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- Volume 60, Issue 1, 2012
Geophysical Prospecting - Volume 60, Issue 1, 2012
Volume 60, Issue 1, 2012
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High‐resolution and super stacking of time‐reversal mirrors in locating seismic sources
Authors W. Cao, S. M. Hanafy, G. T. Schuster, G. Zhan and C. BoonyasiriwatABSTRACTTime reversal mirrors can be used to backpropagate and refocus incident wavefields to their actual source location, with the subsequent benefits of imaging with high‐resolution and super‐stacking properties. These benefits of time reversal mirrors have been previously verified with computer simulations and laboratory experiments but not with exploration‐scale seismic data. We now demonstrate the high‐resolution and the super‐stacking properties in locating seismic sources with field seismic data that include multiple scattering. Tests on both synthetic data and field data show that a time reversal mirror has the potential to exceed the Rayleigh resolution limit by factors of 4 or more. Results also show that a time reversal mirror has a significant resilience to strong Gaussian noise and that accurate imaging of source locations from passive seismic data can be accomplished with traces having signal‐to‐noise ratios as low as 0.001. Synthetic tests also demonstrate that time reversal mirrors can sometimes enhance the signal by a factor proportional to the square root of the product of the number of traces, denoted as N and the number of events in the traces. This enhancement property is denoted as super‐stacking and greatly exceeds the classical signal‐to‐noise enhancement factor of . High‐resolution and super‐stacking are properties also enjoyed by seismic interferometry and reverse‐time migration with the exact velocity model.
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3D post‐stack interval velocity analysis with effective use of datuming
Authors Moshe Reshef, Noam Lipzer, Raanan Dafni and Evgeny LandaABSTRACTInterval velocity analysis using post‐stack data has always been a desire, mainly for 3D data sets. In this study we present a method that uses the unique characteristics of migrated diffractions to enable interval velocity analysis from three‐dimensional zero‐offset time data. The idea is to perform a standard three‐dimensional prestack depth migration on stack cubes and generate three‐dimensional common image gathers that show great sensitivity to velocity errors. An efficient ‘top‐down’ scheme for updating the velocity is used to build the model. The effectiveness of the method is related to the incorporation of wave equation based post‐stack datuming in the model building process. The proposed method relies on the ability to identify diffractions along redatumed zero‐offset data and to analyse their flatness in the migrated local angle domain. The method can be considered as an additional tool for a complete, prestack depth migration based interval velocity analysis.
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Elastic wave mode separation for tilted transverse isotropy media
More LessABSTRACTSeismic waves propagate through the earth as a superposition of different wave modes. Seismic imaging in areas characterized by complex geology requires techniques based on accurate reconstruction of the seismic wavefields. A crucial component of the methods in this category, collectively known as wave‐equation migration, is the imaging condition that extracts information about the discontinuities of physical properties from the reconstructed wavefields at every location in space. Conventional acoustic migration techniques image a scalar wavefield representing the P‐wave mode, in contrast to elastic migration techniques, which image a vector wavefield representing both the P‐ and S‐waves. For elastic imaging, it is desirable that the reconstructed vector fields are decomposed into pure wave modes, such that the imaging condition produces interpretable images, characterizing, for example, PP or PS reflectivity. In anisotropic media, wave mode separation can be achieved by projection of the reconstructed vector fields on the polarization vectors characterizing various wave modes. For heterogeneous media, because polarization directions change with position, wave mode separation needs to be implemented using space‐domain filters. For transversely isotropic media with a tilted symmetry axis, the polarization vectors depend on the elastic material parameters, including the tilt angles. Using these parameters, we separate the wave modes by constructing nine filters corresponding to the nine Cartesian components of the three polarization directions at every grid point. Since the S polarization vectors in transverse isotropic media are not defined in the singular directions, e.g., along the symmetry axes, we construct these vectors by exploiting the orthogonality between the SV and SH polarization vectors, as well as their orthogonality with the P polarization vector. This procedure allows one to separate all three modes, with better preserved P‐wave amplitudes than S‐wave amplitudes. Realistic synthetic examples show that this wave mode separation is effective for both 2D and 3D models with strong heterogeneity and anisotropy.
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On the form of the space‐time domain reflection function for a simple flat‐lying interface
By C.J. ThomsonABSTRACTA description is given of the function R(x, t) forming the kernel of the space‐time domain reflection operator for scalar waves illuminating a single interface in two space dimensions. This reflection function is closely related to migration common‐image gathers and it can be thought of as the set of all plane‐wave reflection coefficients after Fourier synthesis over wavenumber, or slowness, and frequency.
R(x, t) is a generalized function with a complicated form at the origin, or around it when smoothing is applied. Much can be inferred from knowledge of the plane‐wave coefficients themselves and a simple scalar‐ or acoustic‐wave case without density variation displays the essential features. The theoretical predictions are visualized by smoothed numerical examples obtained using a discrete Fourier transform method. The space‐time form of the reflection function for a low‐contrast boundary is also considered, as is, briefly, the transmission function.
Among the characteristics of R(x, t) are that it is essentially V shaped in the (x, t) plane, with linear arms given by the wave cones for the two media involved. These arms carry signals, where H(t) is the Heaviside step function. The time dependence along x = 0 is also relatively simple, being like the delta derivative δ′(t). At or around the origin R(x, t) is intricate due to the fine balancing of its plane‐wave components. This intricacy makes the corresponding reflection operator capable of analysing an arbitrary incident wave to give the reflected wave, including such effects as total reflection and head waves and the reflection of short horizontal wavelength evanescent waves due to a point source very close to the interface.
The results provide a baseline for understanding common‐image gathers used in migration, including those associated with reverse‐time migration and full‐waveform inversion. Clarity about the space‐time form of the reflection operator will hopefully advance the goal of unifying imaging, amplitude‐versus‐offset analysis and waveform inversion. Application to the time‐domain numerical modelling of waves is also conceivable.
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The form of the reflection operator and extended image gathers for a dipping plane reflector in two space dimensions
By C.J. ThomsonABSTRACTThe reflection operator for a simple flat‐lying interface can be thought of as the set of all its plane‐wave reflection coefficients or as the set of virtual surveys with sources and receivers along the interface. When there is dip, however, it is necessary to include the varying effects of propagation between the virtual‐survey level and the interface.
Hence, step one in this paper is to derive the reflection operator for a dipping plane interface as observed at a datum level some distance away. The key assumption is that the aperture at the datum level is sufficient to characterize the reflector properties around a particular point. This translates into an assumption that the dip is moderate, though no explicit small‐angle approximation is required.
The second step is to find the apparent reflection operator that would relate data that have been extrapolated from the datum towards and possibly beyond the reflector using an assumed migration velocity. This apparent reflection operator is closely related to extended common‐image gathers. The apparent reflection operator may be analysed asymptotically in terms of rays and other signals, shedding light on the structure of extended image gathers.
In keeping with the virtual‐survey idea, the results are considered in a subsurface space‐time or slowness‐time domain at various extrapolation levels around the interface. An important distinction is drawn between using subsurface midpoint‐offset coordinates and the wavefield coordinates of the incident and reflected waves. The latter reveal more clearly the effects of dip, because they lead to a more asymmetric apparent reflection operator. Properties such as an up‐dip shift of a traveltime minimum and its associated curvature theoretically provide information about the reflector location and dip and the migration‐velocity error.
The space‐time form of the reflection operator can be highly intricate around the offset‐time origin and it was described for a simple flat interface in a background paper. To avoid a layer of mathematics, the reflection‐operator formulas presented here are in the intermediate space‐frequency domain. They are analysed by considering their stationary‐phase and branch‐point high‐frequency contributions.
There is no Born‐like assumption of weak reflector contrast and so wide‐angle, total reflection and head‐wave effects are included. Snell’s law is an explicit part of the theory. It is hoped that the work will therefore be a step towards the goal of unifying amplitude‐versus‐offset, imaging and waveform inversion.
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Acoustic emissions associated with the formation of fracture sets in sandstone under polyaxial stress conditions‡
Authors M.S. King, W.S. Pettitt, J.R. Haycox and R.P. YoungABSTRACTA polyaxial (true‐triaxial) stress‐loading system, developed originally for determining all nine components of P‐ and S‐wave velocities and attenuation and fluid permeability for 50.8 mm‐side cubic rock specimens tested to failure, has been modified to permit the measurement of acoustic emission events associated with the failure process. Results are reported for Crosland Hill sandstone tested to failure under loading conditions leading to the formation of sets of aligned microcracks, achieved by maintaining the minor principal stress at a low value while increasing the two other principal stresses until failure of the rock. An ultrasonic survey associated with the test has been employed to map the transversely‐isotropic velocity structure created by through‐going parallel fractures resulting from the sets of aligned microcracks. This velocity structure has then been employed to locate acoustic emission events recorded during the test by four acoustic emission sensors located in each of the six specimen loading platens. A selection of acoustic emission events associated with one of the fractures has been processed for moment tensor analysis information, in order to determine the source type and orientation of microcracking as the fracture grows. The mechanisms indicate tensile behaviour during initial fracture propagation. Shear failure, however, appears to dominate as the fracture finally approaches the opposite face of the cubic specimen. The work presented here has, in part, led to the development of new rock testing systems and geophysical monitoring and processing technologies that will enable laboratory study of rock behaviour under conditions better resembling those experienced in situ.
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Thomsen's parameters from p‐wave measurements in a spherical sample‡
Authors Andrej Bóna, Dariush Nadri and Miroslav BrajanovskiABSTRACTThe aim of this paper is to understand the seismic anisotropy of the overburden shale in an oilfield in the North West Shelf of Western Australia. To this end, we first find the orientation of the symmetry axis of a spherical shale sample from measurements of ultrasonic P‐wave velocities in 132 directions at the reservoir pressure. After transforming the data to the symmetry axis coordinates, we find Thomsen's anisotropy parameters δ and ɛ using these measurements and measurements of the shear‐wave velocity along the symmetry axis from a well log. To find these anisotropy parameters, we use a very fast simulated re‐annealing algorithm with an objective function that contains only the measured ray velocities, their numerical derivatives and the unknown elasticity parameters. The results show strong elliptical anisotropy in the overburden shale. This approach produces smaller uncertainty of Thomsen parameter δ than more direct approaches.
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Vp‐Vs relationship and amplitude variation with offset modelling of glauconitic greensand‡
Authors Zakir Hossain, Tapan Mukerji and Ida L. FabriciusABSTRACTThe relationship between Vp and Vs may be used to predict Vs where only Vp is known. Vp/Vs is also used to identify pore fluids from seismic data and amplitude variation with offset analysis. Theoretical, physical, as well as statistical empirical Vp‐Vs relationships have been proposed for reservoir characterization when shear‐wave data are not available. In published work, the focus is primarily on the Vp‐Vs relationship of quartzitic sandstone. In order to broaden the picture we present Vp‐Vs relationships of greensand composed of quartz and glauconite by using data from the Paleocene greensand Nini oil field in the North Sea. A Vp‐Vs relationship derived from modelling is compared with empirical Vp‐Vs regressions from laboratory data as well as from log data. The accuracy of Vs prediction is quantified in terms of root‐mean‐square error. We find that the Vp‐Vs relationship derived from modelling works well for greensand shear‐wave velocity prediction. We model the seismic response of glauconitic greensand by using laboratory data from the Nini field. Our studies here reveal that brine‐saturated glauconitic greensand can have a similar seismic response to that from oil‐saturated quartzitic sandstone and that oil‐saturated strongly cemented greensand can have a similar amplitude variation with offset response to that from brine‐saturated weakly cemented greensand.
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Application of 3D vertical seismic profile multi‐component data to tight gas sands‡
Authors Yousheng Yan, Zengkui Xu, Mingli Yi and Xin WeiABSTRACTDue to the complicated geophysical character of tight gas sands in the Sulige gasfield of China, conventional surface seismic has faced great challenges in reservoir delineation. In order to improve this situation, a large‐scale 3D‐3C vertical seismic profiling (VSP) survey (more than 15 000 shots) was conducted simultaneously with 3D‐3C surface seismic data acquisition in this area in 2005. This paper presents a case study on the delineation of tight gas sands by use of multi‐component 3D VSP technology. Two imaging volumes (PP compressional wave; PSv converted wave) were generated with 3D‐3C VSP data processing. By comparison, the dominant frequencies of the 3D VSP images were 10–15 Hz higher than that of surface seismic images. Delineation of the tight gas sands is achieved by using the multi‐component information in the VSP data leading to reduce uncertainties in data interpretation. We performed a routine data interpretation on these images and developed a new attribute titled ‘Centroid Frequency Ratio of PSv and PP Waves’ for indication of the tight gas sands. The results demonstrated that the new attribute was sensitive to this type of reservoir. By combining geologic, drilling and log data, a comprehensive evaluation based on the 3D VSP data was conducted and a new well location for drilling was proposed. The major results in this paper tell us that successful application of 3D‐3C VSP technologies are only accomplished through a synthesis of many disciplines. We need detailed analysis to evaluate each step in planning, acquisition, processing and interpretation to achieve our objectives. High resolution, successful processing of multi‐component information, combination of PP and PSv volumes to extract useful attributes, receiver depth information and offset/ azimuth‐dependent anisotropy in the 3D VSP data are the major accomplishments derived from our attention to detail in the above steps.
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On the computation of the electrical potential inside a horizontally‐layered half‐space
Authors Alexis Maineult and Jean‐Jacques SchottABSTRACTA new formulation is proposed for the electrical potential developed inside a horizontally‐layered half‐space for a direct current point‐source at the surface. The recursion formula for the kernel coefficient in the potential integral is simpler than the generally used two‐coefficient recursion. The numerical difficulties that may occur during the computation of the integrals and near the source axis are examined and solutions are proposed. The set of equations permits a stable and accurate computation of the tabular potential everywhere in the medium.
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Gravity field and tectonic features of Block L2 in the Lamu basin, Kenya
Authors Bingqiang Yuan, Wensheng Xie, Guihe Liu and Chunguan ZhangABSTRACTA gravity survey on the scale of 1: 250 000 was carried out in Block L2 located in the Lamu basin of south‐east Kenya in order to study tectonic features and find out favourable petroleum prospects in the block. This paper, through data processing and synthetic interpretation of the measured gravity data in the block, discusses characteristics of the gravity field and their geological implications, determines the fault system and the basement depth, analyses features of the main strata, divides structure units and predicts favourable petroleum zones. In the block, the regional gravity anomaly is mainly caused by the inclined Moho surface that rises in the east and subsides in the west topographically and the Bouguer gravity anomaly primarily reflects the superimposition of the gravity effect derived from the Moho surface and the basement relief. Two groups of faults extending NW (NWW) and NE (NEE) respectively are dominant in the block and their activities resulted in the framework of east‐west zoning and south‐north blocking. The basement depth greatly changes in an alternative high and low pattern. The Permian‐Triassic, Jurassic and Tertiary strata are extensively developed, while the Cretaceous is only developed in the east of the block. Structurally, the block can be divided into five units, of which the Tana sag shows excellent source‐reservoir‐seal associations and is a favourable target for future petroleum exploration.
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A data variance technique for automated despiking of magnetotelluric data with a remote reference
More LessABSTRACTThe magnetotelluric method employs co‐located surface measurements of electric and magnetic fields to infer the local electrical structure of the earth. The frequency dependent ‘apparent resistivity’ curves can be inaccurate at long periods if input data are contaminated – even when robust remote reference techniques are employed. Data despiking prior to processing can result in significantly more reliable estimates of long period apparent resistivities. This paper outlines a two‐step method of automatic identification and replacement for spike‐like contamination of magnetotelluric data; based on the simultaneity of natural electric and magnetic field variations at distant sites. This simultaneity is exploited both to identify windows in time when the array data are compromised as well as to generate synthetic data that replace observed transient noise spikes. In the first step windows in data time series that contain spikes are identified according to an intersite comparison of channel ‘activity’– such as the variance of differenced data within each window. In the second step, plausible data for replacement of flagged windows are calculated by Wiener filtering coincident data in clean channels. The Wiener filters – which express the time‐domain relationship between various array channels – are computed using an uncontaminated segment of array training data. Examples are shown where the algorithm is applied to artificially contaminated data and to real field data. In both cases all spikes are successfully identified. In the case of implanted artificial noise, the synthetic replacement time series are very similar to the original recording. In all cases, apparent resistivity and phase curves obtained by processing the despiked data are much improved over curves obtained from raw data.
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Geophones on blocks: a prototype towable geophone system for shallow land seismic investigations
Authors Rui M. Moura and Manuel J. Senos MatiasABSTRACTIn recent years some authors have given a certain amount of attention to towed seismic reflection acquisition systems. Based on some of these works we sought to design and test a system making use of currently available geophones instead of specifically designed sensors as employed in some previous works. Thus, bearing in mind that the geophone's characteristics are achieved in the conditions that they are originally designed to be employed in, i.e., coupled with a spike driven into the ground, we devoted our attention to some of the variables involved in the geophone's performance, namely the total weight, the effect of a spikeless geophone and the surfaces on which the geophone is placed. Previously, we had experimentally verified some variations in the signal response due to coupling geophones in different surface materials, such as hard soil, asphalt and concrete pavement and we noticed that these surface materials were in fact an important factor in the overall response. Hence, these materials, or as we also called them coupling agents, could be employed as a base material in the construction of a mobile seismic acquisition device composed of blocks of a certain size, on which the geophone would be then inserted and thus making it into a spikeless surface towable system. Therefore, various materials were tested in order to select one that could maintain a similar fidelity to that of the spike coupled geophone and thus contribute towards building a more time efficient and towable geophone and block system. Pinging tests revealed variations in the coupling frequency and damping characteristics of each coupling agent and from all of these tested materials one was selected for field comparative tests with the normally planted geophones with spike coupling. Finally a seismic reflection profile was acquired simultaneously with both systems, i.e., spike coupling versus cement block coupled geophones.
This field test showed similar results in terms of signal levels and frequency content and therefore it was possible to observe the presence of the same reflectors and other seismic events in either of the time sections.
With this experiment we propose not only a system that allows a time efficient seismic field operation but we also aim to encourage more research into the response dependency of the coupling agent of which the towable base block is made of.
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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)