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- Volume 10, Issue 6, 2012
Near Surface Geophysics - Volume 10, Issue 6, 2012
Volume 10, Issue 6, 2012
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An overview of the spectral induced polarization method for near‐surface applications
ABSTRACTOver the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IP data for imaging purposes and the increasing application of the method in near‐surface investigations. We summarize here the current state of the science of the SIP method for near‐surface applications and describe which aspects still represent open issues and should be the focus of future research efforts. Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing data base of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has also been made in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IP data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3D, time‐lapse and multi‐frequency IP data, which defines a 5D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.
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Studies with gel‐filled sandstone samples with implications for the origin of induced polarization
Authors Andreas Hördt and Stefanie MildeABSTRACTThe origin of induced polarization (IP) in the pore space of sediments is not completely understood. Existing theories may be separated into two groups. The electrochemical polarization mechanism requires only an electrical double layer around the sediment grains, with the grain size determining the spatial scale. The membrane polarization mechanism is based on a coupling between pores of different sizes and ion mobilities to produce a frequency‐dependent conductivity. 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 conductivity spectra of the gel‐filled sandstone samples are distinctly different from those of water‐filled samples. The phase shifts decrease, the spectrum is flattened and the maximum moves towards higher frequencies. In terms of Cole‐Cole parameters, the gel decreases the chargeabilities and decay times. The effect can be clearly separated from that of increasing water salinity. We conclude that ion mobility in large pores is an important factor for the generation of the IP‐effect. Whereas the decrease in phase shift by the gel can be explained with both electrochemical and membrane polarization, the observed decrease in relaxation times seems to be inconsistent with either type of theories.
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Spectral induced polarization measurements on variably saturated sand‐clay mixtures
Authors K. Breede, A. Kemna, O. Esser, E. Zimmermann, H. Vereecken and J.A. HuismanABSTRACTElectrical spectral induced polarization (SIP) measurements are increasingly being used in environmental studies of the saturated zone. To better understand the mechanisms causing polarization and to extend the range of SIP applications to the vadose zone, it is important to investigate how the SIP response is affected by water saturation. Therefore, sand and sand‐clay mixtures were drained in several steps using a novel measurement set‐up allowing SIP measurements with a high accuracy. The measured SIP spectra were interpreted by Debye decomposition, which provided a relaxation time distribution and a chargeability distribution that was converted to a normalized total chargeability. The results showed that the normalized total chargeability of the fully saturated samples increased with increasing clay content due to the larger specific surface area of the clay minerals. Furthermore, normalized total chargeability first increased and then decreased with decreasing saturation for the pure sand and the 5% sand‐clay mixture. The normalized total chargeability values for the 10% and 20% sand‐clay mixtures only decreased with saturation. The peak relaxation time of the sand‐clay mixtures clearly decreased with decreasing saturation. Existing grain‐size based mechanistic models for SIP are not able to explain the observed behaviour and the observed relationship between relaxation time and saturation suggests that model concepts relying on polarization processes in the pore space are warranted to explain the measurements on variably saturated porous media presented here.
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Influence of physical and chemical properties on the low‐frequency complex conductivity of peat
Authors M. Ponziani, E.C. Slob, H. Vanhala and D.J.M. Ngan‐TillardABSTRACTOrganic layers are heterogeneous in space and their composition changes over time. This poses challenges to ecohydrologists, subsurface hydrologists and ground engineers in characterizing subsurface peat structures and predicting their behaviour over time. Peat deposits can be characterized by performing electrical surveys, provided that the complex conductivity of peat is understood and connected to its physical and chemical properties. Low‐frequency (0.1–1000 Hz) induced polarization measurements were carried out to investigate the correlation between the chemical and physical properties of several peat samples and their electrical properties. A Cole‐Cole model was fit to the peat spectra to obtain the model parameters and study their relationship with the sample properties. All the samples were characterized by analysing their degree of humification, water content, organic content, cation exchange capacity, pH and conductivity of the pore‐fluid. Two significant correlations between physical, chemical and electrical properties are found. The peat bulk conductivity is directly correlated with the pore fluid conductivity, whereas the degree of humification of peat shows an inverse correlation with the phase angle. This study presents results that have implications for peatland characterization with frequency dependent or single frequency analysis of induced polarization measurements.
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Spectral Induced Polarization (SIP) signatures of clayey soils containing toluene
Authors Andrea Ustra, Lee Slater, Dimitrios Ntarlagiannis and Vagner ElisABSTRACTWe performed laboratory experiments to investigate the sensitivity of the Spectral Induced Polarization (SIP) method to toluene contamination in clayey soils. We used mixtures of quartzitic sand and montmorillonite as soil samples, artificially contaminated with varying amounts of toluene. Care was taken to quantify the experimental uncertainty resulting from packing since such effects must be quantified if variations in SIP signatures between samples are to be reliably interpreted in terms of the effects of hydrocarbon concentration. The SIP response of all samples following addition of toluene was monitored for a period of 40 days following sample preparation. Stepwise regression was used to examine the statistical significance of correlations between (i) clay content and (ii) toluene concentration and SIP parameters. Both single‐frequency real and imaginary conductivity measurements, along with the integral chargeability, normalized chargeability, DC conductivity and time constant obtained from a Debye decomposition fitting, were examined in this regression analysis.
The SIP measurements show a clear time dependence following sample preparation, indicating that samples containing toluene may take significant time to reach an equilibrium electrical response. SIP measurements are significantly related to toluene content shortly after sample preparation, when the expected dependence of SIP on clay concentration is apparently suppressed. However, for the state of electrical equilibrium after 40 days (interpreted to indicate surface chemistry at equilibrium) there is no significant relation between SIP measurements and toluene content; instead SIP measurements are then significantly correlated with clay concentration. The total chargeability, normalized chargeability and relaxation time obtained from the Debye decomposition show no correlation with toluene content, indicating that this procedure, which likely integrates over multiple mechanisms, may not be suitable for understanding relationships between SIP and hydrocarbon contamination. We find only small low‐frequency polarization signals observed in relation to toluene concentration (2 mrad at 0.01 Hz), which initially decreases the interfacial polarization. Unlike earlier works, our results do not support the use of the SIP method as a tool for monitoring toluene contamination in clay soils.
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Direct estimation of the distribution of relaxation times from induced‐polarization spectra using a Fourier transform analysis
Authors N. Florsch, C. Camerlynck and A. RevilABSTRACTThe analysis of low‐frequency spectral induced polarization data involves the determination of the distribution of relaxation times either from time‐domain or frequency domain measurements. The classical approach is to assume a simple transfer function (e.g., a Cole‐Cole function) and to determine, by a deterministic or a stochastic fitting procedure, the parameters of this transfer function (for instance the four Cole‐Cole parameters). Some other methods (based on optimization) have been developed recently avoiding the choice of a specific transfer function that can bias data interpretation. We have developed a new approach based on the Fourier transform also avoiding the use of a specific analytical transfer function. The use of the Fourier transform is a classical approach to retrieve the kernel of a Fredholm integral equation of the first kind (especially in potential field theory) and this corresponds exactly to the problem we want to solve. We adapt the Fourier transform approach to retrieve the distribution of the relaxation times (for instance to process low‐frequency induced polarization data). Problems resulting from the use of this approach with noisy data are prevented by using Wiener filtering. As far as induced polarization is concerned, we found that it is necessary to fit the high‐frequency dielectric contribution of the spectra and to remove this contribution from the quadrature conductivity data before inverting the distribution of the relaxation times. Our approach is benchmarked with analytical pair solutions and then tested by using synthetic and experimental data sets.
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2D modelling of induced polarization data with anisotropic complex conductivities
Authors Johannes Kenkel, Andreas Hördt and Andreas KemnaABSTRACTInduced polarization data recorded at the field scale are usually processed using techniques based on isotropy. We study the effect of anisotropic complex conductivities in the frequency domain, with a particular emphasis on the polarization properties (i.e., phase angles). We consider anisotropy to be caused by a thin layering of isotropic media. Based on this, we derive corresponding mixing laws for the phase angles parallel and perpendicular to the layering and show that in cases of anisotropic layering, phase angles can only exist if the complex conductivity magnitudes are also anisotropic. We modified an existing isotropic finite‐element forward modelling code to support anisotropic complex conductivities. Based on an isotropic 2D inversion of a synthetic data set generated for an anisotropic model, we show that ignoring anisotropy may lead to artefacts and poor model recovery. Finally, we investigate a field data set recorded over a fault system in Germany, where anisotropic conductivities are expected. The isotropic 2D inversion of this data set yields a complex image that is difficult to interpret. We suggest an alternative interpretation based on trial‐and‐error forward modelling with anisotropic complex conductivities, using geological a priori information to constrain the model. The obtained model has a much simpler structure, consistent with the anticipated geology but yet is able to explain the measured data.
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Improvement in time‐domain induced polarization data quality with multi‐electrode systems by separating current and potential cables
Authors Torleif Dahlin and Virginie LerouxABSTRACTMeasuring induced polarization in the time domain with relatively compact multi‐channel multi‐electrode systems is attractive because of the simplicity of the procedure and thus its efficiency in the field. However the use of this technique is sometimes discouraged by the bad quality of the measurements in cases of high electrode contact resistances that can render data interpretation infeasible or at least unreliable. It is proposed that capacitive coupling in the multi‐core electrode cables has a significant role in creating this problem.
In such cases separation of current and potential circuits by using separate multi‐conductor cable spreads can yield significant improvement in data quality. The procedure is relatively simple and can be implemented with common resistivity and time‐domain IP equipment.
We show here three field examples from Southern Sweden, all measured as 2D electrical imaging sections. The first one is an example where the use of a single cable spread is sufficient thanks to moderate electrode contact resistance and high signal levels. The following two examples are from sites where induced polarization measurements could not yield consistent results using only a single multi‐conductor cable spread. Useful results were subsequently obtained by using separate cable spreads.
The first example is a 280 m long line measured over an old covered municipal waste deposit where the waste body stands out as a zone of high chargeability. The second example is a 120 m line measured on a sandy glaciofluvial structure that is host to an aquifer of regional importance. The improvement led to discrimination between materials of different grain sizes, with potential bearing for understanding the aquifer. The third example is a 300–400 m line measured across an esker lying on clay till. The improvement led to a clear visualization of the esker and to the identification of a possible fault in the underlying gneissic bedrock.
In all cases pseudosections and examples of chargeability decay curves are shown and discussed as tools for assessing data quality. Inversion results are shown together with background geological information and it is concluded that they are in good agreement.
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Induced polarization 3D tomography of an archaeological direct reduction slag heap
Authors N. Florsch, M. Llubes and F. TéreygeolABSTRACTBoth environmentalists and archaeologists are interested in the detection and quantification of the remains of mine slag heaps from ancient smelters: possible pollution of the slag heaps and information concerning the history of the smelter's mine. Although magnetic surveys can be used to detect subsurface slag accumulations, it is not possible to derive the total amount of material from such surveys or to accurately delineate the heaps through the use of this method. Conversely, ‘Induced Polarization’ (IP) surveying allows a relevant and robust assessment of the volume of slag concentrations to be determined. In the present study, we follow up on results obtained in a previous study, carried out at the 14th Castel‐Minier mine (Ariège, France) site, where we have shown that the chargeability is proportional to the slag concentration in the ground and have used this property to perform 3D tomography of the heap. Based on a previous investigation using spectral induced polarization along a simple profile showing that the phase peak related to the slags is situated at approximately 1 Hz, in the present paper we extend our prospection to 3D by using a Terrameter SAS1000 and measuring temporal chargeability. The set of three‐dimensional data is recorded by means of classical transect measurements along parallel close profiles and is then interpreted using the RES3DINV code. Finally, archaeological and auger soundings confirmed the assessments derived from this geophysical investigation. We also briefly discuss the source of the IP signal, which we suspect to be induced by magnetite particles embedded in the slags.
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Mapping of landfills using time‐domain spectral induced polarization data: the Eskelund case study
Authors A. Gazoty, G. Fiandaca, J. Pedersen, E. Auken and A.V. ChristiansenABSTRACTThis study uses time‐domain induced polarization data for the delineation and characterization of the former landfill site at Eskelund, Denmark. With optimized acquisition parameters combined with a new inversion algorithm, we use the full content of the decay curve and retrieve spectral information from time‐domain IP data. Thirteen IP/DC profiles were collected in the area, supplemented by el‐log drilling for accurate correlation between the geophysics and the lithology. The data were inverted using a laterally constrained 1D inversion considering the full decay curves to retrieve the four Cole‐Cole parameters. For all profiles, the results reveal a highly chargeable unit that shows a very good agreement to the findings from 15 boreholes covering the area, where the extent of the waste deposits was measured. The thickness and depth of surface measurements were furthermore validated by el‐log measurements giving in situ values, for which the Cole‐Cole parameters were computed. The 3D shape of the waste body was pinpointed and well‐defined. The inversion of the IP data also shows a strong correlation with the initial stage of the waste dump and its composition combining an aerial map with acquired results.
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Multi‐measurement integration for near‐surface geological characterization
Authors Andreas Laake and Claudio StrobbiaABSTRACTGeological characterization of the near‐surface is challenging because often shallow layers are unconsolidated and their properties have rapid and significant variations in vertical and lateral directions. This creates challenges for a majority of geophysical methods, which have limitations in the presence of a complex distribution of physical properties and that give optimal results over limited depth intervals, which are different for the different methods. The shallow near‐surface depth region is often problematic for standard seismic exploration methods. As a result, geological characterization through individual measurements is often compromised. The solution proposed here is the integration of multiple measurements of surface and deep subsurface properties, with the goal of generating a geologically meaningful lithostructural model of the near‐surface.
The underlying idea is based on the observation that deep subsurface structural features often leave an impression on the surface. This provides an opportunity for structural mapping at the surface and below the bottom of the near‐surface zone to guide the processing and interpretation of data from the near‐surface.
We show how the integration of multiple data sets from the measurement of different physical properties can be used to generate and enhance specific near‐surface products such as drilling risk maps or P‐wave velocity models for static corrections or for velocity modelling. The analysis proves that integration of multiple data sets can provide a solution when individual measurements, such as refraction traveltimes, do not provide sufficient information to characterize the lithostructure of the near‐surface. Satellite surface imagery and deep seismic data can provide the structural framework for the processing and interpretation of shallow seismic data. Simultaneous joint inversion of shallow P‐ and S‐wave data reveals the vertical and lateral velocity structures in the near‐surface, which are usually concealed when only refraction data are used. Finally, the correlation of multiple data sets in a correct spatial reference provides a quality control for the data set and allows for the lithostructural interpretation of data in the near‐surface.
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A multi‐method geophysical approach based on fuzzy logic for an integrated interpretation of landslides: application to the French Alps
More LessABSTRACTPhysics‐based, such as seismic surveying or electrical resistivity imaging, appear to be efficient in investigating landslide structures and, consequently, for understanding related mechanisms of deformation. They are non‐intrusive probing methods that allow the direct measurement of compressional (P), shear (S) wave velocities and also electrical resistivity, three geophysical parameters that are considered as determinants in defining ground properties and identifying anomalies related to structural (faults, fissures), lithological (sand to clay, or calcareous variations) and hydrological (moisture, water flow) conditions. Both the seismic and resistivity methods are commonly used for landslide investigations, having been tested over the last decade or more on various types of landslides, including mudslides, debris flows, unstable slopes, etc. The common approach in these earlier experiments has been to invert the P‐ and S‐wave velocity fields, plus the electrical resistivity field, using suitable algorithms to produce two‐dimensional P‐ and S‐wave velocities and resistivity tomograms. A coherent and integrated interpretation of the resulting information is, however, not straightforward because each geophysical method is sensitive to different soil properties.
An innovative approach has thus been developed to combine the geophysical parameters imaged on tomograms and convert them into different geological or geomechanical cross‐sections. Knowing that seismic data provide information on variations in fissure density and the presence of sheared materials and that electrical resistivity data provide information on variations in water content, the final cross‐sections are computed by combining different transformation functions able to model the conversion from geophysical parameters to ground properties. The computations are realized in a framework of the fuzzy‐set mathematical theory that maintains a certain level of objectivity and is able to manage uncertainties. The basics of this approach are explained through a brief presentation of the theory of data fusion with emphasis on how uncertainties are taken into account. The results obtained from case studies in the French Alps are then discussed in terms of reliability and compared with available ground reality obtained from surface observations and borehole descriptions.
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Physics of shear‐wave intrinsic dispersion and estimation of in situ soil properties: a synthetic VSP appraisal
Authors Alimzhan Zhubayev and Ranajit GhoseABSTRACTEstimates of in situ porosity and permeability in saturated soils are important in various disciplines. In this research, a recently proposed concept for quantitative integration of dispersive seismic velocity and attenuation in soft soils, based on the underlying physics, is tested on a realistic synthetic shear‐wave (S‐wave) vertical seismic profiling (VSP) dataset. The effects of error in the determination of layer‐specific dispersive velocity and attenuation from VSP data without and with noise, as well as those of error in the used poroelasticity model, on the estimated values of porosity and permeability are investigated. A methodology involving extraction of layer‐specific intrinsic dispersion from VSP data and enhancement of robustness and reliability through use of multiple receivers within the thickness of a given layer, in combination with source stacking, is presented. For the assumed frequency band (50–140 Hz) for S‐wave data, stable values of both porosity and permeability can be obtained for all but a very low‐permeability clayey layer. The results show that if the model error is not large, then both porosity and permeability can be estimated quite accurately even when the data are rather noisy. However, when the model used is grossly inaccurate and there is large noise in the data and hence a large error in the estimated dispersion, then though the absolute error in porosity is still within 2–3%, the permeability can be off by an order of magnitude. In general, in this approach, if the poroelasticity model is so chosen that it explains reasonably well the observed dispersion, then the estimates of in situ porosity and permeability should both be quite accurate.
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Effect of antenna‐medium coupling in the analysis of ground‐penetrating radar data
Authors Sébastien Lambot, Frédéric André, Evert Slob and Harry VereeckenABSTRACTPhysically‐based ground‐penetrating radar (GPR) data processing is essential for quantitative characterization of soils and materials. A novel near‐field GPR antenna model coupled with layered media Green's functions was used to investigate the effect of antenna‐medium coupling in the analysis of GPR data. The radar antennas are modelled using an equivalent set of infinitesimal electric dipoles and characteristic, frequency‐dependent, global reflection and transmission coefficients. These coefficients determine through plane wave decomposition, wave propagation between the radar reference plane, point sources and field points. We calibrated an actual commercial 400 MHz time‐domain antenna, from which synthetic GPR data sets were generated. We observed that, depending on the model configuration, antenna effects may affect the topography of the objective function in full‐waveform inverse problems. In addition, antenna‐medium coupling has a significant impact on the medium surface reflection, whether in terms of amplitude or propagation time (which usually defines the so‐called time zero). We also showed that an effective source cannot be used for simulating near‐field radar data as the antenna‐medium coupling strongly depends on medium properties. In this respect, numerical experiments demonstrated promising perspectives for simultaneous estimates of medium permittivity and conductivity from antenna‐medium coupling.
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Integrated analysis of waveguide dispersed GPR pulses using deterministic and Bayesian inversion methods
Authors Jutta Bikowski, Johan A. Huisman, Jasper A. Vrugt, Harry Vereecken and Jan van der KrukABSTRACTGround‐penetrating radar (GPR) data affected by waveguide dispersion are not straightforward to analyse. Therefore, waveguide dispersed common midpoint measurements are typically interpreted using so‐called dispersion curves, which describe the phase velocity as a function of frequency. These dispersion curves are typically evaluated with deterministic optimization algorithms that derive the dielectric properties of the subsurface as well as the location and depth of the respective layers. However, these methods do not provide estimates of the uncertainty of the inferred subsurface properties. Here, we applied a formal Bayesian inversion methodology using the recently developed DiffeRential Evolution Adaptive Metropolis algorithm. This Markov Chain Monte Carlo simulation method rapidly estimates the (non‐linear) parameter uncertainty and helps to treat the measurement error explicitly. We found that the frequency range used in the inversion has an important influence on the posterior parameter estimates, essentially because parameter sensitivity varies with measurement frequency. Moreover, we established that the measurement error associated with the dispersion curve is frequency dependent and that the estimated model parameters become severely biased if this frequency‐dependent nature of the measurement error is not properly accounted for. We estimated these frequency‐dependent measurement errors together with the model parameters using the algorithm. The posterior distribution of the model parameters derived in this way compared well with inversion results for a reduced frequency bandwidth which is an alternative, yet subjective method to reduce the bias introduced by this frequency‐dependent measurement error. Altogether, our inversion procedure provides an integrated and objective methodology for the analysis of dispersive GPR data and appropriately treats the measurement error and parameter uncertainty.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)