization Measurements on Sand-clay Mixtures in Dependence of Water Saturation
Authors K. Breede, O. Esser, J.A. Huisman, E. Zimmermann and A. KemnaSpectral induced polarization (SIP) measurements are promising for the in-situ determination of soil hydraulic properties. Here, we investigate the influence of water saturation on SIP measurements. Combined electrical and hydraulic measurements were conducted on sand and sand-clay mixtures using a multi-step outflow setup to desaturate the samples with several pressure steps. Our experimental results show that the ohmic conduction decreases with decreasing water content in accordance with Archie’s Law. For the sand-clay mixtures, the phase angle as well as the chargeability increases down to a critical water content and then decreases again in accordance with the short-narrow-pore model. In this model, the chargeability first increases with decreasing water content because the variation in the effective pore radii and the variation in the ion transparency becomes larger. When the saturation decreases further, the chargeability decreases due to a reduction of the thickness of grain-contact water rings resulting in a decrease of effective pore diameters and ion transparency. It is also observed that the phase peak shifts to higher frequencies, i.e. smaller relaxation times, when the saturation decreases, suggesting that electrical relaxation is controlled by the saturated part of the pore space.
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Spectral Induced Polarization: Laboratory Measurements on Saturated Sandy Quartz Samples of Varied Pore Characteristics
Authors K. Koch, A. Kemna and K. HolligerUnderstanding the influence of pore space characteristics on the hydraulic conductivity and spectral induced polarization (SIP) response is critical for establishing relationships between the electrical and hydrological properties of surficial sedimentary deposits. Here, we present the results of laboratory SIP measurements on saturated quartz samples with granulometric characteristics ranging from fine sand to fine gravel. We alter the pore characteristics using three principal methods: (i) variation of the grain sizes, (ii) changing the degree of compaction, and (iii) changing the level of sorting. We then examine how these changes affect both the SIP response and the hydraulic conductivity. In general, the results indicate a clear connection between the applied changes in pore characteristics and the SIP response. In particular, we observe a systematic correlation between the hydraulic conductivity and the relaxation time of the Cole-Cole model describing the observed SIP effect for the whole range of considered grain sizes.
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Studies on the Origin of the IP-effect with Gel-filled Sandstone Samples
More LessThe origin of the induced polarization in the pore space of sediments is not completely understood. There are two groups of theories, which are either based on the grains, or on the structure of the pore space. We have carried out complex electrical conductivity measurements on gel-filled sandstone samples with different gel concentrations and fluid salinities. The idea is to reduce the ion mobility in the pore space, allowing to test hypotheses resulting from the different theories. The electrical conductivity of pure gel increases with gel concentration, indicating an increase in ion density. Thus, adding gel to a saline fluid reduces ion mobility, but does not completely destroy it. The conductivity spectra from the gel-filled sandstone samples are distinctly different from water-filled samples. The phase shifts decrease, the spectrum is flattened, and the maximum moves towards higher frequencies. We conclude that the geometry of the pore space, and the mobilities in the large pores, are important factors for the generation of the IP-effect. The reduction of the IP-effect by a reduction of mobilities is consistent with a quantitative theory which is based on the pore space geometry.
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Detecting Perched Water Bodies Using Surface Seismic Time-lapse Travel-time Tomography
Authors G.S. Baker, D.P. Gaines, S.S. Hubbard, D. Watson and S. BrooksSeismic timelapse methods have typically been constrained to large-scale geologic investigations associated with petroleum exploration and exploitation; however, there is growing interest in using geophysical methods for monitoring near-surface phenomena such as fluid flow in fractured or karstic geologic media, hydraulic infiltration, and near-surface anthropogenic manipulations during environmental remediation. To demonstrate additional utility of surface seismic time-lapse travel-time tomography for near-surface investigations, we monitored a site through time with the objective of attributing increases in seismic P-wave velocity with development of a perched water body in the vadose zone. Our study was conducted at the Oak Ridge National Laboratory, USA, during a multi-disciplinary investigation on the fate and transport of contaminants. Seismic data are processed using a wavepath eikonal traveltime tomography approach, and a modified trend analysis technique is applied to remove the regional velocity component. The final ‘residual’ velocity anomaly images are compared to the wellbore hydrologic data and error analysis, and used to interpret the presence or absence of a perched water body. Our study suggests that velocity estimates, obtained from surface seismic traveltime methods, are effective for indicating spatial and temporal distribution of near-surface perched water bodies.
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New Three-component Landstreamer System Developed for Groundwater and Engineering Applications
Authors K. Martinez, C. Ploug, A. Pugin and J.A. MendozaA new landstreamer system has been developed that provides high resolution imaging from near surface to medium depths. The system is comprised of a combination three-component array to acquire SH data for near surface and a single component array to acquire typical P-wave data at depth. The first application of the system is being carried out for a groundwater mapping project where detailed knowledge of the near surface geological complexity is desired. The landstreamer is also suitable for engineering applications and in difficult to survey urban environments.
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Migration and Instantaneous Phase Combination to Detect Hidden Culverts Characteristics in GPR Data
Authors B. Yousefi, S. Esmaeili and S. JazayeriGround Penetrating Radar (GPR) is a nondestructive testing method used to test the quality of roads, pavements, bridges and buildings. GPR uses high frequency electromagnetic waves at the Microwave or Radio Frequency range to investigate the subsurface. It is used to measure features such as pavement layer thickness, moisture content, horizontal cracks, and voids. The electromagnetic waves reflected from the interfaces in the subsurface are used to analyze the structure of the area under scan. This paper presents two processing methods in hide culverts detection in roads structure that complete each other successfully. At first, we applied Migration method to remove hyperbolas and then by instantaneous phase method, we determined the top depths of culverts on a GPR data that has been collected from a culvert with known characteristics. The results corresponded to visible parameters truly. Then these methods applied on a hide culvert data on a road in Fars providence, Iran. The results satisfied with existed documents of this culvert's construction. The results of the study will be presented in the paper.
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An Infrared Survey on a Concrete Bridge Deck
More LessAn infrared survey was carried out on a concrete bridge deck using a mobile IR-acquisition unit. The survey covered one lane of the bridge and was repeated once per hour during 20 hours. Results show only minor temperature differences between on bridge and off bridge and no influence of structural elements such as girders.
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Some Geophysical Characteristics of the Burning Products in Archaeological and Geological Studies
Authors S.E. Itkis, B.E. Khesin and E.A. VapnikFor the revealing and characterization of archaeological remains that were caused by the burning processes we studied magnetization and corresponding magnetic anomalies over such archaeological features as ovens, kilns and hearths in Israel. Our magnetic and gravity studies of sedimentary rocks subjected to combustion metamorphism were performed in Israel and Jordan. Comparison of obtained data shows general correlation between increase of magnetization and burning temperature, although magnetization decreases due to alteration of burnt matter as well. Magnetic heterogeneity is reflected by ground magnetic anomalies. High-temperature combustion rocks in the Hatrurim basin, Israel show increased density relatively to that of low-temperature rocks. Such density features are displayed in the pattern of residual gravity anomalies.
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Geophysical Researches on Excavations of the Bronze Age Fortified Settlement in Southern Ural Mountains
Authors P.S. Martyshko, V.V. Noskewich, N.V. Fedorova and L.A. MuravievGeophysical researches were carried out on archeological excavations of one of the numerous bronze age fortified settlements discovered in Southern Ural Mountains in 60 - 90th years of XX century. The purpose of the present work was the research of the settlement fragment by non-destroying methods for revealing and mapping separate objects - the remains of dwellings, constructions of economic value, furnaces, economic pits and fortified constructions. This information was required for carrying out more purposeful excavation by archeologists. As the most effective and economic, researches were carried out with magnetometry methods by means of two proton gradiometers and one cesium gradiometer on preliminary spaced network of 0,5х0,5m. For the account of the Earth magnetic field variations the variation station was used. Before survey, clearing of an investigated site from metal debris was performed by metal locator. As a result of survey the map of an anomaly magnetic field was acquired, where the wall with a ditch, remains of walls of separate dwelling, the furnace and economic pits were allocated. After interpretation of anomalies received from a wall its position and a configuration were established.
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Application of Electrical Resistivity Tomography to Detect Archaeological Structures at Tell Al Marj (Homs, Syria)
Authors M. Himi, A. Armendáriz, E. Iriarte, L. Teira, J. Urquijo, J. Ibáñez, M. Haïdar-Boustani, M. Al-Maqdissi, A. Guinea and A. CasasElectrical resistivity tomography has been applied at two different zones in Tell Marj archaeological site (Syria). In both, the electrodes were placed to a distance of 50 cm within each line, being of 1 m the distance between lines. The test in the west zone was carried out in an area where Roman pottery fragments outcrops in the surface. Resistivity measurements were taken from in 11 lines, on a total surface of 235 m2. The test in the east zone was conducted in an area with Neolithic remains (pottery and silex tools). The sampled surface was of 329 m2 (15 lines). In the west zone of Tell Marj, place where in surface they appear abundant fragments of Roman pottery, the inverted resistivity cross-section indicates clearly the presence of two layers: An upper resistive layer 1.5 m thick ness of 1.5 m and below very low resistivity values are observed. The results obtained from 3D resistivity tomography were imaged in two different ways. One corresponds to a general image of the pad diagram, where it is possible to observe the zones of higher electrical resistivity than define an orthogonal architectonic framework. The presence of walls, corridors and stays among 3 and 5 meters of side is observed.
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Examples of Resistivity Tomography for Cultural Heritage Management
Authors V. Compare, M. Cozzolino, E. Di Giovanni and P. MaurielloThe geophysical prospections we present were realized inside the King Ferdinando IV Borbone’ Royal Residence of San Leucio (Caserta, Italy) and in the Archaeological Park of Aeclanum (Mirabella Eclano, Italy).
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Analysis and Representation of 3D GPR Data Acquired across Archaeological Sites
Authors M.T. Jie-A-Looi, M. Hertrich, P. Nagy and A.G. GreenThree-dimensional (3D) ground penetrating radar (GPR) surveying is becoming an increasingly valuable tool in archaeological prospecting. To extract structural details from 3D GPR data usually requires interpretation of multiple vertical cross-sections and/or multiple horizontal slices. Such multiple representations are difficult to incorporate in integrated GIS-based analyses. Using a new 3D GPR data set acquired across the buried remains of a Roman villa complex in Switzerland, we demonstrate the benefits of two new attributes for GIS-based interpretations: (i) the cumulative reflectivities over time intervals containing all significant reflections, and (ii) the times of the amplitude-envelope maxima. When combined, these attributes yield most of the critical summary information in the form of only one or two colour-coded maps.
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Using Ground Penetrating Radar and Induced Polarisation to Detect a Wooden Track Way
Authors A. Weller, A. Fettig, A. Bauerochse and M. EidnerThe detection of wooden relicts embedded in peat is a challenge for geophysics because the contrasts in the relevant physical properties between wood and peat are rather weak. A small section of track way was excavated at a location in the meliorated peatland of Uchter Moor (Germany). Close to the excavation site, a geophysical survey including Ground Penetrating Radar (GPR) and Spectral Induced Polarisation (SIP) was performed to check whether the wooden track way can be identified by geophysical methods. The wooden track way consisting of wooden planks with a length of 2 m and a diameter of 0.2 m is expected at a depth of about 0.9 m. The radar sections measured at different frequencies do not provide a clear evidence of the wooden target. Only additional 3D processing of six parallel profiles by calculating the amplitude of the envelope and visualisation of time slices reveals an extended structure that coincides with the existing track way. The application of SIP is related to the moderate polarisation effects caused by cell membranes of wooden material. The imaginary part of complex electrical resistivity resulting from an inversion of the 12 Hz data provides a clear image of the track way.
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Complex Resistivity Tomography and Spectral Induced Polarization Analysis on Laboratory Device for Soil Characterization
Authors E. Cardarelli and G. De DonnoElectrical Impedance Tomography (EIT) and Spectral Induced Polarization (SIP) are large used techniques for environmental application. The aim of this work is to understand the polarization signatures related to uncontaminated soils using typical subsoil models on laboratory targets and to build a new versatile algorithm suitable for this situations, for a future better estimation of contaminated samples. To this end a new experimental device was build up and SIP measurements were acquired on this new configuration, within 1mHz – 12 kHz frequency range. A couple of frequencies was chosen among the entire frequency range depending on sample configuration, and for each frequency value an inversion of complex resistivity data was carried out through a new algorithm developed in EIDORS environment. SIP measurements have highlighted the spectral response of water, sand and clay, in terms of polarization signatures on the phase spectra, while 2D and 3D complex resistivity inversions have shown the good resolution achieved by the EIDORS algorithm and the importance of the device configuration.
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Application of DC Resistivity and TDEM to the Study of Water Ingress in a Mined Region of South Africa
Authors V. Hallbauer-Zadorozhnaya, E. Chirenje and P. NyabezeThis study investigates the identification of zones of possible ingress of water, taking place above shallow undermining. Water ingress here is associated with the location of buried mine shafts. Here, we evaluate their water bearing probability based on their electrical conductivity signature. Two geophysical methods were used: DC resistivity and time domain electromagnetic sounding (TDEM). Resistivity data were collected using a Syscal-Pro multi-electrode system utilizing 35 electrodes at 10m spacing. For TDEM we used two instruments: TEM-FAST48 and Tsikl-5, with a surface area of the receiver loop equal to 625 m2. In this area TEM-FAST48 allows us to investigate to a depth of 120-150 m, with Tsikl-5 to more than 300 m. 95% of the signals collected by both instruments were distorted by an electro osmosis IP effect. We inverted the TDEM data, resulting in the resistivity, thicknesses of layers, polarizability, and the time constant of decay, of each part of the cross-section. It was shown that the depth of investigation of DC resistivity method is limited at this area, and resolution is low. In contrast, the TDEM surveys revealed a distinct conductive unit which we attribute to an area of high water ingress in this mined region of Johannesburg.
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Ability of the Direct Wave Amplitude of Ground-penetrating Radar for Assessing the Moisture Content Variation of Timber
Authors I. Rodríguez-Abad, R. Martínez-Sala, F. García-García and R. Capuz- LladróThe moisture content (MC) has a significant influence in wood’s physicochemical properties and therefore in most types of wood pathologies. The MC variation leads to modifications of its electromagnetic properties and to specific effects upon waves’ characteristics. Given the advantages of the direct electromagnetic wave (DW) observation, the aim of this work is to asses the ability of this wave to provide access to timber MC characterization. For this purpose, GPR measurements were carried out on timber joists (1.6 GHz central frequency). The dielectric behaviour of wood varies according to the direction of the fiber. Therefore, it is necessary to distinguish between differences attributable to the wood anisotropy and those due to variations in MC. With this aim, prior to the MC analysis, assays were carried out in different directions regarding the fiber. The results of this study show very satisfactory correlations between DW amplitudes and MC in all fiber directions. These results showed, in agreement with theory, that a decrease of MC results in an increase of the DW amplitude. These results demonstrates the GPR efficiency and the innovative application of this technique as a non-destructive evaluation tool for the rapid sounding of timber structures.
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Bedrock Detection from an Integrated Procedure of Refraction Analysis and Tomographic Inversion of the First Arrivals
Authors G. Boehm, R. Francese and M. GiorgiThe aim of this work was to create an automatic procedure to define the near surface structures in the case of seismic acquisition of large quantity of data. The study area is located in North-Eastern Italy, in the Dolomites near the city of Belluno; in particular the target of this study was the definition of the depth and shape of the bedrock horizon. The seismic data used for this analysis are part of a seismic survey acquired in the surface area of Costalta comprising a dozen of 2D lines, sub-parallel to each other in the SW-NE direction, within a rectangular area of 800x600 m (Figure 1). The sources used were the miniVib and the mini-gun. We developed an integrated procedure involving the conventional refraction analysis and joint tomography of the direct (diving) and refracted arrivals (head waves).
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Full Wave Form Time-domain IP Data Acquisition
More LessWe present and discuss some examples of full wave form resistivity-IP data recorded at one site in Sweden and one in Malaysia. The examples show that highly detailed recordings can be made, and that various types of noise can be clearly distinguished. With such detailed information about the character of the signal and noise it is relatively easy to develop algorithms that can remove the noise without biasing the extracted data. Tests using a pure square wave as measuring signal show that information relating to the chargeability of the ground are as clearly visible as in traditional time-domain IP-data, and it should thus lend itself well to extraction of the spectral IP information. An advantage of using square wave for time-domain SIP over frequency domain SIP is that data acquisition can be much faster, furthermore it would be more time efficient than a traditional time-domain IP signal. The results also show that the transmitted current wave form is almost ideal, whereas recorded output voltage varies to a significant extent. This may be due to, for example electro-chemical processes, and a constant voltage transmitter would not have produced current wave forms with such regular shape.
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1D Laterally Constrained Inversion of Towed-array Capacitive Resistivity Imaging Data
Authors O. Kuras, A.V. Christiansen, D. Beamish and R.D. OgilvyTowed-array Capacitive Resistivity Imaging has proved to be a viable, convenient and effective technique for acquiring multi-channel high-resolution Electrical Resistivity Tomography data with dense lateral coverage on engineered surfaces. As resistance data measured with capacitive coupling under quasi-static conditions are equivalent to conventional galvanic data, popular ERT interpretation schemes such as least-squares smoothness-constrained resistivity inversion are fully applicable to CRI data. However, for certain specialised applications, much simpler earth models (e.g. 1D with moderate lateral variations) may be sufficient to adequately reflect the subsurface structure under investigation and a significant amount of a-priori information may exist that can be used to constrain the inverse models. One way to exploit these simplifications is to employ piecewise 1D Laterally Constrained Inversion, which divides the subsurface into a series of 1D earth models connected laterally by model constraints that control the similarity between adjacent models and the propagation of subsurface information along a given interpretational profile. In this study we demonstrate the feasibility of applying 1D-LCI to towed-array CRI data and show the utility of this approach for the analysis of datasets acquired to assess road subgrade condition.
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Selecting Electrical Resistivity Tomography Arrays to Define Geological Structures Aimed by Relative-sensitivity Matrix
More LessResistivity tomography is a widely used geophysical technique. One of the most relevant points while performing ERT surveys is to choose a suitable electrode- array configuration. The best definition of the structure which is under the target depends on it. The proper interpretation of the electrical imaging is essential. On this purpose, criteria for checking the reliability of the model have to be found in order to detect possible artefacts. Structurally well known areas have been studied with ERT using Dipole-Dipole and Wenner-Schlumberger arrays in the NE Spain. The previous knowledge of the structures and the blocks sensitivity imaging has been used as reliability criterions. Also direct model calculations have been performed with a theoretical terrain in order to compare them with the field-results. The results show that the presence of conductive materials at deep position can improve the data acquisition, especially in the case of Dipole-Dipole array. Although this array tends to be noisy and uncertain, it could be the best option in certain structures. Additionally, the imaging of block sensitivity can be useful as criterion to check the coherence between the calculated resistivity and the maximum and minimum sensitivity values.
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Accurate and Robust Evaluation of Singular Boundary Element Integrals for Computing 3D Radiomagnetotelluric Fields
Authors Z. Ren, T. Kalscheuer and H. MaurerWe present a new surface integral formulation designed to efficiently simulate effects of 3D topography and displacement currents on radiomagnetotelluric data. Surface charges, magnetic surface currents and electrical surface currents are introduced to ensure the stability over a wide rage of frequencies. Using a triangular parameterization and the Galerkin approach, the surface integral formulation is transformed into a linear system of equations, in which several double surface integrals must be evaluated. If the triangles in the outer integrals are close to, adjoin or overlap with the triangles in the inner integrals, then these double surface integrals will be weakly or strongly singular. An accurate and robust singularity extraction technique is applied to transform these singular integrals, such that they can be evaluated by analytical expressions. The remaining logarithmic singularities in the outer integrals are removed with a smart strategy of interchanging the order of integration. If the triangles fully overlap, strongly singular integrals can be transformed into weakly singular ones using Gauss' divergence theorem. The efficacy of our singularity extraction technique is validated by means of numerical examples.
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Enhancing the Resolution Power of Least Squares Inversion Results of 2-D Magnetotelluric Data
More LessThe paper presents the results of applying a new approach on 2D inversion of Magnetotelluric (MT) data in order to enhance the resolution and stability of the inversion results. Due to non-linearity and limited coverage of the data acquisition in an MT field campaign, minimizing the error by linearization of the problem in the least squares inversion usually leads to an ill-posed problem. Determination of a suitable Lagrangian multiplier in order to balance minimization of error and model roughness could be a useful approach to achieve both resolution and stability in inversion. We tested the capability of Active Constraint Balancing (ACB) approach in enhancing the resolution power of least-squares inversion results by applying it to 2-D synthetic MT data generated from forward modeling code of Geotools-MT for a model with a conductive anomalous body of 5 Ohm-m beneath the earth. The result of joint inversion of TM and TE mod for the conventional approach with fixed Lagrangian multiplier of 0.06 leads to the rms misfit of 0.036 and reduces to 0.008 when ACB is applied. By comparing the resistivity sections, the anomalous object can be seen much clearer and distinct in the case of ACB approach.
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Incorporation of a 3-D lossy (conductive) Earth into Surface NMR Tomography
Authors J. Lehmann-Horn and M. HertrichThe surface nuclear magnetic resonance (NMR) tomography technique uses electromagnetic (EM) fields to locate and quantify groundwater occurrences. The magnetic fields interact with the spin magnetic moments of water protons situated within the static Earth’s magnetic field. In conventional surface-NMR forward and inverse modeling, semi-analytical solutions for the electromagnetic fields on a layered earth are employed. This simplification restricts the model to regular loop shapes on a flat terrain and 1-D conductivity distributions. To remove such limitations and to extend the analysis to rough terrain (e.g. mountainous regions) and 3-D heterogeneous media incorporating conductive (lossy) zones (e.g. clay lenses), we have developed a hybrid electromagnetic modeling scheme that combines integral equations with the finite-element method. Local mesh refinement enables the sensitivity function to be sampled in an appropriate manner near the principal singularity (i.e. the transmitter). Here, we apply the novel numerical forward and inverse modeling scheme to synthetic data to illustrate its effectiveness.
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2-D Resistivity Imaging for the Crooked Survey Line on the Tough 3-D Topography
Authors M.J. Yi, J.H. Kim, C. Kim, J.S. Son and P. TsourlosIn the 2-D resistivity survey, survey line is sometimes detoured because of geographic or man-made structures. Although the effect of the crooked line is believed to be negligible in most of cases, it should be considered to obtain accurate image of the subsurface. In this study, we examine the effect of crooked survey line on the 2-D resistivity data. A strategy of 2-D resistivity inversion based on the 3-D inversion code with 2-D model structure was suggested to incorporate the crooked line into the inversion, which provided accurate image of the subsurface compared to a conventional 2-D inversion result. As another important practical issue of 2-D resistivity survey, tough topographic variation in the strike direction is also implemented using this strategy to obtain enhanced imaging capability. Field data example of 2-D resistivity survey for mineral exploration at the mountainous area shows that our strategy is very useful in the combined case of crooked survey line on the tough topographic area.
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Frequency Soundings at Low Induction Numbers: Transformation and 1D Inversion Comparing to 2D ERT Inversion
Authors E.V. Balkov and Y.G. KarinThe paper is devoted to an approach for calculating an apparent resistivity and a layered earth parameters from transformation and inversion of FEM soundings data. First part of study presents theoretical estimation of the sounding capability of the multifrequency sensor at simple layered earth model. Second part includes processing of the both FEM and DC ERT data sets collected at the same profiles. The results of transformation and 1D layered inversion of frequency soundings comparing to 2D ERT inversion are considered.
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Related Interrelations Between Filtration and Electric Parameters of Rocks
By S.Z. KozakOn the base of the model consisting of a batch of capillaries part of which is sealed with clayly plugs, the analytical dependencies of electrical parameters (specific electric resistance and polarizability) on filtration coefficient of water-bearing rocks is acquired. It is shown that one of the main factors influencing on the value of electrical and filtration parameters is distribution of clayly particles in the rock. It is determined for water prospecting that polarizability is the most challenging parameter for quantitative evaluation of filtration parameters by geophysical data. Based on the obtained results, an inference is deduced that empirical relations between electrical and hydraulic parameters are to be composed and analised for uniform lithology and genetic aquifers.
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Sensitivity of Experimental Dynamic Stiffness of the Vibrator-Earth System
Authors F. Georgiades and P. ScholtzThe dynamics of the vibrator-earth system is driven by the properties of both the vibrator and the earth. Using a model for the dynamics of this system, we determine theoretically the dynamic stiffness curves that describe the system. In order to define them we need to specify the ground force and the acceleration at the surface of the earth. In our case we use a simplified model of the vibrator to determine the ground force and then assuming that the vibrator baseplate is properly coupled to the surface of the earth (that is very close to the reality due to the hold down force) we use the acceleration of the baseplate to extract these characteristic dynamic stiffness curves of the system. Then we examine, using experimental data, the effect of location, and vibration polarity in dynamic stiffness curves of the system. The sensitivity analysis showed that at low frequencies there is no effect but at medium and high frequencies these curves are very sensitive to the variations. This work is a starting point for the determination of effective earth properties through dynamic stiffness.
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Investigations on Kinematic Refraction Inversion at Different Geological Models
Authors A.N. Paripas and T. OrmosIn this paper a refraction inversion technique developed in the Department of Geophysics, University of Miskolc is investigated for different laterally changing geological models. It is important because the solution of the forward problem uses several approximations for significant reduction of the calculation time and these can affect the inversion results. The effects of these approximations depend on the investigated geological structure. According to our investigations it can be stated that the applied approximation gives very good results in case of slowly changing structures, while even in case of a highly changing model the inversion technique gives a result where the target model is recognizable.
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2D Quantitative Imaging by Elastic Full Waveform Inversion: Application to a Physical Scale Model
Authors F. Bretaudeau, D. Leparoux, R. Brossier, S. Operto and O. AbrahamQuantitative imaging of the first meters of the underground with seismic methods is an important challenge for many applications. This task is difficult because of heterogeneity, strong attenuation, low frequency content and large dominance of surface waves in the data. Tomography, migration, and dispersion analysis of surface wave are not efficient in all contexts. Elastic full waveform inversion was developed for crustal imaging but has a great potential for near surface applications as it could help to image very heterogeneous media by exploiting the whole complexity of the seismograms. We try here to (1) understand how behaves this method on near surface context, and (2) anticipate on the field application. We use for that a simultaneous approach by applying the waveform inversion on a simple three layer medium from synthetic data obtained by numerical modeling, and from experimental data obtained by small scale modeling. We present the small scale model, the acquisition of synthetic and experimental data, and we present an inversion result. Images of the Vp and Vs parameters are reconstructed. Both lateral and vertical variations are well determined. Images from synthetic and experimental data can be compared. The results are similar but we identify differences related to noise.
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Estimation of the Depth and Shape of Intrusive Bodies by 3D Inversion of Ground Magnetic Data
Authors B. Yousefi and B. OskooiThis paper evaluates application of 3D inversion of geomagnetic data for estimating depth and shape of an intrusive body in northeast of Iran. For this purpose, we used an algorithm that has presented by Yaoguo Li and Oldenburge (1996). They have obtained the inverse solution by minimizing an objective of the weighted model subject to fit the observations to the degree determined by the errors. The algorithm discretizes the surveyed area into a large number of vertical rectangular cells and starting from an initial depth, ends with the best fit depths to all the blocks. At last, it determines the best model to show susceptibilities of bodies with an iterative approach. For validating the result, a comparison between the borehole data and predicted model was performed. After comparing the borehole data with the predicted model in well location, a good correlation obtained between thickness of high magnetic susceptibility minerals in borehole data and high magnetic susceptibility parts of predicted model. Finally after 57 iterations, we obtained a model with variable top depths between -20m to -60m and concentration of high magnetic susceptibility increases with depth in northern direction.
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Some Applicability Problems of Euler Deconvolution to the Interpretation of the Results of Microgravity Survey
By S. PorzucekThe author presents some problems arising when Euler deconvolution is applied to microgravity survey. The application of Euler deconvolution requires the values of three orthogonal gravity gradients. These values rather cannot be measured ant the values is calculated from measured gravity values using the Fourier or the Hilbert transformations. In microgravity survey the essential problem for correct calculation of the transformation is the proper selection of sampling step. This paper presents the empirical dependences between characteristic properties of microanomalies and measuring (or calculating) steps, which can be useful for microgravity quantitative interpretation.
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Linearized Electrical Impedance Tomography
More LessWe address the problem of Electrical Impedance Tomography (EIT) as this is encountered in various applications of near surface geophysics. Based on a closed form integral model that relates the boundary observations to the electrical properties of the domain we suggest some extensions to the conventional `linearization--regularization' modeling approaches. Instead of truncating the Taylor series expansion of the forward operator to first-order accuracy, our methodology accounts for the linearization error as a multivariate random variable and uses stochastic simulation to obtain its statistical information.
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Petrophysical and Electrical Study of Soil Properties in Sinaloa, Mexico
Authors V.A. Shevnin, H. Peinado, O. Delgado and A.A. RyjovIn area of study near the channel Valle del Fuerte in Sinaloa state, Mexico, geological and geoelectrical characteristics of soil were measured in seven key areas. In each of seven key areas an electrical resistivity tomography (ERT) profile was fulfilled, three boreholes were made with soil samples collection in each borehole, filtration coefficients of soil, cation exchange capacity, porosity and grain size distribution, groundwater salinity and ionic content in water were determined. ERT data were interpreted and soil resistivity curves versus pore water salinity were measured. Joint analysis of all data obtained was performed to find correlations between them to create petrophysical model of soil. Verification of this model was performed by correlation between the same petrophysical parameters measured in laboratory and obtained from geophysical data.
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Seismic Survey for Characterization of Shallow Depth Materials of Bam City, S.W. Iran
Authors M.A. Riahi and S. Hashemi Tabatabaeisecond and third layers decreases from south west towards the north east of the study area. Attentive to the velocity distribution of the P and S wave velocity, the Poisson’s ratio distribution as well as attenuation coefficient obtained for the identified layers of the area under investigation in the Bam city, it is concluded that the low velocity layer with high thickness and attenuation coefficient of the first layer can be attributed to be the cause of the strong motion vibration of the soil due to Bam earthquake on 26th December 2003 in the city.
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Sensitivity of Electrical Properties to Soil Compaction - Case Study
Authors H. Petersen, W. Rabbel, R. Horn and L. VolkSoil compaction on cropland caused by human activity is realized to be a serious problem. The development of prevention and remediation strategies requires an accurate knowledge of the real areal distribution of soil compaction. Within the project “Interdiscplinary investigations of soil compaction” we performed in-situ compaction experiments on different field sites with Luvisols from loess and glacial till for different tillage systems (with and without use of plough) to investigate the sensitivity of electrical conductivity measured by EMI method to compaction. Results show a clear influence of the compaction on the apparent electrical conductivity, where the conductivity increases with increasing the compaction.
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Velocity and Attenuation Dispersion Relations for the Effective Biot Double Porosity Model: Total Field Formulation
Authors S.A. Greenhalgh, X. Liu and B. ZhouTwo approaches – the host phase fields and the total fields - have been applied to formulate the effective Biot governing equations from the double-porosity, dual permeability model. The host phase formulation, given previously by Pride and Berryman (2003) is made under the assumption that the volume fraction of phase 2 (the inclusions) is very small. The total-field formulation is new and has no such limitation and gives rise to more general equations which cover the host phase field approach as a special case. We have computed the phase velocity and attenuation dispersion curves for the fast and slow Biot compressional waves using both approaches for a sample rock of water-saturated sandstone having sand as the inclusions. The curves are consistent across a wide frequency range for the fast P wave but deviate at higher frequencies and for large volume fractions of the inclusions .For the slow P wave there is a significantly increasing discrepancy in wavespeed as frequency increases, even for low inclusion concentrations
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Seismic Monitoring of a Simulated Radioactive Waste Repository During Water Saturation
Authors E. Manukyan, S. Marelli, H.R. Maurer, A.G. Green and S.A. GreenhalghIn the framework of a radioactive waste disposal and monitoring program as part of the EC Integrated Project ESDRED, we have conducted for the NDA (UK) in co-operation with Nagra (Switzerland) high resolution (0.2 - 4 kHz frequencies) seismic experiments in an anisotropic clay unit at the Mont Terri underground rock laboratory in Switzerland. The objective was to explore the possibilities and limitations of seismic measurements for remotely monitoring, at distances of tens of metres, the changing properties of material filling a small scaled–down version of a repository (1 m diameter tunnel) embedded in a clay formation. Recordings using vertical-component geophones attached to the inside wall of the microtunnel revealed significant waveform variations for different fill materials (empty and sand-filled) and experimental conditions (dry, wet, and pressurised). Initially, the presence of water weakens the clay, but at a later stage the seismic measurements indicate swelling of the clay material, which likely causes healing in the excavation damage zone. On the basis of our results, it is judged worthwhile to develop wireless seismic sensors that could be employed for non-intrusive monitoring during the water saturation of a repository.
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Vertical Shaft Pilot Hole Investigations in Pre-assessment of Orientation, Transmissivity and Continuity of Fractures
Authors E.J. Heikkinen, T. Ahokas, P. Rouhiainen and T. PereSpent nuclear fuel disposal in crystalline bedrock requires knowledge on long and hydraulically conductive fractures near disposal holes and tunnels. It is possible to investigate fractures with different techniques before drilling of pilot holes, before excavation from pilot holes, between tunnels as crosshole survey, or from the surfaces of excavated tunnel. This work included review of materials obtained in a 105 m long section of vertical shaft, investigated before raise-boring, and using single-hole and detailed crosshole investigations performed in and between three drillholes located at 3.2 m distances from each other, within shaft perimeter. The geological mapping of shaft took place after the raise-boring. From the results it was possible to draw conclusions on applicability of these methods in detailed model compilation. Combined use of geological, imaging and geophysical logging will assist detecting the fracture responses and assigning the orientation. The hydraulic difference flow logging will assign the transmissivity. Crosshole hydraulic measurement will detect the hydraulic connections between drillholes. Crosshole mise-à-la-masse survey grounded on fractures and other conductive zones will effectively reveal electrical connections. Single-hole radar survey indicates continuity of conductive features. Explanation of the features will require geological correlation.
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Airborne Electromagnetic Survey of the Groundwater Mineralisation in the Potash Mining District of The Werra River, GER
Authors B. Siemon, A. Ullmann, M. Vasterling, U. Meyer, W.W. Beer and J. PlümacherDissolution of Zechstein salt and discharge of saltwater from potash fertilizer production into the river Werra has led to a considerable mineralisation of ground and surface waters. Injection of waste water into the subsurface was introduced to reduce the discharge directly into the river. The waste water injected displaced the formation water upwards and, in areas where fault zones exist, saltwater rising occurred. In order to map and monitor the near-surface groundwater mineralisation BGR conducted regional airborne geophysical surveys in 1996/97 and 2008. Of particular interest was an area where a test disposal of saline waste water (9.5 million m³) in the Gerstungen syncline took place between 1999 and 2007. Although complicated by the use of different helicopter-borne electromagnetic systems both surveys could be successfully compared with respect to map the near-surface groundwater mineralisation and to outline possible changes due to the injection of saline waste water caused by the potash mining industry. Based on apparent resistivity maps no significant increase in groundwater mineralisation could be substantiated within the achievable exploration depths of about 20 m (saltwater ponds) and 200 m (hard rock)
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Improving the Understanding of Sub Lagoon Hydrogeology with AEM: The Venice Lagoon Example
By A. ViezzoliA comprehensive investigation of the exchange between surficial waters and groundwater in transitional environments, and hence the mixing of waters characterized by a different salt concentration, is an issue of paramount importance considering the ecological, cultural, and socio-economic relevance of transitional environments. Acquiring information, which can improve the process understanding, is often logistically challenging, and generally expensive and slow in these areas. This applies both for punctual, invasive borehole measurements as well as for ground-based non invasive geophysical surveys. Here we investigate the capability of airborne electromagnetics (AEM) to give a significant contribution to the understanding of the hydrogeology within, below and at the margin of the Venice Lagoon, Italy. The quasi-3D modelling of the SkyTEM data by the spatially constrained inversion (SCI) methodology allows to accurately distinguish several hydrogeological features, both underneath the lagoon and the farmland beside it. Hydrogeological features resolved are, for example, the extent of the saltwater intrusion in coastal aquifers and the transition between the upper salt saturated and the underlying fresher sediments below the lagoon bottom, and areas of probable submarine groundwater discharge. While the AEM data show a high degree of coherence with available ancillary information, both form wells and from other geophysical techniques such as high resolution off shore seismic, they improve greatly the understanding of the hydrogeology in the Venice lagoon, both at large scale and in details. The research highlights the great potential of AEM to improve significantly the hydrogeological characterization of subsurface processes in lagoons, wetlands, and deltas worldwide
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Optimizing an Existing Inversion Code
Authors C. Kirkegaard and E. AukenAirborne ElectroMagnetic methods (AEM) have been used for geophysical surveys for several decades, in particular due to its ability to cover large areas on a limited budget and in relatively little time. Ever since the method was first deployed the tendency has been for the typical survey size to be growing at a very rapid rate, in particular during the last decade. AEM surveys of today are often in the order of tens of thousands of flight line kilometres, producing huge datasets that needs to be inverted. Performing a full non-linear inversion of such datasets is a tremendous computational burden and consequently, these datasets are very often inverted using approximate inversion schemes. Several full non-linear inversion codes for AEM data is reported in the literature, however, most of these codes operate on single soundings only and are not optimized for large datasets. We discuss how the existing constrained inversion code em1dinv has been optimized for modern parallel computing and show performance results. The parallel optimized code is capable of handling very large datasets and provides a significant reduction in the total inversion time. Furthermore, we show how the optimized code eliminates inversion artefacts inherent of the previous implementation.
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Seismic Dispersion Analysis Feasibility for the Subgrade Investigation: Measurement, Experimental and Numerical Modeling
Authors D. Leparoux, L. Laguerre, F. Treyssede, P. Côte and V. FerberThe study focuses on the feasibility of the seismic guided waves dispersion analysis for investigating the subgrade layer made of a treated soil, in the case of under construction roads, before the shallower pavement layers are built. Such a non destructive geophysical method could help to qualify the project acceptance concerning this earthworks phase. In this context, the issue deals with a two layers medium case where the investigated subgrade, whom the top is the measurement surface, lays above a low velocity zone, i.e. the natural soil. The first feasibility stage involved field experimental measurements. A clearly main mode was extracted from the dispersion diagram and has been inverted to a S wave velocity profile leading to a global information concerning the subgrade. In a second feasibility stage dispersion diagram has been studied through a reduced scale experimental modeling providing a well controlled data set that have been compared with theoretical results firstly calculated without any source effects and secondly by tacking into account the source effects through a new numerical approach. The results show other modes distinguishable in the diagram that could be taken into account in the case of a dispersion diagram inversion.
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Static Correction Determination Based on Delay Time Inversion
Authors T. Muhammad, H. Perroud and D. RoussetTo obtain a high resolution seismic image,static correction plays a vital role.Static correction is applied to mitigate the effect of irregular topography and weathering velocity.Inadequate static correction will result in seismic data smearing and false anomalies.We present a data driven technique that performs the delay time inversion based on the first arrivals for static correction computation. Based on this technique we do not require the explicit velocity information and the subsurface depth model.So it perform all the necessary steps for static correction computation in time domain thus prevents from certain errors that might propagate otherwise due to time to depth conversion.We applied the time-delay inversion method on the real field data and compared the results with other methods i.e. delay-time and generalized reciprocal method.Generalized reciprocal method requires the explicit velocity information and optimum distance.Change in any one of the parameter will change the apparent reflector velocity and depth model. While using delay time inversion we do not require velocity information and optimum parameter.Moreover delay time technique can be generalised to several weathering layers as compared to the generalised reciprocal method which is valid for only one layer.
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Application of Mobile Geophysical Methods for the Examination of Areas of Landslide Processes Formation and Development
Authors S.P. Levashov, N.A. Yakymchuk, I.N. Korchagin, Y.N. Pischaniy and D.N. BozhezhaThe results of practical application of the non-traditional geoelectric methods of forming a short-pulsed electromagnetic field (FSPEF) and vertical electric-resonance sounding (VERS), as well as georadar sounding on destroyed by the landslide process area of Kiev-Odessa highway are given. It is shown, that the presence of the zones with raised filtering of ground waters is the main reason of roadway erosion and landslides formation. The detection and mapping of underground water flows and areas with the raised soil moistening can be realized operatively by complex of used geophysical methods. The areal survey by FSPEF method allows finding and mapping operatively the underground water flows of natural and man-caused origins and the zones of soils moistening. The bedding depths and thicknesses of water-saturated layers are defined in cross-section by VERS sounding. This complex of geoelectric methods can be used also for regular monitoring of the engineering-geological condition of environment on landslide-dangerous areas of roads.
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3D GPR Measurements for Fractures Detection in Selected Post-mining Regions in Poland
More LessThe paper presents the results of 3D GPR measurements carried out over shallowly located drifts. The aim of georadar investigations was to analyse the influence of mining activity on the rock-mass. Geophysical measurements were conducted in two sites with different geological conditions, i.e. over the drift made in gneiss and over the excavations made in sandstone. All excavations were located shallowly, surrounding geological media had low attenuation and reflection coefficients on the boundaries between hosting media and the voids (drifts) were high. Despite of mentioned facts there were recorded no reflections from the roofs of drifts. This interesting effect was additionally analysed in the paper.
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Investigation of Near-surface Anomalies Using the Refraction Seismic Method
Authors T. Engelsfeld, F. Šumanovac, V. Krstic and N. PavinIn this paper we investigate the influence of near-surface anomalies on the first arrivals of seismic waves. The basic two-layer model is modified with the following anomalies: (1) circular cavity positioned in the upper layer, (2) buried circular object positioned in the upper layer, (3) part of the border between the layers is deformed into concave shape. All anomalies are characterized with the seismic velocities that are lower than the seismic velocity of the neighboring region. First arrivals are calculated by numerical modeling of 2D eikonal equation. Numerically obtained traveltimes are presented in the time-distance graph and are also used to construct the graph of wavefronts. In all cases, the shape of the time-distance graph is characterized by a peak point which is a consequence of slower propagation of seismic waves through the analyzed anomalies.
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Joint Inversion of Seismic Refraction and Electrical Resistivity Tomography to Investigate Sinkholes
Authors B. Gebregziabher, T. Günther and H. WiederholdA joint inversion of seismic and resistivity data was carried out for site investigation to delineate subsurface expression of dissolution induced sinkholes and assess the risk for future development of the study area. The results of the 2D seismic refraction tomography are compared with results of the 2D electrical resistivity tomography and show good compatibility at a depth of about 20 meters. The results of cluster analysis of the joint inversion indicate that the subsurface section of the study area consists of four layers; dry sand, saturated sand, till/clay and saturated chalk layers. The joint inversion results and the lithology from boreholes confirm that sandy sediments are dominant on top of the chalk close to the observed sinkholes and to the discontinuities in reflection seismic sections instead of till/clay which is dominant towards North. Most of the discontinuities and observed sinkholes are located close to the boundary between the dry sand and the shallow till/clay layer. This geologic boundary and the permeability of the sand layer allow for direct groundwater contact with the bedrock (chalk) surface and may lead to the dissolution and formation of sinkholes at the surface.
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Geophysical Prediction and Following Analysis of the Dead Sea Sinkhole Development Through the Ein Gedi – Arugot Area
Authors M.G. Ezersky, L. Eppelbaum and B. MedvedevGeophysical methods - Seismic refraction (SRFR), Electrical resistivity tomography (ERT) and Microgravity were applied to the Dead Sea (DS) sinkhole problem in the Ein Gedi - Wadi Arugot area during the earlier stage of the sinkhole development (1998-2002). They allowed to determine the sinkhole formation mechanism and localize the sinkhole hazardous zones. Following monitoring of the sinkhole development confirmed our suggestions. The numerous boreholes drilling by the Geological Survey of Israel verified the location of the salt edge. The GIS database testifies that during 2003-2009 new sinkholes developed along the salt edge within a narrow 50-70m wide strip oriented approximately in north-south direction (slightly parallel to the shoreline). No promotion in west-east direction (normally to the DS shoreline) was observed. Collapse of sinkholes and their clustering have been occurred within the area of high resistivity anomaly and negative residual gravity anomaly.
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