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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
101 - 116 of 116 results
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Combined S-wave Reflection Seismics and ERT to Characterize Bedrock and DNAPL Presence in the Shallow Subsurface
Authors P.P. Kruiver, G. Diaferia and W.S. WesterhoffThe contamination of soil by DNAPL (solvent, dying residuals etc.) represent a serious threat for the environment. Being heavier than water, DNAPLs can reach deep depth as their migration is controlled by gravity and partially by groundwater flows. As results, those contaminants tend to migrate and accumulate in morphological lows of impermeable layer. Near surface geophysics is often employed for environmental issues in the shallow subsurface. In an area contaminated by DNAPL in Switzerland, Deltares has used a combination of S-wave reflection seismics and Electrical Resistivity Tomography (ERT) to characterize the morphology of the clayey bedrock (observed at around 16 m in existing boreholes in the area) and locate the contaminated portion of the subsurface. By three-dimensional interpretation of seismic data, the clayey bedrock has been extrapolated for the whole area. A channel structure is recognized along the SW-NE direction. These observations are confirmed with the boreholes a priori information. The geoelectrical data confirmed the presence of stiffer, less porous bedrock at around 16 m due to the increase in resistivity in the saturated zone. In a few lines, a shallow high resistive anomaly is observed that can related to a DNAPL plume.
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Cross-Hole Tests at Zelazny Most Tailings Pond, Poland - Highlights and Statistical Interpretation of Results
Authors A. Callerio, K. Janicki, D. Milani, S. Priano and M. SignoriAim of this paper is to describe the peculiarities and innovative aspects of the Cross-Hole (CH) survey carried out for the characterization of the flotation created waste (tailings) deposits of the copper mine located at Zelazny Most, Wroclaw, Poland. The CH survey was performed in two steps, respectively devoted to the energization of compressional (P) and shear (S) modes of propagation. At each depth, a series of energy impulses were performed (10 for the P-wave survey, 20 for the S-wave measurement, considering for the latter case the source polarity inversion). A borehole deviation survey was also performed several times in each borehole, in order to statistically assess the coordinates of the boreholes (i.e. their distances) along their length. Focusing on the P-wave mode, a quantitative evaluation of the repeatability of the signal was carried out by computing the cross-correlation among signals: the 67% confidence interval was considered as an estimator of dispersion of P-waves arrival times. A statistical estimation of boreholes distances was also performed. The uncertainties in the P-wave velocity estimation related to the deviation measurement revealed to be greater than those related to the arrival time estimation. The statistical assessment of the S and P wave profiles at each investigated location permitted to propagate the uncertainties to the estimation of the soil properties of interest.
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Voxel Inversion of Airborne EM Data
Authors G. Fiandaca, E. Auken, A.V. Christiansen and C. KirkegaardWe present a geophysical inversion algorithm working directly in a voxel grid disconnected from the actual measuring points, which allows for straightforward integration of different data types in joint inversion, for informing geological/hydrogeological models directly and for easier incorporation of prior information. Inversion of geophysical data usually refers to a model space being linked to the actual observation points. For airborne surveys the spatial discretization of the model space reflects the flight lines. Often airborne surveys are carried out in areas where other ground-based geophysical data are available. The model space of geophysical inversions is usually referred to the positions of the measurements, and ground-based model positions do not generally coincide with the airborne model positions. Consequently, a model space based on the measuring points is not well suited for jointly inverting airborne and ground-based geophysical data. Furthermore, geological and groundwater models most often refer to a regular voxel grid not correlated to the geophysical model space, and incorporating the geophysical data into the geological/hydrological modelling grids is problematic. We present a voxel grid inversion routine that overcomes these problems and we discuss in detail the algorithm implementation.
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Sharp Spatially Constrained Inversion
Authors G. Vignoli, G. Fiandaca, A.V. Christiansen, C. Kirkegaard and E. AukenWe present sharp reconstruction of multi-layer models using a spatially constrained inversion with minimum gradient support regularization. In particular, its application to airborne electromagnetic data is discussed. Airborne surveys produce extremely large datasets, traditionally inverted by using smoothly varying 1D models. Smoothness is a result of the regularization constraints applied to address the inversion ill-posedness. The standard Occam-type regularized multi-layer inversion produces results where boundaries between layers are smeared. The sharp regularization overcomes this by allowing a reconstruction with a large number of layers, while preserving abrupt changes in the conductivity distribution. Instead of minimizing the norm of the vertical spatial variation of the model, in the focusing approach, it is the number of layers where the variations occur that is minimized. Thus, the results are compatible with the data and, at the same time, favor sharp transitions. The focusing strategy can also be used to constrain the 1D solutions laterally, guaranteeing that lateral sharp transitions are retrieved without losing resolution. By means of real and synthetic datasets, sharp inversions are compared against classical smooth results and available boreholes. With the focusing approach, the obtained blocky results agree with the underlying geology and allow for easier interpretation by the end-user.
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Focused Multi-layer Inversion of Magnetic Resonance Sounding Data
Authors G. Vignoli, G. Fiandaca, A.A. Behroozmand and E. AukenWe present a comparison between two different regularisation schemes for multi-layer inversion of the MRS data. The two regularizations consist of the norm (Occam’s inversion) and the support (focused inversion) of the variation of the water content and relaxation time. The focused regularization provides a sharp inversion as it minimizes the area (the support) where the variations of the model parameters occur, and not the (L2) norm of the variations. Clearly, when sharp vertical transitions of the inversion parameters are present, the focused inversion retrieves the interfaces with higher precision and reconstructs more effectively the physical property values as well. This is a quite common situation in hydrogeophysics, e.g. in correspondence with the water table and/or at the boundary between aquifers and aquitards.
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Experimental Verification of Advanced SNMR Modeling
Authors A. Legchenko, J.F. Girard, N. Perttu, J.M. Vouillamoz, J.M. Baltassat and S. MorlighemWhatever would be the practical implementation of Surface Nuclear Magnetic Resonance (SNMR) survey, inversion is based on forward modeling of the magnetic resonance response. Consequently, the mathematical model is a crucial point for inversion. We have developed advanced mathematical model that allows computing magnetic resonance signal from 3D targets both in FID (free induction decay) and SE (spin echo) modes. This model takes into account both time and lateral variations of the earth’s magnetic field, water temperature and electrical resistivity of the subsurface but also the effect of magnetic rocks on MRS signal. For experimental verification of the modeling routine we used two bulk water reservoirs. One-dimensional (1-D) measurements were performed in winter from the ice surface in the Baltic Sea (Sweden) and 3-D verification was made using an artificial water pool located in the southern part of the New Caledonia Island. Modeling was carried out taking into account reservoir geometry and local conditions obtained from independent measurements. We have found a good correspondence between observed and theoretical signals for both FID and SE measurements. At our knowledge, SNMR modeling was never compared with field data obtained under all mentioned above conditions.
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On the Design of Cross-hole Resistivity Arrays for High-resolution and Cost-effective Storage Reservoir Monitoring
Authors F.M. Wagner, T. Günther, C. Schmidt-Hattenberger and H. MaurerElectrical resistivity tomography (ERT) has received consideration as a tool for permanent monitoring of saline storage reservoirs due to its high sensitivity to compositional pore fluid changes. The information content offered by geoelectrical data is ultimately limited by the electrode arrangement, and consequently, its full exploitation requires a well-conceived experimental design. We present a methodology to estimate an optimum number of electrodes as well as their specific locations along the borehole trajectories based on a maximization of the respective model resolution. Using a synthetic example analogous to the Ketzin site, Germany, we demonstrate that relatively sparse optimized setups with a refinement of the electrode spacings in the target horizon can offer comparable tomographic imaging capabilities with regard to rather dense arrays. The approach presented can assist practitioners with the design of high-resolution and cost-effective down-hole installations at future CO2 storage sites.
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Statistical Estimation of the Depth of Investigation of ERT and IP Surveys Based on a Modified DOI Index Method
Authors J. Deceuster, A. Etienne, T. Robert, F. Nguyen and O. KaufmannSeveral techniques are available to estimate the depth of investigation (DOI) or to identify possible artifacts in resistivity and IP surveys. Commonly, the DOI is mainly estimated using an arbitrarily chosen cut-off value on a selected resolution indicator (resolution, sensitivity or DOI index). Small changes in threshold values may induce strong variations in the estimated DOI. To overcome this problem, we developed a new statistical method to estimate the DOI based on a modified DOI index approach. Three inversions are performed using three strongly different resistivity reference models. We found that the cumulative distribution function of the DOI index values is well fitted by the sum of two normal distributions. We then focused on the evaluation of the mean and standard deviation of the normal distribution linked to the statistically well-constrained cells. We introduced two reliability indexes RI2σ and RI3σ based on confidence intervals, respectively 2σ and 3σ. They are used as alpha transparency values when plotting resistivity and chargeability models to discriminate between well- and poorly-constrained cells. The efficiency of the proposed methodology is assessed on synthetic data. Based on synthetic benchmark analysis, we demonstrated that the selected well-constrained cells are well-reconstructed in size, shape and resistivity.
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Better than the Optimised Traditional ERT Array - The g11n Arrays
Authors S. Szalai and K. SzokoliA new array type - which was until now outside of the interest of the geophysicists - the g11n arrays will be introduced. These arrays belong to the so called quasi null arrays which are - according to their behaviour - between the traditional and the null arrays. It will be shown that these arrays can be more effective than the traditional arrays in detection and characterisation of anomalies have just a small impact on the potential (due to their small size, resistivity contrast to the host rock, and/or their large depth). In many investigations they proved to be even better than the Stummer array which was constructed by an optimisation procedure from the set of the traditional arrays. The g11n arrays proved to be moreover significantly better in horizontal and vertical resolution investigations than even the optimised array. It is already seen that the g11n arrays can be valuable tools for solution of many problems similar to the investigated ones, like e.g.: research of tunnels, caves, cables, abandoned river beds, or discontinuities in the clay layer below a waste deposit. The applications of these arrays shorten moreover the measuring time.
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Anisotropic Inversion of Induced Polarization (IP) Data
More LessThe induced polarization (IP) method is a multi-frequency AC geoelectrical measurement technique that utilizes the phase difference between transmitted and recorded current alongside with the conventional complex electrical conductivity. This information is used to derive the subsurface complex electrical conductivities, usually by means of an inversion algorithm. Presently, these algorithms are based on isotropic complex electrical conductivities. However, it was shown for DC conductivity that assuming this isotropy can yield misleading or unnecessary complex results. We propose an IP inversion algorithm supporting anisotropic complex conductivities. For this purpose, we implemented a numerical forward operator and extended the sensitivity calculation to support these anisotropic complex conductivities. We incorporated a penalty function for the anisotropy as a factor for the individual model cells, which is minimized alongside with the non-smoothness penalty of the original algorithm. We successfully verified the code with synthetic examples targeting the detectability of anisotropy. As a false test, we successfully inverted a synthetic example with isotropic conductivities. First results of the synthetic study indicate that the algorithm is able to resolve anisotropic conductivities while underestimating the original anisotropy factor.
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Global Joint Inversion of Tomographic Data - Appraisal of Model Reconstruction Ambiguity
Authors H.C. Paasche and J. TronickeGeophysical model reconstruction by data inversion is usually ill-posed and suffers ambiguity due to limited number and accuracy of the available observations. Joint inversion of different data sets allows for mutually improved reconstruction of physical parameter models underlying each of the available data sets, but considering the limited number and accuracy of available observations, some ambiguity remains. Here, we use particle swarm optimization to jointly invert synthetic GPR and P-wave crosshole tomographic data sets. Model parameterization is guided by the results of a zonal cooperative inversion based on local search optimization of an initial guess. Global optimization is first done to explore the Pareto front of the joint inverse problem in a very efficient way. Consecutively, the area behind a selected location of the Pareto front is explored to be able to assess the model reconstruction ambiguity inherent to the available data and chosen parameterization.
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Introducing Artificial Bee Colony Optimization to Invert Surface Wave Dispersion Curve
Authors A. Zarean, N. Mirzaei and E. ShabaniSurface waves have been used increasingly as an attractive tool to obtain near-surface shear wave velocity profiles. Inversion of Surface wave dispersion curves is demanding for most local-search methods due to its high nonlinearity and to its multimodality. Among many approaches which have been proposed for surface wave inversion so far, Intelligence Swarm algorithms effectively applied to solve it, and attempt to avoid trapping in local minima. In this paper, we proposed a new approach (Artificial Bee Colony ABC) to overcome above disadvantages. Bee algorithms form another class of algorithms which are closely related to the ant colony optimization. Bee algorithms are inspired by the foraging behavior of honey bees. To evaluate calculation efficiency and stability of ABC to inversion of surface wave data, we first inverted a broadband (1-30 Hz) synthetic dispersion curves from a simple model. After that we extended search space, added noise and ultimately limited the frequency band. Results from all synthetic data demonstrate that ABC can be used for quantitative analysis of Surface wave dispersion curves.
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Airborne Radiometric Data Assessment and Comparison with Topsoil Geochemical U, Th and K
Authors J.A. Hodgson, K.V. Knights, M. Glennon, S. Carey and M.D. TureTellus Border is an EU INTERREG IVA-funded cross-border project which now has newly-acquired airborne geophysical and geochemical data in the six border counties of the Republic of Ireland. This extensive regional baseline geophysical and geochemical dataset provides an excellent opportunity to investigate the relationship between measured radiometric data and equivalent geochemical topsoil concentrations of uranium (U), thorium (Th) and potassium (K). Exploratory data analyses and univariate statistical summaries of both datasets (geophysical and geochemical) were compared across the whole survey area and then by different geological unit and soil type. This allowed trends between different elements and different units to be investigated. Good agreement was found between airborne and topsoil Thorium and Uranium values with similar distributions, means and medians. These values remain generally consistent between different geological units, however, Potassium (k) geochemical values differ significantly with airborne values, being on average 6 times lower. The best agreement between K values occurs over low-K geological regions such as from limestones and shales within carboniferous sediments. High-K regions such as over K-feldspar rich granites show the greatest divergence between geophysical and geochemical data. Generally strong geogenic controls are seen throughout all of the datasets.
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Preliminary Interpretations of the Tellus and Tellus Border Airborne Geophysical Surveys, NI and Ireland
By M. TureAirborne data sets have delineated concealed geological structures, boundaries, dyke swarms, intrusive bodies. The controlling structures for mineral potential of the survey area can be inferred from the data and shows structures generally oriented in NE–SW (Grampian Orogeny) and N–S orientations and are thought to control gold and base metal mineralisation. In general, the airborne data sets provide a wealth of geo-information and require further investigation.
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Interpretation of Airborne Gamma-ray Data of Socorro Island, Mexico
Authors S. Gruber, R. Supper and K. MotschkaIn February 2009, the Geological Survey of Austria performed an airborne geophysical survey on Socorro Island, applying magnetic, electromagnetic and radiometric measurements. Gamma-ray data has been processed with up-to-date radiometric processing software and interpreted with the focus on comparing it with geologic mapping results. Due to the remote position of the island, only few geologic maps were available. Very interesting results could be achieved from the comparison with a map of the island by E.A. Carballido-Sanchez and a map of the easily accessible Lomas Coloradas area by W.A. Bohrson and M.R. Reid. It was largely possible to confirm the studies of W.A. Bohrson and M.R. Reid. In contrast, the radiometric results in some parts disagree with the classification of E.A. Carballido-Sanchez. Main differences are three trachyte domes, a cinder cone and a large lava flow, which all have been assigned to the Lomas Coloradas basalts by E.A. Carballido- Sanchez and which could be interpreted as postcaldera silicic products by means of radiometric data. Interpreting radiometric data of Socorro was particularly interesting, as only few geophysical and geological results existed from the island.
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Comparison between Thermal Airborne Remote Sensing, Multi-depth Electrical Resistivity Profiling and Soil Mapping
Authors A. Tabbagh, C. Pasquier, I. Cousin, M. Seger, M. Dabas and J. ThiessonBrightness temperature of the ground surface measured by airborne remote sensing after a cold wave in December 2002 has a sufficient depth of investigation to inform about the thermal inertia of the subsoil layer. The comparison of this thermal property to the soil map and to resistivity maps confirms this depth of investigation and shows the ability of the thermal property to distinguish between calcisol and cambisol.
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