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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 28 (1980), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The Hankel transform theorem can be applied to the inversion of gravity data for the buried sphere, the horizontal cylinder, and the vertical rod. This new approach leads to exact solutions of the transforms for the assumed bodies. A comparison with the classical procedure by Fourier transform reveals that for the sphere and the vertical rod, the Hankel transform is preferable.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 34 (1986), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A contribution is given to the solution of the problem of filtering noise-degraded Schlumberger sounding curves. It is shown that the transformation to the pole-pole system is actually a smoothing operation that filters high-frequency noise. In the case of residual noise contamination in the transformed pole-pole curve, it is demonstrated that a subsequent application of a conventional rectangular low-pass filter, with cut-off frequency not less than the right-hand frequency limit of the main message pass-band, may satisfactorily solve the problem by leaving a pole-pole curve available for interpretation. An attempt is also made to understand the essential peculiarities of the pole-pole system as far as penetration depth, resolving power and selectivity power are concerned.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 26 (1978), 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 25 (1977), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The polarization content of a medium, in both the time and frequency domains, can be described by parameters which differ in inherent physical meaning and their practical significance.For real situations, general expressions for the apparent parameters, previously determined for both domains, exist for the general case of soundings on a horizontally multi-layered earth.The comparative analysis of these expressions, here restricted to the simple case of a two-layered earth, shows that the theoretical sounding curves of the frequency-domain are different from those of the time-domain. In particular, for every resistivity or chargeability contrasts examined, the apparent frequency-effect curve lies always over the corresponding apparent chargeability curve, but both curves reach the same asymptotical values for shortest and largest spacings.The important conclusions which can be drawn from this result is that both techniques are suitable to investigate subsoil polarizability anomalies. However, from a practical point of view, it is more convenient to adopt the frequency-domain technique when the polarizability increases with depth, while, on the contrary, the time-domain technique is more efficacious when the polarizability decreases with depth.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 23 (1975), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: In this paper a numerical method of direct interpretation of geoelectrical soundings is described. It has similarities with already existing direct methods, but owing to its simplicity and, in particular, to the possibility of applying it also without digital computers, it proves useful mostly in the field, where very often an accurate method for the interpretation of multi-layer curves is required. The direct interpretation system splits up into three steps: i) the evaluation of the resistivity transform after application of Hankel's inversion theorem; ii) the determination of the layer distribution after application of Koefoed's recurrent procedure; iii) the control of the solution. Each step is considered and the practical procedures suggested. Finally two field examples are presented and discussed.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 24 (1976), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Following up our recent study of an indirect procedure for the practical determination of the maximum frequency-effect, defined as fe = 1 − pρ∞/ρdc with ρ∞ the resistivity at infinite frequency, we show at first how, through the Laplace transform theory, ρ∞ can be related to stationary field vectors in the simple form of Ohm's law. Then applying the equation of continuity for stationary currents with a suitable set of boundary conditions, we derive the integral expression of the apparent resistivity at infinite frequency ρ∞,a in the case of a horizontally layered earth. Finally, from the definition of the maximum apparent frequency-effect, analytical expressions of feα are obtained for both Schlumberger and dipole arrays placed on the surface of the multi-layered earth section in the most general situation of vertical changes in induced polarization together with dc resistivity variations not at the same interfaces. Direct interpretation procedures are suggested for obtaining the layering parameters directly from the analysis of the sounding curves.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 22 (1974), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A method is described to transform a dipole sounding curve, obtained with any one of the common dipole arrays over a horizontally layered earth, to the form of a Schlumberger sounding curve. Starting from the general expression which relates the dipole apparent resistivity to the Schlumberger apparent resistivity and its derivative with respect to the spacing, it is possible with some approximations to derive an easy numerical computation procedure in order to perform the transformation. The applicability of the method is discussed briefly.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 44 (1996), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: We investigate the magnetotelluric response of two-dimensional bodies, characterized by the presence of low-frequency dispersion phenomena of the electrical parameters. The Cole-Cole dispersion model is assumed to represent the frequency dependence of the ‘impedivity’ complex function, defined as the inverse of Stoyer's ‘admittivity’ complex parameter. To simulate real geological situations, we consider three structural models, representing a sedimentary basin, a geothermal system and a magma chamber, assumed to be partially or totally dispersive. From a detailed study of the frequency and space behaviours of the magnetotelluric parameters, taking known non-dispersive results as reference, we outline the main peculiarities of the local distortion effects, caused by the presence of dispersion in the target media. Finally, we discuss the interpretative errors which can be made by neglecting the dispersion phenomena. The apparent dispersion function, which was defined in a previous paper to describe similar effects in the one-dimensional case, is again used as a reliable indicator of location, shape and spatial extent of the dispersive bodies. The general result of this study is a marked improvement in the resolution power of the magnetotelluric method.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 42 (1994), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: We investigate, from a theoretical point of view, the possibility of performing marine two-level magnetovariational measurements. An apparent resistivity function is denned and calculated after solving the differential equation governing the behaviour of the natural magnetic field variations inside a one-dimensional earth. In order to generalize the problem, a frequency-dependent resistivity is assumed to characterize the layers and the distortions caused by the polarization effects are carefully analysed. The computation of three-layer amplitude and phase diagrams for the apparent resistivity function shows that, in the case of an intermediate polarizable layer, sandwiched between a non-dispersive overburden and substratum, the H-type sequence results are the most affected by the dispersion phenomenon as it occurs in magnetotellurics. Finally we consider the problem of the sensitivity of the method, since, in practice, it requires top and bottom sensors separated by a vertical finite distance. It is found that in the higher-frequency range, due to the strong attenuation of the relative components of the field, the depth of the bottom sensor must be small enough to guarantee detectable signals, well above the full-scale resolution of the acquisition system. Conversely, in the lower-frequency range such a depth must be large enough to allow the difference between the top and bottom signals to be above the same recording sensitivity threshold.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 34 (1986), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
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