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  • 1
    Electronic Resource
    Electronic Resource
    Decorrugation and removal of directional trends of magnetic fields by the wavelet transform: application to archaeological areas (2003)
    9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Publishing Ltd.
    Geophysical prospecting 51 (2003), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The problem of removing directional trends frequently occurs in the processing of magnetic data and also in the subsequent steps of data interpretation. The so-called corrugations are typical directional trends occurring in levelled data, which may be removed in several ways. Classical techniques are based on high-pass filtering of the data and successively filtering these transformed data with directional cosine filters. Other linear features are due to real sources, such as pipelines in shallow surveys or dike swarms in regional surveys. They should, nevertheless, be considered as noise, due to the fact that their effect is strong and tends to hide the field features related to structures of more interest. We deal with both kinds of problem, presenting the results of a study in an archaeological area of southern Italy. Decorrugation of magnetic field anomalies is performed using a method based on the excellent space–frequency localization properties of wavelet bases, allowing a very sharp filtering of the field along a selected direction. We compare this technique with the classical one in a synthetic case and find that the wavelet decorrugation is simpler and produces low distortion maps. Besides the field decorrugation, the wavelet approach was also shown to be useful in the subsequent enhancement of the measured field. In fact, we show that the wavelet analysis offers a unique framework where various filtering problems (directional, isotropic, global or local as well) may be easily solved. As regards the archaeological case, strong noisy effects from elongated sources (pipelines) were successfully removed in a sharp and local way.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.2003.00373.x
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  • 2
    Electronic Resource
    Electronic Resource
    On the quantitative interpretation of magnetic anomalies by pseudo-gravimetric integration. (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Terra nova 1 (1989), S. 0 
    Details
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Poisson's theorem relates linearly the derivative of the gravity potential (taken along the total magnetization vector direction) and the magnetic potential due to a common, isolated source with constant density and magnetization distributions.From that theorem, two very useful functional transformations for magnetic anomalies were formulated by Baranov, that is the ‘pseudo-gravimetric integration’ and the ‘reduction to the pole'.In this paper, it is shown that the first transformation is, in some sense, more useful than the reduction to the pole and, further, that its utilization presents noticeable advantages as regards the interpretation of magnetic anomalies. In fact, the pseudo-gravimetric integration, in contrast to the reduction to the pole, also provides a smoothing of the anomaly map and, in particular, reduces the degree of non-linearity existing in the mathematical relationship between the data and the source depth.In order to show this last property of the transformation, spectral expansion inversions by 21/2D structures were performed on both magnetic and pseudo-gravimetric synthetic anomalies and the respective speed and precision were compared. The tests clearly indicate that very accurate estimates of the geometric parameters of the known source can be more easily and rapidly calculated by the transformed anomalies. Furthermore, due to its smoothing property, the method seems particularly useful when data are noisy. Tests on real anomalies confirm these results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3121.1989.tb00433.x
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  • 3
    Electronic Resource
    Electronic Resource
    A method to estimate the total magnetization direction from a distortion analysis of magnetic anomalies (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 42 (1994), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Knowledge of the declination and inclination of the total and induced magnetization vectors is normally required for the interpretation and analysis of magnetic anomalies. A new method of estimating the direction of the total magnetization vector of magnetized rocks from magnetic anomalies is proposed. The unknown declination and inclination (D*T and I*T) can be found by applying a reduction-to-the-pole operator to the measured anomalies for different couples of total magnetization direction parameters (DT and IT) and by observing the variation of the anomaly minimum as a function of both DT and I*T.and D*T are estimated using the maximum of this function. Comparing our method to previous methods, one advantage is that our estimates are not zero-level dependent; furthermore, the method allows inclinations to be well estimated, with the same accuracy as declinations; finally declinations are not underestimated. Our method is applied to a real case and meaningful results are obtained; it is shown that the feasibility of the method is improved by removing the low-frequency components.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1994.tb00209.x
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  • 4
    Electronic Resource
    Electronic Resource
    Wavelet analysis for the regional-residual and local separation of potential field anomalies[ Received ] (1998)
    Oxford BSL : Blackwell Publishing Ltd
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A method based on the discrete wavelet transform was applied to the regional-residual separation of potential fields and to the filtering of local anomalies. A specific space-scale wavelet analysis, called multiresolution analysis, allowed decomposition of the signal with respect to a vast range of scales. Different analysing wavelets were applied to anomalies in both synthetic and real cases, but the more appropriate basis needed to be chosen by requiring the maximum compactness for the multiresolution analysis. Moreover, since such analysis was found not to be shift-invariant, the same criterion was applied to choosing the best signal shift. Application of the technique to both synthetic and real cases produced an optimal space-scale representation of the fields and a consistent regional-residual separation. Furthermore, the space localization allowed a variety of filtered signals to be obtained, each one with a specific scale and local area content. Fourier and wavelet analyses were both applied to the filtering out of the intense Etna anomaly from the aeromagnetic field of Sicily. The wavelet method was more powerful, suppressing only the Etna volcano anomaly and leaving the rest of the map practically unchanged.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00105.x
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  • 5
    Electronic Resource
    Electronic Resource
    Estimation of depth and magnetization of magnetic sources from magnetic data: The linearized continuous inverse problem for 21/2D structures (1990)
    Springer
    Pure and applied geophysics 134 (1990), S. 451-471 
    Details
    ISSN: 1420-9136
    Keywords: Magnetic data ; continuous case ; forward problem 21/2D ; inversion ; linearization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The estimation of the depth to the top and bottom of a magnetic source from magnetic data defines a nonlinear inverse problem, while the evaluation of the distribution of magnetization determines a linear inverse problem. In this paper, these interpretation problems are resolved in the continuous case of 21/2D magnetized bodies with lateral magnetization variations. A formulation of the magnetic problem accounting for different directions of remanent and total magnetization vectors and including a more general definition of apparent susceptibility is presented. Differences between 2D and 21/2D formulations are stressed, as regards the anomaly amplitude, shape and zero-level. In order to utilize well-known continuous linear inverse methods, Fréchet derivatives of the data functionals with respect to the depth of the source top and bottom, are analytically described. Thus, using the spectral expansion inverse method (Parker, 1977) and linearizing the problem at several steps of an iterative process, the source depth is obtained within a few iterations, although the starting model is distant from the final solution. The interpretation of an anomaly in the Italian region shows the usefulness of the method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00878742
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    Paper (German National Licenses)
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