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- Volume 47, Issue 6, 1999
Geophysical Prospecting - Volume 47, Issue 6, 1999
Volume 47, Issue 6, 1999
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Local and controlled prestack depth migration in complex areas [Link]
More LessPrestack depth migrations based on wavefield extrapolation may be computationally expensive, especially in 3D. They are also very dependent on the acquisition geometry and are not flexible regarding the geometry of the imaging zone. Moreover, they do not deal with all types of wave, considering only primary reflection events through the model. Integral approaches using precalculated Green's functions, such as Kirchhoff migration and Born‐based imaging, may overcome these problems. In the present paper, both finite‐difference traveltimes and wavefront construction are used to obtain asymptotic Green's functions, and a generalized diffraction tomography is applied as an example of Born‐based acoustic imaging. Target‐orientated imaging is easy to perform, from any type of survey and subselection of shot/receiver pairs. Multifield imaging is possible using Green's functions that take into account, for instance, reflections at model boundaries. This may help to recover parts of complex structures which would be missing using a paraxial wave equation approach. Finally, a numerical evaluation of the resolution, or point‐spread, function at any point of the depth‐migrated section provides valuable information, either at the survey planning stage or for the interpretation.
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Improvement of MT data processing using stationary and coherence tests [Link]
By LamarqueThe great majority of data processing methods for magnetotelluric measurements are based on an average Fourier spectral analysis to estimate either the transfer function or the coherence function. This assumes that the time‐series data are stationary. The aim of this paper is to present a preprocessing method able to extract the stationary and coherent part of the original signals. The practitioner can then apply the magnetotelluric method of his choice to these new data. This preliminary data sorting is done in four steps: (i) slow drifts are eliminated using a high‐pass filter whose cut‐off frequency is determined by an iterative procedure. Based on run tests, this procedure is also able to remove segments with non‐independent samples in the time series; (ii) non‐stationary segments are eliminated after band‐pass filtering; (iii) non‐coherent segments are eliminated before spectral analysis; (iv) the impedance tensor value is then retained, at a given frequency, only if the signals are coherent. This preprocessing method was tested on the simplest, but still used, magnetotelluric method which uses only two field components, and it was found that the average resistivity standard deviation decreased significantly from 14.6 Ωm without sorting to 8.6 Ωm after sorting.
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Marine seismic wavefield measurement to remove sea‐surface multiples [Link]
Authors Ziolkowski, Taylor and JohnstonWe propose a new method for removing sea‐surface multiples from marine seismic reflection data in which, in essence, the reflection response of the earth, referred to a plane just above the sea‐floor, is computed as the ratio of the plane‐wave components of the upgoing wave and the downgoing wave. Using source measurements of the wavefield made during data acquisition, three problems associated with earlier work are solved: (i) the method accommodates source arrays, rather than point sources; (ii) the incident field is removed without simultaneously removing part of the scattered field; and (iii) the minimum‐energy criterion to find a wavelet is eliminated.
Pressure measurements are made in a horizontal plane in the water. The source can be a conventional array of airguns, but must have both in‐line and cross‐line symmetry, and its wavefield must be measured and be repeatable from shot to shot. The problem is formulated for multiple shots in a two‐dimensional configuration for each receiver, and for multiple receivers in a two‐dimensional configuration for each shot. The scattered field is obtained from the measurements by subtracting the incident field, known from measurements at the source. The scattered field response to a single incident plane wave at a single receiver is obtained by transforming the common‐receiver gather to the frequency–wavenumber domain, and a single component of this response is obtained by Fourier transforming over all receiver coordinates. Each scattered field component is separated into an upgoing wave and a downgoing wave using the zero‐pressure condition at the water‐surface. The upgoing wave may then be expressed as a reflection coefficient multiplied by the incident downgoing wave plus a sum of scattered downgoing plane waves, each multiplied by the corresponding reflection coefficient. Keeping the upgoing scattered wave fixed, and using all possible incident plane waves for a given frequency, yields a set of linear simultaneous equations for the reflection coefficients which are solved for each plane wave and for each frequency. To create the shot records that would have been measured if the sea‐surface had been absent, each reflection coefficient is multiplied by complex amplitude and phase factors, for source and receiver terms, before the five‐dimensional Fourier transformation back to the space–time domain.
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Interpretation of shallow refraction seismic data by reflection/refraction tomography [Link]
Authors Pavel Ditmar, Jan Penopp, Rainer Kasig and Jannis MakrisA new algorithm for tomographic inversion of traveltimes of reflected and refracted seismic waves is developed. The inversion gives interface configurations and velocity distributions in layers. The important features of the algorithm are: (a) the inclusion of shot time delays in the list of unknown parameters; (b) the regularization is applied in such a way that the most probable model is characterized by the similarity of neighbouring interfaces. As the problem under consideration is non‐linear, several iterations are necessary in order to obtain the final model. In the case of a very inexact initial model, a ‘layer‐by‐layer’ inversion strategy is recommended as a first inversion step. The inversion program is supplied with a user interface, thanks to which one can: (a) pick interactively and identify seismic traveltimes; (b) build and edit depth/velocity models; and (c) display calculated traveltime curves and compare them with picked traveltimes as well as with the original seismic sections. The efficiency of the inversion software developed is illustrated by a numerical example and a field example in which shallow seismic data are considered. Application to wide‐aperture reflection/refraction profiling (WARRP) data is also possible.
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A multichannel approach to long‐period multiple prediction and attenuation [Link]
Authors Rosenberger, Meyer and ButtkusA method is presented for the prediction and attenuation of long‐period water‐layer multiples based on an adaptive multichannel lattice algorithm. The multichannel approach can be viewed as a generalization of two‐dimensional linear prediction. The multichannel linear least‐squares prediction problem is reviewed briefly and the performance and limitations of the algorithm are demonstrated on two different marine data sets with different properties of the simple and peg‐leg multiple system. The algorithm works well even on problematic data sets and is very easy to apply.
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Surface NMR within a geophysical study of an aquifer at Haldensleben (Germany) [Link]
Authors Ugur Yaramanci, Gerhard Lange and Klaus KnödelThe surface nuclear magnetic resonance (SNMR) method has been tested at a site in Haldensleben, northern Germany, to assess the suitability of this new method for groundwater exploration and environmental investigations. More information is obtained by SNMR, particularly with respect to aquifer parameters, than with other geophysical techniques. SNMR measurements were carried out at three borehole locations, together with 2D and 1D direct current geoelectrics, as well as ground‐penetrating radar, and well logging (induction log, gamma‐ray log and pulsed neutron‐gamma log). Permeabilities were calculated from the grain‐size distributions of core material determined in the laboratory. It is demonstrated that the SNMR method is able to detect groundwater and the results are in good agreement with other geophysical and hydrogeological data. Using the SNMR method, the water content of the unsaturated and saturated zones (i.e. porosity of an aquifer) can be reliably determined. This information and resistivity data permit in situ determination of other aquifer parameters. Comparison of the SNMR results with borehole data clearly shows that the water content determined by SNMR is the free or mobile water in the pores. The permeabilities estimated from the SNMR decay times are similar to those derived from sieve analysis of core material. Thus, the combination of SNMR with geoelectric methods promises to be a powerful tool for studying aquifer properties.
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Grounded‐source TEM modelling of some deep‐seated 3D resistivity structures [Link]
By PoddarLong‐offset transient electromagnetics (LOTEM) is now regarded as a suitable electrical method for deep exploration along with magnetotellurics (MT). In this method, the vertical magnetic‐field impulse response and, occasionally, the horizontal electric‐field step response of a grounded‐wire source on the surface of the earth are measured. Here, these two responses are computed for 3D models of three deep resistivity structures of interest in hydrocarbon exploration: (i) a faulted graben in a resistive basement rock at a depth of 4 km beneath a conductive overburden; (ii) a facies change in a resistive layer buried at a depth of 2 km in the conductive overburden above a resistive basement; and (iii) an anticlinal uplift of a resistive layer at a depth of 1 km in the conductive overburden above a resistive basement. The results show that the sensitivity of the electric‐field response to model perturbation is generally greater than that of the magnetic‐voltage response. Further, the electric‐field sensitivity is confined to early and intermediate times while that of the magnetic‐voltage response is confined to intermediate and late times. Hence it is recommended that both electric and magnetic recordings are made in a LOTEM survey so that the final results can be presented as apparent‐resistivity curves derived from the two responses jointly as well as separately.
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Analysis of Thomsen parameters for finely layered VTI media [Link]
Authors James G. Berryman, Vladimir Y. Grechka and Patricia A. BergeSince the work of Postma and Backus, much has been learned about elastic constants in vertical transversely isotropic (VTI) media when the anisotropy is due to fine layering of isotropic elastic materials. Nevertheless, there has continued to be some uncertainty about the possible range of Thomsen's anisotropy parameters ε and δ for such media. We use both Monte Carlo studies and detailed analysis of Backus' equations for both two‐ and three‐component layered media to establish the results presented. We show that ε lies in the range −3/8 ε ½[〈v2p〉〈v−2p〉−1], for finely layered media; smaller positive and all negative values of ε occur for media with large fluctuations in the Lamé parameter λ in the component layers. We show that δ can also be either positive or negative, and that for constant density media, sign (δ) = sign (〈v−2p〉 − 〈v−2s〉〈v2s/v2p〉). Monte Carlo simulations show that among all theoretically possible random media, positive and negative δ are equally likely in finely layered media. (Of course, the δs associated with real earth materials may span some smaller subset of those that are theoretically possible, but answering this important question is beyond our present scope.) Layered media having large fluctuations in λ are those most likely to have positive δ. This is somewhat surprising since ε is often negative or a small positive number for such media, and we have the general constraint that ε − δ > 0 for layered VTI media. Since Gassmann's results for fluid‐saturated porous media show that the mechanical effects of fluids influence only the Lamé parameter λ, not the shear modulus μ, these results suggest that small positive δ occurring together with small positive ε (but somewhat larger than δ) may be indicative of changing fluid content in a layered earth.
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Gravity changes and natural gas extraction in Groningen
Authors Martin Van Gelderen, Roger Haagmans and Mirjam BilkerGravity observations made over the Groningen gasfield from 1978 onwards are available. The existing gravity data were reanalysed and a new survey was carried out. The observed gravity changes obtained during four surveys, spanning an 18‐year period, were compared with the gravity effect due to natural gas extraction computed from the reservoir model and the production data. The random error in the gravity values is small enough to detect the effect of gas extraction after a few years (up to 2 μgal/year). The trends obtained from observed and calculated gravity changes agree well within their expected error margins after statistical data snooping. Due to the inadequate measurement set‐up of the previous surveys, systematic errors present in the gravity data hampered the use of this data for refinement of the reservoir and the production models. However, with improved gravity monitoring, in particular a very systematic network set‐up and well‐calibrated instruments, this should now be feasible. By using the network analysis from the 1996 measurements for the planning of a network in space and time, most of the current problems can be avoided.
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Geophone orientation and coupling in three‐component sea‐floor data: a case study [Link]
Authors Xiang‐Yang Li and Jianxin YuanWe analyse the geophone orientation and coupling in a data set from the North Sea. Based on the polarization of the water‐break on the sea‐floor, we have derived processing algorithms for determining the receiver orientation for gimballed and non‐gimballed geophone systems. For a gimballed system, the problem reduces to a simple horizontal rotation. However, for a non‐gimballed system, where all three geophone axes may vary due to varying acquisition conditions such as dipping sea‐floor, twisting of recording cable, etc., the three orientation angles cannot be found directly from the recorded displacement vectors. Using the data redundancy within a common‐receiver gather, a robust two‐stage method is derived for the non‐gimballed system in which all three orientations can initially be unknown. Testing on the North Sea data set acquired with a gimballed system shows that the three‐component geophones in the data set are orientated satisfactorily within an error of 5°. However, there are some undesirable cross‐couplings between the vertical and horizontal geophones, which results in leakage of shear‐wave energy from the horizontal components to the vertical components.
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GPR modelling by the Fourier method: improvement of the algorithm [Link]
More LessWe improve two aspects of the modelling scheme for the simulation of electromagnetic radio waves, based on the Fourier pseudospectral method.
When there are large contrasts in the material properties, use of the standard algorithm (regular grid) causes a series of artefacts, as, for instance, ringing and acausal events. These problems, due to the non‐locality of the differential operator, are solved by using the staggered Fourier method (staggered grid).
Realistic radiation patterns can be obtained from simple combinations of magnetic and electric sources. If the directivity pattern of the antenna is known, from either a finite‐difference simulation or an analytic evaluation or an experimental characterization, it can then be simulated by a composite‐source concept. This effective source is implemented in the modelling algorithm by means of a perturbation technique, which first computes the intensity and directional spectra of the single electromagnetic sources. Their location is optimized to obtain the best fit with a minimum number of sources. The approach is, in principle, valid for the far‐field radiation pattern of the antenna.
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Fracture‐frequency prediction from borehole wireline logs using artificial neural networks [Link]
Authors Elaine M. FitzGerald, Christopher J. Bean and Ronan ReillyBorehole‐wall imaging is currently the most reliable means of mapping discontinuities within boreholes. As these imaging techniques are expensive and thus not always included in a logging run, a method of predicting fracture frequency directly from traditional logging tool responses would be very useful and cost effective. Artificial neural networks (ANNs) show great potential in this area. ANNs are computational systems that attempt to mimic natural biological neural networks. They have the ability to recognize patterns and develop their own generalizations about a given data set. Neural networks are trained on data sets for which the solution is known and tested on data not previously seen in order to validate the network result. We show that artificial neural networks, due to their pattern recognition capabilities, are able to assess the signal strength of fracture‐related heterogeneity in a borehole log and thus fracture frequency within a borehole. A combination of wireline logs (neutron porosity, bulk density, P‐sonic, S‐sonic, deep resistivity and shallow resistivity) were used as input parameters to the ANN. Fracture frequency calculated from borehole televiewer data was used as the single output parameter. The ANN was trained using a back‐propagation algorithm with a momentum learning function. In addition to fracture frequency within a single borehole, an ANN trained on a subset of boreholes in an area could be used for prediction over the entire set of boreholes, thus allowing the lateral correlation of fracture zones.
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On the aperture effect in 3D Kirchhoff‐type migration
By Jianguo SunIt is well known that the migrated image given by a Kirchhoff‐type (diffraction‐stack) migration with limited aperture is always accompanied by some events which depend on the migration aperture. Although these events may severely affect the quality of migration, they have been studied only in 2D cases. Here, the events due to the migration aperture in 3D situations are investigated using a new method of analysing the reconstructed wavefield. It is found that a finite migration aperture results in a reconstructed wavefield with two components. One comes from the tangent points and curves between the traveltime surfaces of reflected and point‐diffracted rays and is independent of the migration aperture, and the other is from the boundary of the migration aperture and depends strongly on the location and size as well as on the shape of the migration aperture. It is this last component that describes the aperture effect in migration. If the migration aperture is not sufficiently large, and if the input for migration is not zero on the boundary of the migration aperture, the boundary component may partially or totally cancel the migration signal. Furthermore, for synthetic data, the aperture effect cannot be eliminated by enlarging the migration aperture because, except for the common‐shotpoint data, the aperture effect always exists however large the migration aperture becomes. This leads to the conclusion that the published Kirchhoff‐type operators are not the exact inverse operators of the Fresnel–Kirchhoff integral if the input data are synthetic.
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Multifold borehole radar acquisition and processing [Link]
Authors Fabrice Hollender, Sylvie Tillard and Laurent CorinThe multifold acquisition principle was applied to a borehole radar survey, performed in a granitic site (Grimsel Test Site, Switzerland). Two multifold coverage acquisitions (40‐fold and 20‐fold) were carried out in a subhorizontal borehole. Instrumental drifts (transmission time and sampling frequency fluctuations) were corrected in order to remove shifts observed on CMP gathers and to optimize velocity analysis and trace stacking. Computation of velocity spectra was adapted in order to take into account the features of the medium investigated (homogeneous velocity, various reflector orientations). The NMO velocities were then interpreted as angles between reflectors and the survey line. The processing, based on the computation of several constant velocity stacked sections performed with different NMO velocities, leads to better results than the standard DMO + NMO processing. The signal‐to‐noise ratio of the stacked profile is improved in comparison with the single‐fold section, which results from a standard acquisition. From a practical point of view, the implementation of a multifold radar survey within a borehole is difficult but a greater investigation range is obtained, more reflectors are detected and the mapping of geological discontinuities is improved.
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Use of magnetotelluric signals from 50 Hz power lines for resistivity mapping of geothermal fields in New Zealand [Link]
More LessElectromagnetic (EM) fields radiated from the transmission lines of the New Zealand electricity grid have been digitally recorded at test sites near the Tokaanu geothermal field. Amplitudes and phases of the 50 Hz signals (and the odd harmonics up to 450 Hz) were determined using a software implementation of a phase‐locked filter. These data were then analysed to determine the components of the magnetotelluric impedance tensor and the corresponding apparent resistivities and phases. At most sites, there was sufficient variation in the elliptical polarization of the EM fields to enable the impedance tensors to be determined in full. Sites where the EM data had been affected by near‐source effects were identified by having large vertical magnetic field components and by being closer to a power line source than about 3–5 skin depths. With the test measurements, the north‐eastern part of the Tokaanu geothermal field was successfully delineated giving low resistivities (< 5 Ωm) on the inside and higher resistivities on the outside, in agreement with the Schlumberger array DC apparent resistivities. The small size of the 50 Hz magnetotelluric equipment and its portable nature make this method of resistivity measurement suitable for reconnaissance resistivity mapping in places with difficult access.
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Source/receiver array directivity effects on marine seismic attenuation measurements
Authors Bernhard Hustedt and Roger A. ClarkAttenuation of seismic waves, quantified by the seismic quality factor Q, holds important information for seismic interpretation, due to its sensitivity to rock and fluid properties. A recently published study of Q, based on surface seismic reflection data, used a modified spectral ratio approach (QVO), but both source and receiver responses were treated as isotropic, based on simple raypath arguments. Here, this assumption has been tested by computing apparent attenuation generated by frequency‐dependent directivity of typical marine source and receiver arrays and acquisition geometries. Synthetic wavelet spectra were computed for reflected rays, summed over the first Fresnel zone, from the base of a single interval, 50–3000 m thick and velocity 2000 m/s, overlying a 2200 m/s half‐space, and for offsets of 71–2071 m. The source and receiver geometry were those of an actual survey.
The modelled spectra are clearly affected by directivity, most strongly because of surface ghosts. In general, the strong high‐frequency component, produced by the array design, leads to apparently negative attenuation in individual reflection events, though this is dependent on offset and target depth. For shallow targets (less than 400–500 ms two‐way traveltime (TWT) depth), apparent Q‐values as extreme as −50 to −100 were obtained. For deeper target depths, the directivity effect is far smaller. The implications of the model study were tested on real data. QVO was applied to 20 true‐spectrum‐processed CMPs, in a shallow (405–730 ms TWT) and a deeper (1000–1300 ms TWT) interval, firstly using a measured far‐field source signature (effectively isotropic), and secondly using computed directivity effects instead. Mean interval Q−1‐values for the deeper interval, 0.029 ± 0.011 and 0.027 ± 0.018 for conventional and directional processing, respectively, suggested no directivity influence on attenuation estimation. For the shallow interval (despite poor spectral signal‐to‐noise ratios and hence scattered attenuation estimates), directional processing removed directivity‐generated irregularities from the spectral ratios, resulting in an improvement from Q−1int = −0.036 ± 0.130 to a realistic Q−1int = 0.012 ± 0.030: different at 94% confidence level. Equivalent Q‐values are: for the deeper interval, 35 and 37 for conventional and directional processing, respectively, and −28 and 86 for the shallow interval.
These results support the conclusions of the model studies, i.e. that source/receiver directivity has a negligible effect except for shallow targets (e.g. TWT depth ≤ 500 ms) imaged with conventional acquisition geometry. In such cases directivity corrections to spectra are strongly recommended.
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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)