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Near Surface Geoscience 2014 - 20th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 14-18, 2014
- Location: Athens, Greece
- Published: 08 September 2014
81 - 100 of 185 results
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Simple and Efficient Approaches to Integrate Data into Geophysical Inversion Problems - Examples from Se
Authors M.K. Karaoulis, P.I. Tsourlos, M.P.E. Kleine and P.P. KruiverSummaryData integration in geophysics provides additional information to elucidate subsurface structure and reduce non-uniqueness of inverted models.
In this work we demonstrate two alternative and easy methods to integrate data. A) the use of image-guided inversion, where structural information is taken directly from a high-resolution geophysical image (e.g. ground penetrating radar or seismic reflection) or from a geological cross-section. This structural information is introduced into the inverse problem through a weighted smoothing matrix, where it correlates and favors formations related to a specific structural feature and not just uniformly across the entire model. B) Definition of spatially weighted distribution of the Lagrange parameter, where the inversion solution favors model that incorporates naturally a-priori information from different data types. It is important to notice that in both scenarios, no petrophysical expressions are necessary and the modification of existing codes is limited to the stabilazation term.
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3D Airborne Electromagnetic Inversion in the Areas with High Conductivity Contrasts and Rugged Topography
Authors M.S. Zhdanov and L. CoxSummaryThis paper introduces an approach to 3D modeling and inversion of the airborne electromagnetics (AEM) that is suited to arbitrarily complex earth models with very high conductivity contrasts and rugged topography, yet is fast enough to consider large surveys. We use a hybrid FE-IE method, which directly avoids errors associated with numerical differentiation and interpolation of the electric vector potentials at the edges of the elements containing the receiver. This approach is stable and accurate and for conductivity contrasts in excess of 10 :1, as is typically required for practical AEM interpretation. We incorporate the moving sensitivity domain method into this modeling framework to increase the modeling speed for an entire survey by several orders of magnitude. A case study for the 3D inversion of 90 line km of DIGHEM data from the Reid-Mahaffy test site is presented to demonstrate the efficacy of our method.
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Combined Inversion of Seismic Waveforms and Traveltimes for Acoustic Waves
Authors E. Manukyan, H. Maurer, A. Nuber and R.G. PrattSummarySeismic full waveform inversion (FWI) is a powerful method for obtaining high resolution subsurface images, but the objective function of waveform data typically contains many local minima that may lead to inappropriate solutions. This can be avoided by choosing an initial model that is close to the true model, and by incorporating only low frequencies at an early stage of an inversion run. Here, we demonstrate that the local minima problem can also be mitigated by simultaneously inverting waveforms and first arrival travel times. Using a simple two parameter example we show, how the addition of travel time data can remove a local minimum present in the waveform misfit function. Inversions of synthetic 2D acoustic data show that our novel combined travel time and waveform inversion approach leads to superior results compared with a pure FWI inversion.
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Inversion to Recover Electrode position Displacements on 3D Electrical Resistivity Tomography Monitoring Grids
Authors P.B. Wilkinson, S.S. Uhlemann, J.E. Chambers and P.I. MeldrumSummaryElectrodes installed on active landslides and vulnerable earthworks to monitor changes in resistivity associated with moisture dynamics can be subject to movement. This affects the geoelectrical data and leads to errors in the resulting Electrical Resistivity Tomography (ERT) images. This paper demonstrates the selection of appropriate ERT measurements to provide sensitivity to electrode displacements in both directions on a plane. A combination of linear and equatorial dipole-dipole measurements are considered, which permit use on rectangular grids of any aspect ratio. A Gauss-Newton inversion scheme is developed that allows for the incorporation of constraints based on the magnitude and direction of movement. The effects of the constraints are demonstrated using synthetic data designed to simulate a planned laboratory experiment. In the presence of realistic levels of data noise and uncertainty in the preferred directions of movement, the inversion method is able to recover the displacements of the electrodes with a root-mean-square misfit of less than 1% of the electrode spacing.
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Hybrid Approach for Travel Time Seismic Tomography in Elliptical Anisotropic Media
Authors E. Cardarelli and A. CerretoSummaryHybrid approach for seismic travel time tomography is proposed in the case of elliptical anisotropic media. A sequential scheme is presented that combines simulating annealing with linear least square inversion. Simulated annealing was implemented to obtain a velocity model that can be used as initial guess for linear least square inversion. Linear Traveltime Interpolation raytracing approximation is used to trace rays and calculate traveltimes. The procedure is tested both for a synthetic model and a real case. The real case comes from a previous study, that was solved by linear least square inversion. In both synthetic and real cases comparison is performed between the linear least square inversion results and hybrid approach; in the field study we compare the old results with those inferred by sequential approach.
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2D Cole Cole Inversion of Time Domain Induced Polarization Data - Model Studies and Field Measurements
By B. TezkanSummaryA 2D inversion algorithm for time domain induced polarization data has been developed. The IP transients are inverted into 2D Cole Cole models including resistivity, chargeability, relaxation time, and frequency constants. In order to speed up the inversion procedure, an approximate forward algorithm has been used which is highly accurate and its validity was tested with exact solutions up to 3D. In the 2D inversion procedure, a 3D DC forward algorithm has been used as forward operator. The method requires a conventional 2D DC inversion and a homogenous half space Cole-Cole inversion to derive the Cole Cole parameters of each cell. Synthetic model studies and a field example will be demonstrated.
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1D Joint Inversion of Rayleigh Fundamental Mode and Electric Sounding Curves - Synthetic Tests
Authors M. Senkaya and H. KarslıSummaryLayer thickness is important model parameter as far as S-wave velocity that contributes to Rayleigh phase velocity. But, S-wave velocity is only inverted parameters in the traditional Rayleigh-wave inversion process. Thus, using erroneous thickness information in inversion process will cause unreliable or non-unique S-wave velocity profile. However, layer thickness can be incorporated into process by the joint inversion approach to increase accuracy of sub-surface model. This study presents 1D joint inversion of Rayleigh fundamental mode curve with electric sounding data, to use layer thickness as variable in the inversion process, contrary from separate inversion of Rayleigh dispersion curve. The proposed method was tested on noise-free and Gaussian noisy synthetic data. Results demonstrate that reliable vertical shear velocity profile can be obtained by the joint inversion approach, even there is no a priori.
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Combined Onshore and Offshore Seismic Investigations at the Geothermal Field of Aedipsos-Yaltra, Central Greece
Authors V.K. Karastathis and E. MouzakiotisSummaryA clear picture of the fault that facilitates the hydrothermal flux in one the most important geothermal fields of the Greek mainland, in the North Euboean Gulf, has been acquired through the implementation of a combined onshore and offshore seismic survey.
The results of the seismic investigations accurately map the structural characteristics of the fault providing us with precise estimations of its dip and throw.
An efficient exploitation of the field can be achieved by selecting the proper drilling sites according with the results of the seismic surveys.
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Surface-wave Dispersion Inversion versus SH-wave Refraction Tomography in Saturated and Poorly Dispersive Quick Clays
Authors S. Pasquet, G. Sauvin, M.R. Andriamboavonjy, L. Bodet, I. Lecomte and R. GuérinSummaryA seismic survey involving two distinct acquisition setups, with vertical and horizontal component geophones, has been carried out along the same line on a site presenting a simple vertical structure (peat, quick clays and bedrock) and no strong lateral variations. SH-wave refraction tomography and Rayleigh-wave dispersion inversion provided the same shear-wave velocity gradient in the quick clays. SH-wave tomography failed to correctly depict the peat layer and to reach the bedrock. A collection of Rayleigh-wave dispersion images were extracted along the line using windowing and stacking techniques. A thorough analysis of these images made it possible to give a complete description of the site velocity structure, illustrating the complementarity of both methods.
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Real-time or Full-precision CRS Imaging Using a Cloud Computing Portal - Multi-offset GPR and Shear-wave Reflection Data
Authors Z. Heilmann, H.P. Müller, G. Satta and G.P. DeiddaSummaryThe presented cloud-computing portal allows automated imaging of near-surface structures either in full-precision using a global simultaneous search for the best fitting spatial CRS stacking operator or in realtime using a pragmatic sequence of line searches. The simultaneous search even though computationally very expensive becomes necessary when the pragmatic search fails due to strongly varying velocities, low CMP fold and high noise levels. In our first case study we apply real-time imaging by reprocessing multi-offset Ground Penetrating-Radar data with a good signal/noise ratio. Generating stacked and prestack time migrated sections, consistent with conventional results, required less than 10 minutes. In our second case study we analyze a challenging SH-wave seismic reflection data, recorded in an urban environment, where a high level of ambient noise and landfills below the sealed surface down to 2 m depth resulting from recent reconstruction work hampered seismic data quality severely. Here, we successfully applied our newly developed full-precision CRS imaging approach in addition to conventional CMP stack aided by VSP. In this urban environment, the relevance of applying both, high resolution SH-wave seismic and VSP surveys was demonstrated. Two recently drilled wells confirm the aquiclude to be found as predicted by the seismic forecast.
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Distorting Effects of the Near-surface Layer on Seismic Imaging - Physical Model Investigations
Authors Y.K. Mo, S.A. Greenhalgh, J.O.A. Robertsson and H. KaramanSummaryThe 2D ultrasonic model facility at ETH Zurich was used to investigate how a variable near-surface weathered layer can adversely affect the imaging of reflection data. The simulated geological structures are constructed from 2 mm thick metal and plastic sheets, cut and bonded together into various shapes. The experiments entailed the use of a piezoelectric source driven by a pulse amplifier at ultrasonic frequencies to generate Lamb waves in the plate, which are detected by piezoelectric receivers and recorded digitally on a National Instruments recording system, under SignalExpress software control. High quality reflection seismic data were collected. However, the pre-stack migrated image for the deeper reflectors deteriorates as the near-surface becomes more complex and as block anomalies are inserted within the layers. Crosshole and VSP seismic data were also acquired, and first arrival travel-time tomography used to reliably reconstruct the velocity distribution. The 2D physical scale model system provides a valuable complement to numerical modeling to study various aspects of wave propagation and subsurface imaging.
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Long-term Geoelectrical Monitoring to Support Nuclear Decommissioning at the Sellafield Site, UK
Authors O. Kuras, P.B. Wilkinson, P.I. Meldrum, L.S. Oxby, S.S. Uhlemann, J.E. Chambers, A. Binley, J. Graham, G. Dewey and N. AthertonSummaryThe reduction of risk and hazard associated with the UK’s civil nuclear legacy is a strategic priority for the UK’s Nuclear Decommissioning Authority (NDA). A full-scale field trial using 4D ERT has been completed successfully at the Magnox Swarf Storage Silos at the Sellafield Site, which has monitored a controlled injection experiment. This paper summarises the achievements of the trial and lays out future plans for geoelectrical monitoring at Sellafield. The potential of ERT as a long-term monitoring tool is explored, and electrode longevity estimates are made based on the results of the MSSS trial.
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Field Application of Resistivity and Spectral Time Domain IP to Investigate Geoelectrical Signatures of Free-phase PCE
Authors S. Johansson, G. Fiandaca and T. DahlinSummarySoils beneath former dry cleaning sites are often contaminated with the chlorinated hydrocarbons PCE and its degradation products, which are highly toxic and cancerogenic. In this study, resistivity and time domain induced polarization (DCIP) were measured at a former dry cleaning site in order to investigate if the source contamination of PCE could be detected. 80 meter long 2D-profiles were measured across the site with the ABEM Terrameter LS instrument. The DC and full IP decay data were inverted using the 2-D DC/IP inversion algorithm developed by Fiandaca et al. (2013) , giving access to the spectral information contained in the IP decays. The inversion parameters DC resistivity (ρ), chargeability (m0), the relaxation time (τ) and the frequency factor (C) were compared to reference data from boreholes. The reference data consisted of geological classification and concentration of free-phase chlorinated hydrocarbons, obtained from soil samples. A high resistive anomaly was observed at a location along the profile where soil sampling have confirmed high concentrations of free-phase chlorinated hydrocarbons. High IP-effects, in terms of the spectral parameters m0 and C, was seen next to this area. It is suggested that the IP-effects arise in a zone next to the free-phase source plume.
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Complex Conductivity Measurements of Silver Nanoparticles in Unsaturated Porous Media
Authors E A Atekwana, G.Z. Abdel Aal and D.D. WerkemaSummaryThe emergence of engineered nano-materials (ENMs) in the global marketplace and their accidental introduction into the subsurface pose a potential risk to the environment and public health. There is a need for the development of techniques to detect their presence and transport in the subsurface. Complex conductivity measurements have been shown to be sensitive to the presence of metals. Laboratory experiments were conducted to investigate the complex conductivity response of silver (Ag) nanoparticles in a sand column under different nanoparticle concentrations (0, 2, 4, 6, 8 and 10 mg/g) and moisture content (5%, 10%, 15%, 20% and 30%). Complex conductivity measurements were obtained between 0.1–1000 Hz. The results showed that at different water saturation the magnitude of the imaginary conductivity component increased by one order with increasing concentration of Ag nanoparticles with minimal changes in the real conductivity component. On the other hand, at different concentrations of Ag nanoparticles the magnitude of the imaginary conductivity and real conductivity components increased by one and a half order and one order of magnitude, respectively. Our results demonstrate that complex conductivity measurements are sensitive to the presence of nanoparticles in unsaturated porous media which potentially could be used in guiding the detection and remediation processes of such contaminants within the vadose zone.
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Distribution of 137Cs in Forest Components of Selected Areas in the Czech Republic
Authors S. Takahashi, I. Suchara, K. Okamoto, K. Umegaki and R. FujiyoshiSummaryMigration process of 137Cs in soil has been investigated by measuring its activity concentration with depth. However, surface portion of soil (organic components) have various properties with different thickness. It is necessary to classify the distribution of 137Cs into each forest component.
Therefore, we investigated forest components (litter, fermentation, humus, Ah horizon, upper and lower part of mineral soil) in several forest areas of the Czech Republic, and tried to clarify the present situation about 30 years after the Chernobyl accident.
The highest value of 137Cs activity concentration was obtained to be 1.60 kBq/kg in fermentation.
Cesium-137 is therefore still present in organic layer. We’ve got previous results on distribution of 137Cs in humus horizon at the same forest area. Apparent half-life of 137Cs in humus horizon was estimated to be 20.2 year. This value gave initial deposition of 137Cs in humus layer to be 3.9 kBq/kg, comparable with that (4.3 kBq/kg) derived from the apparent burial rate of 137Cs. This is a new finding on initial deposition of 137Cs in humus layer with different ways.
In order to predict future situation in forest components affected by nuclear accidents, long term monitoring is found to be important.
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3D Geophysical Model of the Vajont Rockslide from Seismic and Geoelectric Surveys
Authors G. Boehm, R. Francese and M. GiorgiSummaryThe Vajont rockslide represents the most catastrophic massive failure of a mountain slope ever reported in historical times. The reconstruction of the geometry and of some key physical properties represents a major step forward in the understanding of the slide mechanism and of the collapse dynamics.
The landslide body and the surroundings were then targeted with a large 3D geophysical campaign aimed to define the seismic and the electrical properties of the various geological units comprising its body.
Data analysis and interpretation was constrained by a series of additional seismic and electrical measurements collected on the outcropping rocks below the village of Casso.
The larger values of P- and S-wave velocities as well as terrain resistivity appear to be located in the eastern block while in the western one these parameters are much lower values. These variations in the rock mass coherency are somewhat related to the larger stress field acting on the western block before and during the failure. Furthermore the slide displacement of the eastern block is lower than the western one due a small amount of rotation during the collapse.
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Delineation of Clay Layers in a Landslide Area in Norway Using Frequency Domain Helicopter-borne EM Survey
Authors V.C. Baranwal, E. Dalsegg, H. Dretvik, J.S. Rønning, J.F. Tønnesen and I.L. SolbergSummaryThere was a quick-clay landslide in Byneset on 1st January 2012. The landslide area is surrounded mostly by agricultural lands. The geology in the area consists of old ocean floor with outcropped bedrocks at several places. Prehistorically, the sea-level was ∼160 m higher than the present sea-level. The sediment in the area mainly consists of marine clay. Subsequent leaching by fresh groundwater alters the chemical composition of the pore water and “quick clay” may develop. Quick clay completely liquefies when remolded under stress and results in large landslides.
Resistivity survey is a powerful tool to investigate such clay layers but it is time consuming. Airborne EM survey can be a faster way to investigate large areas. Seismic refraction survey is useful to delineate depth of bedrock. Therefore frequency domain helicopter EM (FHEM), resistivity and refraction seismic surveys were performed in the area last year. There is a good agreement of the results from all the geophysical surveys. Resistivity and FHEM data show various clay layer boundaries well in case of thin marine clay deposits. However, FHEM data is poor to resolve geological boundaries in case of thicker marine clay deposits due to low skin depth in a conductive environment.
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Improved Time-lapse ERT Monitoring of Dense Non-aqueous phase Liquids (DNPLs) with Surface-to-horizontal Borehole Arrays
Authors C. Power, J. Gerhard, P. Tsourlos, P. Soupios, K. Simyrdanis and M. KaraoulisSummaryElectrical resistivity tomography (ERT) has long been recognized to exhibit the potential to significantly improve characterization of sites contaminated with dense non-aqueous phase liquids (DNAPLs).
However, the technique has not become a common tool for mapping such contaminants due to the complexity of the DNAPL target coupled with the inherent limitations of the commonly used surface and cross-hole ERT configurations. The objective of this work is to evaluate the potential of surface-to-horizontal borehole (S2HB) ERT for mapping the spatio-temporal evolution of DNAPL mass during remediation. A coupled DNAPL-ERT model was employed to provide an initial, theoretical evaluation of S2HB ERT, relative to surface ERT, for monitoring the remediation of a realistic, field-scale DNAPL source zone. A laboratory experiment was then performed to demonstrate the S2HB ERT approach for a real system involving a changing NAPL distribution over time. Four-dimensional ERT inversion was employed for both numerical and experimental data to generate time-lapse resistivity images of the subsurface regions experiencing NAPL changes. Results demonstrate significantly improved resistivity imaging with S2HB ERT and its potential as a non-invasive DNAPL site tool for mapping DNAPL mass changes during remediation.
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Mapping of Olive Oil Mills’ Wastes (OOMW) through ERT - A Case Study from Alikianos Site in Eastern Crete (Greece)
Authors N.G. Papadopoulos, P. Soupios, J.H. Kim, K. Simyrdanis, S. Kirkou and P. TsourlosSummaryThe production of olive oil in Greece and in Mediterranean is of major economic importance and goes hand in hand with a large food-processing industry. Thus it constitutes an important source of employment, especially in regions where it still remains today one of the major financial activities. However, these activities produce large waste amounts which are disposed in the environment without control. What is needed, apart from the adoption of specific policies regarding waste management, is the development and application of automated systems that will allow the rapid identification of environment degradation incidents and thus assist the decision making and the determination of specific measures and actions to be taken in order to protect and improve degraded areas. To this direction Electrical Resistivity Tomography was tested in an Olive Oil Mill Waste (OOMW) site in western Crete trying to explore its resolvable capabilities to image for such pollutants. The results indicate that OOMW can be mapped due to their conductive signature in relation to their hosting material rendering ERT a significant tool in the direction of providing a specific environmental tool in order to define a strategic framework for addressing this problem.
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Simultaneous 4D Cross-borehole ERT Inversion to Guide Leak Monitoring in Nuclear Decommissioning
Authors L. Oxby, O. Kuras, P.B. Wilkinson and P.I. MeldrumSummaryNuclear decommissioning efforts at the Sellafield site, UK, are increasingly supported by near-surface geophysical monitoring. Electrical Resistivity Tomography (ERT) is being applied to help manage the increased risk of leakage from the Magnox Swarf Storage Silos (MSSS) during planned waste retrievals.
A full-scale field trial of ERT has been undertaken in the immediate vicinity of the MSSS site. Six borehole electrode arrays were installed to monitor a subsurface volume equal in size to a proposed imaging cell under the MSSS structure. An automated geoelectrical imaging system was used to observe dynamic resistivity changes within the ground caused by simulated leaks.
Controlled injections into the vadose zone of environmentally benign conductive simulants were designed to test a number of realistic leak scenarios. The complex geology and low conductivity contrasts provided a significant challenge for ERT to detect and characterise the simulated leaks.
We show that full, simultaneous 4D ERT inversion successfully resolved the pathways adopted by the tracer fluid; these were found to be compatible with historic contamination detected in cores retrieved from the boreholes. The trial has demonstrated the value of ERT for nuclear decommissioning by enhancing our conceptualisation of likely behaviour of potential future leakage at MSSS.
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