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- Volume 26, Issue 1, 1978
Geophysical Prospecting - Volume 26, Issue 1, 1978
Volume 26, Issue 1, 1978
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THE INFLUENCE OF OBLIQUELY DIPPING DISCONTINUITIES ON THE USE OF RAYLEIGH CHANNEL WAVES FOR THE IN‐SEAM SEISMIC REFLECTION METHOD *
Authors S. FREYSTÄTTER and L. DRESENAbstractThe geological sequence rock‐coal‐rock represents a seismic low‐velocity channel. Channel waves generated in a coal seam and their reflections from discontinuities can be used for proving the minability of the seam. To investigate the process of reflection, two‐dimensional models of the sequence rock‐coal‐rock have been investigated by means of the ultrasonic transducer technique. Two‐dimensional models have the advantage that the wave field can be observed at an arbitrarily chosen point of the model plate. Thus, by means of these models the direct and the reflected Rayleigh channel wave can be observed along their path of propagation.
From the geophysical point of view the various types of discontinuities of a coal seam can be divided into two basic types: one is restricted to the seam, the other includes in addition a fractured zone in the adjoining rock.
The investigation of the symmetrical Rayleigh channel wave reflected by a discontinuity yields the following results:
For dip angles γ between approximately 90° and approximately 60° the reflectivity is virtually independent of the type of discontinuity. This does not hold for the limiting case of γ= 90° (vertical dip) for which the reflectivity increases with increasing influence of the fractured zone. For dip angles γ between approximately 60° and approximately 40° the reflectivity is still independent of the fractured zone but the shape of the reflected wave deteriorates with increasing influence of the fractured zone.
For dip angles γ below approximately 40° the reflected wave deteriorates such that the application of the in‐seam seismic reflection method will be difficult or even impossible.
The conversion of the direct wave of the symmetrical fundamental mode into a reflected wave of the antisymmetrical fundamental mode has been observed.
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THE PROPAGATION OF ATTENUATED SH CHANNEL WAVES*
More LessAbstractThe problem of detecting discontinuities which interrupt coal seams is of great importance to the coal mining industry. One possible method of detecting such discontinuities is that of in‐seam seismology where both source and detectors are placed under‐ ground on the coal face. In this paper the propagation of SH waves from a line source in the seam is investigated. There exists a particular set of waves–channel waves–, confined to the coal seam. These waves are dispersive and have an associated Airy phase whose geometrical attenuation is least. However, if absorption of energy within the coal is included then the Airy phase is the dominant wavegroup only for distances less than a certain maximum. If a detection criterion proposed by Dresen and Freystätter applies, then there is a lower limit to the frequency that should be used for exploration. This requirement may be in conflict with attenuation considerations.
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INVESTIGATION OF THE SEISMIC RESPONSE OF CYCLICALLY LAYERED CARBONIFEROUS ROCK BY MEANS OF SYNTHETIC SEISMOGRAMS *
Authors H. RÜTER and R. SCHEPERSAbstractSeismic exploration for coal as well as basic scientific research indicate the existence of unsolved problems. These problems arise partly because the requirements are different from those in exploration for gas and oil and are partly due to the geological situation. The medium to be investigated is composed of cyclically changing layers with extremely high velocity and density contrast. Furthermore, the structure of the carboniferous rock is highly fractured and folded. This leads to difficulties in interpreting the seismic response of carboniferous rock. To overcome these difficulties synthetic seismograms are a useful tool.
Calculating synthetic seismograms the carboniferous rock has been modelled as a sequence of seams and rock. The following results have been obtained
- (i) A single seam gives rise to a distinct reflection signal even for a thickness of 1/50 of the wavelenght.
- (ii) Individual reflections are not visible from a sequence of layers containing a great number of seams and interfaces. Due to constructive interference only a few high amplitudes appear. These high amplitudes are labelled “interference reflections”.
- (iii) With increasing travel time the interference reflections are mainly composed of short lag multiples such that the primary reflections have no significant influence.
- (iv) The sequence of seams acts on the reflected seismic signal as a high pass filter and on the transmitted signal as a low pass filter. The cut‐off frequencies are determined by the average seam thickness, and the steepness of the slopes increases with increasing number of seams.
- (v) The interference reflections can be used for determining the geological structure at least for the upper part of the sequence.
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TRANSFORMATION OF TIME‐DISTANCE CURVES OF DIVING WAVES INTO A MULTILAYERED MODEL*
By J. D. LASKIAbstractA method to compute the parameters of a multilayered medium with dipping interfaces is described. The input data are time‐distance curves of diving waves. The output data are the thickness of the layers, dips, and parameters characterizing the increase of velocity with depth. Three examples are presented.
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REFLECTIONS AND REFRACTIONS FROM CURVED INTERFACES: MODEL STUDY *
By S. K. GUPTAAbstractReflections and refractions from curved interfaces were studied on two dimensional scale models. Time of arrival, amplitude and character of reflected, converted, and refracted waves were mainly used for this study. Some reflected refractions, refracted reflections and diffractions were also considered. It was possible to separate PS and SP waves and to study their amplitude and character separately. From the amplitude study of refracted arrivals it was concluded that the refracted ray path penetrates into the high velocity layer rather than propagating along the interface. Although most of the results are interesting from the theoretical point of view, a few applications to exploration problems are suggested.
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SOME APPLICATIONS OF IP‐TECHNIQUE FOR DIFFERENT GEOPHYSICAL PROSPECTING PURPOSES *
By L. MALMQVISTAbstractThe usefulness of IP‐technique for different geophysical purposes is discussed on the basis of field examples. IP and resistivity results are not always correlated, and the IP‐information may be unique in prospecting for very low grade sulphide mineralizations. The IP‐technique seems to be useful sometimes in prospecting for disseminated Ni‐Cu‐ mineralizations. A massive sulphide mineralization could be localized using IP.
The disturbances from different kinds of geological objects other than prospecting targets generally are smaller in IP‐work than in resistivity work. It was possible to discriminate between apparent resistivity anomalies caused by shear zones and by sulphide mineralizations by means of IP‐measurements. The influence of magnetite on IP‐ measurements is briefly discussed. The relation between ground magnetic measurements ΔZ and the IP‐parameter is in many areas weak and does not seriously influence the interpretation of the IP‐measurements. A high correlation between ΔZ and IP‐measurements has been found on a titaniferous iron ore. This ore body gives strong IP‐anomalies indicating that it is a good IP‐target. Finally, the application of small moving arrays in very detailed IP‐measurements is found useful for detailed mapping of a mineralized zone under a thin overburden and for determination of the dip of mineralized zones.
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GEOELEKTRISCHE SONDIERUNGEN IN GRADIENTMEDIEN UND ÄQUIVALENTEN SCHICHTSYSTEMEN *
Authors R. RÖSLER and M. SCHALLARAbstractAnalytical solutions exist for the potential of a Point Source in media with depth dependent conductivity of the form σ(z) =σ0(1 +αz)n.
Of particular importance is the case of the linear gradient (n= 1). More general distributions of conductivity with depth can be approximated by equivalent sequences of homogeneous layers. Comparison between such approximations and the analytical solution for media with linear gradient of conductivity indicates that already a coarse step function yields a reasonably good approximation.
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RESISTIVITY SOUNDING ON A MULTI‐LAYERED EARTH WITH TRANSITIONAL LAYERS. PART II: THEORETICAL AND FIELD EXAMPLES *
By D. PATELLAAbstractThis final part of our study of the resistivity sounding method in the presence of transitional layers deals with the direct application of the theoretical developments, previously reported, to both theoretical and field examples. The former are used to explain in more detail all steps of the direct procedure of interpreting resistivity measurements, when transitional layers with different dispositions are present in the assumed earth section. An alternate sequence of uniform and transitional layers is at first considered, then the case of a random sequence of these layers, and finally a sequence of only transitional layers. Then we give some logical considerations about the best fields of application of the new interpretation theory, with particular reference to hydrogeological, geological‐ structural, and mining exploration problems. Representative field examples are finally presented and discussed in detail for each of the exploration fields above listed. The comparison of the geophysical models obtained with the old interpretative scheme and the new system shows the greater resolving power of the new theory, provided that preliminary geological considerations, and where existing–borehole evidence justify the use of the new approach.
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CORRELATION BETWEEN SEISMIC REFLECTION AMPLITUDE AND WELL PRODUCTIVITY – A CASE STUDY *
By F. HunAbstractThe results of specific processing of seismic sections on an actual oil field have proved that it is possible to find a link between a seismic parameter (amplitude of a reflecting horizon) and petrophysical characteristics of a reservoir whose changes modify well productivity.
A good correlation appears between the values of good oil productivity and the weakness of the amplitudes of one horizon close to the oil producing layers.
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CALCULATION OF VELOCITY FROM SEISMIC REFLECTION AMPLITUDE *
By S. BOISSEAbstractIt is not possible to determine accurate geological velocities from seismic velocity analysis for thin layers or complex structural features, especially under an unconformity. Instead, we can approach the problem of interval velocity with seismic amplitudes analysis and compute the reflection coefficient along the unconformity surface.
An error estimation has been made on a model to test the possibility of such a method and to choose the best parameters to be used. The method has been applied on an actual case: the computed interval velocities show good correlation with the values obtained by a sonic log.
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QUASI‐STATIC ELECTROMAGNETIC FIELDS OF A HORIZONTAL ELECTRICAL DIPOLE ON THE SURFACE OF A POLARIZABLE HALF‐SPACE *
By B. S. RATHORAbstractThe expressions for quasi‐static electromagnetic fields of a horizontal electrical dipole placed on the surface of a polarizable half‐space have been derived for low and high values of induced polarization parameter and presented along with numerical results. It has been observed that the polarity of the mutual impedance function is negative for low values and positive for high values of the induced polarization parameter in the entire time domain, whereas the induced voltage function is positive for low values of induced polarization parameter and becomes negative for high values. In the case of low values of induced polarization parameter the transient electromagnetic field in the beginning increases with time, later on it starts to decay with time. The present study shows that the decay rate of electromagnetic field is highly dependent upon the induced polarization parameter. In the light of the present study the conductivity of the polarizable medium may be determined more accurately.
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ENCODING TECHNIQUES FOR THE VIBROSEIS * SYSTEM **
Authors T. BERNHARDT and J. H. PEACOCKAbstractThe use of certain codes for the Vibroseis system of exploration has been investigated. The codes considered are well‐known in communication theory. The effect of coding the Vibroseis input signal is the suppression of correlation noise and therefore an improvement of the signal to noise ratio.
It is believed that, during the course of research, a new property of one of the codes used has been discovered.
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SIGNATURE AND AMPLITUDE OF LINEAR AIRGUN ARRAYS *
More LessSummaryDuring the last few years many airgun arrays have been designed with the objective of generating a short signature of high amplitude. For linear arrays of non‐interacting airguns two rules have been derived that may help in the design or evaluation of airgun arrays. To achieve a short pressure pulse, the total available air volume has to be distributed over the individual guns in such a way that the tail of the signal, owing to the added bubble signals, becomes as flat as possible.
When we think of ordering airguns according to volume, this flat signal tail can be achieved by designing the volumes such that the difference in bubble times of two adjacent guns is proportional to their volume to the power 2/3.
The amplitude expected from a linear array of non‐interacting airguns is limited by the physical length of the array.
A graph of measured values tends to confirm this relation.
No relation has been found between the total volume of an array and its amplitude.
The graph also detects inefficient use of available array length of existing arrays.
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VERTICAL FREQUENCY EFFECT SOUNDING IN INDUCED POLARIZATION AND GALVANIC RESISTIVITY METHODS *
By A. KEÇELIAbstractIt has been shown that for a polarizable layer as a transmission medium there is an indirect proportionality between the frequency effect (fe) in induced polarization (IP) and the wave number of the electrical induced field. Making use of this relationship for a two layered earth a polarization transform function has been obtained. Since the mathematical expression for the polarization transform function is the same as that of the resistivity transform function, it is possible to make direct interpretation for IP frequency effect field curves. Thus, AA or QQ type resistivity sequences can be interpreted from induced polarization response of a horizontally stratified earth without resistivity extrema.
A depth factor has been defined in order to obtain the true depth using the apparent depth. In this way, some electromagnetic effects between horizontal layers with different polarizabilities can partly be eliminated.
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A NOTE ON CONVOLUTION AND PADDING FOR TWO‐DIMENSIONAL DATA*
Authors V. K. GUPTA and N. RAMANIAbstractGupta, V. K., and Ramani, N., 1978, A Note on Convolution and Padding for Two‐Dimensional Data, Geophysical Prospecting 26, 214–217.
Filtering of gridded data by means of two‐dimensional generalized convolution results in a loss of data along the edges of the map. This loss can be counteracted by ‘padding’– filling the surrounding of the input map with some kind of data. Padding by algebraic extrapolation is assumed to be better than surrounding the maps with zeroes. A Føsrtran IV routine for padding by algebraic extrapolation is available from the authors.
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CARTOGRAPHIE DES “PSEUDO‐RESISTIVITES APPARENTES’ PAR PROFILAGE TELLURIQUE‐TELLURIQUE ASSOCIE A LA MAGNETO‐TELLURIQUE *
Authors PHAM VAN NGOC, D. BOYER and ET M. CHOUTEAUAbstractThe magneto‐telluric and telluric‐telluric profiling techniques are presented, and then a new method which combines these two techniques for mapping a parameter defined as the “apparent pseudo‐resistivity”.
The results obtained for the uranium structures in the Cluff Lake district (Saskatchewan Canada) indicate the use of these different techniques in the detection of zones of conduction for surface zones and for deeper zones, and demonstrate the advantages of these methods over the usual electrical methods in problems of structural research.
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INVERSION OF GRAVITY PROFILES BY POLYNOMIAL METHOD *
Authors P. BALDI and M. UNGUENDOLIAbstractThe inversion of a gravity profile associated with a twodimensional distribution of mass with constant difference in density is dealt with, in an initial approximation, by using a polynomial of appropriate degree. The method can be applied successfully, for example, to sedimentary basins.
Some examples confirm the applicability of the method, which can be readily programmed on a computer.
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SELF‐MATCHING DECONVOLUTION IN THE FREQUENCY DOMAIN *
More LessAbstractIn a previous paper the author showed how, by computing an inverse filter in the frequency domain, an automatic compromise could be made between the conflicting requirements to spike a wavelet and to keep the attendant noise amplification within bounds. This paper extends the technique to take account of errors in the estimated shape of the wavelet defined to the deconvolution process. The drastic effects which such errors can have if they are ignored are demonstrated. A novel form of filter–called the “self‐matching filter”–is defined which allows the user to limit not only the noise amplification but also the sensitivity of the filter to random uncertainties in the estimated wavelet. This is achieved by whitening the spectrum only within automatically selected pass bands whilst suppressing other noise‐dominated or uncertainly defined frequency components. Conventional Wiener filtering is shown to be a special case of this more general filter, namely one in which the wavelet uncertainty is completely ignored. The type of phase spectrum which the output pulse should be designed to possess (e.g. zero phase or minimum phase) is briefly discussed.
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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)