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- Volume 69, Issue 7, 2021
Geophysical Prospecting - Volume 69, Issue 7, 2021
Volume 69, Issue 7, 2021
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Imaging complex three‐dimensional structures, applying three‐component vector migration to synthetic vertical seismic profiling data
Authors Jakob B.U. Haldorsen and Leif JahrenABSTRACTBy applying a ray‐based three‐dimensional vector migration process to fully elastic three‐component synthetic Vertical Seismic Profiling data, we are able to demonstrate that the process gives an accurate compressional reflection image of the complex formation without much visible artefacts from spatial aliasing, or from ‘migration smiles’ caused by the limited aperture of the receiver array. We demonstrate that the process reconstructs images of both steeply dipping salt flanks, and the sediments that tend to be dragged along by the moving salt and truncated against the sides of the salt body. The process uses raw unseparated data as input, and the wavefield separation of compressional and shear waves are done by projection operators inside the kernel of the migration process. The compressional reflection image appear to have no visible artefacts from the abundance of converted shear.
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An artificial neural network approach for the inversion of surface wave dispersion curves
More LessABSTRACTWe describe a new algorithm for the inversion of one‐dimensional shear‐wave velocity profiles from dispersion curves of the fundamental mode of Rayleigh surface waves. The novelties of our approach are that the layer velocities and thicknesses are set as unknowns, and an artificial neural network is proposed to solve the inverse problem. We suggest that training data should be calculated for a set of random synthetic velocity layered models, while layer thicknesses and velocities should be set to fixed intervals, with ranges estimated based on the systematic application of empirical relations between Rayleigh and S‐wave velocities to the dispersion data. Our main challenge is a total overhaul of the artificial neural network, which includes selecting the optimal artificial neural network architecture and parameters by performing a large number of numerical experiments. Our synthetic results show that the accuracy of the proposed approach outperforms that of the Monte Carlo approach. We illustrate our proposed method with West Siberia data processing obtained from an area of approximately 800 . From a user perspective, the main strength of our method is the computationally efficient processing of large amounts of dispersion data, which make it well suited for four‐dimensional near‐surface monitoring.
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An illumination‐compensated Gaussian beam migration for enhancing subsalt imaging
Authors Shaoyong Liu, Zhe Yan, Wenting Zhu, Bingkai Han, Hanming Gu and Shanzheng HuABSTRACTSeismic images of Earth's subsurface are essential for oil and gas exploration. Gaussian beam migration is popular for seismic imaging owing to its flexibility and efficiency of implementation. However, the practical use of classic Gaussian beam migration is limited for complicated structures such as subsalt because of poor seismic illumination in those areas. We propose an illumination‐compensated Gaussian beam migration under the framework of least‐squares migration, enhancing the subsalt imaging effectively. A novel scheme based on the Born modelling and migration of Gaussian beam is developed to estimate the point spreading function, with which the illumination compensation can be efficiently implemented in the local wavenumber domain. As the aim of least‐squares migration, the proposed illumination‐compensated Gaussian beam migration produces true‐amplitude images in subsalt areas, which facilitates the seismic interpretation of subsalt structures. The total computational cost of the proposed method includes one Born modelling process and two conventional Gaussian beam migrations, and thus it is much more efficient than the classic least‐squares migration which requires multiple iterations. Numerical examples have demonstrated the effectiveness of the adaptive strategies, manifesting applicable potential in oil and gas exploration of subsalt targets.
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Extended full waveform inversion with matching filter
Authors Yuanyuan Li and Tariq AlkhalifahABSTRACTFull waveform inversion has shown its huge potentials in recovering a high‐resolution subsurface model. However, conventional full waveform inversion usually suffers from cycle skipping, resulting in an inaccurate local minimum model. Extended waveform inversion provides an effective way to mitigate cycle skipping. A matching filter between the predicted and observed data can provide an additional degree of freedom to improve the data fitting and avoid the cycle skipping. We extend the search space to treat the matching filter as an independent variable that we use to bring the compared data within a half cycle to obtain the accurate direction of velocity updates. We formulate the objective function using the penalty method by linearly combining a data‐misfit term and a penalty term. The objective function with a reasonable penalty parameter has a larger region of convergence compared to conventional full waveform inversion. We search for the optimal solution over the extended model by updating the matching filter and the velocity in a nested way. The normalization of the data can bring us an equivalent normalization to the filter and a more effective convergence. In the synthetic Marmousi model, the proposed inversion method recovers the velocity model stably and accurately starting from a linearly increasing model in the case of lack of low frequencies below 3 Hz, in which conventional full waveform inversion suffers from cycle skipping. We also use a marine field data to demonstrate the effectiveness and practicality of the proposed method.
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Simultaneous joint migration inversion as a high‐resolution time‐lapse imaging method: Feasibility and robustness study
Authors Shan Qu and Dirk Jacob VerschuurABSTRACTThe conventional time‐lapse processing workflow is usually sensitive to the non‐repeatable uncertainties between different vintages caused by noise, acquisition designs and independent processing. Therefore, in order to reduce these non‐repeatable uncertainties, all the datasets are usually acquired from well‐sampled and well‐repeated acquisition surveys, and the independent processing is always carefully tailored to maximally reduce the non‐repeatable uncertainties during processing. Moreover, the conventional time‐lapse analysis method, based on a time‐shift map, is not always a good indicator of the actual velocity differences due to its local one‐dimensional subsurface assumption. In order to relax these rigid requirements and have a better velocity change indicator, a robust high‐resolution simultaneous joint migration inversion was proposed as an effective time‐lapse tool for reservoir monitoring. The method combines a simultaneous data‐processing strategy with the joint migration inversion method. Joint migration inversion is a full wavefield inversion method with a parameterization in terms of reflectivity and propagation velocity. The simultaneous strategy allows the baseline and monitor parameters to communicate and compensate with each other dynamically during inversion, thus, suppressing the non‐repeatable uncertainties during the time‐lapse processing. To investigate the feasibility of using high‐resolution simultaneous joint migration inversion in practice, some numerical experiments are conducted to test the dependence of high‐resolution simultaneous joint migration inversion on the quality of the time‐lapse datasets including the following aspects: random noise; sparse surveys; ocean bottom node versus streamer (different types of monitoring surveys); non‐repeated sources, including source positioning errors and non‐repeated source wavelets; spatial weighting operators in the L2‐norm penalty terms; and sensitivity to weak time‐lapse effects. All the experiments are carried on the basis of a realistic synthetic time‐lapse model based on the Grane field offshore Norway. These experiments show that high‐resolution simultaneous joint migration inversion is robust to random noise, survey sparsity, survey non‐repeatability, source positioning errors and source wavelet discrepancies. Furthermore, high‐resolution simultaneous joint migration inversion remains effective when the spatial weighting operators in the L2‐norm penalty terms are largely relaxed, and high‐resolution simultaneous joint migration inversion is capable of detecting weak time‐lapse changes (e.g. velocity changes down to 35 m/s).
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Seismic erratic noise attenuation using unsupervised anomaly detection
Authors Woodon Jeong, Mohammed S. Almubarak and Constantinos TsingasABSTRACTThis study introduces a new attribute to identify seismic erratic noise, i.e. outlier, in the context of unsupervised anomaly detection and is defined as local outlier probabilities. The local outlier probabilities calculate scores of degrees of isolation, i.e. outlier‐ness, for each object in a data set, which represents how far an object is deviated from its surrounding objects. Since the local outlier probabilities combines a density‐based outlier detection method with a statistically oriented scheme, its scoring system provides regularized outlier‐ness, which is an outlier probability, to be used for making a binary decision to do inclusion or exclusion of an object; such a decision only requires a simple and straightforward threshold on a probability. Based on the binary decision that flags outliers versus non‐outliers, local outlier probabilities‐denoising workflows are developed by combining multiple steps to complete an application of the local outlier probabilities to attenuate seismic erratic noise. Higher stability and improved robustness in the detection and rejection of seismic erratic noise have been achieved by implementing moving windows and decision tree‐based processes. To avoid loss of useful signal energy, signal enhancement applications are additionally suggested. Numerical experiments on synthetic data investigate the applicability of the proposed algorithms to seismic erratic noise attenuation. Field data examples demonstrate the feasibility of a local outlier probabilities‐denoising application as an effective tool in seismic denoising portfolio.
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Quality control for the geophone reorientation of ocean bottom seismic data using k‐means clustering
Authors Woodon Jeong, Mohammed S. Almubarak and Constantinos TsingasABSTRACTDuring ocean bottom seismic acquisition, seafloor multicomponent geophones located in rugose and sloping water bottom can be affected by skewed energy distribution, such as leaked shear energy on the vertical geophones and leaked compressional energy on the horizontal geophones. To correct for the tilted energy distribution, which is one of most effective preprocessing steps, a geophone reorientation step is applied. This is a simple and straightforward process that applies a 3‐dimensional rotation matrix with respect to the orientation angles. Since the reorientation process highly affects the outcome of the entire data processing workflow, it has to be accompanied by a careful quality control process to verify its validity for the whole survey area. In this study, we propose a quality control workflow for the geophone reorientation by using unsupervised machine learning. A correlation analysis is employed to compare numerical versus analytical solutions of both the azimuth and the incidence angles for the direct arrivals. A comparison of both solutions aims to generate correlation coefficients that are indicative of the accuracy of geophone orientation. The correlation coefficients are subsequently investigated by the k‐means clustering algorithm to differentiate and identify normally and abnormally deployed/reoriented geophones. Numerical experiments on a field ocean bottom seismic data set confirm that the proposed workflow effectively provides reliable labels for normally and abnormally deployed/reoriented geophones. The labelling assigned by the proposed quality control workflow is a suitable indicator for abnormalities in the geophone reorientation step and will be helpful for further investigation, such as re‐correction or removal of abnormally reoriented geophones.
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Not all faults are conductive: the effects of seasonal weather changes on the near‐surface expression of faults
Authors David C. Nobes and Sharon M. HornblowABSTRACTThe near‐surface electrical properties of faults can change with seasonal climate, depending on the nature of the materials filling the faults, e.g. fault gouge vs fractured rock, and be subject to seasonal weather changes. A period of dry weather, especially in a hot climate, can dry out the near‐surface sections of faults that are filled with porous materials, thus increasing the electrical resistivity, yet may not affect a fault that is filled with fine‐grained material that retains moisture well. Conversely, a sustained period of wet weather can fill porous faults with water, thus decreasing the resistivity. To illustrate the impact of seasonal weather changes on the near‐surface properties of faults, we conducted electrical imaging and ground‐penetrating radar surveys during the late spring, summer, and early autumn on the Glen Lyon and Benmore segments of the Ostler Fault, respectively, in the South Canterbury and Central Otago provinces of the South Island of New Zealand. The changes in the response are clear: seasonal weather changes can affect the near‐surface properties of faults, and the geophysical responses from the near‐surface faults can vary depending on the nature of the material filling the fault.
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Analytical model for rock effective elastic properties with aligned elliptical cracks embedded in transversely isotropic background
Authors Junxin Guo, Luanxiao Zhao, Zhifang Yang and Hongbing LiABSTRACTSeismic methods are often used for fracture detection and characterization, for which many analytical rock physics models have been developed to link fracture parameters to rock elastic properties. However, most of these models are limited to isotropic background and circular (penny‐shaped) fractures. In this work, we derived a more general analytical model for effective elastic properties of rocks embedded with aligned elliptical cracks in transversely isotropic background. The solutions for the case with dry cracks parallel to isotropic plane of transversely isotropic background were first derived using the corresponding Eshelby's tensor. Then, the results were extended to the case with inclined and rotated cracks under the assumption of constant crack opening displacement tensor. Finally, to obtain the results for fluid‐filled crack case, the Brown–Korringa equation was applied. To investigate the controlling factors on rock elastic properties, we studied one fractured tight sandstone sample with transversely isotropic background. The results show that rock elastic properties are insensitive to crack spin angle, but sensitive to crack inclination angle. Depending on crack inclination angle and fluid saturation, background anisotropy can either amplify or reduce the anisotropy of seismic wave velocity. Crack density and aspect ratio are found to be controlling crack parameters for rock elastic properties. The crack plane shape has negligible direct influence on rock elastic properties, whereas it has indirect influence by affecting values of crack density and aspect ratio. Compared with the numerical model and the previous analytical model for the special case with circular cracks, our model agrees well with them, which validates the accuracy of our model.
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Impedance loading an acoustic source in a borehole located in a fluid‐saturated porous medium
Authors I. Markova, M. Markov, J. Ramírez Sabag and G. Ronquillo JarilloABSTRACTConventional sonic logging tools are destined mainly for the determination of elastic waves’ velocities. In this paper, we have considered a different type of sonic logging tool that is destined for the registration of parameters of reflected waves in a borehole. We have calculated the mechanical impedance loading an acoustic source located on the axis of a fluid‐filled borehole. The problem was solved for a borehole drilled in a poroelastic formation. The solution was obtained in the framework of the Biot theory of poroelasticity. We have considered two limiting cases with permeable and impermeable (mudcake at the boundary between the borehole and porous rock) borehole wall. We have obtained the frequency dependences of the mechanical impedance. It was shown that the acoustic system ‘logging tool – fluid‐filled borehole – porous rock’ presents several resonance frequencies. These resonances’ frequencies are close to the resonance frequencies of a liquid layer between two rigid cylinders with radii equal to the source and borehole radii. The mechanical impedance calculated at the resonance frequency depends on the porosity and permeability that allows one to use impedance measurements in the frequency range near the resonance to determine the filtration properties of highly permeable rocks (of order higher than 10 mD). We have calculated the mechanical impedance in the low‐frequency range. The results obtained have shown that the use of low‐frequency impedance measurements provides a good way to evaluate the permeability of porous media.
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Sequential joint inversion of gravity and magnetic data via the cross‐gradient constraint
Authors Mojtaba Tavakoli, Ali Nejati Kalateh, Mohammad Rezaie, Lutz Gross and Maurizio FediABSTRACTDifferent geophysical methods use different model parameterizations and inversion algorithms. Thus, combining these different inversion systems and yet adding the nonlinear cross‐gradient constraint in a joint inversion framework might be a big challenge, for instance, as explained further by Moorkamp et al. in 2011, there is a complex interaction between the data misfit terms, regularization and cross‐gradient terms and an imperfect fit to the data is expected. In this paper, we use a sequential algorithm for a two‐dimensional joint inversion of gravity and magnetic data, which tries to avoid these issues by decoupling the gravity inversion, the magnetic inversion and the cross‐gradient minimization processes. The efficiency of the algorithm and developed code is demonstrated by the joint inversion of noisy synthetic data. The results show a significant improvement in the respective models obtained by introducing the cross‐gradient joint inversion over the models obtained by separate inversions for synthetic data and then for field data targeting potash ore source in the AjiChai salt deposit in north‐western Iran.
In this application case, the lower density of salt minerals such as potash, compared to its surrounding sedimentary sequences, motivates a gravity study. Furthermore, the relative lower susceptibility of these salt minerals, alongside their diamagnetic effect, makes them a suitable target for magnetic surveys. Separate gravity and magnetic studies had been performed over the deposit; however, a constitutive relationship between density and magnetization within the area of interest supporting a joint inversion had not been established. In this paper, we apply the sequential cross‐gradient approach to perform the first full joint inversion for the AjiChai salt deposit. The magnetic inversion here is performed to recover the magnetization amplitudes rather than the magnetization vector. In fact, we assume there is no remanent magnetization and, therefore, that the magnetization vector is constant and parallel to the geomagnetic field direction. The constructed density and magnetization models are of high concordance with available geological information and previous studies including drilling results. In addition, unlike previous separate inversion models, the models are structurally and geometrically similar.
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Research Note: Automated data‐driven alignment of near‐bit and top‐drive vibration sensors for seismic while drilling and beyond
Authors Anton Egorov, Ilya Silvestrov, Andrey Bakulin and Pavel GolikovABSTRACTDownhole near‐bit vibration sensors are used to record the pilot signal for seismic while drilling and identify drilling dysfunctions for drilling optimization. The typical downhole sensor is a memory‐based device with an inexpensive internal clock of limited accuracy that is not synchronized to surface data. Integrating downhole vibrations with other time‐based data such as surface drilling parameters or seismic recordings requires accurate time alignment between all data sets. We present a novel automated two‐step alignment procedure that uses a Global Positioning System‐synchronized top‐drive vibration sensor as a reference. The accuracy of the new method satisfies the most demanding requirements of seismic while drilling. The first step finds the delay time and linear drift using global optimization. The second step estimates nonlinear drift using time‐variant cross‐correlation. Alignment precision of a few seconds after the first step becomes a few milliseconds after the second step, as demanded by seismic‐while‐drilling. The field data example shows that the proposed methodology successfully aligns top‐drive and downhole data using a fully unsynchronized near‐bit vibration data set. After alignment, the near‐bit sensor delivers a usable pilot for seismic‐while‐drilling processing, resulting in better quality correlated seismic data than those obtained with the top‐drive surface pilot.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)