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- Volume 16, Issue 1, 1968
Geophysical Prospecting - Volume 16, Issue 1, 1968
Volume 16, Issue 1, 1968
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THE EAEG'S CHOICE OF UNITS*
By R. GREENABSTRACTThe use of MKS system units in preference to the cgs system facilitates accurate numerical calculation in magnetostatic problems in geophysics and the practice of stating the precise dimensions of every unit guards against confusion. Suggestions are made for the unique definition of quantities such as magnetic potential, etc. for which the undesirable circumstance of arbitrary alternatives still persists.
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DETERMINATION DE L'IMPULSION SISMIQUE*
By P. BOISABSTRACTDetermining the shape of a pulse generated by an explosion solely from the data provided by the recorded seismic trace is a difficult and even ambitious task.
Knowledge of parameters such as length and number of “arches” of the pulse under study is, in fact, indispensable in solving this problem.
These parameters cannot be found directly in the seismic trace, which nevertheless contains a great amount of information. Autocorrelation, with its mathematical and statistical properties, is an efficient way of making the best of this information.
We compute all the autocorrelations of reflections having a given number of arches which fulfil certain conditions determined in advance. Then, after statistical testing of some parameters pertaining to the autocorrelations (abcissae of zeros, of extrema …), we select only those with a maximum likelihood. It is sufficient to consider only the reflections whose autocorrelations have been selected and to arrange them in groups according to their shape and arch number in order to obtain average pulses.
In so doing several solutions are arrived at, but when considering a given number of traces, a single record for instance, it is possible by comparing these results with each other to considerably reduce their number.
In the last part of the paper the nature of the impulse obtained with our method is examined in order to find out whether it is “minimum phase” for carrying out deconvolutions.
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PNEUMATIC ACOUSTIC ENERGY SOURCE*
By BEN F. GILESABSTRACTIn recent years considerable work has been done to devise a satisfactory non‐dynamite seismic system that would replace dynamite in offshore areas. Prior to the advent of digital recording and processing, the non‐dynamite sources have generally not provided the depth of penetration or the resolution required for satisfactory seismic interpretation.
More recent developments in non‐dynamite offshore marine sources include adaptation of the Vibroseis from a land unit to a marine unit, and adaptation of the Dinoseis unit from a land to a marine unit. The SUE (Seismic Underwater Explorer) system is a thermodynamic non‐dynamite source utilizing a mixture of propane and oxygen detonated in a special chamber approximately 15 feet below the water surface. This source gives penetration to more than 4 sec in areas typified by Gulf of Mexico type geology and shows deeper penetration than had previously been obtained by dynamite along the western United States in areas with 20 lb charge limitations. A pneumatic source, the airgun, has been in production use in the United States since June 1966. This non‐dynamite source provides an intriguing amount of versatility and can be expanded to provide additional energy as necessary to obtain the penetration desired. Tests using systems comprised of from eight to twenty‐three airguns show penetration in excess of 5 seconds in many areas. Power spectra comparisons both in amplitude and frequency content demonstrate that this is a controlled source generating a controlled seismic wavelet and a controlled frequency spectrum that can be tailored to fit requirements of particular areas. Sample sections obtained in the Gulf of Mexico and the Pacific Ocean offshore California show adequate penetration to 5.0 seconds reflection time.
Quantitative measurements with the airguns demonstrate the effect of:
- 1 Variation of the number of guns in the system;
- 2 Shaping the frequency spectrum by using different sizes of airguns in the system;
- 3 Effects on signal‐to‐noise ratios as a result of stacking several small energy sources together;
- 4 Reproducibility of the initial pulse wavelet from shot to shot.
The improvement in record quality as a result of advanced digital processing with non‐dynamite sources is comparable to that obtained with dynamite sources. Non‐dynamite sources make additional improvements possible where high source multiplicity is advantageous. Excellent dynamic correlations yield accurate velocity control as well as definitions of apparent velocities attributable to multiples and primary‐to‐multiple amplitude relationships.
Non‐dynamite sources are being used more and more extensively in offshore exploration. The advent of digital recording and processing provides a means for improving depth of penetration and resolution of many non‐dynamite sources.
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DEEP SEISMIC SOUNDING OF THE EARTH'S CRUST IN THE AREA OF THE DINARIDES AND THE ADRIATIC SEA*
Authors T. DRAGAŠEVIÍ and B. ANDRICAbstractDeep seismic sounding was performed along two profiles which cross at the Dinarides area right angles. One of the profiles goes far into the Adriatic Sea.
Besides considerations on the lithophysical conditions, characteristics of the registered waves are analysed. The amplitude curves and curves of amplitude ratios are shown. Special attention was paid to the frequency of the registered waves.
In order to obtain a better knowledge of the registered wave pattern three‐component recordings of waves were carried out. The analysis of the records obtained is given, with particular regard to the possibility of creating converted waves.
The Earth's crust structure along the profiles II and III is given.
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ANALYSIS OF GRAVITY ANOMALIES OF TWO‐DIMENSIONAL FAULTS USING FOURIER TRANSFORMS*
Authors BIJON SHARMA and L. P. GELDARTABSTRACTThe Fourier transform formula for a two‐dimensional fault truncating a horizontal bed at an arbitrary angle of inclination is derived. The amplitude spectrum of the Fourier transform is found to give information about the depth to the top of the upper part of the faulted bed and the inclination of the fault‐plane. Under suitable conditions the thickness and the displacement of the bed involved can be obtained. With actual field data, these transforms can be obtained at discrete points by a Fourier analysis of the gravity anomaly. A field example from the Logan fault area near Montreal, Que., Canada, is given.
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ÜBER DIE GENAUIGKEIT GEOMAGNETISCHER FELDREGISTRIERSTATIONEN*
Authors G. FANSELAU, E. RITTER and H. WOLTERABSTRACTThe accuracy of the magnetic registrations of a new field station constructed at Niemegk Observatory is described. The most important aspects for the construction were good mechanical stability, a good constancy of all physical data and the fact, that the station is ready in the field for solid registration ere long. The apparatus was proved during geomagnetic survey in the field, especially in view of the variability of the geomagnetic variations. The results obtained by field work and by comparison at observatories are good. It is planned to enlarge the registrations by means of photocellcompensators.
Der Plan zur Konstruktion einer transportablen und schnell aufstellbaren geomagnetischen Feldregistrierstation geht schon auf das Jahr 1939 zurück. In diesem Jahr wurde von Fanselau ein Plan für eine solche Station den Askania‐Werken in Berlin‐Friedenau unterbreitet und zur Konstruktion empfohlen. Der Krieg verhinderte eine rasche Durchführung der Fertigungsarbeiten, so daß erst nach Beendigung des Krieges die Askania‐Werke den auf diesen Vorschlägen beruhenden Magnetographen fertigen konnten. Am Adolf‐Schmidt‐Observatorium für Erdmagnetismus in Niemegk wurden dann weitere geomagnetische Feldregistrierstationen entwickelt [1, 2, 3]. Bei der Konstruktion des Gerätes wurde bewußt auf Robustheit und Stabilität Wert gelegt, während der Gesichtspunkt, ein möglichst leichtes und kleines Gerät zu haben, nicht so sehr im Vordergrund stand. Von dem neuen Typ der Station sind inzwischen schon eine ganze Reihe von Exemplaren in Betrieb genommen worden, so daß es an der Zeit ist, einige Bemerkungen über Zuverlässigkeit in der Konstanz der Skalen‐ und Basiswerte sowie über andere instrumentelle Daten der Geräte zu machen. Auf Grund eingehender Messungen am Observatorium und im Gelände liegt genügend umfangreiches Material vor, um diese Fragen sicher beantworten zu können.
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EIN AUTOMATISCHES VERFAHREN ZUR INTERPRETATION MAGNETISCHER ANOMALIEN NACH DER METHODE DER KLEINSTEN QUADRATE*
By W. BOSUMABSTRACTThe interpretation of magnetic anomalies on the basis of model bodies is preferably done by making use of “trial and error” methods. These manual methods are tedious and time consuming but they can be transferred to the computer by making the required adjustments by way of the method of least squares.
The general principles of the method are described. Essential presumptions are the following:
- 1 the assumption of definite model bodies
- 2 the existence of approximation values of the unknown quantities (position, dip, magnetization, etc.)
- 3 a sufficiently large number of measuring values, so that the process of adjustment can be carried out.
The advantages of the method are the following:
- 1 substantial automatization and a quick procedure by using computers
- 2 determination of the errors of the unknown quantities.
The method was applied to the interpretation of two‐dimensional ΔZ‐ and ΔT‐anomalies.
Three types of model bodies are taken as basis of the computer program, viz. the thin dyke, infinite resp. finite in its extension downward and the circular cylinder. Only the measuring values are given to the computer. The interpretation proceeds in the following steps:
- 1 calculation of approximation values
- 2 determination of the model body of best fit
- 3 iteration in the case of the model body of best fit.
The computer produces the end values of the unknown quantities, their mean errors, and the pertaining theoretical anomalies. These end results are given to a plotting machine, which draws the measured curve, the theoretical curve and the model bodies.
Interpretation examples are given.
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NOMOGRAMS FOR SOLVING EQUATIONS IN MULTILAYER AND DIPPING LAYER CASES*
More LessAbstractNomograms for solving equations in multilayer and dipping layer cases are presented. The nomograms constructed are used to solve the following equations: I. Intercept‐time formula. 2. Critical distance formula. 3. Critical angle formula. 4. Critical angle and dip angle formula. 5. Vertical depth formula.
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THE ELECTRICAL CURRENT PATTERN INDUCED BY AN OSCILLATING MAGNETIC DIPOLE IN A SEMI‐INFINITE THIN PLATE OF INFINITESIMAL RESISTIVITY*
Authors O. KOEFOED and G. KEGGEABSTRACTSeveral papers have been published in which the electromagnetic anomalies are described that are produced by conductive ore bodies of different shapes. No publications are available, however, in which the electrical current pattern is described that is induced in these ore bodies. Yet an insight in this electrical current pattern would be valuable in order to assess the possibilities of different electromagnetic techniques, for instance with regard to the determination of the dip and of the depth extent of plate shaped ore bodies.
In the present paper computations are given of the electrical current pattern induced by an oscillating magnetic dipole in a semi infinite plate shaped orebody of infinitesimal thickness, in which the penetration depth of the current is infinitesimal to a higher order than the thickness of the plate. The computations are based upon an equation derived by Wesley for the magnetic field produced in these conditions, combined with the relation between the electrical current density in a laminar sheet and the magnetic field produced by this current at the surface of the sheet.
The results of the computations show that, if the horizontal distance between the dipole source and the sheet is sufficiently small, the maximum current density of the return current may occur at a depth below the upper edge of the sheet which is appreciably smaller than the depth of the upper edge of the sheet below the surface. The depth of the return current becomes large when the horizontal distance between the source and the sheet is large.
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BOOK REVIEWS
Book reviewed in this article:
S. A. Eiby, “Earthquakes”, Frederick Muller Ltd.
N. P. Yermakov et al, Research on the Nature of Mineral‐Forming Solutions
P. Bormann, Registrierung und Auswertung seismischer Ereignisse (Grundlagen, Stand und Entwicklungstendenzen), Recording and interpretation of seismic events (principles, present state and tendencies of development).
St. N. Davis and R. J. M. DE Wiest, Hydrogeology
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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 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)