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  • Articles  (3,011)
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  • 1955-1959  (3,011)
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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The 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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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Seismic 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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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Changes 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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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A method of refraction computing is described which establishes the two points of incidence at the surface of the ray-paths from any point on the refractor when observed from opposing directions. The distance between these points is used to determine the radius and the centre of a circle to which the refractor must be tangential. The matching of forward and reverse data is intrinsic in the method so that the resulting profile is automatically positioned in depth. It is shown that if the effects of dip are neglected, errors are introduced in the derived radius and also in the position of the centre but these errors are shown to cancel each other almost exactly.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The wells Parentis 10 and 12 which are located on the eastern edge of the Parentis anticline were found to be considerably lower at the top of the reservoir series than had been expected.Unfortunately the village of Parentis is located in this important zone and no more seismic results could be obtained which might help locate faults and give a better picture of the area. To help in our understanding of the area, we attempted to investigate the subsurface conditions surrounding the wells by shooting shaped charges or primacord in the well and recording the returning seismic energy along profiles located on the surface. Thus far, we have used this method in three wells: Parentis 10, Parentis 12 and Lugos 2.With the data given by a velocity survey in each well, time-distance curves have been calculated for several typical possibilities: horizontal layers, dipping layers and the effect of a fault. A comparison of the calculated and experimental curves shows anomalies which correspond to a change of the average velocity along the travel path, which can correspond to geological changes such as: a sudden change of dip, a fault or a thickening of the section.The anomalies have been studied on the different profiles from each well and a possible interpretation of the subsurface conditions has been given. The directions of the major accidents determined by this method are in general agreement with those found by the regular seismic shooting.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 6 (1958), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The directional sensitivities of an array of geophones depend as much on the size of the array as on the directional sensitivities of the single geophones forming the array. In a former paper (Celminš 1956) the effect of the size of the array was investigated. In addition to this the influence of the directional sensitivities of the individual geophones on the sensitivities of an array of geophones is investigated here.In δ 2 some general formulae for the directional sensitivities are derived. In addition the directional sensitivities of a single geophone according to P- and S-waves are investigated (δ3 and δ4). In δ5 the'directional sensitivities for the most important cases of arrays of parallel geophones are computed. The results of the computation (shown in fig. 4 and 5) give an idea of the types of directional sensitivities which occur in the use of geophone arrays.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 3 (1955), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The paper discusses the variations of Elevation Correction Factors (E.C.F.) across various stratigraphical and structural occurrences with particular reference to the dipping bed, fault and horst. Graphs are prepared giving E.C.F. variations for various angles of dip and a method is suggested for application in the field. Errors in the linear variation that is commonly used for small angles of dip are discussed.The second part of the paper consists of the application to interpretation of (I) Change of gravity dip and (2) The gravity inversion. It is shown how from a gravity map the dip, position of outcrop and thickness of a dipping bed can be determined. The last part shows how anticlines can give rise to negative anomalies and that in the direction of dip similar structures can at one point have no surface expression and afterwards invert to positive anomalies. The danger of attempting to predict the size of structures in such areas is emphasised.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 3 (1955), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: When gravimetric prospecting is concerned with a region featuring broken relief the calculation of topographical corrections becomes vitally important, for these rectify apparent anomalies completely. It is essential that they should be calculated as exactly as possible. By the classic method this takes an extremely long time and may in the long run involve an outlay actually higher than that expended on operations on the terrain.It has thus been felt necessary to seek methods which would speed up calculation, and, bearing that in mind, the use of a polar integrator has been envisaged. With this the contour lines of a map can be followed between two fixed radii, the pole corresponding to the location of the position. Two integrator roulettes will give the values of the first two terms of the expansion in terms of the quotient of the height divided by the distance, the second of the terms resulting from the curvature of the earth. Unless there are escarpments very close to the position, the use of these two terms will be sufficient.Polar integration will deal successively with different contour lines and a subsidiary graph will be plotted, giving the value of each of the integrals in terms of the height. By means of this subsidiary graph the values of the corrections will be obtained with a planimeter and a moment integrator. It is envisaged that the same operation could be repeated in the case of two maps with different scales, for instance, 1–20,000 and 1–200,000 the effect of the areas in the immediate neighbourhood of the position being taken into account by the observer. It is hoped that in this way the calculation of topographical corrections will be made quicker, more exact and less laborious.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 3 (1955), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The problems connected with reflexion surveys in areas of rough topography consist of choosing proper equipment and of making careful near-surface corrections. This paper illustrates some solutions to these problems that have been adopted in a survey which has recently started in a hily coastal area, in Southern Italy.As the area is characterized by a youthful and very actively eroded topography of Post-Miocene clays and litoral clastic sediments, both rotary and percussion drilling were necessary whilst all the seismic equipment had to be light to allow straight lines to be shot as far as possible. Details are given of the truckmounted, light equipment which has been designed for this special purpose.In connection with the youthful topography of the area being surveyed, the criteria for the proper computation of the near-surface corrections is discussed. The great differences in elevation, combined with variations in the surface and near-surface materials, necessitated a careful analysis before deciding the thickness of the correction zone. The results are shown of the preliminary tests carried out in order to determine the average thickness of the low-velocity zone. The solution adopted for drawing the reference surface determining the base of the correction zone for the whole of the area is subseqently explained.Reference datum is drawn so as to follow gently, and with segments of constant slopes, the general features of the rough topographic relief at a depth ranging from 30 to 60 metres from the surface. Criteria for the routine checking of velocity variations in the correction zone are illustrated. Such checks are based on both uphole time analyses and refraction “first breaks” plots, with deeper shot holes at constant intervals.Unsatisfactory results necessitate longer spreads for “low velocity tests” and uphole shooting from deeper holes and the reference datum is deepened for the particular area where this becomes necessary.The final cross-sections are shown and although, as yet, no deep holes, are available to check the seismic data, the general appearance of the cross-sections seems to indicate that subsurface data are free from the influence of topography and of the velocities in the near-surface formations.
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