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- Volume 5, Issue 4, 1957
Geophysical Prospecting - Volume 5, Issue 4, 1957
Volume 5, Issue 4, 1957
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MULTIPLY REFLECTED REFRACTIONS IN A SHALLOW LAYER.
More LessAbstractA strong late arrival, with several cycles, has been observed in line refraction shooting, for distances ranging from 5000 to 30,000 feet. Using equations given by Officer (1953) it has been possible to establish that this event is a multiply reflected refraction. The conclusive factor was its very large amplitude which was about 60 times that expected for the simple head wave, at the distances involved. The wave‐guide was bounded by the surface of the earth and by the base of the Low Velocity layer, at a depth of about 80 feet.
An earlier paper (O'Brien, 1957a) gave a study of the head wave pulse refracted from this interface and the conclusions in the two papers are altogether compatible.
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THE PULSED VIBRATOR AS A SEISMIC SOURCE*
By F. F. EVISONABSTRACTNon‐explosive seismic sources, notably the falling weight and the eccentric machine, have occasionally proved advantageous in experimental and practical seismology in spite of their relative feebleness.
Another alternative is the pulsed vibrator, which offers the advantage of a completely controllable waveform. Recent theory shows that such a source is quite well adapted to the radiation of high frequency waves from the surface of the ground; it is estimated, for example, that in typical circumstances reflections should be obtainable from depths of the order of 200 metres. The theory also shows how resonances may be avoided so as to radiate the impulsive waveform unimpaired.
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QUELQUES EXEMPLES DE DIFFRACTIONS EN SISMIQUE‐REFRACTION ET LEUR APPLICATION A LA DETERMINATION DES VITESSES VERTICALES*
By Y. LEDOUXABSTRACTThe progress in the registration of refraction shots obtained in the last years permits a more systematic study of the secondary arrivals. In zones with disturbed tectonics, a noticeable portion of the received energy derives from diffraction phenomena caused by velocity anomalies in the marker bed or in the overburden (faults, local heterogeneities). Some characteristic examples are presented and schematically explained.
A more advanced analysis of the branches of the travel time curve caused by diffraction leads, in favourable cases, to a new method of calculating the offsets and the vertical velocities, independent of the usual methods for the determination of these parameters.
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ÉTUDE DE L'ATTÉNUATION DE L'INFLUENCE DES ANOMALIES SUPERFICIELLES DANS LE CALCUL DE LA DÉRIVÉE SECONDE DE LA GRAVITÉ*)
Authors Jean GOGUEL and Jeannine LEMOINEABSTRACTIn order to eliminate the effect of smoothing due to the use of a finite number of grid points, the second derivative is computed by integrating the product of g with a convenient continuous function, which yields the second derivative to the extent to which the first terms of the Taylor expansion of g represent its value correctly. By applying this method to the anomaly caused by an isolated mass, and to that caused by a homogeneous half plane, it is shown that, if the result obtained is interpreted as if it really were a second derivative, erroneous values for the depth and the mass are obtained. If the real depth of the mass is small, a too large apparent depth is obtained. In the case of a half plane the use of a system of grid points gives the same result. These considerations permit the rational choice of the method of computing the second derivative, such that the effects of too shallow mass irregularities are attenuated.
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SUR ĽINTERPRÉTATION DES ANOMALIES GRAVIMÉTRIQUES ET MAGNÉTIQUES *)
Authors Jérôme CHASTENET DE GERY and Henri NAUDYABSTRACTFor the interpretation of gravimetric and magnetic maps two types of master curves are proposed: firstly those which apply both to the vertical gradient of g and to the pseudo gravimetric gradient obtained by reduction to the pole of magnetic maps; and secondly those which apply direct to magnetic anomalies.
With regard to the first type of master curve, the principle and the advantages of the reduction to the pole are discussed. In the last part of the paper it is shown how a set of master curves, calculated for two‐dimensional east‐west striking structures, can also be used for the interpretation of two‐dimensional structures of arbitrary direction of strike. Practical examples of application are given.
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DENSITY DETERMINATION ON NEAR‐SURFACE LAYERS BY GAMMA ABSORPTION*)
Authors J. HOMILIUS and S. LORCHABSTRACTFrom gamma ray absorption the density of soil to a depth of i m. can be determined in situ, without taking samples, by a method which has been described in a previous paper (Wendt 1954) and which has been checked with satisfactory results (mean error about 1%). A nearly vertical steel rod with a gamma ray source at its lower end is pushed into the ground, and the gamma radiation is measured at the surface with two counters placed symmetrically with respect to the rod.
As a first step in interpreting the observed effects, the assumption had been made that counters are infinitesimally small. This paper shows how a theory can be developed without this assumption, the finite dimensions of the counters being taken into account by an integral expression. For this purpose a determination of the dependence of the counting rate on the angle of incidence of the gamma rays is required. A comparison of the experimental results with the theoretical ones demonstrates that the integral expression is correct.
The influence of deviations from the normal geometry of the arrangement (e.g. caused by bending of the probe) on the results is investigated, as well as the influence of variations of the density with depth.
The treatment of the basic problems has been carried through in such a manner that the results obtained can be applied to other absorption methods as well.
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SOME PROBLEMS OF THE AEROMAGNETIC SURVEYS1)
By W. DOMZALSKIABSTRACTA brief description of the procedure usually adopted for ascertaining the accuracy o aeromagnetic data is given as a background to the discussion of factors which affect this accuracy. These factors fall mainly into two groups: spatial positioning, i.e. relation to the ground surface, and effects contributing to the observed relative magnetic values. The evaluation of the observed anomalies in terms of the most probable causes and the anomalies themselves are directly influenced by the amount of the available control of factors mentioned above. The effects of various factors, such as plan positioning, height keeping, diurnal drift, are discussed, the arguments being based on theoretical and practical premises.
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BOOK REVIEW
Book review in this article
Gedenkboek F. A. Veiling Meinesz, Verhandelingen van het Koninklijk Nederlandsch Geologisch‐Mijnbouwkundig Genootschap, Geologische Serie, Vol. XVIII, May, 1957.
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Volumes & issues
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Volume 72 (2023 - 2024)
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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 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)