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  • Articles  (5,528)
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  • Articles  (5,528)
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  • 1
    Electronic Resource
    Electronic Resource
    SOME ASPECTS OF HANDLING VELOCITY INVERSION AND HIDDEN LAYER PROBLEMS IN SEISMIC REFRACTION WORK (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: There are two types of masked layers in seismic refraction work: the velocity reversal (low-velocity layer) and the hidden layer (insufficient velocity contrast or layer thickness). On the basis of an analytical formulation of the general case of a masked layer under an overburden of plane and parallel multiple refractors the two limiting cases are discussed: the solution resulting from an uncritical interpretation of the measured time-distance curve and the blind zone solution. Between these two limiting cases there is a variety of possible masked layer solutions. These no-blind zone solutions—as well as the blind zone solution itself—are formulated separately for the velocity inversion and the hidden layer case.For the evaluation of some no-blind zone solution a diagram is presented which can be used for any case of multiple refractors in the overburden of the masked layer. However, it is only for the three- and the four-layer case that a blind zone interpretation by use of diagrams is advisable. Such diagrams are presented together with the basic sets of formulae which contain as parameters only ratios of velocities and layer thicknesses. As the velocity of the masked layer is usually unknown the diagrams are principally constructed to show the dependence on the masked layer velocity. This is useful for estimation of the largest possible error.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01336.x
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  • 2
    Electronic Resource
    Electronic Resource
    OFFSET CONTINUATION FOR SEISMIC STACKING (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Wave equation migration techniques have shown the limits of traditional stacking methods with data from tectonically complicated areas. An improved stack can be obtained utilizing the dip-moveout correction technique based on offset continuation. The properties and the limits of the algorithms used are summarized briefly.Several synthetic and real data examples are shown and compared with the results obtained using conventional processing in order to show the focusing effects and the strong improvement in signal-to-noise ratios, both at the stacked and migrated section level. The possibility of exploiting this technique to transform multiple coverage into increased spatial resolution is illustrated with examples.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01341.x
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  • 3
    Electronic Resource
    Electronic Resource
    OPTIMIZATION OF SHORT DIGITAL LINEAR FILTERS FOR INCREASED ACCURACY (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The accuracy of short length digital linear filter operators can be substantially increased if the sampling interval as well as the abscissa shift are properly adjusted. This may be done by a trial and error process of adjustment of these parameters until the error made by the filter operator, applied to a suitably chosen test function, is smallest.As an illustration of the application of this method, 7-, 11- and 19-point filters for the calculation of Schlumberger apparent resistivity from a known resistivity transform are designed. Errors with the new 7-point filter are seen to be less than those with a 19-point filter of conventional design. The errors with the new 19-point filter are two to three orders of magnitude smaller than those made by the conventional 19-point filter.The new method should provide digital linear operators that allow significant improvements in accuracy for comparable computation efforts, or substantial reduction in computation for comparable accuracy of results, or something of both.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01320.x
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  • 4
    Electronic Resource
    Electronic Resource
    TIME-TO-DEPTH MIGRATION USING WAVEFRONT CURVATURE (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A well-known technique for the migration of normal-incidence two-way travel-time maps is extended to common-source-point travel-time data. The travel time and the travel-time gradient are used to compute the parameters defining the tangent plane of the reflecting interface. It is also shown how the curvature matrix of the received wavefront can be used to compute the curvature of the reflecting interface. The method is initially derived for common-source-point data and then extended to common-midpoint data.In a three-dimensional medium the wavefront curvature matrix is computed by solving a 2 × 2 symmetric matrix Riccati equation. In a two-dimensional medium and in a medium with constant velocity gradient, the wavefront curvature matrix is computed by solving a scalar Riccati equation and two linear equations. The migration procedures are also simplified.When the velocity function is unknown, the migration procedures cannot be used. An inverse modeling algorithm which simultaneously performs the migration and estimates the velocity function must then be applied. Two different inversion schemes are discussed briefly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01305.x
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  • 5
    Electronic Resource
    Electronic Resource
    AN APPROACH TO THE IDENTIFICATION OF FINE SEDIMENTS BY INDUCED POLARIZATION LABORATORY MEASUREMENTS (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Time-domain-induced polarization (IP) laboratory measurements were performed on about 200 fine sediment samples with varying water content. The results permitted an analysis of IP properties of clays, loams, silts, and sands.Particular emphasis has been given to the analysis of the chargeability m as a function of lithotype and the water content.By analyzing decay curves, a new parameter was identified. It is a statistically specific characteristic of the lithotype and is independent of the water content. Therefore, it provides a diagnostic parameter for lithotype identification. In association with the values of chargeability and electrical resistivity, this parameter permits a reliable evaluation of water content and yields useful information about the porosity and permeability of the lithotype.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01310.x
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  • 6
    Electronic Resource
    Electronic Resource
    THE MAGNETOTELLURIC IMPEDANCE TENSOR—ITS RANDOM AND BIAS ERRORS (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Approaches to the reduction of bias in the computation of the elements of the magnetotelluric impedance tensor have been proposed in the past by several authors. In this paper a clear distinction is made between random errors and bias errors. No effort is made to reduce either, but the emphasis is on their estimation. Both types of errors depend critically upon the polarization of the magnetic field. The random error increases with increasing noise-to-signal ratio in the electrical field, and it is rather insensitive to noise in the magnetic field. The bias error increases with increasing noise-to-signal ratio in the magnetic field. Expressions for random errors and maximum bias errors are developed and discussed using a single station set-up. Random errors with a reference station set-up are also calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01298.x
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  • 7
    Electronic Resource
    Electronic Resource
    REPLY TO COMMENT BY A. T. BASOKUR (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01303.x
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  • 8
    Electronic Resource
    Electronic Resource
    A NOTE ON “ASPECTS OF VERTICAL SEISMIC RESOLUTION” BY O. KOEFOED (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01321.x
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  • 9
    Electronic Resource
    Electronic Resource
    A NUMERICAL METHOD TO COMPUTE THE RESISTIVITY TRANSFORM FROM WENNER SOUNDING DATA (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A numerical technique to compute the resistivity transform directly from the observed Wenner sounding data has been developed. In principle, the procedure is based on a decomposition method and consists of two steps: the first step determines a function that approximates the apparent resistivity data and the second step transforms this function into the corresponding kernel by an analytical operation. The proposed method is tested on some theoretical master curves. A high degree of precision is achieved with very little computer time. The applicability is shown on two field examples.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01346.x
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  • 10
    Electronic Resource
    Electronic Resource
    APPLICATION OF BOREHOLE GEOPHYSICS AT AN EXPERIMENTAL WASTE STORAGE SITE (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A suite of electrical, radiation, and mechanical borehole probes were run in a 76-mm-diameter borehole drilled to a slant depth of 380 m in leptite and granite. The hole is located in Precambrian bedrock in central Sweden where a site is dedicated to in-situ experiments pertaining to the disposal of radioactive wastes. The challenge to borehole logging methods for such site investigations is to resolve geological features and fluid flow parameters in geological sites which are initially chosen for their homogeneity, low porosity, and minimal fracturing. The Stripa borehole is characterized by high electrical resistivity values in the 20–100 kΩm range, by acoustic velocities around 5800 m s-1 (which is close to laboratory values on intact specimens), and by total porosity of around one volume percent. In this context, probe resolution was adequate to produce interpretable information on almost all of the logs.Two principal rock types were encountered in the hole: granite, of quartz monzonitic composition, and leptite. The granite and leptite intercepts are subdivided into units characterized by mafic mineral content, sulfide mineral content, and electrical and radiation properties. Iron-rich zones in the leptite are highly anomalous on the gamma-gamma and neutron logs; thin mafic zones in the granite can also be distinguished. Occurrences of a few percent pyrite are detected by the electrical, gamma-gamma, and neutron logs. Although overall porosity is quite low throughout the hole, analysis of the resistivity and neutron logs indicates the porosity increases by a few volume percent at fracture zones. The differential resistance and caliper probes detect borehole diameter roughness of less than 1 mm, helping to confirm acoustic waveform anomalies which are indicative of fracture zones. Compres-sional wave transit time and shear-wave interference patterns usually occur coincident with open fractures observed in core, the correlation being especially good at major fracture zones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01347.x
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