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- Volume 8, Issue 2, 1960
Geophysical Prospecting - Volume 8, Issue 2, 1960
Volume 8, Issue 2, 1960
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ANOMALE ULTRASCHALL‐GESCHWINDIGKEITEN IN GESTEINSBOHRKERNEN *
By H. BAULEAbstractThe velocity of supersonic pulses in cylindrical drill cores and rock samples is measured as a function of the frequency or wave length λ of the pulses. If λ is very much lower than the diameter D of the drill core –λ < D– the higher velocity of the longitudinal waves vl appears; on the other side we get the lower velocity of the rod waves vd (Stabdehnungswellen) if λ > D.
Some drill cores from the carbon were found the velocities of which for sonic and supersonic pulses do not follow the above rule. It is shown that these cores have a constant velocity both for short and for long wave lengths in a frequency range between 5 kHz and 2 MHz.
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IMPROVEMENT IN THE QUALITY OF DEEP REFLECTIONS BY UNIFORMLY LINEAR SHOTPOINT ARRAYS*
More LessAbstractBy means of demonstration material, the author reports on the successful application of a shooting technique for seismic reflection operations, which has proved to work satisfactorily in those parts of the North‐West German prospective oil area which are unfavourable to reflections. The shooting technique is characterized by uniformly linear shotpoint arrangements which are parallel with the geophone spread and reach from one end of the spread to the opposite. All charges are detonated simultaneously.
Technical details of the method are discussed, particularly the problem of the most favourable geophone spreads in continuous profiling as well as the position of the uphole geophone. The difference in quality of reflections obtained by the normal pattern shooting and the linear shotpoint arrangement is demonstrated by seismograms. The special problems involved in the interpretation of the seismic records are referred to.
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COMPARATIVE CONSIDERATIONS ON THE ENERGY CONTENT OF SEISMIC WAVES IN CENTRAL AND LINEAR PATTERN SHOOTING*
Authors H. PIERAU and O. ROSENBACHAbstractIn continuation of a publication by Pierau and Mueller (1960), theoretical and experimental comparisons are made about the energy content of seismic waves for central and linear shotpoint arrangements. The following results are obtained:
Using shotpoint arrangements as chosen for the investigation, the energy content of deep reflections is greater in linear than in central pattern shooting, provided that the total quantity of charge is the same in both cases. Furthermore the seismograms obtained with linear shotpoint arrays are less disturbed by surface waves than those obtained in central pattern shooting. Both effects are superimposed and improve considerably the signal‐to‐noise ratio, thus explaining the improvement in the quality of deep reflections by uniformly linear shotpoint arrangements.
ZUSAMMENFASSUNGIn Fortführung einer Publikation von Pierau und Müller (1960) werden theoretische und experimentelle Vergleiche über den Energiegehalt seismischer Wellen bei Zentralund Reihenschüssen durchgeführt, die folgendes Ergebnís liefern:
Bei den gewählten Anordnungen der Schussbohrungen und unter der Voraussetzung gleicher Gesamtladungen sind die tiefen Reflexionen beim Reihenschuss energiereicher als beim Zentralschuss. Ausserdem sind die Seismogramme von Reihenschüssen weniger durch Oberflächenwellen gestöit als diejenigen von Zentralschüssen. Beide Effekte zusammen verbessern wesentlich das Verhältnis von Nutz‐ zu Stör‐Energie und erklären die Verbesserung der Qualität tiefer Reflexionen durch gleichmässig lineare Anordnung der Schussbohrungen.
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SEISMIC WAVES IN TRANSITION LAYERS*
More LessAbstractA transition layer is a layer in which the velocity changes linearly with depth from va to ve and we consider it embedded between two layers of constant velocity va and ve, respectively. A plane displacement wave of arbitrary shape is supposed to come out of the upper v”‐layer, and to meet the transition layer at vertical incidence. Using methods of elementary calculus only, the subsequent events are derived for both inside and outside the transition layer.
The transition layer can be regarded as the limit of a sequence of constant‐velocity‐layer divisions. Thus, after computing the first few multiples of the incident wave for these divisions, the respective multiples of the transition layer are obtained as the limit of these multiples. Then, with the help of recurrence formulae, the multiples of the transition layer of any order are computed. The sum of all multiples defines the complete response of the transition layer and satisfies the differential equations of the problem.
The solution has the form of a superposition integral, and it is seen that the transition layer has the properties of a linear filter. The superposition integral is built up out of the incident wave and a conglomerate of Bessel functions, the latter being the corresponding response to an incident spike impulse in a mathematical sense. The reflected and transmitted responses to the spike impulse are shown for two values of the velocity ratio. For large values of this ratio, both these responses have a‘wave‐like’shape and it is seen that the transition layer may effect a serious change of shape of incident waves.
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ON THE SOLUTION OF ONE DIMENSIONAL ELASTIC WAVE PROPAGATION PROBLEMS IN STRATIFIED MEDIA BY THE METHOD OF CHARACTERISTICS
Authors M. C. BLACK, E. W. CARPENTER and A. J. M. SPENCERAbstractThis paper describes a numerical method of solution for wave propagation in a medium whose elastic parameters and density vary with depth in any specified way. Results for a simple two layer problem are given to illustrate the method, and the extension to problems of current geophysical interest is briefly discussed.
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SYMPOSIUM ON SYNTHETIC SEISMOGRAMS*
More LessAbstractThis paper is intended as a basic introduction to synthetic seismogram techniques. Their principles, uses and limitations are discussed and the relative advantages of the different methods are compared.
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ATTACKING THE PROBLEMS OF THE SYNTHETIC SEISMOGRAM
By N. A. ANSTEYAbstractThe four problems considered are the definition and simulation of the filtering effects which occur in the earth between the explosive and the case of the geophone, the problem of errors in the time scale of the synthetic record, the simulation of multiple reflection effects, and the difficulty of finding a representative comparison trace on the field record. A partial solution to the first of these is offered by Siran, examples of whose operation are presented. A method of assessing allowable errors in the time scale is discussed. Multiple reflections may be introduced by the preparation of multiple‐corrected reflection‐coefficient logs on a digital computer such as the IBM 709. The particular problems of multiple reflections from the base of the weathered layer (including “ghost” reflections) are discussed. The preparation of a “composite” trace is shown to be a partial solution to the problem of the comparison with the field record.
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SISMOGRAMMES SYNTHETIQUES; POSSIBILITES, TECHNIQUES DE REALISATION ET LIMITATIONS*.
Authors P. BOIS, J. CHAUVEAU, G. GRAU and M. LAVERGNEAbstract1°– Synthetic seismograms have been computed with and without multiples by exact and by approximate methods. By means of these seismograms, some studies have been made on the various multiple reflections, the character of the films, the effect of the surface in the formation of multiples, as well as on the shape of the pulse and the spectra of seismograms. Refraction synthetic seismograms have also been realized on schematic examples with fault and pinch‐out.
2°– Various techniques for synthesizing seismograms are described which make use of analogue computing equipment such as a bar with variable diameter, or of digital computers. The orders of magnitude of the prices of these various methods are shown.
3°– Some of the limitations are described which are imposed on synthetic seismograms by the hypotheses and the inaccuracies of our knowledge of the conditions which prevail underground.
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FILM SYNTHETIQUE AVEC REFLEXIONS MULTIPLES THEORIE ET CALCUL PRATIQUE*
Authors V. BARANOV and G. KUNETZAbstractOur purpose is to give a short summary of the theory of synthetic seismograms, including all multiple reflections and to show the method of their construction with the use of an electronic computer.
The waves to be considered in reflection seismic being approximately plane and horizontal it is generally admitted that in most cases the propagation phenomena can be described with the equation
(u, displacement; V(z), velocity; p, density). Moreover, for all practical purposes, the velocity V (z) is not a continuous function of the depth z. In fact, the earth can be divided up into more or less thin layers, with constant velocity inside each layer and sudden variations at each interface. It is therefore reasonable to substitute to the single equation (1) a series of simple propagation equations with constant coefficients
provided a set of boundary conditions is adjoined to them in order to ensure the continuity of displacement and tension.
As with all seismic problems, this is essentially a transient system and a very convenient method to resolve equation (2) is to resort to the Laplace transformation, by writing
Then the general integral of (2) is:
A and B being two constants. This expression is valid inside of a layer, including the two faces. If the propagation velocity in an adjoining layer is V we get an equation similar to (4), say (4′), but with different constants C and D. At a point on the interface, both expressions are valid. Consequently, if we take three points M, P and N into consideration, respectively at the depths z– V T, z and z + V T, we can write four expressions (4) and
(4′). The continuity of the tension gives a fifth expression. The constants A, B, C and D can be eliminated from these equations. The result of the elimination is
is the reflection coefficient. Going back to the original functions we find a recurrence expression with four terms
In order to make use of this expression, we set out from curve
which divides the plane Ozt into two domains:
1) a domain contiguous to the axis Oz where u is identical to zero;
2) the remainder of the plane where for z = o, u(t, o) =s(t)–“the signal”–is given in a narrow interval in the proximity of the origin.
The numerical calculation is carried out at the intersection points of two sets of straight lines:
1) equidistant parallels to Oz (with spacing x) and
2) parallels to Ot through the intersection of the curve T with the straights of the first family.
Computations having been carried out for all points (z, t) of the second domain, they lead finally to the values of the function u at the surface, u(t, o), outside the interval where the signal is given. This function u(t, o) is the requested synthetic seismogram.
The shape of the signal enters into the calculations. As a matter of fact it is always necessary to try several signals, hence to construct several synthetic seismograms. However, the operation consisting in the modification of the response is simpler than the calculation of the initial film. This leads to the notion of the synthetic impulse seismogram, which is constructed by assuming that the signal is a pure impulse. This impulse seismogram being calculated, it is easy to construct as many synthetic records as there are signals to be taken into account.
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Ľ ANALYSE DES FILMS REELS ET LEUR COMPARAISON AVEC LES FILMS SYNTETIQUES
Authors J. DELAPLANCHE and Y. LEDOUXAbstractThe practical use of synthetic records involves previous studies on field records from an original point of view; these studies consist essentially in analysing and cancelling noise, in bringing out the form of pulses and the character of seismic events, in detecting multiple arrivals. Various tools have been tested for this purpose: high multiplicity shooting, analysis of conventional well velocity surveys, mixing or selecting traces, varied filters; all these operations are facilitated by the use of magnetic recording.
After several examples of these methods are given, it is shown how the results of such studies can help to make and discuss synthetic records with and without multiples. The share of multiple events and their effect on the efficiency of the later comparisons are especially treated.
The authors finally present some specific examples of successful comparisons between synthetic records and field records treated beforehand. They insist on the discrepancies and on the importance of finding their origin, for the most tangible results in seismic interpretation generally appear during the analysis of such discrepancies.
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BOOK REVIEWS
Book reviews in this article
P. Lasfargues, Prospection Electrique par courants continus,
Milton B. Dobrin, Introduction to Geophysical Prospecting,
M. M. Slotnick, Lessons in seismic Computing,
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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 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)