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- Volume 6, Issue 4, 1958
Geophysical Prospecting - Volume 6, Issue 4, 1958
Volume 6, Issue 4, 1958
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THE CAUSES OF GEOPHYSICAL ACCIDENTS*
By H. RICHARDAbstractThe attention of everyone has already been drawn to the part which accident reports must play (Geophysical Prospecting, March, 1957, PP‐ 1 to 8). We intend to show here, with supporting examples, that the systematic analysis of the causes, does allow one to obtain practical conclusions.
To begin with, it is advisable to widen the notion of geophysical accident as much as possible. This being done, about 200 reports distributed over a long period and a sufficient number of parties, are dealt with. The collected reports are sufficiently numerous to draw conclusions in a general way bearing on the headings: drilling, transport, outbreaks of fire, explosives, falls, stings, shocks, miscellaneous causes.
All the quoted figures are referred to the number of accidents reported. Action must be taken to encourage european geophysicists to study accident reports and to analyse the causes. This may be done in such a way that secret information, such as whereabouts of their personnel, is not disclosed.
Efforts to promote “Security” must in the first place bear on the quest for the causes, the definitions of ‘accident’ and of ‘geophysicist’ being taken at their widest possible meaning.
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SOME MODEL EXPERIMENTS RELATING TO ELECTROMAGNETIC PROSPECTING WITH SPECIAL REFERENCE TO AIRBORNE WORK*
Authors E. H. HEDSTRÖM and D. S. PARASNISAbstractAfter a discussion of the underlying principles, some electromagnetic model experiments made primarily in connection with the development of the ABEM airborne method are described. The experiments were made on thin vertical and horizontal conductors of “infinite” extent with coil arrangements involving one transmitter and one receiver unit. The in‐phase and out‐of‐phase components of the field picked up by the receiver were measured in percent of the amplitude of the normal field at the receiver. In this case 1° phase‐shift corresponds to 1.75 % of the normal field amplitude. The experiments were made on a scale 600–2000 times smaller than the natural scale. The ores and overburdens normally encountered in the field were simulated by sheets of Cu, Al, Zn and Pb of varying thicknesses. The frequencies used were 500, 880 and 1500 c/s. The variation of the secondary fields with the thickness, resistivity and depths of the conductors causing them is discussed in some detail. The bearing of the laboratory work on ground and especially airborne electromagnetic methods is indicated. A few miscellaneous experiments are also described. The results of an airborne survey over a known ore body and those of the corresponding model experiments are given. The ABEM airborne electromagnetic method and the so‐called “Canadian” method are briefly compared in the light of the model experiments described.
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ON THE THEORY OF GAMMA RAY SCATTERING IN BOREHOLES*
Authors J. HOMILIUS and S. LORCHABSTRACTThis paper deals with the theory of the gamma ray scattering method, which is applied in determinations of density, mainly in boreholes. In this method, the gamma radiation from a radioactive source is scattered round the borehole, and recorded by a detector which is protected against direct radiation by lead.
The theory is based on the scattering and absorption properties of matter with regard to gamma radiation, as described by the Klein‐Nishina formula in terms of the quantum theory. The single scattered portion of radiation, which is of particular importance, is calculated. In doing this an absorption dependent on the scattering angle, and a counter efficiency dependent on the energy of radiation and the angle of incidence, are taken into account. The range of the detector can also be considered.
For homogeneous soil, the dependence of the counting rate on the distance between source and detector, on the density, and on the screening of radiation by the lead absorber, is investigated.
Furthermore, the scattering range for various soil densities and distances between source and detector is also determined. The existence of a substantial degree of agreement between theory and experimental findings is demonstrated. Results are compared with those of the Djadkin diffusion theory, and are illustrated by means of examples of the gamma gamma probe and the gamma density meter for surface measurements.
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ON REFLECTED REFRACTION WAVES*
By W. BRAUCHABSTRACTAs an introduction the various ray paths of a refraction wave, which is reflected at a fault, are discussed for the case of an arbitrary angle between the refracting horizon and the fault. Simple geometric considerations lead to the conclusion that the best chances for recording these pulses are encountered, if the angle between the refracting horizon and the fault is either 90° or the critical angle of refraction. In both cases identical travel times of the pulses are to be expected.
The case of a fault perpendicular to the refracting horizon is considered in detail for dipping beds. Formulas for the shot point travel time curve and the time contour map are derived. Computed time contour maps show considerable differences between the direction of strike of the contour lines and the strike of the fault, as well as between the recorded apparent velocity and the true velocity of the refracting horizon. Finally, alignment charts and computing procedures are given by which the position of the fault and the velocity of the refracting horizon can be obtained from the recorded shot point travel times or the time contour map.
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SEISMIC MODEL EXPERIMENTS CONCERNING REFLECTED REFRACTIONS*
Authors O. KOEFOED, J. G. VAN EWYK and W. T. BAKKERABSTRACTSeismic model experiments are described in which long strips of plexiglass were used as models. One end of the strip was sawn off at an oblique angle and, at the opposite end, the strip was excited by means of a barium titanate transducer. The experiments showed that, if the width of the strip was sufficiently small, an anomalous reflection against the oblique end occurred which travelled in the longitudinal direction of the strip. This anomalous reflection did not occur when the width of the strip was large. These results are explained on the basis of Fresnel's theory. It is inferred that, in the subsurface, refracted waves may be reflected against fault planes without the law of reflection being satisfied, provided that the refracted wave is propagated in a sufficiently thin high velocity layer.
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SYMPOSIUM ON CHANGES OF SHAPE OF SEISMIC PULSES*
By N. A. ANSTEYABSTRACTTheoretical work on seismic pulse propagation must, of necessity, use advanced mathematical methods. To the general reader these researches may seem far removed from the necessities of commercial geophysics; this paper attempts to show in non‐mathematical terms that the basic potential of the seismic method permits the obtaining of considerably more information than is currently expected, and that, of all the advances which are required to realise that potential, a knowledge of the laws governing the transmission of the seismic pulse is most desired.
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THE INFLUENCE OF A LAYER COMPLYING WITH A LINEAR VELOCITY LAW ON THE SHAPE OF SEISMIC PULSES
Authors H. MENZEL and O. ROSENBACHABSTRACTA layer with parallel plane boundaries is assumed to have a constant density and exhibit a velocity of propagation of seismic waves which increases linearly with the distance from one boundary plane. The influence of this layer on the shape of seismic pulses is investigated in two different cases, in which:
1. The layer is embedded between two media each of which has a constant density and velocity of propagation.
2. The layer is adjacent to one medium of constant density and velocity; i.e. one boundary plane of the layer is the free surface of a two‐layered elastic half space.
Through one medium with constant velocity a plane compressional wave impinges at normal incidence on the layer complying with the linear velocity law. The incident seismic pulse is therefore split up into reflected and transmitted parts, the elastic motions of which are studied in the neighbourhood of the layer. The mathematical solution can be deduced for a general pulse by using the Laplace‐Transformation. The general solution reveals that the layer following the linear velocity law influences the shape of the reflected and transmitted pulses. This influence is discussed in detail by demonstrating some numerical examples.
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A NOTE ON THE SEISMIC PULSE RECORDED FROM A MINE EXPLOSION
By N. A. ANSTEYABSTRACTIt is not easy to record a seismic pulse at distances of interest to oil prospectors without there being reflected or refracted pulses superimposed on the direct arrival. Accordingly the record illustrated is considered worth publishing, although it was taken fortuitously during a normal survey and cannot claim to be a controlled experiment. A comparison with the filtered pulse to be expected from a theoretical Ricker‐type wavelet is presented.
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CHANGES OF SHAPE OF SEISMIC IMPULSES IN HOMOGENEOUS VISCOELASTIC MEDIA
By G. W. POSTMAABSTRACTChanges of shape of seismic waves provide information on the properties of the material in which the waves propagate. Ricker (1953) has attempted to explain the changes of shape on the basis of a simple viscoelastic theory. His conclusions are at variance with those of others who find a dependence of the attenuation on frequency which could be explained only by a much more complicated linear theory or by nonlinear theories.
To provide a basis for discussion, the essentials of the theory of viscoelasticity are briefly reviewed. If a relaxation spectrum, rather than one or very few relaxation times, is admitted, a great variety of experimental results can be described by the linear theory of viscoelasticity. A linear theory is indicated when no obvious violations of the principle of superposition occur.
Ricker's theory is presented with some modifications which allow for a finite duration of the initial pulse and for the approximate character of his basic assumptions. There do not appear to be serious discrepancies between his theory and his experimental results. Some of the objections to his theory can be met by assuming a finite duration of the initial pulse. However, more direct measurements made under similar circumstances by McDonal et al. (1958) at the same location lead to a conclusion on the nature of the material not in accordance with Ricker's. This casts doubt on the sensitivity of his method.
Laboratory measurements usually yield results which are not explainable in terms of simple viscoelastic models. Whether a linear theory with a relaxation spectrum or a nonlinear theory should apply depends much on the experimental conditions. We must also consider the possibility of nonlinear mechanisms which are active at small amplitudes. No stand is taken in this controversy, but it is pointed out that the question linear or nonlinear could be decided experimentally without considering the details of the theories.
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BOOK REVIEW
Book reviewed in this articles:
Principles of Geodynamics, by A. E. Scheidegger.
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Volume 72 (2023 - 2024)
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Volume 42 (1994)
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Volume 40 (1992)
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Volume 34 (1986)
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Volume 32 (1984)
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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)