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- Volume 8, Issue 3, 1960
Geophysical Prospecting - Volume 8, Issue 3, 1960
Volume 8, Issue 3, 1960
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EXPERIMENTS WITH STATIC‐RESISTANT BLASTING CAPS*
Authors J. H. BANVILLE and H. RICHARDAbstractPremature detonation of seismic blasting‐caps may be caused by the accumulation of an electrostatic charge. This is especially true in desert areas where there are many sandstorms.
Two makes of static resistant cap have been tested in the laboratory and the results compared with those for non‐resistant caps made by the same manufacturers. The full results are given in the table, the breakdown voltage for the resistant caps is in the range 20–30,000 volts as compared with the range 10–15,000 volts for the ordinary cap.
It is strongly recommended the static resistant caps should always be used and that experiments on premature detonation from this cause should bemade in the field.
In January 1960, 20 crews in the Sahara used a total of 146,150 static‐resistant blasting caps.
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DIFFRACTION PROBLEMS IN FAULT INTERPRETATION*
More LessAbstractDistinguishing between diffractions and true reflections is often difficult and may lead to misinterpretations. In the Molasse zone of Upper Austria, numerous faults were established by seismic surveying. Diffractions were observed at several antithetic faults but not at synthetic faults. As an example, a seismic record section of the Steindlberg structure is shown. The reflections from the base of Tertiary arid from the Cretaceous‐Jurassic contact run parallel over long distances, and so do the less important reflections lying above and between. If, contrary to the general trend, the reflection from the base of the Tertiary approaches the underlying reflection from the Cretaceous‐Jurassic contact, or if the latter diverges from the former, this is considered a criterion for a diffraction.
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DEVELOPMENT OF NEW TECHNIQUES APPLIED TO THE SEARCH FOR STRATIGRAPHIC TRAPS*
Authors L. CARISSIMO and O. D'AGOSTINOAbstractThe “Summarizer” is a new device designed and manufactured by the Agip Mineraria Electronic Laboratories which gives an output which is a function of the total energy detected by the geophone spread. This paper gives some examples of its use in stratigraphic mapping.
Test surveys have been carried out in an area of the Po basin where the formations show many facies variations. Three gas fields of known geological features were considered. The usefulness of various methods of representation was evaluated as well as the contribution given by the new device. It was noticed that the results given by the “Summarizer”, when added to those obtained by standard and “variable area” sections, supply the interpreter with new useful elements. Lateral facies variations between gravels and shales have been recognized and localized.
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THE USE OF AMPLITUDES IN REFRACTION SHOOTING – A CASE HISTORY*
More LessAbstractOn a group of seismic refraction records there occurs a late arrival with the same apparent velocity as an earlier arrival. It was first thought that the late arrival travelled along the same refractor as the earlier one but had been delayed in the overburden either by multiple reflection or by P‐S (dilatation to shear) conversion. Either of these two mechanisms could be made to fit the time‐distance data. A study of the ratio of the amplitudes of the two events showed that the observed ratio was about ten times that to be expected on either of these hypotheses. Also, the rate of attenuation of the earlier arrival (3.7 ± 1.0 db/1000 ft) was appropriate to a thin layer while the rate for the later arrival (0.74 ± 0.30 db/1000 ft) was appropriate to a thick layer. Accordingly, the later arrival was identified as a refraction from a deeper layer.
The subsequent depth section agreed very well with that found by drilling.
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ELASTISCHE WELLEN IM ANISOTROPEN MEDIUM NACH EINER MAKROSEISMISCHEN UNTERTAGE‐MESSUNG*
By S. J. DUDAAbstractIn some mines of the Siegerland, besides dilatational and shear waves, an additional direct wave has sometimes been found the velocity of which was somewhat lower than that of the shear wave. Investigations were carried out with the object to establish the directions of oscillations and the velocities of all three kinds of waves. It was the purpose to draw conclusions as to the character and the causes of the appearence of the third wave.
By these investigations the wave having the lowest velocity can be identified as a second shear wave. This points to the fact that double refraction of the shear wave is taking place, the wave of higher velocity being a pure transverse wave and the wave of lower velocity being a quasi‐transverse wave.
As a result of these measurements the examined medium is shown to have a transverse isotropy due to stratification. It is, however, not possible to decide whether there exists also an anisotropy due to schistosity.
Some further calculations show that the wave of lowest velocity can not be explained as a surface wave propagated on the walls of the gallery.
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GENERAL METHOD FOR CONSTRUCTION OF TRAJECTORIES AND WAVE‐FRONTS DIAGRAM FOR ANY VELOCITY DISTRIBUTION
More LessAbstractA graphical method is presented for construction of trajectories and wave‐front diagrams for any velocity distribution.
The analytical solution is given for the case when the time‐depth curve shows a flexure, that is, when the the velocity increases down to a certain depth and, after reaching a maximum, begins to decrease.
A numerical example is considered for the computation of a trajectory.
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A GENERALIZED CAGNIARD GRAPH FOR THE INTERPRETATION OF GEOELECTRICAL SOUNDING DATA
By O. KOEFOEDAbstractThe Cagniard method of interpreting geoelectrical sounding data is based on the use of an empirical graph which expresses the relation between the resistivity and thickness of the second subsurface layer on one side, and on the other side the replacement resistivity and replacement thickness which are derived from the two‐layer sounding curve which is asymptotic to the second branch of the observed sounding curve. In his method separate graphs of this type are required for different values of the resistivity of the third subsurface layer. In the present paper a generalized Cagniard graph is derived which has the advantage of being independent of the value of the resistivity of the third subsurface layer. Finally, the accuracy of the method is discussed, and a procedure is described for extending its use to cases where more than three subsurface layers intervene.
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NOTE ON THE IMPEDANCE OF A GROUNDED WIRE
More LessAbstractThe self‐inductance of a straight grounded wire lying on the surface of the earth is computed from a (slightly corrected) formula given by A. Wolf and compared to approximate formulae.
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AN EXAMPLE OF THE IMPORTANCE OF NATURAL REMANENT MAGNETIZATION IN THE INTERPRETATION OF MAGNETIC ANOMALIES
Authors R. W. GIRDLER and G. PETERAbstractAn example is given from the Gulf of Aden of a total intensity magnetic anomaly which cannot be explained by a body assumed to be magnetized in the direction of the Earth's present magnetic field. It is enferred that the natural remanent magnetization is much greater than the induced magnetization and computations have been made which suggest the presence of igneous rocks with a reverse magnetization. The example illustrates that it is important to know the ratio of remanent to induced magnetization and the direction of remanent magnetization for the correct interpretation of magnetic anomalies.
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BOOK REVIEWS
Book Reviewed in this article:
J. W. Horton, Fundamentals of Sonar.
F. J. Turner & J. Verhoogen, Igneous and metamorphic Petrology.
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Volume 72 (2023 - 2024)
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Volume 69 (2021)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 52 (2004)
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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)